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BIL 118 (28)


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BIL 118
O' Sullivan

Capillaries  Smallest, most numerous blood vessels o Carry blood from arteries to veins o Blood flows from arterioles into the capillaries, then from them into venules o Size of lumen is roughly equal to diameter of erythrocytes o Thin wall is composed of endothelium (single layer of overlapping flat cells)  Function: exchange of materials between blood and tissue cells (e.g. O2, CO2, nutrients, wastes)  Capillary distribution varies with metabolic activity of body tissues o Skeletal muscle, liver, and kidney have extensive capillary networks o They are metabolically active and require an abundant supply of oxygen and nutrients o Connective tissue have less abundant supply of capillaries o Epidermis of skin and lens, and cornea of eye completely lack a capillary network  Flow of blood is controlled by structures called precapillary sphincters o Located between arterioles and capillaries o Muscle fibres allow them to contract  Hydrostatic pressure is created by the heart which pumps blood into arteries  At the arteriole end o Hydrostatic pressure > water potential o As plasma proteins lower water potential o H2O, small molecules, fluid are forced out through the permeable capillary wall o Plasma proteins are not forced out as they are too large  At the venule end o Water potential > hydrostatic pressure (due to lower volume) o Fluid tends to flow back into the blood with waste products produced by cells Lungs and Respiration Diffusion and gas exchange  All organisms exchange food, waste, gases, heat with their surroundings by diffusion  Rate of diffusion is given by Fick's law and depends on o Thickness of the membrane the molecules must diffuse across, o Surface area for gas exchange o Mass and solubilit
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