Exchange of materials in the capillaries

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  • The Exchange of Materials between Capillaries and Body Cells.
    • The blood carries many materials around the body.
      • Oxygen.
      • Products of digestion- these are transported to the liver to be processed or stored.
      • Urea- a waste product formed from the breakdown of excess proteins in the liver.
      • Hormones from the sight of production.
    • There are two ways in which materials are exchanged between capillaries and body cells.
      • 1. Passive Diffusion
        • The movement of soluble molecules from areas of high concentration to areas of low concentration
        • Body cells are surrounded with tissue fluid (interstitial fluid), which acts as an intermediary
        • Tissue fluid essentially takes on the same composition of small, soluble molecules as the arterial blood travelling throught the capilllary.
      • 2. Bulk flow
        • Occurs when the pressure inside the capillary is lower than the pressure outside and causes the liquid component of blood (plasma) to be forced out through pores in the capilllary walls.
        • Sometimes called forced filtration or ultrafiltration.
        • Most proteins are retained in the capillarys they are too large to fit through the pores.
    • Tissue fluid supplies cells with glucose, oxygen and other soluble molecules (e.g. amino acids; ions etc.).
      • CO2 and other metabolic wastes diffuse out of the cells, through the tissue fluid and into the capillaries to be excreted through the circulatory system.
      • Tissue fluid volume is regulated as excess fluid returns to the blood either through the capillary walls or through the lymphatic system.


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