Capillaries plasma colloid osmotic pressure

FIGURE 10-5. Operative forces at the capillary membrane tending to move fluid either outward or inward through the capillary membrane. In hypovolemic shock, one therapeutic strategy is the administration of colloids that can sustain and/or draw fluid from the interstitial space by increasing the plasma colloid osmotic pressure. (Reprinted from Guyton AC, Hall JE. Textbook of Medical Physiology. 8th ed. Philadelphia Saunders,... 

Plasma colloid osmotic pressure is generated by proteins in the plasma that cannot cross the capillary wall. These proteins exert an osmotic force, pulling fluid into the capillary. In fact, the plasma colloid osmotic pressure, which is about 28 mmHg, is the only force holding fluid within the capillaries. Interstitial fluid colloid osmotic pressure is generated by the small amount of plasma proteins that leaks into the interstitial space. Because these proteins... 

Figure 15.7 Starling principle a summary of forces determining the bulk flow of fluid across the wall of a capillary. Hydrostatic forces include capillary pressure (Pc) and interstitial fluid pressure (PJ. Capillary pressure pushes fluid out of the capillary. Interstitial fluid pressure is negative and acts as a suction pulling fluid out of the capillary. Osmotic forces include plasma colloid osmotic pressure (np) and interstitial fluid colloid osmotic pressure (n,). These forces are caused by proteins that pull fluid toward them. The sum of these four forces results in net filtration of fluid at the arteriolar end of the capillary (where Pc is high) and net reabsorption of fluid at the venular end of the capillary (where Pc is low).

Although the interstitial fluid hydrostatic pressure is "negative," it causes fluid to be pulled out of the capillary, so this pressure is "added" to the other outward forces. The only force pulling fluid into the capillary is the plasma colloid osmotic pressure ... 

Albumin is the most abundant (about 55%) of the plasma proteins. An important function of albumin is to bind with various molecules in the blood and serve as a carrier protein, transporting these substances throughout the circulation. Substances that bind with albumin include hormones amino acids fatty acids bile salts and vitamins. Albumin also serves as an osmotic regulator. Because capillary walls are impermeable to plasma proteins, these molecules exert a powerful osmotic force on water in the blood. In fact, the plasma colloid osmotic pressure exerted by plasma proteins is the only force that retains water within the vascular compartment and therefore maintains blood volume (see Chapter 15). Albumin is synthesized in the liver. 

Increased capillary permeability may allow plasma proteins to leak into the interstitial spaces of a tissue. The presence of excess protein in these spaces causes an increase in interstitial fluid colloid osmotic pressure and pulls more fluid out of the capillaries. Mediators of inflammation such as histamine and bradykinin, which are active following tissue injury and during allergic reactions, increase capillary permeability and cause swelling. 

Marasmus is considered to be due to inadequate food intake. It is not usually the quantity but the quality of the food that is deficient, e.g. low nutritional value of bulky vegetables. Kwashiorkor is considered to be caused, more specifically, by a low-protein diet. This condition frequently develops at the time of weaning when protein-rich milk is replaced by protein-deficient solid food. It did not appear in the medical literature until 1934 when it was reported by Cicely Williams who studied the condition while she was working among tribes of Western Africa. She gave it the name kwashiorkor, which was used by the Ga tribe to describe the condition that develops when the baby is taken away from mother s breast, usually because another baby has been bom. It has generally been held that the oedema is a consequence of a low plasma albumin concentration and a reduction in the colloid osmotic pressure which reduces the movement of water from tissue fluid back into capillaries. The low albumin level results from a decreased rate of synthesis of albumin by the liver. However, if marasmus is due entirely to lack of energy... 

The main function of albumin in the plasma is to provide colloid osmotic pressure. It is of major importance in maintaining blood volume and in the exchange of fluid between blood and the tissues. Heavy proteinuria may involve the loss of >3.5 g of albumin per day and this, in turn, causes a reduction in plasma oncotic pressure. When plasma oncotic pressure is reduced, fluid is not completely reabsorbed from the tissues at the venous end of capillaries. The fluid is retained within the tissues, causing oedema. The effects of gravity on fluid accumulation in the body causes oedema to be more marked in the lower body than in the upper parts, so oedema is often noticed first around the ankles. 

Apparently damage of the capillaries in the affected areas is the primary cause of the exudation. If the development of exudate was merely a consequence of lowered colloid osmotic pressure in the plasma, the exudate fluid would be much lower in protein content. In encephalomalacia the albumin globulin ratio of the plasma remains normal after the onset of the disease. 

The effective osmotic pressure of the blood across capillary walls. The greatest contribution to the colloid osmotic pressure comes from plasma proteins, which, unlike the plasma ions, cannot move through the capillary walls. The oncotic pressure balances the effect of capillary blood pressure which tends to force water into the interstitial spaces. 

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