Читать книгу Understanding Anatomy and Physiology in Nursing - John Knight - Страница 110

When the pre-capillary sphincters open, blood flows into the capillary beds under high pressure (around 35 mmHg) directly from the arterioles. Each individual capillary is composed of a tube of squamous epithelial cells.

Capillaries are just wide enough to allow erythrocytes to squeeze through and travel along their length. Erythrocytes themselves are deformable because of their biconcave structure (Chapter 9); this allows the membranes of each erythrocyte to be in close proximity to the capillary wall, increasing the efficiency of oxygen diffusion into the tissues.

The adjacent cells in a capillary have regular tiny slits/gaps in their junctions which function as crude mechanical filters. When blood is forced into these porous vessels, fluid containing low-molecular-weight molecules such as oxygen, salts (sodium, potassium calcium, chloride), amino acids and sugars such as glucose is driven out through the vessel wall by a process called filtration. This fluid is termed interstitial or tissue fluid and is continually being produced to act as a medium to deliver useful molecules to the local cells. Most cells are continually bathed in a thin layer of this interstitial fluid, which also forms a medium into which waste materials such as carbon dioxide and urea can be discharged.

During the process of filtration larger molecules such as plasma proteins are too big to fit through the porous capillary walls and are therefore retained in the capillary blood. This retention increases the osmotic potential of the blood towards the venous end of the capillary bed, which serves to pull tissue fluid, now rich in dissolved waste products, back in through the capillary walls.