Cardiovascular: Controlling regional blood flow L9 Flashcards
Why is there a distribution of cardiac output?
Organs and tissues in the body have differing metabolic requirements for oxygen and nutrients. Blood carries these vital substances, so that each organ requires a different amount of blood. Therefore, blood is distributed in varying amounts to the different organs.
Simply put, how is it possible to create a distribution in cardiac output?
Blood can be distributed to different parts of the body easily because the systemic circuit is in parallel. Because our mean arterial blood pressure is high, this allows good control of amount of flow. Q=ΔP/R. As pressure is a factor in flow, the fact that we have high arterial pressure and lower pressure in resistance vessels and capillaries, allows a large set pressure difference giving a lot of control over flow that can be created with slight changes in total peripheral resistance.
Describe how the overall flow of blood changes as it distributes throughout the body, at rest.
Cardiac output is 5L/min at rest. When blood divides to supply different organs, flow also divides depending on metabolic needs of tissues. After blood has flown through the organs and entered the venous system, veins join together and flow ‘adds’ back together so that flow exiting the organs add to 5L/min.
How is the distribution of cardiac output effected in exercise?
During exercise, the body is in ‘fight or flight’ mode, whereby the body distributes nutrients to tissues that work during fight or flight, and less nutrients to processes that occur more predominantly in ‘rest and digest’. Overall, cardiac output increases, but not every organ realises this increase.
In moderate exercise, there are increases in blood flow to the skeletal muscles (to perform the exercise), increases to the skin (to cool off), and increase to the heart (so nutrients can be transported faster to meet the overall increased metabolic needs). There are decreases in blood flow in systems that aren’t immediately necessary for example the gastrointestinal tract, kidneys and renal area.
In strenuous exercise, there are still further increases in blood flow to the skeletal muscle and heart to compensate for this high demand of nutrients, but there is no longer increased blood flow to the skin, this is to maintain flow to the vital areas.
Notably, the amount of blood flow to the brain is constant and independent to the amount of exercise occurring, because the brain is constantly important.
What changes in the vessels if there was a change in cardiac output during exercise assuming MABP has to remain relatively constant? Give an example of this.
MABP = CO x TPR, mean arterial blood pressure is constantly regulated to be maintained, therefore, resistance changes allow for this change in cardiac output. Different organs will have different resistances at their vessels to direct flow and match their metabolic needs. Resistance in the kidneys and gut will increase in order to reduce flow. Resistance in the muscles, heart and skin will decrease in order to increase flow.
What are the flow rates at varying exercising states?
Rest = 5L/min
Moderate 13 L/min
Intense =25L/min
How does resistance change? What are the terms used to describe this change?
Resistance is altered by changing the radius of the lumen of the vessel. Blood will rather flow down the path in which is easiest i.e the larger radius. Vasconstriction is the narrowing of the radius of the lumen, causing a decrease in blood flow. Vasodilation is the widening of the radius of the lumen, causing an increase in blood flow.
What is the relationship between resistance and radius? What is significant about this relationship?
Inversely proportion:
R=1/(r^4)
This means a tiny increase/decrease of radius, can cause huge relative changes in resistance i.e if radius was doubled, resistance would decrease 16 fold.
What is vascular tone?
The amount of contraction of the arterioles, which have a high proportion of smooth muscle that is susceptible to vasoconstriction or vasodilation.
Explain how the following mechanisms influence vascular tone: Neural, Hormonal, Mechanical, Endothelial, Metabolic.
Many mechanisms influence vascular tone in arterioles:
Neural: Sympathetic nerves causes diffuse vasoconstriction.
Hormonal: for example, adrenaline, causes blood to be distributed in a way appropriate for ‘fight or flight’ (some arterioles will constrict, others dilate)
Mechanical: smooth muscle in arterioles ‘autoregulates’ (adjusts automatically) to change blood flow with normal variations in blood pressure e.g change in posture.
Endothelial: the cells lining the lumen of the blood vessels release chemicals to cause changes in tone. These chemicals allow propagation of changes in tone upstream to larger vessels. They can also act locally to allow more blood into tissues when they have been damaged (remember inflammatory response of immune system).
Metabolic: tissues and blood vessels that are working harder and require more blood release vasodilating chemicals, and tissues and blood vessels that are not working as hard release vasoconstricting chemicals.
