Coordinated Responses of the CVS - Gravity & Exercise Flashcards
Describe the three main changes that occur during orthostasis
On standing up (orthostasis), the cardiovascular system changes according to the effect of gravity:
- Blood pressure falls at first - postural hypotension, lack of blood flow to the brain - faint
- Quickly recovers - due to homeostasis mechanisms such as baroreflex
- Baroreflex integrates three smaller changes by increasing heart rate, force of contraction and total peripheral resistance
What are the 3 changes that the baroreflex causes
Increasing heart rate, force of contraction and total peripheral resistance
Describe the effects of gravity on blood pressures during orthostasis
When laying flat, the arterial and venous pressures are higher at the head and feet and arterial is higher near the heart whereas venous is lower nearer the heart e.g. arterial - 95 mmHg at head and feet and 100 mmHg at the heart and venous - 10 mmHg at head and feet and 3-5 mmHg at heart
When standing up the effect of gravity means that the arterial pressure at the head decreases e.g. to 60 mmHg from 95 mmHg, the arterial pressure at the heart also decreases slightly e.g. 100 mmHg to 95 mmHg and at the feet it becomes much higher e.g. 180 mmHg from 95 mmHg
The venous pressure also changes so that the pressure in the head is close to or at 0 mmHg , the venous pressure near the heart is also low at 0-5 mmHg and the pressure at the feet has increased from 10 mmHg to 90 mmHg
Describe how the heart is still able to pump blood to the head and feet despite gravity
- Blood is still able to flow up to the head because the arterial pressure near the heart is higher than in the head so the blood flows down the pressure gradient
- But in the feet the blood would have to flow against the pressure gradient - based on bernoulli’s law
(blood flow = pressure energy + potential energy + kinetic energy)
- Although the pressure gradient is against blood flow to the feet the potential and kinetic energy is in favour of it and so these areas are still perfused
Describe how gravity induces high venous blood pressure
- Gravity causes venous pressure in the feet to increase
- This is because of hydrostatic pressure
- The pressure is higher at the lower end of a person and the extent the pressure increases is dependent on the height of that person, the density of the fluid (blood) and gravitational forces (pressure = fluid density x height x gravitational acceleration constant)
Describe how orthostasis causes hypotension
- Blood pools in the legs due to the compliance of the veins which reduces the amount of blood returning to the heart - there is also transmural pressure which can cause exudation again reducing venous return
- This causes central venous pressure to decrease which then decreases the end diastolic volume
- As less blood is in the heart each time it contracts the stroke volume is decreased which decreases cardiac output
- This then leads to poor perfusion of the brain and so can lead to dizziness/fainting
Describe the reflex response to orthostasis
- Less stimulation (unloading) of baroreceptors
- Lower afferent fibre activity
- Less NTS switches off inhibitory nerves that go from caudal ventrolateral medulla (CVLM) to rostral ventrolateral medulla (RVLM)
- Results in RVLM being more active sending efferent signals to heart and arterioles
- Increased sympathetic drive to SA node and increased HR. Myocardium increased contractility. Vasoconstriction (arterioles, veins) increases TPR
State some factors that can make postural hypotension worse
- Alpha adrenergic blockades or general sympathetic blockades/other drugs that reduce vascular tone e.g. side effect of Ca2+ channel blockers used to treat hypertension and angina
- Varicose veins - impairs venous return
- Lack of skeletal muscle activity - due to paralysis or forced inactivity e.g. long term bed rest
- Reduced circulating blood volume e.g. haemorrhage
- Increased core temperature - causes peripheral vasodilation so there is less blood volume available
Describe the effect of microgravity on the CVS
Redistribution of blood into chest region
Initially: Blood not pooling in feet, it returns to the heart easily, increases atria/ventricle volume and so preload and cardiac output. Sensed by cardiac mechanoreceptors leading to a reduction in sympathetic activity.
This reduces ADH and increases atrial natriuretic peptide (ANP), there is increased glomerular filtration rate (GFR) and reduced RAAS. Overall reduction in blood volume by 20%.
Long-term: Less blood volume, reduced stress on heart, heart reduces in muscle mass, general drop in BP.
On return to gravity: Severe postural hypotension, due to much lower blood volume and smaller heart. Baroreceptor reflex can not compensate.
What are the CVS responses to exercise
- Integrated by central command in the brain - anticipation of exercise causes some of these changes to be initiated
- Once exercise occurs there is feedback from muscle via mechanoreceptors and metaboreceptors
- Increased lung oxygen uptake - transports oxygen around the body and supply it to exercising muscle - increases heart rate and the force of contraction - can increase by 10-15 x during strenuous exercise
- Control of blood pressure - despite huge changes in cardiac output and resistance - protects the heart from excessive afterload that reduces cardiac output
- Coordinated dilation/constriction of vascular beds - selectively target areas where oxygen is delivered
State the 3 components changed to increase oxygen uptake by the pulmonary circulation and by how much they increase
- Heart rate - increases by 3x
- Stroke volume - increases by 1.5x
- Arteriovenous oxygen difference - increases by 3x
Describe how oxygen uptake and cardiac output change during different difficulties of exercise and why those changes are necessary
- During light exercise there is only a small change in pulmonary blood flow (cardiac output (l/min) but there is a big change in arteriovenous oxygen difference (ml O2 /l)
- As exercise becomes moderate and then heavy, the efficiency at which oxygen is supplied is limited as its concentration can only get so high so it plateaus - hence to maintain adequate oxygen supply during exercise there is increased blood flow and a greater oxygen gradient allowing increased oxygen intake in the lungs
Describe how heart rate and stroke volume change during different difficulties of exercise and why those changes are necessary
- During light exercise the stroke volume (ml) increases significantly but the heart rate (bpm) stays relatively the same
- As exercise becomes moderate and then heavy, the increase in stroke volume reaches in maximum value and maximum contractility is reached so to maintain higher cardiac output the heart rate increases significantly
Describe how exercise induces tachycardia
- Brain central command - occurs as the body prepares for exercise and from feedback signals from muscle mechanoreceptors
- There is decreased signals sent down the vagus nerve to the SA and AV nodes which means there is increased sympathetic activity in these
- Heart rate increases - up to 3x the normal resting rate
Describe how exercise increases stroke volume
- Sympathetic activity increases stroke volume
- There is increased end diastolic volume - increased sympathetic activity causes venoconstriction which increases venous return/central venous pressure - activates starling’s law so preload is increased
- Faster ejection - increased sympathetic activation of beta 1 receptors causes faster ejection due to inotropic increases in Ca2+ that increases systole
- Decreased end systolic volume (increased ejection) - increased contractility by sympathetic activation of beta 1 receptors and increased stretching e.g. starling’s law
