Capillary exchange

Capillary exchange is the movement of gases, nutrients, wastes, and fluid between blood in capillaries and surrounding tissues. It occurs across thin capillary walls through diffusion, filtration, and reabsorption.

Diffusion: The net movement of particles from a region of higher concentration to one of lower concentration. Oxygen, carbon dioxide, and many small solutes cross capillary walls down concentration gradients.
Capillary: A microscopic blood vessel connecting arterioles and venules, with a wall usually one endothelial cell thick. Capillaries are the vessels across whose walls the exchange occurs.
Tissue perfusion: The passage of blood through the small vessels of a tissue. Perfusion brings oxygen and nutrients to capillaries where they can enter tissues.
Continuous capillary: A capillary with an uninterrupted endothelial lining and basement membrane. Its relatively tight barrier supports selective exchange in muscle, skin, and the central nervous system.
Starling equation: An equation that estimates fluid movement across a semipermeable membrane from hydrostatic and oncotic pressures. It combines pressure forces that drive filtration and reabsorption across capillary walls.
Endothelium: A layer of cells lining the inside of blood vessels and lymphatic vessels. Capillary endothelial cells form the principal barrier between blood and tissue.
Oxygen delivery: The amount of oxygen transported by blood to tissues per unit time. Oxygen must diffuse from capillary blood into tissue cells to support metabolism.
Fenestrated capillary: A capillary whose endothelial cells contain pores that increase permeability to water and small solutes. Its pores enable rapid exchange in tissues such as the kidney and endocrine glands.
Hydrostatic pressure: The pressure a fluid exerts on the walls of its container. Blood hydrostatic pressure pushes fluid outward from capillaries into interstitial spaces.
Interstitial fluid: The extracellular fluid occupying spaces between tissue cells. It receives filtered fluid and dissolved substances before they reach cells.

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