Why is plasma colloid osmotic pressure a resistance

Plasma colloid osmotic pressure is an important resistance factor in maintaining fluid balance inside and outside blood vessels, mainly determined by the concentration of plasma proteins, especially albumin. Its resistance effect is reflected in five aspects: combating capillary hydrostatic pressure, preventing excessive tissue fluid production, regulating renal water and sodium reabsorption, affecting lymphatic reflux efficiency, and participating in inflammation response regulation.

1. Anti static water pressure:

The static water pressure at the end of the capillary artery is about 30mmHg, which will push the liquid to filter out into the tissue gap. The normal value of plasma colloid osmotic pressure is 25-30mmHg, and about 80% of the static water pressure is counteracted by reverse force to maintain the net filtration overpressure at a low level. When the plasma protein concentration drops below 20g/L, insufficient colloid osmotic pressure will lead to edema.

2. Preventing tissue fluid accumulation:

About 0.5ml of plasma flowing through capillaries is filtered out to form tissue fluid per 100ml. Colloidal osmotic pressure attracts water from interstitial spaces back into blood vessels, creating a pressure gradient of approximately 5mmHg with the colloid osmotic pressure of tissue fluid. When albumin synthesis is reduced in patients with cirrhosis, the disruption of this gradient can lead to ascites.

3. Regulating renal function:

The glomerular capillary colloid osmotic pressure increases from 25mmHg at the inlet to 35mmHg at the outlet, and this increasing resistance ensures that only 20% of plasma components are filtered. A large amount of proteinuria in patients with nephrotic syndrome leads to the decrease of plasma colloid osmotic pressure, which instead causes compensatory retention of water and sodium. 4. Impact on lymphatic reflux: Edema occurs when the amount of tissue fluid produced exceeds the compensatory capacity of the lymphatic system by about 2-4L/day. The decrease of colloid osmotic pressure will make the production rate of tissue fluid exceed the limit of lymphatic return, typically in patients with upper limb lymphedema and hypoproteinemia after breast cancer surgery.

5. Participate in inflammation regulation:

During infection, increased vascular permeability leads to protein extravasation, and local colloid osmotic pressure decreases, forming a "resistance gap" that promotes inflammatory exudation. Clinical infusion of albumin can restore osmotic pressure gradient and alleviate tissue edema in burn or sepsis patients. Maintaining normal plasma colloid osmotic pressure requires a daily intake of 1.2-1.5g/kg of high-quality protein, with priority given to fish, egg white, and whey protein. Patients with combined renal insufficiency should control the proportion of plant protein to avoid exacerbating azotemia. Regular aerobic exercise can enhance liver protein synthesis function, and it is recommended to engage in 150 minutes of moderate intensity exercise per week. Monitoring serum albumin levels should be included in routine physical examinations, with at least one test per year for individuals over 40 years old, and shortened to 3-6 months for patients with cirrhosis or kidney disease. When lower limb depression edema or morning eyelid edema occurs, 24-hour urine protein quantification and liver function should be checked in a timely manner.