4.6 Factors Affecting Blood Flow Flashcards
Describe vasodilation and vasoconstriction
Vasodilation is the increase in the internal diameter of a blood vessel that results from relaxation of smooth muscle within the wall of the vessel. This causes an increase in blood flow, but a decrease in systemic vascular resistance and a decrease in pressure.
Vasoconstriction is the decrease in the internal diameter of a blood vessel that results from contraction of smooth muscle within the wall of the vessel. This causes a decrease in blood flow, but an increase in systemic vascular resistance and an increase in pressure.
Describe the aim of blood flow distribution in the body.
Blood flow distribution must be balanced so that essential organs (i.e brain,heart,muscle) have a continuous supply no matter what. Blood pressure needs to stay almost constant, so changes in total flow is done by changing cardiac output. However, the heart is not capable of supplying blood to match excessive demand in all tissues, therefore blood is redirected to meet demand according to the needs to the tissue (so differing metabolic demands of different tissues can be met).
How is blood flow through the capillaries regulated
Blood flow through capillaries are regulated through resistance vessels (arterioles) so that blood flow and tissue oxygen consumption can be closely matched
Describe the relationship in a blood vessel between pressure, tension and flow
Transmural pressure is the pressure exerted across the vessel wall, this pressure causes the smooth muscle cells to be stretched causing tension in the wall. Flow is then caused by the difference in pressure across a vessel/bed (e.g p=100 at aorta and p=20 at capillary)-hence a pressure drop across the vascular system causes blood flow
What is autoregulation
Autoregulation is the ability of vessels to maintain a stable blood flow despite changes in blood pressure. Autoregulation is myogenic (originates from the heart)
Describe the hierarchy in control of vessel blood flow
The CNS controls blood pressure and body temperature. Hence the neural regulation of the blood flow is almost all from continuously active sympathetic nerves causing vasoconstriction. But the CNS can be locally over-ridden in essential vascular beds by substances released by the endothelium.
What are some different mechanisms vascular beds have to override neural control and maintain appropriate blood flow.
Hyperaemia is increased blood flow. Active and reactive hyperaemia occur when products of metabolism build up, causing vasodilation and increased blood flow. These are useful for tissues whose metabolism changes often like skeletal muscle, the heart and parts of the brain. Many different metabolites have been identified as potential candidates to regulating re/active hyperaemia, adenosine being one main one (others include K+, phosphate and prostaglandins). Adrenaline can also contribute.
Flow mediated vasodilation is when increased blood flow leads to vasodilation to accommodate this higher flow
Myogenic tone is where increased blood pressure against the vessel walls causes a stretch in the vessel walls that results in a vasoconstriction reflex. This vasoconstriction gives vascular tone in order to reduce the blood flow to maintain autoregulation despite the increased blood pressure.
Give some general compounds that can stimulate metabolic autoregulation
adenosine, O2, ATP, ADP, lactic acid, CO2, histamine, K+ and H+
