3.5.7 Capillaries & Tissue Fluid

Capillaries:

• Smallest blood vessels, found in close contact with tissues.
• Site of metabolic exchange where substances such as oxygen and nutrients move between the blood and cells.
• Structure:
  • One cell thick walls: Provides a short diffusion pathway for efficient exchange.
  • Narrow diameter: Slows blood flow, allowing time for diffusion.
  • Gaps between endothelial cells: Enable movement of small molecules.

Tissue Fluid:

• Tissue fluid is the liquid that surrounds cells in tissues.
• It contains dissolved oxygen, glucose, amino acids, fatty acids, and ions to supply cells with nutrients.
• Also facilitates the removal of waste products, such as carbon dioxide and urea, from cells.

Formation of Tissue Fluid:

1. Hydrostatic Pressure:

• Blood is pumped along arteries → arterioles → capillaries, creating hydrostatic pressure in the capillaries.
• This pressure forces plasma (fluid) and small molecules out of the capillaries into the surrounding tissue.

2. Selective Permeability:

• Only small molecules like glucose, amino acids, oxygen, and ions can leave the capillaries.
• Large molecules like plasma proteins and red blood cells remain in the blood as they are too large to pass through the capillary walls.

3. Tissue Fluid:

• The fluid forced out forms the tissue fluid, which bathes the cells and provides a medium for the exchange of nutrients and waste.

Reabsorption of Tissue Fluid:

4. Water Potential Gradient:

• Plasma proteins remaining in the capillaries lower the water potential of the blood.
• The water potential of the tissue fluid is higher than that of the blood.
• Water re-enters the capillaries by osmosis towards the venous end of the capillary bed, where hydrostatic pressure is lower.

5. Hydrostatic Pressure:

• As blood reaches the venous end of the capillaries, the hydrostatic pressure in the capillaries drops.
• This allows some tissue fluid to flow back into the capillaries.

Excess Tissue Fluid and the Lymphatic System:

1. Excess Fluid:

• Not all tissue fluid returns to the capillaries.
• The remaining fluid enters the lymphatic system.

2. Lymphatic System:

• A network of vessels that drain excess tissue fluid.
• Fluid within the lymphatic system is called lymph.
• Lymph contains fewer nutrients and oxygen than tissue fluid but carries waste products and pathogens.
• Lymph is returned to the bloodstream via the subclavian vein.

3. Lymph Nodes:

• Found in the lymphatic system, they filter the lymph.
• Contain lymphocytes, which help destroy pathogens as part of the immune response.

Key Terms:

• Tissue Fluid: Fluid that surrounds cells, providing nutrients and removing waste.
• Hydrostatic Pressure: The pressure exerted by blood plasma against the walls of capillaries.
• Water Potential Gradient: The difference in water potential that drives osmosis.
• Lymphatic System: A network that collects and returns excess tissue fluid to the bloodstream.

Summary of Tissue Fluid Formation and Reabsorption:

Step	Action
At the arterial end of capillaries	High hydrostatic pressure forces fluid and small molecules out of the capillaries.
In the tissue	Tissue fluid bathes cells, delivering nutrients and removing waste.
At the venous end of capillaries	Low hydrostatic pressure and high oncotic pressure (due to plasma proteins) draw water back in via osmosis.
Excess fluid	Drains into the lymphatic system and returns to the bloodstream.