Circulatory System and Capillary Exchange

Circulatory System Purpose

  • Primary function: Deliver oxygen and nutrients to body tissues; remove waste products.
  • Exchange location: Capillaries (smallest blood vessels).
  • Capillary wall composition: Single layer of endothelial cells.

Substance Movement Across Capillaries

  • Several mechanisms facilitate the movement of substances between blood and surrounding tissues:

Diffusion Through Endothelial Cell Membranes

  • Permeability:
    • High for small lipid-soluble molecules and gases due to the hydrophobic nature of the cell membrane.
    • Oxygen: Moves from blood to tissue along the concentration gradient.
    • Carbon dioxide: Moves from tissue to blood along the concentration gradient.
  • Facilitated Diffusion:
    • Glucose and small water-soluble molecules utilize specific channels (transporters) to cross the cell membrane.
  • Osmosis:
    • Water moves via osmosis.

Transcellular Vesicle Transport (Transcytosis)

  • Process:
    • Proteins and hormones are packaged into lipid vesicles.
    • Transport occurs through endothelial cells via endocytosis and exocytosis.

Bulk Exchange Through Intercellular Clefts and Fenestrated Capillaries

  • Intercellular Clefts:
    • Gaps between endothelial cells facilitate bulk exchange in most tissues.
  • Fenestrated Capillaries:
    • Contain pores that allow for exchange in some tissues.
  • Bulk Flow:
    • Blood plasma (containing nutrients) exits capillaries at the arterial end (filtration).
    • Tissue fluid (containing wastes) re-enters capillaries at the venous end (reabsorption).

Forces Driving Bulk Flow

  • Balance between hydrostatic and osmotic forces:

Hydrostatic Force

  • Definition: Pressure of fluids in a closed space.
  • Inside capillaries: Corresponds to capillary blood pressure.
  • Tissues: Generally have lower fluid content, resulting in lower hydrostatic pressure compared to capillaries.
  • Effect: Drives fluids and blood solutes out of the capillaries.

Osmotic Force

  • Also known as oncotic pressure.
  • Generation: Primarily due to differences in protein concentrations (especially albumin) between blood and interstitial tissue.
  • Blood: Higher protein content draws water into blood vessels.

Net Flow and Pressure Differences

  • Arterial End:
    • Higher capillary blood pressure and, consequently, hydrostatic pressure due to proximity to the heart.
    • Net outward flow.
  • Venous End:
    • Osmotic pressure remains constant.
    • Net inward flow.
  • Overall:
    • Net outward filtration exceeds net inward reabsorption.
    • Approximately 15% of fluid remains in tissues and is collected by the lymphatic system.

Edema

  • Definition: Abnormal accumulation of excess fluid in tissues, manifesting as swelling or enlarged organs.

  • Principal Causes:

    • Increased Filtration:
    • Caused by elevated blood pressure or increased capillary permeability.
    • Decreased Reabsorption:
    • Results from reduced blood albumin concentrations.
    • Obstruction of Lymphatic Drainage.

  • Consequences:

    • Hinders nutrient/waste exchange and gas exchange.
    • May lead to tissue necrosis.
    • Severe edema may reduce blood volume, potentially causing circulatory shock.