Flashcards in Responses to CVS Stress Deck (19)
Why is gravity a problem for the cardiovascular system?
Gravity pushes a column of blood down from the head to the toes causing distention in the veins lower down in the body and increasing the volume of blood in the venous reservoir.
Why is blood pressure measurement taken from the arm?
Because it is level with the heart. When standing up, the blood pressure will decrease above the heart and increase below the heart so at the level of the heart, gravity would cause less of a change in pressure.
How does standing up affect blood pressure and why?
The blood pressure increases below the heart and decreases above the heart because gravity pushes a column of blood down from the head to the toes.
How does the change in posture affect fluid movement?
As gravity pushes blood down the body, there is an increase in capillary hydrostatic pressure meaning that more fluid leaves the capillaries and enters the interstitial compartment. This means that there is reduction in effective circulating blood volume so there is decreased venous return to the heart.
Explain how standing up affect venous return to the heart?
Blood pools in the distensible veins and fluid leaves the circulatory system due to the increased hydrostatic pressure so there is a decrease in the venous return to the heart and hence less ventricular filling and less stretching of the muscle fibres (preload) so the force of contraction is decreased (Starling's Law).
What is haemorrhage?
Reduction in actual circulating blood volume.
What are the three main responses to haemorrhage?
Increased heart rate
Increased heart contractility
Organ specific vasoconstriction
What is the compensatory mechanism that takes place in the capillaries to preserve fluid volume?
Autotransfusion - more fluid is retained in the capillaries (because the hydrostatic pressure is decreased due to the decrease in fluid volume but the colloid osmotic pressure is the same) - this means that less fluid leaves the capillaries and more fluid is reabsorbed.
State three hormones that are important in the response to haemorrhage?
What is the action of angiotensin II?
Angiotensin II is a powerful vasoconstrictor - it will decrease the amount of blood flowing to the kidneys and hence it will decrease urine output
Where do aldosterone and vasopressin act?
Kidney Collecting Duct
What are the effects of aldosterone?
Aldosterone causes the loss of K+ into the urine and the retention of Na+ and water
What are the overall effects of these three hormones?
Reduced blood flow to the kidneys causing reduced urinary output
Increased fluid retention to preserve blood volume and pressure
What percentage fluid loss can these compensatory mechanisms cope with?
Describe a mechanism that causes local vasodilation.
Active Hyperemia - if there is a lot of metabolism going on in a tissue, it will use up a lot of oxygen and glucose. The uptake of oxygen and glucose is detected by the tissue and it causes an increase in vessel radius.
What are the two afferent inputs to the medullary cardiovascular centre?
Preprogrammed pattern - autonomic response in anticipation of exercise
Chemoreceptors in the muscle - detect a changing environment
How does exercise affect TPR? What is the overall change?
Exercise causes a lot of vasodilation (e.g. to the skeletal muscle and the skin (to radiate heat away)) but there is some organ-specific vasoconstriction (spleen, GI tract). The overall effect is a DECREASE in TPR.
How does exercise affect CO? What is the overall change?
Increased sympathetic activity increases heart rate and contractility and the muscle activity causes increased venous return to the heart and so increased preload. However, there is an increase in hydrostatic pressure so more fluid is lost to the interstitium and fluid is lost as sweat. Overall there is an INCREASE in CO.