The distribution of nutrient materials to, and the uptake of waste products from, the tissues occurs only in the capillary bed. Arteriolar resistance ensures that blood enters the capillaries at a pressure only slightly higher than that of the colloid osmotic pressure of the blood. Arteriolar resistance is normally under the influence of a sparse sympathetic vasoconstrictor discharge superintended by the activity of ‘vasomotor centres’ in the brain stem. The activity of these ‘vasomotor centres’ is itself modified appropriately by afferent impulses from the sino-aortic baroreceptors.
Veins subserve mainly a capacity role but are also involved in veno-constrictor reflexes.
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