Coordinated Responses of the CVS - Gravity & Exercise (Physiology)

♡ Upon standing up, the cardiovascular system responds to the effect of gravity

♡ Initially, blood pressure falls, leading to postural hypotension, which can result in a lack of blood flow to the brain and fainting

♡ However, the cardiovascular system quickly recovers due to homeostatic mechanisms such as the baroreflex

The baroreflex integrates three smaller changes by increasing:

♡ Heart rate

♡ Force of contraction

♡ Total peripheral resistance

-This helps to quickly restore blood pressure and prevent fainting after orthostasis

♡ When standing up, gravity causes changes in blood pressure.

♡ Initially, there is a decrease in blood pressure, which can lead to postural hypotension and potential fainting.

♡ However, homeostatic mechanisms like the baroreflex help the cardiovascular system quickly recover and stabilize blood pressure.

♡ To get arterial blood flow from the heart to the feet when standing up, there needs to be an arterial pressure gradient

♡ The arterial pressure gradient typically ranges from 95 to 180 mmHg

Darcy’s law and Bernoulli’s law explain how blood flow is maintained:

♡ According to Darcy’s law, blood flow depends on the pressure gradient and resistance

♡ Bernoulli’s law states that blood flow involves pressure energy, potential energy, and kinetic energy

♡ With increased potential energy at heart level compared to the feet and increased kinetic energy of ejected blood, blood flows from the heart to the feet

Pressure=p x h x g

♡ High pressure in the venous system at the feet is primarily due to hydrostatic pressure

♡ According to hydrostatic principles, pressure (P) is higher at the bottom of a fluid column

♡ The magnitude of pressure depends on factors such as the height of the fluid column, the density of the fluid, and gravity

♡ When standing up, there's less stretch stimulation of baroreceptors

♡ This reduces signals to the brainstem, particularly the Nucleus Tractus Solitarius (NTS)

♡ Decreased NTS activity means less inhibition of the caudal ventrolateral medulla (CVLM), which then reduces inhibition of the rostral ventrolateral medulla (RVLM)

♡ With reduced inhibition, the RVLM becomes more active and sends signals to increase sympathetic activity

♡ Increased sympathetic activity leads to higher heart rate, stronger heart contractions, and vasoconstriction, all to maintain blood pressure

♡ Less stretch stimulation of baroreceptors leads to reduced signals to the brainstem

♡ Decreased activity in the NTS results in less inhibition of the CVLM and subsequently the RVLM

♡ The RVLM becomes more active, increasing sympathetic activity

♡ This leads to higher heart rate, stronger heart contractions, and vasoconstriction to maintain blood pressure

♡ Drugs that reduce vascular tone, such as α-adrenergic blockers or certain calcium channel blockers used for hypertension or angina

♡ Conditions that impair venous return, like varicose veins

♡ Lack of skeletal muscle activity due to paralysis or forced inactivity, such as long-term bed rest or standing for long periods

♡ Decreased circulating blood volume, such as from haemorrhage

♡ Increased core temperature, which causes peripheral vasodilation and less available blood volume, like standing up after a hot bath

♡ In microgravity, there's a redistribution of blood into the chest region.

♡ Initially, blood doesn't pool in the feet, easily returns to the heart, increases atrial and ventricle volume, preload, and cardiac output.

♡ Cardiac mechanoreceptors sense this and reduce sympathetic activity, leading to decreased ADH and increased ANP.

♡ Long-term effects include reduced blood volume, less stress on the heart, decreased heart muscle mass, and a general drop in blood pressure.

♡ Upon return to gravity, severe postural hypotension occurs due to lower blood volume and a smaller heart, which the baroreceptor reflex can't fully compensate for.

♡ Microgravity refers to the condition experienced in space where the force of gravity is significantly reduced, approaching zero

♡ It occurs when an object is in a state of free fall or orbit around a larger body, such as a planet or a celestial body

♡ In microgravity, objects appear to float because there is very little gravitational force acting on them

♡ Cardiovascular responses to exercise are initiated by central command in the brain.

♡ Even the anticipation of exercise triggers some of these changes

♡ During exercise, muscles provide feedback through mechanoreceptors and metaboreceptors

♡ This feedback further influences cardiovascular responses.

♡ Exercise increases lung oxygen uptake, transport around the body, and supply to exercising muscles.

♡ It also leads to increased heart rate (HR) and force of contraction

♡ Despite significant changes in cardiac output and resistance during exercise, blood pressure is carefully regulated

♡ This regulation protects the heart from excessive afterload, which could otherwise reduce cardiac output

-Vascular beds undergo coordinated dilation and constriction during exercise

-This selective targeting ensures that oxygen delivery is prioritized to areas where it is needed most

♡ Sympathetic activity increases stroke volume by approximately 1.5 times

This increase is achieved through:

♡ Increased end-diastolic volume due to venoconstriction and increased venous return

♡ Faster ejection facilitated by sympathetic activation of β1 receptors, leading to an inotropic increase in calcium ions

♡ Decreased end-systolic volume due to increased contractility from sympathetic activation of β1 receptors and increased stretching, as per Starling's law

♡ During exercise, cardiac output increases approximately 4.5 times, from around 5 to 22 liters per minute.

♡ This increase results from multiplying heart rate (which triples) by stroke volume (which increases by 1.5 times)

♡ The brain's central command prepares the body for exercise

♡ Muscle mechanoreceptors provide fast feedback to the brain regarding the commencement of exercise

This leads to:

♡ Decreased signal transmission down the vagus nerve to the sinoatrial (SA) and atrioventricular (AV) nodes

Increased sympathetic activity to the SA and AV nodes

♡ The maximum heart rate during exercise is approximately calculated as 220 minus age.

♡ For example, for a 30-year-old individual, the approximate maximum heart rate during exercise would be 190 beats per minute

♡ The increase in cardiac output leads to a fall in local resistance due to metabolic hyperaemia vasodilation

♡ There's also a local sympathetic response and β2-mediated vasodilation, facilitated by circulating adrenaline

♡ β2 receptor expression is high in skeletal muscle and coronary arteries, contributing to vasodilation in these areas

♡ Compensatory vasoconstriction of non-essential circulations occurs to prevent a drop in blood pressure

♡ This vasoconstriction targets inactive or unrequired tissues such as the kidneys, gastrointestinal (GI) tract, and inactive muscle.

♡ The central control mechanism involves the rostral ventrolateral medulla (RVLM), which regulates specific pre-ganglionic sympathetic nerves in the spinal cord. These nerves then send out post-ganglionic nerves to specific tissues.

Additional Information:

♡ Splanchnic circulation refers to blood flow to abdominal organs such as the stomach, liver, spleen, pancreas, and intestines.

♡ Metaboreceptors are small-diameter sensory fibers found in skeletal muscle

♡ They are chemosensitive, meaning they respond to increased levels of substances like potassium (K+), hydrogen ions (H+), and lactate, which occur during muscle exercise

♡ Metaboreceptors trigger reflex responses during exercise:

♡ Tachycardia, through increased sympathetic activity.

Increased blood pressure

♡ Pressor response to exercise, especially prominent during isometric exercise (increased muscle load)

♡ Metaboreceptors play a crucial role in the pressor response to exercise, particularly during isometric exercise

♡ Static (isometric) exercise raises blood pressure more than dynamic (isotonic) exercise

♡ Elevated blood pressure helps maintain blood flow to contracted muscles, attempting to force blood into the muscle

♡ Selective metabolic hyperaemia refers to the dilation of resistance vessels supplying contracted muscles due to metabolic activity

♡ It ensures adequate blood flow to active muscles during exercise

♡ Dynamic exercise involves constant shortening and relaxing of muscle groups with various movements.

♡ Static exercise focuses on working a specific muscle group without constant movement

♡ Dynamic exercise typically results in lower blood pressure compared to static exercise.

♡ This is because dynamic exercise involves multiple muscle groups and movements, leading to lower sympathetic tone and less impact on blood pressure

♡ Static exercise raises blood pressure more due to local metabolic hyperaemia

♡ During static exercise, the specific muscle group being worked experiences increased metabolic activity, leading to localized vasodilation and higher blood pressure