Colloidal osmotic pressure

Plasma protein fractions include human plasma protein fraction 5% and normal serum albumin 5% (Albuminar-5, Buminate 5%) and 25% (Albuminar-25, Buminate 25%). Plasma protein fraction 5% is an IV solution containing 5% human plasma proteins. Serum albumin is obtained from donated whole blood and is a protein found in plasma The albumin fraction of human blood acts to maintain plasma colloid osmotic pressure and as a carrier of intermediate metabolites in the transport and exchange of tissue products. It is critical in regulating the volume of circulating blood. When blood is lost from shock, such as in hemorrhage, there is a reduced plasma volume. When blood volume is reduced, albumin quickly restores the volume in most situations. 

Understanding the effects of colloid administration on circulating blood volume necessitates a review of those physiologic forces that determine fluid movement between capillaries and the interstitial space throughout the circulation (Fig. 10—5).4 Relative hydrostatic pressure between the capillary lumen and the interstitial space is one of the major determinants of net fluid flow into or out of the circulation. The other major determinant is the relative colloid osmotic pressure between the two spaces. Administration of exogenous colloids results in an increase in the intravascular colloid osmotic pressure. In the case of isosomotic colloids (5% albumin, 6% hetastarch, and dextran products), initial expansion of the intravascular space is essentially that of the volume of colloid administered. In the case of hyperoncotic solutions such as 25% albumin, fluid is pulled from the interstitial space into the vasculature... 

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 ... 

The plasma colloid osmotic pressure remains constant ... 

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. 

A decrease in the concentration of plasma proteins causes a decrease in the plasma colloid osmotic pressure. As a result, filtration is increased, reabsorption is decreased, and fluid accumulates in the tissue. Most plasma proteins are made in the liver therefore, a decrease in protein synthesis due to liver failure is an important cause of this condition. Malnutrition may also impair protein synthesis. Finally, kidney disease leading to proteinuria (protein loss in the urine) decreases the concentration of plasma proteins. 

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. 

Plasma colloid osmotic pressure nGC) is generated by the plasma proteins. These proteins exert an osmotic force on the fluid, which opposes filtration... 

HSA is the single most abundant protein in blood (Table 12.7). Its normal concentration is approximately 42 g 1 1, representing 60 per cent of total plasma protein. The vascular system of an average adult thus contains in the region of 150 g of albumin. HSA is responsible for over 80 per cent of the colloidal osmotic pressure of human blood. More than any other plasma constituent, HSA is thus responsible for retaining sufficient fluid within blood vessels. It has been aptly described as the protein that makes blood thicker than water. 

Grabowski, C.T. 1981. Plasma proteins and colloid osmotic pressure of blood of rat fetuses prenatally exposed to mirex. Jour. Toxicol. Environ. Health 7 705-714. 

Serum albumin is the most abundant protein in blood plasma. Its primary function is to control the colloidal osmotic pressure in blood, but is also important for its buffering capacity and for its ability to transport fatty acids and bilirubin, as well as xenobiotic molecules. The physiological implications of its esterase-like activity are unknown (see Sect. 3.7.5). 

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... 

A dehciency of amino acids resnlts in decreased production of albnmin in the liver, lowering its concentration in the plasma and hence the colloid osmotic pressure. Consequently, fluid is lost from the blood and its viscosity increased, so that the heart has to work harder to pnmp the same quantity of blood and eventually it may not be able to cope, especially as cardiac muscle is lost in prolonged starvation (Table 16.6). Death will then resnlt from cardiac failure. 

The most frequent protein in the plasma, at around 45 g is albumin. Due to its high concentration, it plays a crucial role in maintaining the blood s colloid osmotic pressure and represents an important amino acid reserve for the body. Albumin has binding sites for apolar substances and therefore functions as a transport protein for long-chain fatty acids, bilirubin, drugs, and some steroid hormones and vitamins. In addition, serum albumin binds Ca "" and Mg "" ions. It is the only important plasma protein that is not glycosylated. 

Human plasma has a colloid osmotic pressure of 3.6 kPa, of which 2.8 kPa is contributed by albumin. Volume-for-volume, 4.5% albumin is approximately four times more effective in expanding the plasma volume than crystalloid solutions, and the effect lasts 6-8 hours, compared to only 15-20 min with crystalloids. Although popular in the past as volume expanders, albumin solutions have fallen into disfavour. They are prepared from pooled human plasma, with all the inherent risks of pooled blood products. Albumin can cause adverse reactions, similar to other transfusion reactions, such as chills, urticaria, and vasodilatation. These may be caused by organic or inorganic substances formed during the processing... 

Albumin 50-65 Colloid osmotic pressure, binds drugs, fatty acids, hormones... 

The colloid osmotic pressure of aqueous dextran solutions can be regulated by molecular weight and concentration of the solute [52]. Dissolved dextran in low concentrations possesses Newtonian flow characteristics [45]. The relationship between viscosity and concentration is shown in Fig. 6 for different dextran fractions [52]. The molecular weight dependence of the intrinsic viscosity can be estimated by several equations [37,46,53]. 

To determine the protein concentration Ce in the efferent blood we assume that filtration equilibrium is established before the blood leaves the glomerular capillaries, i.e., that the glomerular hydrostatic pressure minus the efferent colloid osmotic pressure Posm equals the tubular pressure. The experimentally determined relation between the colloid osmotic pressure and the protein concentration C can be described as [16] ... 

Because of the implicit manner in which the glomerular pressure is related to the efferent colloid osmotic pressure and the filtration rate [Eqs. (5)—(8)], direct solution of the set of coupled algebraic equations for the two-nephron system becomes... 

Increased portal blood pressure Blood flow in the portal system is often obstructed in cirrhosis, resulting in an increased portal blood pressure. Further, colloidal osmotic pressure of the blood is decreased as a result of impaired synthesis of plasma proteins by the diseased liver. Increased portal blood pressure and low osmo-larity of the blood cause fluid to escape from the portal vascular system and collect in the abdomen. 

Fein IA, Rackow EC, Sprung CL, Grodman R Relation of colloid osmotic pressure to arterial hypoxemia and cerebral edema during crystalloid volume loading of patients with diabetic ketoacidosis. Ann Intern Med 96 570-575,1982. 

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. 

Binding and transport. There are four binding sites on the albumin molecule, with varying specificity for different snbstances (snch as fatty acids and bilirubin). Competitive binding of drugs may occur at the same site, with displacement of the originally bound substance. Maintenance of colloid osmotic pressure. Albumin is responsible for 75-80 % of osmotic pressure. 

Filtration is driven by both hydrostatic and oncotic (colloid osmotic pressure) transport. 

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