Coordinated cardiovascular responses Flashcards
What 3 things drives the flow of blood through the body?
pressure
gravitational energy
kinetic energy.
How does the gravitational gradient change when lying down vs. standing?
Lying down: Small gravitational gradient due to small height difference.
Standing: Greater gravitational gradient between blood at head (high energy) and feet (low energy).
what is the siphon principle
How does the siphon principle explain blood flow in rigid tubes?
The flow depends only on the energy gradient between the inlet (LV) and outlet (RV), not the path blood takes. Gravity’s upward and downward effects cancel each other out.
Why does blood still flow when standing despite gravity?
Pressure energy drives blood flow. The arterial pressure remains higher than venous pressure, ensuring flow continues through the closed system.
What happens to blood flow when standing in real blood vessels?
Blood flow decreases because veins are not rigid. Gravity affects venous flow more significantly, reducing return to the heart.
How does the body minimize reduced blood flow when standing?
The cardiovascular system has a coordinated response to orthostasis (standing up) to compensate for gravitational effects on blood flow.
What does the siphon diagram demonstrate?
Blood flows from the high-pressure LV (95 mmHg) to the low-pressure RV (4 mmHg) regardless of the gravitational path due to the energy gradient.
Why does the siphon principle not fully apply to human vasculature?
Blood vessels, especially veins, are not rigid. This causes flow reductions when standing due to gravitational effects on venous return.
What is orthostasis?
Orthostasis refers to the effect of standing upright on blood pressure distribution in the body due to gravity.
What is the mean capillary pressure when supine (lying down)?
Approximately 25 mmHg.
How much do vascular pressures increase in the feet when standing upright?
Vascular pressures increase by about 90 mmHg.
What are the arterial and venous pressures in the feet when upright?
Artery: 185 mmHg
Vein: 105 mmHg
What are the arterial and venous pressures in the feet when lying down?
Artery: 90 mmHg
Vein: 10 mmHg
What is the pressure gradient across the foot capillary bed during standing?
80 mmHg (185 mmHg in artery - 105 mmHg in vein).
Why do feet swell when standing for long periods?
Foot capillary pressure rises, leading to increased filtration, causing fluid to leak into tissues, resulting in feet swelling.
Is blood flow directly affected by gravity?
No, the pressure gradient across the vascular bed is unchanged, so flow is not directly affected by gravity.
BUT it is indirectly affected (e.g., increased pressure in feet).
Why do veins play a significant role in blood redistribution when standing?
Veins are compliant (distensible), meaning they expand when pressure increases, causing blood to pool in the lower body when standing.
What happens to venous valves when you stand?
Increased pressure causes blood to accumulate below the heart, closing the one-way valves in the legs and briefly reducing venous return to the heart.
Where does blood redistribute when standing?
Upper legs: Most compliant, major blood pooling.
Abdomen: Increased venous blood volume.
Calves, ankles, feet: Less compliant, smaller pooling.
Arteries in the legs: Slight volume increase.
How much blood volume accumulates in the veins below the heart after standing?
Approximately 300-600 ml of blood accumulates in distended veins.
Why does central venous pressure (CVP) fall when standing?
Central venous pressure is reduced (by ~3 mmHg), and therefore, by the Frank-Starling mechanism, cardiac output falls.
How long does it take for veins below the heart to distend with blood after standing?
Approximately 45 seconds.
What are the key events when standing upright?
Blood pools in compliant veins (upper legs, abdomen).
Venous valves close, reducing venous return.
Veins fill, valves reopen, and flow stabilizes.
CVP and cardiac output fall due to Frank-Starling mechanism.
What happens to stroke volume (SV) and cardiac output (CO) during orthostasis?
Both stroke volume and cardiac output decrease because of reduced venous return.
How does orthostasis affect blood flow to the brain and mean arterial blood pressure (MABP)?
Blood flow to the brain and MABP in the upper body decrease due to reduced cardiac output.
What receptors are activated in response to decreased blood pressure during orthostasis?
Baroreceptors and volume receptors are activated to detect reduced blood pressure.
What mechanisms are triggered to compensate for orthostasis?
Increased heart rate (HR)
Vasoconstriction in arteries and veins
Increased total peripheral resistance (TPR)
How does blood pressure change during orthostasis?
Systolic blood pressure slightly decreases or stabilizes, while diastolic blood pressure increases due to vasoconstriction.
What are the two mechanisms that reduce blood flow to the lower extremities on standing?
Reflex sympathetic vasoconstriction via baroreceptors.
A local sympathetic axon reflex (veno-arteriolar reflex).
How does the baroreceptor reflex reduce capillary pressure?
Baroreceptor reflex constricts arteries and arterioles in the lower extremities, reducing blood flow and hydrostatic pressure in the capillaries.
Why is arteriolar constriction important during orthostasis?
It minimizes filtration in the capillaries of the lower extremities, helping to prevent excessive fluid leakage into tissues.
What triggers the local veno-arteriolar reflex?
Increased blood volume and pressure in veins stretch their walls, activating sympathetic sensory fibers
How does the veno-arteriolar reflex work?
Stretch in vein walls triggers action potentials in sympathetic fibers, which stimulate nearby arterioles to constrict, reducing blood flow further.
What is the combined effect of the baroreceptor reflex and veno-arteriolar reflex?
Both mechanisms act to:
Reduce capillary pressure.
Minimize fluid filtration in the lower extremities during standing
What happens to arteriolar pressures in the lower extremities due to vasoconstriction?
Arteriolar pressures decrease, helping to reduce capillary hydrostatic pressure and prevent swelling.
How do sympathetic sensory fibers connect veins and arterioles?
Stretch-activated fibers in vein walls innervate arterioles, triggering vasoconstriction through the local veno-arteriolar reflex.
What is the mean ABP and CVP when lying down?
CVP = 4 mmHg
mean ABP = 95mmHg
What is the mean ABP and CVP when standing up?
CVP = 1 mmHg
Mean ABP = 95mmHg
What is the role of the skeletal muscle pump in orthostasis?
The skeletal muscle pump helps the cardiovascular system cope with orthostasis by pumping blood in veins towards the heart, preventing blood pooling in the lower extremities.
What happens during muscle contraction in the skeletal muscle pump?
Contraction increases pressure in the vein segment.
Blood is pushed forward through the upper valve.
The lower valve closes, preventing backflow.
What happens when the muscle relaxes in the skeletal muscle pump?
Pressure in the vein segment decreases.
The lower valve opens, allowing blood to fill the segment from below.
Blood flows forward towards the heart.
What is the “milking” action of veins?
Repeated muscle contractions exert a pumping action, pushing blood through the veins in a forward direction.
How does the skeletal muscle pump affect venous pressure in the feet?
During walking, it can lower venous pressure in the feet to 20-30 mmHg.
What is the role of the skeletal muscle pump during exercise?
It acts like an ancillary heart, contributing up to half of the energy needed to increase blood flow in the cardiovascular system.
What is the venous pressure in the foot when standing still?
Approximately 120 cm H₂O (88 mmHg) due to the effect of gravity.
What happens when valves in superficial veins fail?
Valve failure exposes superficial veins to chronic high pressures, leading to varicose veins.
What are varicose veins, and how do they appear?
Varicose veins are swollen, twisted veins that bulge out from the skin, often blue or dark purple in color, caused by valve failure in superficial veins.
What are tributary veins?
Veins under the surface of the skin.
What happens to venous pressures above the heart when standing?
Gravity causes venous pressures above the heart to fall, and veins outside the cranium partially collapse a few centimeters above the heart.
Why do veins outside the cranium collapse when standing?
Collapsing prevents internal pressures from falling below zero while still allowing blood to flow through the margins of the veins.
Do veins within the cranium collapse?
No, cranial veins do not collapse, their pressure falls to about -10 mmHg, maintaining the arterial-to-venous pressure gradient for brain blood flow.
How is blood flow to the brain affected by standing?
Blood flow to the brain decreases by ~20% due to the fall in cardiac output.
Why can prolonged standing cause fainting?
Blood pooling in the lower extremities reduces brain perfusion, potentially leading to fainting. Falling or lying down restores blood flow to the brain.
What is the significance of cranial vein pressures falling below zero?
The negative pressure (-10 mmHg) helps maintain the pressure gradient driving blood flow through the brain.
How does fainting restore brain blood flow?
Falling or lying down reduces gravitational effects, improving venous return and perfusion to the brain.
Why don’t veins within the cranium collapse when standing?
How does venous pooling affect central blood volume when moving from supine to upright?
decreases by ~400 mL
What happens to central venous pressure (CVP) when standing up?
decreases by ~3 mmHg.
How does stroke volume change when transitioning from supine to upright?
decreases by 40%.
what is a vasovagal syncope?
fancy way of saying fainting due to standing, from fall in blood pressure
What causes sudden syncope (fainting) during prolonged standing
What is concealed haemorrhage?
Bleeding that is not visible and occurs within the body, often due to trauma or internal conditions.
What are the signs, symptoms and consequences of shock?
What are the average blood volumes for men and women?
How does the WHO classify haemorrhage severity?
Based on percentage of blood loss:
How does the rate of blood loss affect severity?
What are the 3 key reflex responses to haemorrhage?
What are the accompanying effects of reflex responses?
Why is the sympathetic system important in the immediate period after haemorrhage?
increased ADH in Cardio-pulmonary stretch receptors cause what
increased adrenaline in Cardio-pulmonary stretch receptors cause what
How does sympathetic vasoconstriction affect blood flow during moderate haemorrhage?
In moderate haemorrhage the vasoconstriction is directed to the gut, kidney, the muscle and the skin as they are not crucial to keep you alive.
where does vascular resistance increase/ decrease during moderate haemorrhage?
How do cardiac output (CO) and blood pressure (BP) change with increasing blood loss?
CO decreases more rapidly than BP.
BP is better preserved initially to protect tissue perfusion.
When does the CNS ischaemic response occur?
When blood pressure drops to ≤50 mmHg, triggering profound vasoconstriction via the sympathetic nervous system.
How long does it take to restore the blood volume in a moderate haemorrhage?
It takes around a day, generally with a haemorrhage of around 25%, to restore the blood volume.
what are the 3 slower mechanisms to help restore blood volume (takes days):
What are the renal mechanisms to restore blood volume?
- There’s a fall in blood pressure and a fall in blood volume.
- This acts on the baroreceptors and the cardiopulmonary receptors.
- These report to the brainstem.
- This leads to thirst.
- The decrease in atrial stretch leads to the decreased release of atrial natriuretic peptide (ANP) - this promotes Na+ and water reabsorption.
- Along with thin the input into the hypothalamus and brain stem increases the release of antidiuretic hormone, which conserves water and decreases diuresis.
- The input also increases the sympathetic renal nerve activity which increases renin, angiotensin 2 and aldosterone which all helps to promote Na+ and water reabsorption.
- This helps to restore the blood volume.
- At the same time, the fall in blood pressure and the fall in blood volume are direct stimuli for the RAAS which again increases the reabsorption of Na+ and water.
Describe how the quality of blood is restored using a graph.
Gradually over time the haemodilution is reversed and that requires the provision of new blood cells and that’s what takes the longest time.
Gradually over time the haemodilution is reversed and that requires the provision of new blood cells and that’s what takes the longest time.
And the kidneys increase their synthesis and release of erythropoietin because they’re hypoxic due to the haemorrhage and lack of blood flow.
This stimulates production of new RBCs and during this period there are a lot of immature RBCs in the blood, which is an indication that cells are being released at a higher rate than usual from the bone marrow.
What is non-progressive shock?
- If a haemorrhage isn’t very large or if they’re transfused quickly enough the patient will develop non-progressive shock.
its a form of shock that resolves without treatment/ transfusion; so a fit young person can lose < ~20% blood volume without progressive shock (e.g. when donating blood ≈ 10%, you don’t go into shock).
How long does it take for blood volume and cardiac output to recover in non-progressive shock?
Recovery occurs over 16–24 hours as compensatory mechanisms restore blood volume and cardiac output.
How can transfusion impact the reversibility of shock?
Reversible shock: CO improves if transfusion is given in time.
Irreversible shock: Delayed transfusion results in cardiac damage, causing a permanent decline in CO.
describe the cycle of progressive shock
Orthostasis summary
Haemorrhage Summary
