Ramchandra: Regulation of Vascular Function Flashcards
What is the normal cardiac output at rest?
5L/min
Most of the cardiac output goes to the kidneys, liver, muscle and brain.
What is the cardiac output of an individual during exercise? How does it increase?
25L/min - increased through increased heart rate as well as stroke volume, the second due to the frank starling law
Most of the cardiac output goes to the skin and muscles. Percentage of CO going to the brain drops but the volume stays the same. Drop caused by the increased CO.
Muscle - Increased O2 for increased ATP generation.
Skin - Increased blood flow to release heat; reducing protein damage.
What is the consequence of constant ultramarathon running?
Increased periods of reduced internal organ perfusion leading to internal organ damage.
How does the innervation vary across the vascular tree?
Sympathetic Nerves supply all of the vascular beds. However, arteries and arterioles are densely innervated, whereas the veins are more sparsely innervated.
Innervation of the veins increases as the vessel size increases.
Capillary beds receive no direct innervation.
How do blood vessel dimensions change?
Smooth Muscle Cells that surround the endothelium of the capillaries contract when stimulated by sympathetic nerves.
The highest proportion of vascular smooth muscle found in small arteries and arterioles.
How does vasoconstriction affect pressure?
Vasoconstriction increases the precapillary resistance and therefore blood flow is reduced.
- However, this also leads to a greater dissipation of energy as blood flows through the small arteries and arterioles.
- This further leads to an increased pressure gradient and therefore reduced hydrostatic pressure in the capillaries.
Postcapillary constriction can occur to some level but is not as effective as precapillary
What is autoregulation?
This is where perfusion pressure is altered to a specific organ and the neural input is ablated. It uses constriction to adjust the flow in order to counteract pressure changes. (eg. if pressure increases then the precapillary vessels constrict allowing less flow thus returning perfusion to its constant)
- Acute Reduction in Perfusion Pressure = Increase to constant over time.
- Acute Increase in Perfusion Pressure = Decrease to constant over time.
What is reactive hyperaemia?
This is the phenomenon that demonstrates the link between metabolism and blood flow. It occurs after a blood vessel is occluded for a short period. When the occlusion is released, blood flow rises above the pre-occlusion level and this hyperaemia is maintained for a period roughly proportional to the duration of the occlusion.
What is the myogenic hypothesis?
Is linked to the understanding/explanation of autoregulation
- Increased perfusion pressure increases vascular pressures throughout the circulation.
- Increased transmural pressure leads to vascular distension.
- Stretch elicits smooth muscle contraction.
What is the metabolic hypothesis of local control?
If a given organ is working harder it will demand more flow and cause vasodilation of the arteries supplying it. It does this through release of metabolites that cause vasodilation.
- For Example: If the heart is asked to work harder (through an increase in heart rate) it will result in coronary vasodilation. In this example, the effects of norepinephrine that binds to alpha receptors causing vasoconstriction is overridden.
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What are the mediators of the metabolic hypothesis?
- Decreased pH - Especially in exercising muscle - lactic acid
- Increased potassium - As more ATP is utilised.
- Adenosine and Adenine Nucleotides.
Where are the effects of the metabolites most strong?
The metabolites have their strongest effect in causing vasodilation in the terminal of the pre-capillary vessels but is limited in the post-capillary vessels.
The effects of these metabolites do not explain the coordinated vasodilation throughout pre-capillary distribution circuit
How is NO formed?
Shear stress on vascular endothelium - caused by increased blood flow - leads to the catalysation of L-Arginine to citrulline and NO by NO synthase (endothelial form of the enz.)
What are the two other integrative models?
- ATP released in microcirculation by red blood cells due to reduced pO2 acts directly on endothelium.
- Electrically activated vasodilation conducted rapidly upstream via the endothelium.
The diagram on the integrated model of local control…
What occurs in postganglionic sympathetic nerves in response to their depolarization?
- Calcium enters the nerve terminal via calcium channels.
- Calcium causes vesicles containing norepinephrine to fuse to the nerve membrane,
- Norepinephrine then released out into the synaptic cleft
- It then binds to a receptor or difuses into the blood stream