Describe the transfer of substances between blood in capillaries, tissue fluid and body cells

Sourav PanNovember 3, 2024

Answered

The transfer of substances between blood in capillaries, tissue fluid, and body cells is a critical process that facilitates the exchange of nutrients, gases, and waste products necessary for cellular function. This process primarily occurs through the capillary walls and involves mechanisms such as filtration, diffusion, and osmosis.

Formation of Tissue Fluid

1. Filtration:
   • At the arterial end of capillaries, blood is under high hydrostatic pressure (approximately 32 mm Hg). This pressure forces water and small solutes (such as oxygen, glucose, and ions) out of the capillaries into the surrounding tissue spaces, forming tissue fluid (also known as interstitial fluid).
   • Larger molecules, particularly plasma proteins, remain in the blood due to their size and contribute to colloidal osmotic pressure (around 25 mm Hg), which opposes filtration. However, at this point, hydrostatic pressure exceeds osmotic pressure, allowing for net fluid movement out of the capillaries.
2. Composition of Tissue Fluid:
   • Tissue fluid contains water, nutrients (like glucose and amino acids), ions, and gases (such as oxygen), but it has significantly fewer proteins compared to blood plasma due to the selective permeability of capillary walls.

Transfer of Substances

1. Diffusion:
   • Once formed, tissue fluid bathes the body cells. Substances move between the blood and cells primarily through diffusion, where molecules move from areas of higher concentration to areas of lower concentration.
   • Oxygen and nutrients diffuse from the capillaries into the tissue fluid and then into body cells. Conversely, carbon dioxide and metabolic waste products produced by cells diffuse into the tissue fluid and then back into the capillaries.
2. Reabsorption:
   • At the venous end of the capillaries, hydrostatic pressure decreases (around 15 mm Hg), while osmotic pressure remains relatively high due to the presence of plasma proteins. This pressure differential encourages reabsorption of tissue fluid back into the capillaries.
   • Approximately 90% of the filtered fluid returns to circulation through this reabsorption process; however, about 10% is drained into lymphatic vessels for immune function and fluid balance .

Summary of Substance Transfer

• Oxygen Transport: Oxygen moves from blood in capillaries to tissue fluid and then diffuses into body cells.
• Nutrient Transport: Nutrients such as glucose and amino acids also move from blood to tissue fluid before entering cells.
• Waste Removal: Carbon dioxide and other waste products produced by cells diffuse into tissue fluid and are transported back into capillaries for removal.