Capillary Exchange

The cardiovascular system's chief role is to disseminate gases, nutrients, waste, and other substances to the body's cells. Small molecules like gases, lipids, and lipid-soluble substances directly diffuse through capillary wall endothelial cell membranes. Glucose, amino acids, and ions, including sodium, potassium, calcium, and chloride, use transporters for facilitated diffusion via membrane-specific channels. Glucose, ions, and bigger molecules may also pass through intercellular clefts. Larger molecules can transit through fenestrated capillary pores, and sizeable plasma proteins can go through the large gaps in the sinusoids. Some large proteins in the blood plasma can enter and exit the endothelial cells in vesicles via endocytosis and exocytosis. Water moves by the process of osmosis.

Bulk flow, more efficient than mere diffusion, drives fluids into and out of capillary beds. This movement involves two pressure-induced mechanisms: filtration, where fluid moves from a higher-pressure area in a capillary bed to a lower-pressure area in the tissues, and reabsorption, where fluid moves from a high-pressure area in the tissues into a low-pressure area in the capillaries. Both these mechanisms involve the interaction of hydrostatic and osmotic pressures.

Hydrostatic pressure, defined as the pressure of any fluid in an enclosed space, is the primary force driving fluid transport between capillaries and tissues. When fluid exits a capillary and enters tissues, the hydrostatic pressure in the interstitial fluid increases. This opposing hydrostatic pressure is known as the interstitial fluid hydrostatic pressure (IFHP). Generally, the capillary hydrostatic pressure (CHP) from the arterial pathways is much higher than the IFHP, as lymphatic vessels continuously absorb excess fluid from the tissues. Consequently, fluid typically moves out of the capillary and into the interstitial fluid, a process termed filtration.

Osmotic pressure drives reabsorption—the movement of fluid from the interstitial fluid back into the capillaries. While hydrostatic pressure forces fluid out of the capillary, osmotic pressure draws it back in. Osmotic pressure is influenced by osmotic concentration gradients, that is, the concentration difference of the solute-to-water composition between the blood and tissue fluid. A region with a higher solute concentration (and lower water concentration) draws water from a region with a higher water concentration (and lower solute concentration) across a semipermeable membrane.

The pressure created by the concentration of colloidal proteins in the blood is called the blood colloidal osmotic pressure (BCOP). It influences capillary exchange and results in the reabsorption of water. These plasma proteins are suspended in blood and cannot move across the semipermeable capillary cell membrane. This means that they stay in the plasma, giving the blood a higher colloidal concentration and lower water concentration than tissue fluid. Water is drawn back into the capillary, carrying dissolved molecules with it. This difference in colloidal osmotic pressure results in reabsorption.

The process of fluid transfer across the capillary wall is controlled by the net filtration pressure (NFP), which is dictated by the balance between hydrostatic and osmotic pressures. When fluid is being reabsorbed, the NFP is negative. The NFP isn't constant along the capillary bed; it varies at different points. Near the arterial side of the capillary, the NFP is about 10 mm Hg, calculated by subtracting the blood colloid osmotic pressure (BCOP) of 25 mm Hg from the capillary hydrostatic pressure (CHP) of 35 mm Hg. The pressures of the interstitial fluid are practically zero. Hence, this NFP of 10 mm Hg promotes the fluid's net movement out of the capillary at the arterial end. Around the mid-point of the capillary, the CHP equals the BCOP of 25 mm Hg, which makes the NFP fall to zero. That means the volume remains constant - fluid exits and enters the capillary at the same rate. Close to the venous side of the capillary, the CHP decreases to about 18 mm Hg due to fluid loss, while the BCOP stays at 25 mm Hg. This results in water being pulled into the capillary, indicating reabsorption. In other words, at the venous capillary end, the NFP is −7 mm Hg.