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Overview of Human Anatomy and Physiology

Learning Objectives

At the completion of this chapter the student will be able to:

1.Define anatomy and physiology, and list the structural levels of organization in the human body.

2.Differentiate and understand the characteristics of living organisms.

3.Describe the role of homeostasis in the human body, and provide specific examples.

4.Identify the major organ systems of the body and describe their purpose.

5.Use proper anatomical terminology to describe the body and relative positions.

6.Discern and appreciate the types of movement of the body and their proper names.

Case Study Introduction

It’s a cold, rainy Monday night and you just have one of those bad feelings. You are a Paramedic, and this feeling comes all too often. Suddenly, you and your partner are called to the scene of an automobile accident that has just occurred on the major highway leading into town.

As you arrive on scene, you find the driver unconscious and still in the car. On primary assessment, you notice that she is still breathing, though with difficulty, on her own. You see blood around her nose and mouth, and you hear a gurgling sound as she breathes. You also notice that she is wearing her seatbelt, but the vehicle air bags did not deploy.

Another person who witnessed the accident tells you that she saw the driver try to steer away from a truck that had lost control in the rain. However, the driver of the car, in the midst of maneuvering, also lost control of her vehicle and ended up veering off the highway, spinning out of control, and hitting the guardrail directly head-on before coming to a stop.

As your partner sets up the medical equipment, you take a secondary assessment. The victim’s breathing is shallow and rapid, with multiple lacerations on her face. She has a thready radial pulse of 150 beats per minute, and her blood pressure in 90/50. Respirations are at 28 breaths a minute, diminished on the right side.

Your partner hands you a cervical collar and you place it on the victim, stabilizing her cervical spine. You crew arrives and together the team pulls the victim out of the vehicle and places her on a backboard, transporting her to the ambulance. Inside the ambulance, you begin a more detailed survey, monitoring her vitals and looking for more injuries. You notice a large contusion on her sternum and several lacerations superior to the antecubital fossa, and on the medial aspect of her right arm. She also has a deformity on the lateral aspect of the left thigh. While you start all life-saving protocols, including IV fluids and an ECG, your partner calls the nearest hospital with the report you have given him.

 ■ Introduction

When studying the human body as a whole, a basic and step-by-step progression is the most logical approach. The human body is a complex and dynamic system composed of many different structures and functions. Therefore, a necessary and thorough understanding of the human body’s anatomy and physiology is essential.

It is wise to always begin the study of any subject by first defining that subject. What is anatomy and physiology? Anatomy, by definition, is simply the study of a living organism’s structure or its form, while physiology is the study of its function. More simply, anatomy is the study of what an organism looks like, and physiology is the study of how that living organism operates. Remember this: throughout the course of Anatomy and Physiology, you are simply learning form and function.

Anatomy is further divided into gross anatomy and microscopic anatomy. Gross anatomy is studied through the dissection of anatomical structures visible to the naked eye. Microscopic anatomy, true to its namesake, is the study of very small anatomical structures that require the assistance of a microscope to be adequately seen. Moreover, it becomes sensible to classify the human body from its most simple to its most complex level of structural organization.

Figure 1-1 Organ systems.

Here we have one of the most basic, yet, extremely important aspects to the study of anatomy and physiology, (and hence a favorite test question for instructors and examiners worldwide); the order of the levels of structural organization of the human body, from the simplest to the most complex as follows:

The following is an overly simplified, yet accurate example of the above: Atoms of hydrogen and oxygen form and function together to produce a molecule of hydroxide. Molecules (for example, hydroxide and phosphate) form and function together to produce the phospholipid bilayer of a cell.

A group of cells known as stratified squamous cells form and function together to produce a tissue called epithelium.

The epithelial tissue forms and functions together to produce an organ called skin. The skin, hair, and nails form and function together to produce an organ system called the integumentary system. The integumentary system forms and functions together with ten other organ systems to produce an organism – the human being.

Next, to fully understand the human as an organism, we must first look at the big picture, an overview if you will, of the body’s form and function. One of the most fundamental concepts to keep in mind during your investigation of the human body it to remember that the body is a dynamic, living organism – that is the human body’s structure and functioning must be maintained at all times via self-sustaining processes.

 ■ Characteristics of Living Organisms

As previously noted, when studying the human body, it’s imperative to remember that it’s a living organism. The human body must be distinguished from non-living objects, not simply for mere classification purposes, but for a proper and systematic understanding of the dynamics regarding the actual anatomy and physiology involved in the human body.

Six characteristics said to be required of livings organisms are discussed below.

All living things are composed of one or more cells. All organisms start from a single cell, which will divide several times until differentiation is completed, where the cell’s form and function change – producing various kinds of cells to carry out diverse purposes within the organism.

Note: Viruses are a debatable omission; they are not composed of cells, but rather exist and function in a “host” cell. However, they are still classified as living organisms.

All living things are organized.1 The complexity of a living organism is assembled from microscopic, albeit structured levels, grouping common forms and functions together. Macroscopically, they become observable as the cells are structured to allow for the most physiological sense of the organism. Here, another essential premise in anatomy and physiology should be made: Form always follows function, meaning that organisms are structured conducive and according to how they work – not vice versa.

All living organisms require the use of energy.2 A chemical process by which nutrients are absorbed and converted into energy to be used at the cellular level is called metabolism. This energy is needed to perform each and all of the cell’s functions. The human body cannot produce all of the required nutrients organically, and therefore, must ingest nutrition from other sources for this purpose.

All living organisms grow and develop.3 Each cell undergoes a cell cycle, where it grows and divides to form another indistinguishable, duplicate cell. Following specific instructions from the organism’s DNA, or genetic code, differentiation transforms an organism’s cells into different types, making a more complex organism. Development is the growth, maturation, and transformation of an organism.

All living organisms reproduce. Reproduction is essential to the prolongation of a species existence. In sexual reproduction, there is a joining of the DNA of two organisms at the cellular level. Sexual reproduction is the form in which the human species reproduce.

All living organisms must possess a normal and stable internal environment; this process is called homeostasis.4 The internal environment is a matter of physiologic components that include temperature, water regulation, pH balance, heartbeat, sleep, energy, and blood pressure, as well as other conditions. Homeostasis is preserved through a complex system of checks and balances in human beings which is discussed further.

Homeostasis

Homeostasis is defined as maintaining a normal, stable, internal environment. The human body directs a multitude of highly complex interactions in order to sustain balance or to return its operating organ systems to their normal, standard level of functioning. These complex interactions facilitate compensatory changes accommodating the physical and psychological functioning needed for survival.

All homeostatic mechanisms have at least three separate but codependent modules for regulating and controlling the variables involved in their respective homeostatic processes.5

The “receptor” is the sensing module that oversees and reacts to changes in the internal environment. When the receptor senses a stimulus, it responds by sending the appropriate information to a second module, the “control center” (the brain in humans).

The control center sets the scope of which a variable is maintained and regulates an appropriate response to the stimulus, which signals the third module, an “effector”, to correct the abnormality by either augmenting it with positive feedback or diminishing it through negative feedback.

Figure 1-2 Homeostasis.

For example, when body temperature rises due to external environment, the nervous system triggers blood vessels to dilate and sweat glands to secrete. Under comfortable condition, sweat glands might secrete half a liter of sweat daily. On a hot day, sweat glands can secrete as much as 12 liters per day. It’s the evaporation of sweat from the surface of the skin that cools the body through dissipation of body heat.

Mechanisms involved in homeostatic controls, such as body temperature controls, are known as positive or negative feedback.

Positive feedback

Positive feedback mechanisms are intended to promote or enhance the body’s response to a stimulus that has already been activated. Contrary to negative feedback mechanisms, which initiate a response to return physiological functions within the body’s set and normal range, the positive feedback mechanisms are actually designed to force and keep physiologic functions out of normal ranges.

Figure 1-3 Wound healing.

In order to accomplish this, a sequence of events triggers a respective physiological process through a cascading progression that enhances the effect of the stimulus.

One example of a positive feedback loop mechanism that can be observed in the body is platelet accumulation during a blood clotting episode in response to a break or cut in the lining of blood vessels.

Another example is the release of the hormone oxytocin, which triggers uterine contractions in a female in order to promote the delivery of a newborn that takes place during childbirth. Oxytocin also influences breast milk secretions.

Negative feedback

Negative feedback mechanisms exist and operate to decrease activity of any organ or organ system in an effort to revert it to its normal range of functioning.6 A great example and common method of doing this is the regulation of blood pressure. Stretch receptors in blood vessels can sense an increase in the resistance of blood flow against the walls during a period of increased blood pressure.

The blood vessels, acting as receptors, receive the increased pressure as a stimulus and signal this message to the brain, the control center. The brain then transmits a message to the heart and blood vessels, both of which then respond as effectors. The heart rate decreases and vasodilation (expansion in blood vessel diameter) occurs. The combination of this physiologic response would cause the blood pressure to decrease and return within its normal range.

The opposite occurs during a sudden decrease in blood pressure; blood vessels now sense the decrease in resistance and signal the brain, which relays the message back to the heart and blood vessels. The heart rate would increase, and vasoconstriction (narrowing of the blood vessels) will occur – ultimately raising blood pressure back to its normal physiological range.

Another excellent example of the negative feedback loop mechanism is seen when the human body is deprived of nutrition. The body, in protective mode, will reset the metabolic rate to a point much lower than its norm. This approach allows the body to continue to function and complete all of its normal necessary physiological functions, albeit at a slower rate, even though the body is starving.

This is why people who drastically reduce their caloric intake while trying to lose weight find it easy to lose the weight initially, but notice it becomes much harder to lose more weight after some time passes. This is due to the body readjusting itself to function at a lower metabolic set point in order to allow for survival than with a lower than normal supply of energy. Exercise increases the body’s caloric expenditure, and can alter this effect by exogenously increasing the metabolic demand.

One simpler, yet effective example of the negative feedback mechanism is temperature regulation. The hypothalamus, which monitors the body’s temperature, is highly proficient at detecting even the slightest deviation of normal body temperature (37°C/98.6°F). The response to such deviation would be stimulation of sweat glands to produce sweat in an effort to reduce temperature by the cooling effect of evaporation, or signaling various muscles in the body to contract rapidly, or shiver, in an effort to produce heat and increase body temperature.

Both feedback mechanisms are correspondingly essential for normal healthy functioning of the human body. Complications, disease, aging processes, and even death can occur if either of the two feedback mechanisms are distorted in any way.

 ■Organization of the Human Body

The human body contains two major body cavities – the ventral cavity and the dorsal cavity.

These two cavities are further subdivided and structured to organize and classify the body’s internal organs.7

Body cavities

The dorsal cavity contains two sub-cavities, the cranial cavity and the spinal cavity. Technically speaking there is no anatomical division as the cranial cavity flows directly into the spinal cavity. However, they are separated into two distinct cavities to assist in study and analysis.

Figure 1-4 Muscle, nerve, and blood vessel connections.

The cranial cavity is a fluid-filled spaced inside the skull occupied by the brain. The spinal cavity encompasses the spinal cord, which travels down the posterior aspect of the body and is continuous with the cranial cavity (the spinal cord is attached directly to the brainstem), extending down toward the base of the spine.

The ventral cavity also includes two sub-cavities, the thoracic cavity and the abdominopelvic cavity. These two cavities are separated anatomically by the diaphragm, with the thoracic cavity residing above the abdominopelvic cavity.

The thoracic cavity holds the heart, lungs, thymus, lower one-third of the esophagus, and an irregularly shaped, central compartment called the mediastinum.

The abdominopelvic cavity is further subdivided in some texts into the abdominal cavity and the pelvic cavity, with the caveat that there is no real anatomical partition separating the two cavities.

The abdominal portion contains both the main and accessory organs of digestion (stomach, small and large intestines, liver, pancreas, and gallbladder), as well as the kidneys and ureters. The pelvic portion is surrounded and guarded by the pelvic girdle, and encloses mainly the reproductive organs and the bladder.

Serous membranes

At this point, discussion of serous membranes, also called serosa, of the ventral cavity is appropriate. A membrane is a thin, flexible lining of tissues that secretes a lubricating fluid. Serous membranes form the lining that insulates each body cavity and all internal organs.8 The membrane itself is composed of two types of tissue – a superficial epithelial layer, called the mesothelium, and a deeper layer composed of connective tissue.

The epithelial layer is a single layer of simple squamous epithelial cells that have no blood supply. This layer is responsible for secreting the lubricating serous fluid. The deeper, second layer is composed of connective tissue, appropriately named the connective tissue layer, which provides vascular and nerve supply to the epithelial layer. The connective tissue layer also functions to bind the membrane to organs and cavities.

When a serous membrane covers and lines an organ, it is generally called a visceral membrane. When it covers and lines a body cavity, it is generally called a parietal membrane.

Note: Serous fluid is secreted and lubricates the space that exists between these two membranes.

Specific names are given to serous membranes in the ventral cavity when the membrane covers and lines a particular organ or cavity. For example, the serous membrane that coats the thoracic cavity is called the parietal pleura. Parietal distinguishes that it is referring to a cavity, and pleura distinguishes that it is referring to the thoracic cavity. Moreover, the serous membrane that lines the surface of each lung, which of course is an organ, is called the visceral pleura.

The term visceral distinguishes that it is referring to a particular organ – the lung – the term pleura again distinguishing that it remains located in the thoracic cavity. The area flanked by these two cavities, called the pleural space, is lubricated with serous fluid to protect the linings from the friction caused by the expansion and contraction of the lungs during breathing. This is similar to engine oil lubricating the cylinder within which a piston pumps up and down during internal combustion of a gasoline engine.

Another specific serous membrane is the lining or covering of the heart, called the visceral pericardium. The serous membrane that coats the pericardial cavity (the area that encases the heart) is called the parietal pericardium. (Remember that the term visceral defines the membrane that is covering an internal organ, and the term parietal means that the membrane is lining a cavity.)

In an effort to reduce the friction caused by the heart pumping inside its cavity, there exists a space joining these two membranes where another serous fluid is secreted.

The abdominal cavity is lined with a serous membrane called the peritoneum. The membrane overlaying each abdominal visceral organ is called a visceral peritoneum, whereas the parietal peritoneum is the membrane that insulates the entire abdominal cavity. The region conjoining these two membranes is called the peritoneal cavity, which too, is lined with serous fluid for lubrication purposes.

Figure 1-5 Circulation overview.

 ■Organ Systems Overview

The human body is made up of several organ systems that function together as one complex unit. Traditionally, the body is divided and studied in eleven different organ systems. Below is an overview of those organ systems, each with the respective structures involved and a brief description of the functions they perform.

Circulatory system

The body’s circulatory system, sometimes called the cardiovascular system (in an effort to distinguish it from some texts who include the lymphatic system as part of the circulatory system), is formed by the heart, blood, and blood vessels.

For our purposes, the anatomy and physiology of the body’s lymphatic system will be discussed separately in a later chapter.

The cardiovascular system’s main function is to pump blood, delivering oxygen and essential nutrients to all living cells via the channels of blood vessels throughout the body.

The heart’s cardiac output, or volume of blood that is pumped per minute, is an excellent way of measuring how well the heart is functioning.

The three main types of blood vessels, which are discussed in greater detail later, are the arteries, veins, and capillaries. The main component of the human body’s circulatory system, the cardiovascular system, is a closed organization, meaning that the blood never leaves the network of blood vessels. Instead, nutrients, gases, and hormones diffuse across the membrane of the cells in the capillaries and flow into the interstitial fluid (essentially lubricates cellular structures, filled with components such as amino and fatty acids, sugars, and regulatory substances), where they are passed along to target tissues and ultimately the target cells.

Figure 1-6 Digestive organs.

Digestive system

The digestive system, or gastrointestinal system as it’s sometimes called, is formed by the major organs of digestion – the stomach, small and large intestines, and rectum plus the accessory organs of digestion – the teeth, salivary glands, liver, gallbladder, and pancreas. Each of these organs will be described in greater detail in Chapter 10: The Digestive System. Also included are the mouth (oral cavity), esophagus, and anus. Together, these structures function to digest food and excrete waste.

Digestion is the mechanical and chemical breakdown, or catabolism, of food into smaller macronutrients so that they are more readily absorbed.

Mechanical digestion is initially performed by the teeth through chewing. Chemical digestion is initially started by the secretion of saliva, which contains various enzymes (such as amylase) that begin to break down food as soon as it enters the oral cavity.

As food is passed into the esophagus and into the stomach, hydrochloric acid and enzymes continue the chemical digestion process of food breaking it down into a thick liquid known as chyme. Chyme will eventually make its way to the small intestine, where a majority of the nutrients are absorbed, then pass through the large intestine, and finally be excreted as waste material from the rectum and anus by defecation.

Endocrine system

The endocrine system is an organization of glands which regulate various functions in the human body through the use of chemical compounds.9 These chemical compounds or messengers are called hormones. They act as an information signaling system similar to the body’s nervous system.

In contrast to the nervous system, the endocrine system’s effects are initiated in a much slower and gradual manner though their effects are longer lasting, sometimes prolonged over a period of weeks.

The endocrine system, which is ductless, must be distinguished from the exocrine system, which secretes its chemicals through ducts. Exocrine glands secrete onto the skin or into a body cavity, such as sweat.

Hormones are released from endocrine glands directly into the bloodstream and travel to target tissues to generate a specific response. They regulate metabolism, growth and development, tissue function, sexual development, and mood.

Figure 1-7 Hormone (endocrine) system.

The physiological levels of hormones and the functions they perform are age-dependent and change over the course of an individual’s lifespan.

Figure 1-8 Skin anatomy.

Specific glands and secretions will be further detailed in Chapter 13: The Endocrine System. Some of the glands involved in the endocrine system include the:

•Pituitary gland

•Pineal gland

•Thyroid gland

•Parathyroid glands

•Adrenal glands

•Hypothalamus

In addition to these glands, the body contains many other organs that have secondary endocrine functions, such as the heart, kidneys, reproductive organs, stomach, pancreas, liver, and intestines.

Integumentary system

The integumentary system is composed of the skin, hair, and nails. The name is derived from its Latin origin integumentum, which meant “to cover”. The skin, hair, and nails do in fact cover the body, as one of its major functions is to serve as a protective barrier against germs, heat or cold, and help cushion internal organs against injury. The integumentary system also functions to:

•regulate temperature

•receive external stimuli such as pressure, pain, and vibration

•aid in the synthesis of vitamin D from sunlight exposure

The skin is known to be the largest organ of the human body, as it comprises approximately fifteen percent of total body weight.10 The skin is organized into two layers, called the epidermis and the dermis.

The outer layer, the epidermis, is a major abneural and avascular layer of stratified squamous epithelial cells, which are further organized into five minor layers, or strata11:

•Stratum corneum

•Stratum lucidum

•Stratum granulosum

•Stratum spinosum

•Stratum basale

It receives nourishment from the lower major layer, the dermis.

The dermis is composed of what anatomists refer to dense irregular connective tissue, called collagen and elastin.

These tissues allow for both the integrity and the flexibility of the skin. The dermis also has a nerve and vascular supply, and is the base for the other structures in the integumentary system, such as the hair and nails.

It should be noted here that some texts refer to the skin as having “three” layers. These texts are referring to a layer deep to the dermis, called the hypodermis. Technically speaking, however, the hypodermis should not be included as a segment or layer of the skin, nor part of the integumentary system.

The hypodermis, also called the subcutaneous layer, is primarily composed of adipose tissue or body fat. Its primary function is simply insulation and storage of energy.

Immune system

The immune system is a multifaceted biological system of structures and processes that serve to protect the human body from disease.12 The primary structures that form the immune system are the bone marrow and the thymus gland. Organs and functions of the immune system will be discussed in further detail in Chapter 8: The Lymphatic and Immune Systems. The secondary structures are:

•Lymph nodes

•Tonsils

•Adenoids

•Spleen

•Leukocytes (white blood cells)

Figure 1-9 Immune system cells.

These structures function together to produce a wide array of defensive mechanisms that serve to protect the human body from a diverse population of pathogens, or infectious agents. The human body’s immune system can be classified into two components: natural immunity and acquired immunity.

Natural immunity, also termed the innate immune system is the dominant defense component of the human body’s immune system.

It is a general, non-specific defense system that does not target specific pathogens, but rather responds to invading pathogens in an immediate and universal “attack-all” approach.

Individual elements of the innate immune system include the body’s inflammatory response, the “complement system”, and leukocytes, or white blood cells. Acquired immunity, also called the adaptive or specific immune system, is activated by the innate immune system to mount a highly specialized immune response, capable of recognizing, targeting, and remembering specific pathogens. This system is dynamic and adaptable, and has the ability to mount a stronger, more specific immune response due to its immunological memory.

Cells of the adaptive immune system “remember” specific pathogens. If such pathogens enter the body more than once, these “memory cells” quickly recognize and target the respective pathogens and eliminate it. Individual constituents of the adaptive immune system include specific leukocytes called lymphocytes (B cells and T cells), as well as antiantibodies, also called immunoglobulins. For example, vaccines induce adaptive immunity.

While an exceptionally complex and highly specialized system, the human body’s immune system is not without fault; disorders of the immune system are the reason for allergies, hypersensitivity reactions, autoimmune disorders, tumors, and other devastating diseases.

Muscular system

The muscular system is the organ system that allows the body to move, keep balance, control posture, and provide heat to keep the body warm.

Three distinct types of muscle tissue – skeletal muscle, cardiac muscle, and smooth muscle are common in the human body.

Each muscle type has a different form, and therefore provides a different, yet essential function(s). Skeletal muscle and cardiac muscle tissue have striations, or a series of linear markings, visible under a microscope. Smooth muscle, as its name suggests, does not. Smooth muscles are employed to automatically regulate the secretion and release of substances, such as acid, enzymes, and bile from the lumen of hollow organs such as the stomach, intestines, and gallbladder.

Skeletal muscle, also called voluntary muscle, is under conscience control. Muscles are innervated by nerves and in direct communication with the peripheral nervous system to receive electrical impulses from the brain, telling the muscles to contract.

Skeletal muscles are connected to bones, organized in opposing groups centered on a joint, which allows for a variety of movements by the human body.

Figure 1-10 Types of muscle tissue.

Cardiac muscle, like smooth muscle, is not under conscious control and is only found in the heart. Its main function is to contract the chambers of the heart to circulate blood throughout the body. A unique feature of cardiac muscle are intercalated discs, observable under a microscope, that allow for the synchronized contraction of cardiac tissue necessary for normal blood flow to occur.

Figure 1-11 Skeletal and muscle structure.

Nervous system

The nervous system is an organ system which, through a network of highly specialized cells called neurons and unique chemicals called neurotransmitters. Neurotransmitters transmit nerve impulses. Their function is to completely manage and direct the actions of the human body.

The form and function of the nervous system is divided into two main arrangements:

•Central nervous system (CNS)

•Peripheral nervous system (PNS)

The CNS is formed by the brain and spinal cord which receive and process information from the body. The PNS is formed by the cranial nerves, spinal nerves, and ganglia (groups of cell bodies) that function to transmit information between the body and the CNS.

Figure 1-12 Neurons.

The PNS is further divided into the autonomic nervous system (ANS), which regulates and controls the body below the level of consciousness (think “automatic” nervous system) and the somatic nervous system (SNS) which is under conscious or voluntary control. The ANS is also further subdivided into the sympathetic nervous system and the parasympathetic nervous system. A simpler way of appreciating this is through the visual aid of a flow chart:

Figure 1-13 Structures of the nervous system.

The CNS and PNS both function to transmit and receive electronic impulses between the central and peripheral divisions of the body. This is accomplished through the use of cells and nerves. Neurons and glial cells comprise the two main types of cells in the nervous system. Neurons, or nerve cells, are responsible for “communicating” with each other through connections known as synapses, or cell-to-cell junctions that rapidly transmit and receive chemical or electrical signals. Types of neurons and respective structures will be discussed in detail in Chapter 14: The Nervous System.

Glial cells (named from their Greek origin meaning “glue”) are supporting cells in the nervous system. Glial cells maintain nerve cells by gluing them in place, supplying them with nutrients, and removing pathogens and damaged or dead neurons.

One very important function of a particular glial cell, called an oligodendrocyte in the CNS and a Schwann cell in the PNS, is the production of myelin – a fatty sheath wrapped around the axons of neurons, electrically insulating them and allowing for more rapid and efficient transmission of impulses. The axons, long and slender projections of a neuron, are arranged and travel in bundles, making up the bulk of a nerve. Nerves as well as the individual parts of a nerve cell (the axon, cell body, and dendrites) will also be discussed in greater detail later.

Figure 1-14 Male reproductive organs.

Reproductive system

The reproductive system or genital system is a coordination of structures, glands, and hormones that function to together for the sole purpose of reproduction.

Human reproduction is a form of sexual reproduction conventionally by way of sexual intercourse, also called copulation or coitus, between a male and female.3

During sexual intercourse, a man’s erect penis is inserted into a female’s vagina until ejaculation, or the release or semen, takes place. Sperm from the semen travels up the vagina where it reaches the cervix and enters the uterus and eventually the fallopian tubes, where internal fertilization of the female egg cell, or ovum, occurs.

The process of internal fertilization and embryogenesis will be discussed in greater detail later. Nevertheless, unique from other organ systems, the reproductive systems allow for significant disparity between the two sexes.

All male sex organs are considered to be external genitalia and include the:

•testes

•vas deferens

•epididymis

•seminal vesicles

•prostate

•penis

The main male sex hormone associated is testosterone.

The female sex organs can be classified as either interior or exterior. The interior organs are the:

•ovaries

•fallopian tubes

•uterus

•cervix

•vagina

•mammary glands

Figure 1-15 Female reproductive system.

The exterior genitalia (collectively called the vulva) are the introitus, the labia minora and majora, the clitoris, and Bartholin’s glands.

The major female sex hormones associated with the reproductive system are estrogen and progesterone.

These respective hormones allow for the development of proper secondary sexual characteristics (to be discussed in greater detail later) for the female and male.

Organs and functions of the male and female reproductive systems will be discussed in further detail in Chapter 15: The Reproductive System.

Respiratory system

The respiratory system is an anatomical assembly of structures, commonly separated into an upper and lower system that are responsible for breathing.

The structures included are the nose, pharynx, larynx, trachea, bronchi, lungs, and diaphragm. Breathing, more accurately termed respiration and ventilation, is the introduction and release of gases, oxygen and carbon dioxide, respectively, into and out of the body.

The human body requires oxygen for the survival of all living cells, and the byproduct of each cell’s use of oxygen is carbon dioxide, which must be released as it becomes toxic at certain levels.

Oxygen is exchanged for carbon dioxide in the respiratory system, hence is given the name – “gas exchange”.

Gas exchange is responsible for regulating an appropriate acid-base balance in the body, as part of homeostasis.

Respiration is under the direct control of the autonomic nervous system and is physiologically functional in the medulla oblongata and the pons, parts of the brain stem. Housed in this part of the brain stem is a series of interconnected neurons that make up the body’s respiration regulatory center. Inspiration, also called inhalation, is an active process initiated by the body’s diaphragm.

Figure 1-16 The respiratory system.

The organs and processes involving respiration will be discussed in more detail in Chapter 9: The Respiratory System. For now, a brief description provides a foundation for the basic functions of the breathing process.

When the diaphragm contracts, it forces contents of the abdominal cavity downward, allowing the ribcage to be expanded, which creates room for lung expansion. This generates increased “thoracic pressure” (remember that term for future discussion) and allows air to flow into the lungs from the atmosphere. Oxygen is then passed down through to tiny sacs called alveoli, which allow for the exchange of oxygen with the uptake of carbon dioxide from the surrounding pulmonary capillaries.

Carbon dioxide is then released from the body via exhalation. Exhalation, also called expiration, is generally a passive process, meaning no muscular contraction is needed in order to it to occur. The lungs have natural elasticity, similar to a rubber band. As the lungs are filled, air stretches the lungs until their natural recoil threshold is met, causing the remaining gases to be exhaled until the lungs reach a state of equilibrium with the atmospheric pressure. Disorders of the respiratory system are studied by a branch of medicine called pulmonology.

Skeletal system

The skeletal system is an organ system composed primarily of bones responsible for the structural framework and support of the human body. An infant’s structure is not identical to an adult skeleton.13 At birth, an infant has more bones than an adult but over time, some of these bones mature and fuse together.

An adult human body has 206 bones that make up approximately 50% of the body’s total weight.14

The human skeleton is classified into two units: the axial skeleton and the appendicular skeleton.

The axial skeleton, named so because they refer to the bones in the vertical axis, is composed of 80 bones that include the vertebral (spinal) column, the rib cage, and the skull.15 The appendicular skeleton, named after the term appendage, is made up of the remaining 126 bones, including the bones of the shoulder girdle, the pelvic girdle, and the arms, legs, hands, and feet.16

Figure 1-17 X-rays of skeletal bones and joints.

Along with some auxiliary structures such as ligaments, tendons, joints, and cartilage – the skeletal system functions as a platform permitting movement, protection of organs, providing support and stability, storage of minerals such as calcium and iron, and production of red blood cells or hematopoiesis. These processes will be discussed in much greater detail in Chapter 4: The Skeletal System.

Urinary system

The urinary system functions as a whole to remove excess, unnecessary, and sometimes harmful substances, including toxins, fluids, and metabolic byproducts from the body through the production and excretion of urine.

Figure 1-18 Urinary system components.

The urinary system, also called the excretory system, is the organ system responsible for the formation, storage, and excretion of urine. In the human body, the macroscopic structures include the kidneys, ureters, bladder, and urethra. The kidneys are “bean-shaped” organs that float just beneath the ribcage in the abdominal cavity. They are retro-peritoneal, meaning they are located behind the peritoneum.

Although a relatively small structure, (each kidney is approximately four to six inches long and three inches wide), the kidneys receive anywhere from 20–25% (approximately 1200 ml of the total blood pumped from the heart each and every minute!17

The kidney’s prime function is to filter excess fluid, toxins, and byproducts from the blood, and expel it form the body. Once urine leaves the kidneys, it is carried to the bladder by two hollow tube-like projections, called ureters. From the bladder, urine is expelled from the body via the urethra.

Other functions of the kidneys include salt and water regulation, pH balance, medication clearance, and erythropoietin production. Several other structures play a significant role within the urinary system and will be discussed in greater detail later. Some of these structures include the glomerulus, the renal corpuscle, Bowman’s capsule, the nephron, and the renal tubules.

The urinary system will be discussed in greater detail in Chapter 12: The Urinary System.

 ■Anatomical Terminology

In order to avoid ambiguity and confusion, standard anatomical terminology is used universally when studying the science of any organism, and with specific regards to its anatomy. Because an organism can change position with respect to the environment, and because parts of the organism (head, arms, legs, etc.) can change position with respect to the organism’s body itself, it is fundamental that descriptive and positional terms refer to the organism when it is in standard anatomical position.

Standard anatomical position is assumed when the body is standing erect, facing forward, feet slightly apart, with the arms resting at the sides – slightly rotated outward so that the palms face forward and the thumbs are pointed away from the body.

While studying anatomy, one should become familiar with some of the vocabulary used to describe various aspects of the human body. Any and all directional and descriptive terms are made with reference to the body in its standard anatomical position.

Anatomical directional terms can be used like the directions of a map to precisely communicate the positions of structures, abnormalities, and lesions of the body. This ensures a scientific method of communication that prevents incorrect and possibly life-threatening mistakes.Likewise, anatomical directional terms are also directed to cuts or sections of the body. Body planes, as they are also referred to, are a valuable way to explain and isolate distinctive views of the body.

Some of the most universally applied anatomical directional terms are listed below, followed by body planes and sections.

Superior – above

Inferior – below

Anterior/Ventral – front

Posterior/Dorsal – back

Medial – towards the middle/midline

Lateral – away from the middle/midline

Cephalad – toward the head

Caudal – toward the tail (bone)

Proximal – toward the trunk

Distal – away from the trunk

Superficial – towards the surface

Deep – towards internal structures

Ipsilateral – same side of the midline

Contralateral – opposite sides of the midline

Note: In humans, the terms “anterior” and “ventral”, as well as “posterior” and “dorsal” are interchangeable because human beings stand erect in anatomical position. Therefore, these terms become identical. Using directional terms such as “proximal” and “distal” is reserved for reference in describing the limbs.

Let’s take a look at some specific examples:

The head is superior to the heart.

The leg is inferior to the thigh.

The nose is anterior/ventral to the occipital bone.

The tibia is medial to the fibula.

The radius is lateral to the ulna.

The thorax is cephalad to the umbilicus.

The inguinal region is caudad to the umbilicus.

The elbow is proximal to the wrist.

The ankle is distal to the knee.

The skin is superficial to muscle tissue.

The bones are deep to muscle tissue.

The spleen and left lung are ipsilateral to each other.

The ears are contralateral to each other.

Body planes/sections

It is also important to become familiar with the different sections or planes of the body, as this will facilitate the study of anatomy. The first step is to imagine a series of horizontal and vertical lines passing through the body. These lines, called planes, divide the body according to their location. Three main types of planes should be noted:

Sagittal Plane – This is an imaginary vertical line that runs from the top of the body all the down to the bottom, cutting the body into two halves, left and right. A subtype of plane that often confuses people is the distinction between a sagittal plane and a mid-sagittal plane. The only difference is that a mid-sagittal plane or cut divides the body into two equal left and right halves, whereas a sagittal plane simply divides the body into any two-unequal left and right sides.

Frontal/Coronal Plane – This is an imaginary vertical line that runs down the epicenter of your body from one side to the other, splitting the body into a ventral (anterior) and dorsal (posterior) partition. (It helps to know and remember the location of the coronal suture in your skull, after which this plane was named.)

Transverse Plane – This is an imaginary horizontal line cutting through the middle of your body, separating it into an upper portion and lower portion. This plane is usually the easiest to imagine, and hence remember.

Anatomical landmarks: anterior (front) vs. posterior (rear)

Figure 1-19 Anterior/Posterior views.

Anterior landmarks

Antebrachial –forearm	Antecubitalfossa –anterior elbow	Axillary –armpit	Brachial – upper arm
Buccal –cheek	Carpal –wrist	Cephalic –head	Cervical –neck
Coxal –hip	Cranium –skull	Crural –leg	Digit –finger
Facial –face	Femoral –thigh	Frontal –forehead	Inguinal –groin
Mammary –breast	Mental –chin	Metacarpal –hand	Nasal –nose
Oral –mouth	Orbital –eye	Otic –ear	Patella –knee
Pubic –pelvic	Tarsal –ankle	Umbilicus –navel

Posterior landmarks

Acromion – shoulder	Calcaneus – heel	Gluteus – buttocks	Lumbar – lower back
Occipital – back of head	Olecranon – elbow	Pedal – foot	Plantar – sole
Popliteal – back of knee	Sacral – between the hips	Vertebral – spinal column

The abdomen and pelvis

Medical professionals divide the abdomen into regions and/or quadrants to assist in analysis and diagnoses. Two separate divisions are commonly noted, one in which the abdomen is divided into nine regions, and another where it is divided into four quadrants.

The nine regions are divided into relatively smaller areas than the four quadrants, and allow for more specific analysis of each individual area.

Figure 1-20 Abdominal regions.

The specific areas included are the:

•right hypochondriac

•right lumbar

•right iliac

•epigastric

•umbilical

•hypogastric

•left hypochondriac

•left lumbar

•left iliac

The right and left hypochondriac regions are the most lateral and superior segments, and sit directly beneath the ribcages on their respective sides.

The epigastric lies between the two hypochondriac regions, directly in the midline and above the umbilical region. The lateral and central sections are the right lumbar and left lumbar regions, with the umbilical region lying between. The most inferior sections are the right and left iliac regions with the hypogastric area in between.

The four abdominopelvic quadrants are used to divide the entire region of the trunk below the diaphragm into four equal sections.

Figure 1-21 Body systems work together.

The quadrants are named simply by their relative locations:

•Left lower quadrant (LLQ)

•Right lower quadrant (RLQ)

•Left upper quadrant (LUQ)

•Right upper quadrant (RUQ)

In the RUQ, a majority of the liver can be found, as well as the gallbladder and right kidney.

The LUQ is host to the left lobe of the liver, the left kidney, spleen, and a major portion of the stomach.

In the RLQ sits the cecum and appendix.

The LLQ contains the parts of the small intestine and the descending colon.

Anatomically organizing the abdomen into these rewgions and/or quadrants offers medical professionals a rapid and efficient way of forming a differential diagnosis, especially critical in the cases of an acute abdomen.

 ■Types of Movement

The human body must also be appreciated for the many types of movements it is capable of. The body is able to perform an extensive range of movements, subject to the location and more specifically the joint, at which pivotal point movement occurs.

Movement takes place due to the contraction of skeletal muscles over joints in the body.

Depending on the location, mobility, and type of joint at which the contraction occurs, a direct and specific movement will be observed.

Accordingly, these forms of movements have specialized names, and are often described in pairs of opposed actions.

The first pair of opposite movements in our discussion is called flexion and extension.

Flexion occurs when a muscular contraction causes the angle of a joint to decrease. For example, the humerus bone of the upper arm joins with the ulna and radius bones of the lower arm at the elbow joint. During contraction of the biceps muscle, the bones of the lower arm are pulled toward the upper arm, thereby decreasing the angle of the elbow.

The opposite type of movement is called extension, where the angle of a joint increases. Using the above example, when the triceps muscle contracts, it pulls the bones of the lower arm farther away from the upper arm, increasing the angle of the elbow joint. Another example of flexion/extension occurs at the knee joint.

During contraction of the quadriceps muscle in the thigh, the lower leg is pulled away from the thigh, straightening the entire leg, resulting in extension, or an increase in the angle of the knee joint. During contraction of the biceps femoris, semitendinosus, and semimembranosus muscles, (commonly referred to as the hamstrings), the lower leg is brought closer to the thigh resulting in flexion, or a decrease of the angle of the knee joint.

The second pair or opposite movements to be discussed are called abduction and adduction. Abduction is the term given to describe the movement of a limb away from the midline of the body. An easy way to picture this is to imagine someone doing jumping jacks.

The opposite movement, called adduction, is to bring the limbs back toward the midline of the body, just as when you stand still with your feet together and your arms pressed against your sides.

Figure 1-22 Adduction and abduction.

The next pair of opposing movements is called supination and pronation. These movements are unique to the elbow joint in rotating the wrist. Pronation is demonstrated by turning the palm of the hand over from anatomical position to face toward the back of the body. Supination is rotating the palm forward, up, or down and returning it to anatomical position.

Another pair of opposing movements is internal rotation (also called medial rotation) and external rotation (also called lateral rotation). Internal rotation can be demonstrated by contraction of the pectoralis major, commonly referred to as the “pecs” or chest muscle, which causes an internal rotation of the humerus; essentially drawing the arm near the body. External rotation is just the opposite. The infraspinatus and teres minor muscles (small muscles on the posterior aspect of the body) contract to externally rotate the humerus, causing the upper arm to move away from the body.

Elevation and depression is another pair of opposite movements. Elevation is movement in a superior direction. The only prominent example of this is contraction of the upper fibers in the trapezius muscle, which causes an elevation of the shoulder girdle (such as extending the arms outward at shoulder level, as if pretending to be an airplane). The opposite movement occurs when the lower fibers of the trapezius muscle are contracted (arms back to the sides), resulting in depression of the shoulder girdle.

Another pair of opposite movements is dorsiflexion and plantar flexion. Dorsiflexion is a subtype of flexion movement, where contraction of the tibialis anterior, or “shin muscle”, causes a decrease of the angle between the dorsum (top) of the foot and the lower leg. This movement can be demonstrated by observing someone remove their foot from a gas pedal of an automobile. The opposite movement, caused by contraction of the gastrocnemius, or “calf muscle”, is called plantar flexion.

This is actually a misnomer; since the movement actually increases the angle between the dorsum of the foot and the lower leg. Rather, it should be appropriately named plantar extension. Nonetheless, this movement can be demonstrated by pressing down on a gas pedal or simply standing on one’s toes.

Eversion and inversion are another paired and opposite movement unique to the foot. Eversion is the movement of the sole of the foot away from the midline of the body, which occurs at the subtalar joint, (also called the talocalcaneal joint), more commonly referred to as the ankle. The opposite movement is called inversion, where the sole of the foot is moved toward the midline of the body. Inversion also occurs at the subtalar joint, and excessive or forceful inversion is seen in “twisting the ankle”.

Yet another pair of opposite movements is called protrusion and retrusion. Protrusion is simply any anterior movement of body part, although it is commonly reserved for the forward movement of the mandible, or jaw. Retrusion is the posterior movement of a body part, as in returning the mandible to its normal position after protrusion.

The final pair of opposite movements in our discussion are protraction and retraction. Protraction is the proper anatomical term for the specific anterior movement of the shoulder girdle, whereas retraction (being the opposite) is the anatomical term for the specific posterior movement of the shoulder girdle.

Consequently, we have some movements that do not have clear opposites. Nevertheless, they are specific anatomical movements that require acknowledgement.

Rotation is a movement that occurs when a bone moves around a central axis, without displacing the axis. The bone may rotate around its own longitudinal axis, or the axis of rotation may lie in a completely separate bone, such as in the case of rotating the head in a “no” fashion. In this movement, for example, the point of pivot is formed by the odontoid process of the axis (the second cervical vertebra) around which the atlas (the first cervical vertebra) rotates.

Circumduction is the conical movement of a body part and results from a combination of movements including flexion, extension, abduction, and adduction.

An example of circumduction can be achieved by circling your arms around in a “windmill” fashion, or drawing imaginary circles in the air with your entire leg in extension.

Finally, a highly-specialized type of movement of which the human body is capable is called opposition. This is a grasping motion where the thumb can be pressed against the remaining other four digits of the hand, either altogether (as in the simulation of a “claw hand”) or one digit at a time, as seen when one is physically counting on his/her fingers.

It should also be noted that the above movements do not belong to an all-inclusive or exhaustive list. The movements listed are basically the major types of movements possible across a variety of joints. Furthermore, the above-mentioned movements are often combined at various joints, producing what seems like an infinite variety of motions that can be performed by the human body.

 ■Conclusion

Anatomy and physiology has been studied for hundreds of years. It is one of the most basic and fundamental, yet imperative courses for anyone studying the human body. The study of the human body’s structure and its function provides a great deal of information for health and medical professionals alike.

The human body is a dynamic, complex, living organism with a wide variety of structures and an even wider breadth of functions. The elementary concepts presented in the overview of human anatomy and physiology is just the beginning to an enormous amount of information. Every student must make a strong effort to thoroughly investigate the basic principles that govern the human body–and anatomy and physiology does just that.

Case Study Conclusion

Upon arrival at the ER, the physician’s main concern is to replace the patient’s blood volume. A fluid bolus is administered, with lung sounds and BP checked every 250 cc until normal range is achieved.

The doctor then orders cervical, spine and chest x-rays with ABGs to determine PaCO2 and PaO2 levels.

He also orders an x-ray of her left femur. No signs of spinal injury are noted. A CT of the head shows no brain trauma or subdural hematomas. While the patient did sustain fractures to ribs 4 and 5 from forceful impact, no signs of splintering are noted. A CT scan confirmed injury to her liver, which caused internal bleeding, hence the low blood pressure (hypotension).

Patient is stabilized and transported to the surgical suite, where her liver was repaired. An orthopedic surgeon was called to address the confirmed transverse midline femoral shaft fracture, which was repaired via external fixation. Deemed superficial, her facial and forearm lacerations were sutured. Following surgery, she was placed in the ICU and monitored for blood clots as well as to reduce risk of development of pulmonary embolism.

Barring any further medical emergencies, patient will be transferred from the ICU to a hospital room to recover. She will then be assessed by a physical therapist who will determine appropriate care plan for ambulatory weight-bearing and mobility.

 ■Questions

1.Which of the following is the best description of homeostasis?

A.The balance of nutritional compounds that will support a living organism’s optimal function.

B.Stable internal environment comprised of physiologic components including blood pressure, temperature and pH balance.

C.A very simple process of fluid, chemical and blood volume balances.

D.A mechanism that has at least two co-dependent regulatory processes.

Answer: B. Homeostasis is achieved through complex interactions of checks and balances in the body able to adapt to and compensate for ever-changing physiological interactions that regulate numerous body functions and processes.

Learning Objective: 3

2.Define the difference between gross and microscopic anatomy.

A.Gross anatomy is the study of muscles while microscopic anatomy is the study of molecular structures.

B.Gross anatomy is basically visible to the naked eye while microscopic anatomy is the study of structures in the body invisible to the naked eye.

C.Gross anatomy defines dissection of the body while microscopic anatomy defines anything that is observed under the microscope.

D.Gross anatomy is the study of living organisms while microscopic anatomy is the study of how organ systems work.

Answer: B. Gross anatomy is the study of any area of the body visible to the naked eye; bone, muscle, or an organ for example, while microscopic anatomy pertains to cells and structures or even functions of an organ only visible through microscopic viewing or analysis.

Learning Objective: 1

3.Define the overall purpose of anatomical terminology and positioning for medical personnel.

A.Ensures a static and scientific method that is used around the world to correctly ascertain anatomical positioning, i.e. anterior/posterior or proximal/distal to denote precise physical position.

B.It’s a process that is used in the U.S. that divides the body into upper and lower sections for easier study by medical students.

C.Anatomical terminology changes with physiological aspects of the body caused by changes in any of the body’s organisms and aids in diagnostics.

D.Anatomical terminology and positioning cues serve as an aid to medical personnel when it comes to imaging studies and results.

Answer: A. Anatomical terminology and positioning is an essential and universal aspect of medical education that serves as a fundamental and precise use of terms to refer to specific locations of the body.

Learning Objective: 5

4.Which of the following adequately describe the six major characteristics of living organisms?

A.Positive feedback, negative feedback, homeostasis, oxygen, one-celled organism, reproduction.

B.Multicellular, contain RNA, negative pH balance, non-structured form, serve as a host cell, growth.

C.One or more cells, use energy, are organized, grow, produce, have a stable environment.

D.Undergo mitosis, have only one function, always mature, reproduce, have receptors, use energy.

Answer: C. All living organisms require six characteristics: are unicellular or multicellular in structure, are organized and undergo metabolism. They must also grow and develop and reproduce, and finally, must have a stable internal environment (homeostasis).

Learning Objective: 2

 ■References

1.			North Hunterdon-Voorhees Regional High School District [Internet] “All Living Things Share Certain Characteristics”. Accessed March 30, 2017. Available from http://www.nhvweb.net/nhhs/science/wking/files/2011/09/IntroAnatomy-wking-v1-Compatibility-Mode.pdf
2.			Lumen Boundless Biology [internet] “Energy and Metabolism”. Accessed March 2017. Available from https://www.boundless.com/biology/textbooks/boundless-biology-textbook/metabolism-6/energy-and-metabolism-68/the-role-of-energy-and-metabolism/
3.			Elaine Marieb and Katja Hoehn, Human Anatomy & Physiology. 10th ed. (San Francisco, CA: Pearson, 2016), 4 p.
4.			Khan Academy [Internet] “Homeostasis”, accessed April 2917. Available from https://www.khanacademy.org/science/biology/principles-of-physiology/body-structure-and-homeostasis/a/homeostasis
5.			Elaine Marieb and Katja Hoehn, Human Anatomy & Physiology. 10th ed. (San Francisco, CA: Pearson, 2016), 9 p.
6.			Martini. Anatomy & Physiology. [Internet] Accessed April 2017. Available from http://wps.aw.com/wps/media/objects/451/462581/CH01/html/ch1_4_1.html. Pearson Education, Inc. 2003
7.			Carolee Sormunen, Terminology for Allied Health Professionals. 5th ed. (Canada: Thomson, 2003), 19 p.
8.			Elaine Marieb and Katja Hoehn, Human Anatomy & Physiology. 10th ed. (San Francisco: Pearson, 2016), 18 p.
9.			Carolee Sormunen, Terminology for Allied Health Professionals. 5th ed. (Canada: Thomson, 2003), 498 p.
10.			The Physics Factbook. An encyclopedia of scientific essays [Internet] “Surface Area of Human Skin”. Accessed April 2017. Available from http://hypertextbook.com/facts/2001/IgorFridman.shtml
11.			Elaine Marieb and Katja Hoehn, Human Anatomy & Physiology. 10th ed. (San Francisco: Pearson, 2016), 153 p.
12.			Ibid. 772 p.
13.			Medscape [Internet] “Skeletal System Anatomy in Children and Toddlers. Accessed April 2017. Available from http://emedicine.medscape.com/article/1899256-overview. Skeletal System Anatomy in Children and Toddlers.
14.			Wilma Phipps, Frances Monahan, Judith Sands, Jane Marek, Marianne Neighbors. Medical-Surgical Nursing Health and Illness Perspectives. (St. Louis: Mosby, 2003), 1437 p.
15.			Ibid.
16.			Ibid.
17.			Elaine Marieb and Katja Hoehn, Human Anatomy & Physiology. 10th ed. (San Francisco: Pearson, 2016) 971 p.