coordinated responses of the CVS Flashcards
Describe the effect of gravity on the flow of blood in the blood vessels
Gravity means there is a higher blood pressure at the feet than there is at the head
Describe how complication can arise in the feet
Blood is able to pool in the veins due to due to the compliant walls, leading to an increased hydrostatic pressure, followed by other complications
What law is used to describe how the blood is able to get back from the feet —> heart
Darcy’s law is not enough to explain how the blood will be able to get from the feet back to the heart.
Therefore, we have to use Bernoulli’s Law which takes into consideration pressure, kinetic and potential energy
What 3 things does Bernoulli’s law take into consideration?
- Pressure
- Kinetic energy
- Potential energy
What is orthostasis hypotension?
Orthostasis: upright standing position
Hypotension: low BP
Orthostatic hypotension (aka postural hypotension) occurs when a person’s blood pressure falls when suddenly standing up from lying/sitting down
What causes a person to feel faint in orthostatic hypotension?
On standing up (orthostasis), the cardiovascular system changes according to the effects of gravity.
The central venous pressure falls at first. According to Starling’s Law, this causes a decreased stroke volume, which decreases cardiac output, which leads to decreased blood pressure. Thus, there is poor perfusion of the brain, leading to postural hypotension, leaving the person feeling very faint.
How does someone recover from orthostatic hypotension?
However, the person is able to quickly recover due to homeostatic mechanisms such as baroreflex. The baroreflex integrates three small changes: an increased heart rate, increased heart contractility and increased total peripheral resistance.
Describe the reflex response to orthostatic hypotension?
1) There is a decreased stimulation (unloading) of the baroreceptors (because the heart is less stretched).
2) There is decreased afferent fibre activity.
3) NTSf switches off inhibitory nerves that go from the CVLM to the RVLM = RVLM being more active, sending efferent heart signals to the heart and arterioles.
4) There is increased sympathetic drive to the SA node, increasing the heart rate. Myocardium contractility also increases. Vasoconstriction of the arterioles and veins occurs, increasing the TPR; there is decreased vagal parasympathetic activity to the SA node. These together increase BP.
NTS: nucleus tractus solitarius
CVLM: caudal ventrolateral medulla
RVLM: rostral ventrolateral medulla
What makes postural hypotension worse?
• Alpha-adrenergic blockade or generalised sympathetic blockade
Drugs that reduce vascular tone
• Varicose veins
Impairs venous return
• Lack of skeletal muscle due to paralysis
Long term bed rest, soldiers on guard etc
• Reduced circulating blood volume
Eg-haemorrhage
• Increased core temperature
Peripheral vasodilation, less blood volume available (eg- standing up after a bath)
Describe the effect of microgravity of space on the CVS
Blood shifts towards the head
There is LESS need for the ANS, RAAS, ADH, and ANP systems to control blood pressure
What actually happens:
• There is an increase in preload = increase in atrial/ventricular volume
• This is sensed by mechanoreceptors
• There is then a:
Decreased sympathetic nerve activity
Reduction is RAAS, ADH
Increase GFR, ANP and diuresis
This all LEADS TO A 20% REDUCTION IN BLOOD VOLUME
What effect does static exercise have on the CVS compared to dynamic exercise?
Static exercise raises the blood pressure more than dynamic exercise. This is because static exercise is constant contraction of a small number of muscles, so there is a higher load. With dynamic exercise, there is a shortening/lengthening of many muscles, which is a low load.
What are the general CVS responses to exercise?
- Increase lung oxygen uptake, which is transported around the body and supplied to exercising muscle
- Controlled BP, despite huge changed in CO and TPR (this is to protect the heart from excessive afterload)
Increase mechanoreceptor and metaboreceptor stimulation
Describe how the integration of several small adaptations create an overall large response to exercise
The oxygen uptake by the pulmonary circulation can increase 10-15 times during strenuous exercise. This BIG change is brought about by 3 small changes:
1. Increased heart rate (3x) 2. Increased stroke volume (1.5x) 3. Increased arteriovenous oxygen difference (3x)
Describe what occurs during exercise-induced tachycardia, and how stroke volume is affected
There is an increased stimulation of the brain central command, along with the increased stimulation of the muscle mechanoreceptors.
There is a decrease in parasympathetic stimulation, and in increase in sympathetic stimulation, both affecting the SA and AV nodes.
There is an increased stroke volume, due to an increase in sympathetic activity. There are factors that contribute to the increased stroke volume:
1. Increased end diastolic volume
2. Faster ejection
3. Decrease end systolic volume
Describe the changes in cardiac output and flow changes during exercise
There is a fall in local resistance due to metabolic hyperaemia (excess blood) vasodilation. There is also increased sympathetic activity and B2 mediated vasodilation via the circulating adrenaline. There is a HIGH EXPRESSION of B2 receptors in skeletal muscle and coronary arteries. This increases cardiac output and flow changes.
