Regulates blood flow by changes in tissue blood vessels
Vascular material remodelling occurs in response to tissue demand. This remodelling occurs rapidly within a few days in young animals. It also occurs rapidly in new tissues such as scar tissue, cancer tissue.
The main mechanism of blood flow regulation is to alter tissue perfusion. For example, if tissue metabolism is increased during prolonged periods of increased perfusion, the process is called angiogenesis. If metabolism decreases, perfusion decreases. The figure shows a large increase in the number of capillaries on electron microscopy to contract over a short period of a few days to 30 days, compared with unstimulated muscle in the other leg of the animal.
Thus, vascular remodelling occurs in response to the needs of the tissue. This remodelling occurs rapidly within a few days in young animals. It also occurs rapidly in newly grown tissues such as scar tissue and cancerous tissue, but it occurs much more slowly in mature, solid tissues. Therefore, the time required for long-term correction to take place can be as few days in infants or as long as months in older adults. Furthermore, the degree of response was much better in young tissue than in old tissue. Thus, in young tissues, perfusion will adjust to almost match the tissue’s needs exactly, but in older tissues, perfusion is often much slower than tissue demand.
Figure. A large increase in the number of capillaries (white dots) in the anterior tibial muscle of rats, which was electrically stimulated to contract every day for 30 days (B) compared with unstimulated muscle (A) in 30 Intermittent electrical stimulation was modified mainly by rapid contraction, the anterior tibial muscle changed mainly by slow contraction, and oxidative muscle with an increase in the number of capillaries and a decrease in myofibril diameter as shown.
The role of oxygen in long-term regulation
Oxygen is important but not only for the immediate regulatory mechanism but also for the long-term control mechanism. Examples include increased tissue perfusion in animals living at high altitudes where atmospheric oxygen is low. In preterm infants receiving tent oxygen for therapeutic purposes, excessive oxygen causes immediate cessation of growth of new vessels in the retina of the newborn eye and may even cause degeneration of some small circuit that has been shaped. When infants are stopped from tent oxygen, the explosive growth of new blood vessels occurs to counteract the sudden drop in blood oxygen. Indeed, when overgrowth occurs, blood vessels in the retina grow from the retina into the vitreous of the eye, eventually causing blindness, a condition known as retinal fibrosis.
Importance of angiogenesis factors in new blood vessel formation
Many factors that promote new blood vessel growth have been identified, almost all of which are peptides. The four most well-described factors are endothelial vascular growth factor VEGF, fibroblast growth factor, platelet-derived growth factor PDGF, and angiogenin, each of which is cleaved from tissue that has a supply disproportionately. perhaps it is due to a lack of tissue oxygen or nutrients or both, leading to the formation of angiogenic factors (called “angiogenic factors”).
Angiogenesis begins with new blood vessels sprouting from other blood vessels. The first step is the dissolution of the basal membrane of the endothelial cell at the point of emergence. Following this step is the reproduction of new vascular endothelial cells that flow outward through the vessel wall on pathways extending directly toward the source of the angiogenic factors. The cells on each cord continue to divide and rapidly coat the inside of the tube. Next, the tube connects to other tubes that grow from new blood vessels (other arterioles). If the blood flow is greater, the smooth muscle cells eventually spread into the vessel wall, so some new blood vessels develop in the arterioles or veins or possibly in the larger vessels. Thus, angiogenesis explains the ways in which local tissue metabolites can induce the growth of new blood vessels.
Other substances such as some steroid hormones have opposite effects on small blood vessels, sometimes even causing vascular wall cells to die and blood vessels to disappear. therefore, blood vessels may also disappear when they are not needed. Tissue-produced peptides can block the growth of new blood vessels. for example, angiostatin, the protein fragment, is a natural inhibitor of angiogenesis. Endostatin is also an angiogenic antagonist, which is isolated from collagen breakdown type XVII. Although the biological functions of these angiogenic antagonists are unknown, they are of great importance in their application to stop tumour haematopoiesis and, therefore, to prevent rapid blood flow. needed to choose nutrition for rapidly growing tumours.
Perfusion was determined by maximal blood flow demand, not average demand
The most important feature of long-term blood flow control is that perfusion is determined primarily by maximal blood flow demand rather than average. For example, during high-energy exercise, blood flow requirements for the whole body usually increase 6 to 8 times with blood flow at rest. Exceeding this blood flow limit may not be required for more than a few minutes per day. However, even this short-term need can cause elements of the angiogenic mechanism to be produced by the muscle to increase the amount of blood required. if it's not available, every time a person tries hard exercise, the muscle will get the lack of nutrients it needs, especially oxygen and so the muscle won't contract.
However, after the newly formed blood vessel develops, the additional blood vessel is usually kept in a state of contraction, only opening to allow more blood flow when there is an appropriate local stimulus such as hypoxia, stimulation from the nerve causing the blood to flow through. Vasodilation, or other stimulation of the blood flow, is required.