Blood Composition and Components

Study Notes

Blood is a vital component of the extracellular fluid, circulating throughout the body via the cardiovascular system.
It accounts for 8% of body weight, approximately 5.6 liters in a typical adult male.

Blood consists of two primary components: plasma (55%) and the cellular part (45%).
Plasma is the fluid portion of blood, while the cellular part comprises red blood cells, white blood cells, and platelets.

Plasma is a clear, yellowish fluid, making up about 3.5 liters (5% of body weight).
Plasma clots when allowed to stand, leaving behind a remnant called serum.

Water (90%): The primary constituent of plasma.
Organic Substances (9.1%):
- Plasma proteins (7.2-7.4 g/dl) play crucial roles in various physiological functions.
- Lipids, glucose, amino acids, vitamins, enzymes, and waste products are also present.
Inorganic Constituents (0.9%):
- Sodium (Na+) is the main inorganic cation.
- Chloride (Cl-) is the primary inorganic anion.
- Bicarbonate (HCO3-), phosphates, and sulfates are also present.
Blood Gases (O2, CO2, N2): Essential for gas exchange and respiration.

Types: Several plasma proteins exist, with albumin, globulin, fibrinogen, and prothrombin being the most significant.
- Albumin: 3.5-5 g/dl, molecular weight 69,000.
- Globulin: 2.5 g/dl, molecular weight 90,000-156,000.
- Fibrinogen: 0.4 g/dl, molecular weight 340,000.
- Prothrombin: 0.01 g/dl, molecular weight 68,700.
Sites of Formation:
- Albumin, fibrinogen, and prothrombin are synthesized in the liver.
- Globulins are produced both in the liver (50%) and in plasma cells of the reticuloendothelial system (50%), a network of cells found primarily in the liver, spleen, lymph nodes, and bone marrow.

Osmotic Function:
- Plasma proteins contribute to the total osmotic pressure of plasma (25 mmHg), also known as colloidal osmotic pressure or oncotic pressure.
- This pressure aids in regulating the exchange of fluids between interstitial fluid and blood through capillary membranes, influencing blood volume.
Transport Function:
- Albumin and globulins (α and β) act as carriers for vital substances like hormones, vitamins, lipids, and minerals.
Defensive Function:
- γ-globulins, also termed immunoglobulins or antibodies, play a critical role in defending the body against microorganisms and their toxins.
Blood Clotting Function:
- Prothrombin and fibrinogen are essential for the process of blood clotting.
Viscosity:
- Plasma proteins contribute to the viscosity of blood, making it 3-5 times as viscous as water. Plasma alone is 1.5 times as viscous as water.
Capillary Function:
- Plasma proteins influence capillary permeability by partially blocking pores in the capillary membrane.
Buffer Function:
- Plasma proteins act as weak acids or bases, maintaining the pH of blood at a stable 7.4.
Source of Tissue Proteins:
- Plasma proteins can be broken down into amino acids in the liver and macrophages, providing a source of rapid protein replacement for depleted tissues.