Some diseases are closely linked to life-style

Capillaries

• Smallest, most numerous blood vessels
  • Carry blood from arteries to veins
  • Blood flows from arterioles → capillaries → venules
  • Size of lumen is ≈diameter of erythrocytes
  • Thin wall is composed of endothelium (single layer of overlapping flat cells)
• Function: exchange materials between blood and tissue cells (O2, CO2, nutrients, wastes)
• Capillary distribution varies with metabolic activity of body tissues
  • Skeletal muscle, liver, and kidney have extensive capillary networks
  • They are metabolically active and require an abundant supply of oxygen and nutrients
  • Connective tissue have less abundant supply of capillaries
  • Epidermis of skin and lens, and cornea of eye lack capillaries
• Flow of blood is controlled by precapillary sphincters
  • Found between arterioles and capillaries
  • Smooth muscle allows them to contract and reduce blood flow
• Hydrostatic pressure is created by the heart which pumps blood into arteries
• At the arteriole end
  • Hydrostatic pressure > water potential
  • Plasma proteins lower water potential
  • H2O, small molecules, and fluid are forced out through permeable capillary wall
  • Plasma proteins are not forced out as they are too large
• At the venule end
  • Water potential > hydrostatic pressure (due to lower volume)
  • Fluid flows back into blood with waste products produced by cells

Blood Plasma

• Plasma is a liquid containing proteins, inorganic salts, amino acids, vitamins, hormones
• Main plasma proteins are albumin, globulins, and fibrinogen
• Albumin maintains osmotic pressure and acts as a transport protein for various substances
• Globulins are mainly antibodies
• Fibrinogen is involved in clotting process

Oedema

• Accumulation of tissue fluid in interstitial space
• ↑capillary hydrostatic pressure due to increased blood volume
  • Heart failure
  • Impaired venous valves of leg (backflow of blood)
• ↑capillary permeability due to inflammation
• Impaired fluid return to blood due to low protein levels in blood plasma
  • This ↓colloid osmotic pressure of plasma
    • Fluid fails to return to blood at venous end
    • Fluid accumulates in interstitial space
  • Caused by
    • Low protein absorption from gut
    • Liver disease (proteins not produced)
    • Kidney disease (proteins leak into urine)
• Blockage of lymphatic vessels
  • Small amounts of plasma protein leak out of blood
  • Usually transported back into the circulation by lymphatic vessels
  • Blocked vessels causes accumulation of protein in interstitial space
  • This ↑colloid osmotic pressure
  • Fluid diffuses from blood into interstitial space
• Oedema increases diffusion pathway of nutrients and oxygen between blood and cells