Local Blood Flow Regulation
- Blood flow through tissues is not only critical for delivery of oxygen, nutrients, and hormones, but also to carry away metabolic waste products. However, blood volume and the energy required for its circulation through the vasculature is limited and precious. Consequently, long-term homeostasis requires that the quantity of blood flow to a particular tissue is precisely matched to and does not unnecessarily exceed its metabolic and functional requirements. It should be pointed out that certain specialized organs may receive blood flow in excess of their own individual metabolic demands in order to achieve their specialized physiological functions. For example, the kidneys receive an enormous amount of the total body blood flow in order properly regulate plasma fluid and electrolyte levels.
- Short Term Regulation
- Short-term changes of blood flow to particular tissues is regulated by arterioles and pre-capillary sphincters which lie upstream of capillary beds. By contracting or dilating, these segments of the vasculature can substantially change their resistance to blood flow. Increased resistance via contraction will reduce blood flow into the downstream capillary bed whereas reduced resistance via dilation will increase blood flow. The processes which regulate arteriolar and pre-capillary sphincter are coordinated by certain mechanisms that operate intrinsically within the tissues and others which are coordinated by extrinsic, neuroendocrine mechanisms. These are discussed in detail in the subsections below.
- Long-term Regulation
- Chronic under-use or chronic over-use of tissues may require long-term changes in local blood flow. Long-term increments or decrements in blood flow are largely the result of changes in the amount of tissue vascularity which are coordinated by complex networks of growth factors. We will not discuss long-term changes in local blood flow further, but be aware that such physiological mechanisms do exist.