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9 Tissues

Figure 9.1 Levels of organisation.

Figure 9.2 Types of cells.

Figure 9.3 Human body tissues.

There are many types of cells in the body, organised into four distinct categories of tissues: epithelial, connective, nervous and muscle. Within these four categories, there are several subdivisions. Each of these categories is characterised by specific functions, contributing to the overall health and maintenance of the body. Generally, tissue types are composed of similar cells carrying out associated functions; for example, the epidermis of the face and the buccal mucosa (lining of the mouth) are the same tissue type with related functions but their appearance is very different to the naked eye. Blood and bone look very different yet both are classified in the same tissue type. Tissues, made up of large numbers of cells, are classified according to their size, shape and functions (Figures 9.1–9.3).

Epithelial tissue

Epithelial tissue is located in the covering of external and internal surfaces of the body, and the hollow organs and tubes; it is also found in the glands. The overall function of the epithelium is to offer protection and impermeability (or selective permeability) to the covered structure. Cells are closely packed and the matrix (the intracellular substance) is minimal. Usually there is a basement membrane on which the cells lie. The epithelial tissue can be simple (a single layer of cells); subdivided into squamous epithelium (forms the lining of the heart, blood vessels, lymph vessels, alveoli of the lungs, lining of the collecting ducts of the nephrons); or stratified where there are several layers of tissue, composed of several layers of these cells. Keratinised stratified epithelium is found on dry surfaces exposed to wear and tear such as the skin, hair and nails. Non‐keratinised epithelium protects those moist surfaces that are subjected to wear and tear, such as the conjunctiva, the linings of the mouth and the vagina. The urinary bladder is lined with transitional epithelium which permits the bladder to stretch as it fills.

Nervous tissue

This is made up of neurons and glial cells. Its function is to receive and transmit neural impulses (reception and transmission of information). Two types of tissue are found in the nervous system: excitable cells (the neurons – initiate, receive, conduct and transmit information) and non‐excitable cells (the glial cells – supporting the neurons). A neuron is made up of two major parts: the cell body, containing the neuron’s nucleus, and cytoplasm and other organelles. Nerve processes are ‘finger‐like’ projections arising from the cell body and can conduct and transmit signals. There are two types: axons carrying signals away from the cell body and dendrites carrying signals toward the cell body. Neurons usually have one axon (this can be branched). Axons normally terminate at a synapse through which the signal is sent to the next cell, typically through a dendrite.

Connective tissue

There are many kinds of connective tissue and it is the most abundant type of tissue; connective tissue is typically used to fill empty spaces between other body tissues. The cells of connective tissue secrete substances that compose extracellular material, such as collagen and elastic fibres, creating a considerable spacing between these cells. Other important biological features include substance transportation, protection of the organism and insulation. Connective tissue (excluding blood) is found in organs supporting specialised tissues.

The matrix of areolar connective tissue is semi‐solid, containing adipocytes, mast cells and macrophages. Where there is a need to provide elasticity and tensile strength, areola tissue is present, for example under the skin, between muscles and in the alimentary canal. Adipose tissue supports the kidneys, brain and eyes and is related to energy intake and expenditure. Lymphoid tissue contains reticular cells and white blood cells, and is located in lymph tissue in the lymph nodes and lymphatic organs. Dense connective tissue (fibrous tissue composed of closely packed collagen fibres with little matrix) is found in ligaments, periosteum, muscle fascia and tendons. Blood is a fluid connective tissue. Cartilage is found as hyaline cartilage on the ends of the bones that form joints, the costal cartilage attaching the ribs to the sternum, and in the trachea, larynx and bronchi. Bone cells are surrounded by a matrix of collagen fibres with added strength provided by calcium and phosphate.

Muscle tissue

Muscle tissues are made of cells that permit contractions, generating movement. The function of muscle tissue is to pull bones (skeletal striated muscle), to contract and move viscera and vessels (smooth muscle), and make the heart beat (cardiac striated muscle). Muscle is involved each time we move, breathe, ingest food or urinate. The muscle cells comprise internal structures, called sarcomeres, in which myosin and actin molecules work to create contraction and movement.

There are three kinds of muscle in the body: skeletal, cardiac and smooth muscle. Skeletal muscle (striated muscle) is a voluntary muscle. Cells in skeletal muscle are long and thin with multiple nuclei. Cardiac muscle is only found in the heart, with the muscle fibres interlocking with each other to ensure that as one aspect of the muscle is stimulated, all other stimulated fibres contract sequentially. Cardiac muscle is not under voluntary control; the special cells of the sinoatrial node are responsible for sending out impulses resulting in cardiac contraction. Smooth muscle is involuntary, held together by connective tissue with bands of elastic protein wrapped around them. Smooth muscle is seen in the walls of hollow structures and vessels: blood vessels, ureters, urinary bladder, parts of the respiratory tract, ducts and glands of the alimentary tract.

Clinical practice point

A disruption of the structure of tissue is a sign of injury or disease. These changes can be detected through histology, which is the microscopic study of tissue appearance, organisation and function. When learning to care for people safely and effectively, this requires the nurse to appreciate the microscopic and macroscopic aspects of the human. Exploring how the body’s tissues function permits you to appreciate what can happen when cells, tissues and organ systems fail.