Difficulty: Intermediate to upper-intermediate | Prerequisites: Parts 1 and 2 of these notes (cardiac electrophysiology, cardiac output, stroke volume). You need to understand CO = HR × SV and the autonomic effects on the heart before working through blood pressure regulation.
This material covers everything downstream of the heart: the blood vessels themselves, how blood flows through them, what determines blood pressure, and how the body regulates pressure on a beat-to-beat and minute-to-minute basis. The key insight is that mean arterial pressure depends on just two variables (cardiac output and total peripheral resistance), and the body uses baroreceptor reflexes, local metabolic signals, and Starling forces at the capillaries to keep MAP stable while directing blood where it is needed. Nearly every clinical scenario involving exercise, haemorrhage, standing up, or drug effects on blood pressure draws on these concepts.
MAP = CO × TPR. Arterioles are the primary resistance vessels and the main site of pressure drop. Veins hold most of the blood volume. The baroreceptor reflex detects changes in MAP and adjusts sympathetic/parasympathetic output to correct it. Locally, active tissues produce metabolic vasodilators (CO2, H+, adenosine) that increase their own blood flow. At the capillaries, Starling forces (hydrostatic pressure pushing fluid out, oncotic pressure pulling it back in) govern filtration and reabsorption; when plasma protein drops, filtration wins and oedema results.
Mean arterial pressure (MAP)
The average blood pressure during one cardiac cycle. Calculated as CO × TPR (or estimated as diastolic pressure + 1/3 pulse pressure). MAP is the driving pressure that pushes blood through the organs. Think of it as the "operating pressure" of the entire systemic circulation.
Total peripheral resistance (TPR)
The combined resistance of all the systemic blood vessels, dominated by the arterioles. In simple terms, it is how much the vasculature as a whole resists blood flow. Vasoconstriction raises TPR; vasodilation lowers it.
Arterioles
Small-diameter vessels with thick smooth-muscle walls. They are the primary resistance vessels and the site of the greatest pressure drop in the systemic circulation. By constricting or dilating, arterioles control both local blood flow to tissues and overall TPR.
Capillaries
The thinnest-walled vessels where exchange of gases, nutrients, and waste occurs. They have the largest total cross-sectional area of any vessel type, which means blood velocity is slowest here, maximising exchange time.
Veins (capacitance vessels)
Thin-walled, highly compliant vessels that hold roughly 60-70% of total blood volume at rest. They serve as blood reservoirs. Venoconstriction can shift blood from the venous reservoir back to the heart, increasing venous return and preload.
Baroreceptors
Stretch-sensitive receptors located primarily in the carotid sinus and aortic arch. They detect changes in arterial pressure and send signals to the cardiovascular centre in the medulla, which adjusts sympathetic and parasympathetic output to restore MAP.
Active hyperaemia
The increase in local blood flow to a tissue in response to increased metabolic activity. Caused by local accumulation of metabolic vasodilators (CO2, H+, adenosine, K+). This is what happens in exercising skeletal muscle.
Reactive hyperaemia
The temporary increase in blood flow to a tissue after a period of ischaemia (blood flow restriction). Metabolic vasodilators accumulate during the blockage and cause a burst of increased flow when the blockage is removed.
Starling forces (capillary exchange)
The four pressures governing fluid movement across capillary walls: capillary hydrostatic pressure (pushes fluid out), interstitial hydrostatic pressure (pushes fluid in), plasma colloid osmotic/oncotic pressure (pulls fluid in), and interstitial osmotic pressure (pulls fluid out). The balance determines net filtration or reabsorption.
Oedema (edema)
Excessive fluid accumulation in the interstitial space. Occurs when net filtration exceeds the lymphatic system's ability to drain the excess, commonly due to increased capillary hydrostatic pressure or decreased plasma oncotic pressure (as in liver failure or nephrotic syndrome).
Arteries (elastic/conducting arteries): large vessels near the heart with lots of elastic tissue. They stretch during systole and recoil during diastole, smoothing pulsatile flow.
Arterioles (resistance vessels): the key control point. Their smooth muscle can constrict or dilate to regulate both local blood flow and total peripheral resistance. The greatest pressure drop in the systemic circulation occurs across the arterioles.
Capillaries (exchange vessels): single-layer endothelium. Largest total cross-sectional area of any vessel type, so blood velocity is slowest here, maximising time for diffusion.
Veins (capacitance vessels): hold 60-70% of total blood volume. High compliance (stretch easily). The skeletal muscle pump and respiratory pump help push venous blood back to the heart.
Flow = Pressure gradient / Resistance (analogous to Ohm's law: I = V/R).
Blood flow through an organ is directly proportional to the pressure gradient across it and inversely proportional to resistance.
Resistance depends on vessel radius more than any other factor. Because resistance is inversely proportional to the fourth power of the radius (Poiseuille's law), a small change in radius produces a very large change in resistance. Halving the radius increases resistance roughly 16-fold.
Blood viscosity also affects resistance, but radius is the dominant and most actively regulated variable.
Baroreceptors in the carotid sinus and aortic arch detect the degree of arterial wall stretch, which correlates with MAP.
When MAP drops (e.g. haemorrhage, standing up): baroreceptor firing decreases → the cardiovascular centre in the medulla increases sympathetic output and decreases parasympathetic output → heart rate and contractility increase, arterioles constrict (raising TPR), veins constrict (increasing venous return) → MAP is restored.
When MAP rises (e.g. high blood pressure): baroreceptor firing increases → sympathetic output decreases, parasympathetic output increases → heart rate drops, arterioles dilate → MAP is brought back down.
This reflex operates on a second-to-second timescale. It is why you do not faint every time you stand up.
When a patient stands suddenly, gravity pools blood in the lower limbs → venous return decreases → EDV drops → SV and CO drop → MAP falls momentarily.
Baroreceptors detect the drop and trigger increased sympathetic activity: heart rate rises, arterioles constrict, venoconstriction mobilises pooled blood.
The sequence is: decreased venous return (first change) → baroreceptor reflex → increased sympathetic activity (compensatory response).
Sudden blood loss reduces blood volume → decreased venous return → decreased preload → decreased SV and CO → decreased MAP.
Baroreceptor firing drops → increased sympathetic activity → increased HR, increased contractility, arteriolar vasoconstriction, venoconstriction.
The compensatory response is: increased heart rate and vasoconstriction to restore MAP.
Intrinsic (local) control matches blood flow to tissue metabolic needs.
Active hyperaemia: increased metabolism (e.g. exercising muscle) produces CO2, H+, adenosine, K+, and decreased O2 locally. These metabolic by-products cause local arteriolar vasodilation, increasing blood flow to the active tissue.
Reactive hyperaemia: after a temporary blockage, accumulated metabolic vasodilators cause a burst of increased blood flow when the blockage is released.
Extrinsic control (sympathetic nervous system) primarily regulates MAP by adjusting TPR across many vascular beds simultaneously. It can override local control when systemic needs (maintaining blood pressure) outweigh local needs.
At the arterial end of a capillary, capillary hydrostatic pressure is high, favouring filtration (fluid moves out into the interstitium).
At the venous end, capillary hydrostatic pressure has dropped, and plasma colloid osmotic (oncotic) pressure now dominates, favouring reabsorption (fluid moves back into the capillary).
Plasma colloid osmotic pressure is the primary force opposing filtration. It is generated by plasma proteins (mainly albumin) that cannot cross the capillary wall.
Increased capillary hydrostatic pressure → increased filtration.
Decreased plasma protein (hypoproteinaemia, as in liver failure or nephrotic syndrome) → decreased oncotic pressure → increased net filtration → oedema.
During dynamic (aerobic) exercise: CO increases (via increased HR and SV), TPR decreases (because of massive vasodilation in active skeletal muscle), MAP increases modestly.
The combination is: ↑CO, ↓TPR. MAP rises slightly because the increase in CO outweighs the drop in TPR.
Venous return increases during exercise because of the skeletal muscle pump (rhythmic contraction of leg muscles compresses veins and pushes blood towards the heart) and the respiratory pump (increased breathing creates pressure changes that aid venous return).
A drug causing generalised arteriolar vasodilation → decreased TPR → if CO does not change, MAP falls (because MAP = CO × TPR).
A drug causing generalised venoconstriction → shifts blood from the venous reservoir towards the heart → increased venous return → increased preload → increased SV.
Mean arterial pressure: MAP = CO × TPR
Flow equation: Flow = ΔP / R (pressure gradient divided by resistance)
Poiseuille's law (conceptual): Resistance is inversely proportional to r⁴ (radius to the fourth power). Doubling the radius decreases resistance by a factor of 16.
If flow doubles and pressure gradient stays constant: resistance must have halved (from the flow equation rearranged: R = ΔP / Flow).
MAP estimate from blood pressure: MAP ≈ diastolic + 1/3 (systolic - diastolic)
The baroreceptor reflex is the reason orthostatic hypotension (dizziness on standing) occurs in patients with autonomic dysfunction: their reflex arc cannot compensate quickly enough for the gravitational shift in blood volume. Compression stockings work by reducing venous pooling in the legs, effectively mimicking what the skeletal muscle pump and sympathetic venoconstriction normally do. Clinical oedema from liver cirrhosis is a direct application of Starling forces: the failing liver cannot produce enough albumin, plasma oncotic pressure drops, and fluid leaks into the interstitium.
Students often think capillaries provide the greatest resistance to blood flow. They do not. Arterioles are the primary resistance vessels and the site of the greatest pressure drop.
Students often confuse cross-sectional area with individual vessel size. Capillaries are tiny individually, but their enormous number gives them the largest total cross-sectional area of any vessel type.
Students sometimes think the baroreceptor reflex to a drop in MAP involves increased parasympathetic activity. The opposite is true: parasympathetic output decreases and sympathetic output increases.
Students often assume that during exercise, TPR increases. In dynamic exercise, TPR actually decreases because of massive vasodilation in working muscles, even though sympathetic activity constricts vessels elsewhere.
⚠️ MAP = CO × TPR is the other guaranteed equation. Know how to predict what happens to MAP when CO or TPR changes independently.
⚠️ Arterioles as resistance vessels and the site of the greatest pressure drop are tested repeatedly. Do not confuse them with capillaries.
⚠️ Baroreceptor reflex scenarios are common: "Patient stands up, what happens first?" (Decreased venous return.) "What is the reflex response?" (Increased sympathetic activity.)
⚠️ Starling forces and oedema: "Patient has hypoproteinaemia, what happens?" (Decreased plasma oncotic pressure → increased filtration → oedema.)
⚠️ Local vasodilation during exercise (CO2, H+, adenosine) is a favourite question. Expect to distinguish between local (intrinsic) and extrinsic (sympathetic) control.
⚠️ Vessel radius and resistance (Poiseuille's law): a small decrease in radius greatly increases resistance. This is tested conceptually.
Fill in the blank: MAP = ____ × ____. (CO × TPR)
True or false: Capillaries are the primary resistance vessels. (False. Arterioles are.)
Fill in the blank: The greatest percentage of total blood volume is found in the ____. (veins)
True or false: A sudden decrease in MAP triggers increased parasympathetic activity. (False. It triggers increased sympathetic activity and decreased parasympathetic activity.)
Fill in the blank: The Starling force that primarily opposes capillary filtration is ____. (plasma colloid osmotic / oncotic pressure)
Q: Which blood vessel contributes the greatest resistance to blood flow?
A: Arterioles. Their small diameter and thick smooth-muscle walls make them the primary determinant of TPR.
Q: The greatest pressure drop in the systemic circulation normally occurs across which vessels?
A: The arterioles.
Q: Which vessel type contains the largest total cross-sectional area?
A: Capillaries. Although each is tiny, their enormous combined number gives the largest total area.
Q: Blood flow through an organ is directly proportional to what?
A: The pressure gradient across that organ (Flow = ΔP / R).
Q: During exercise, increased CO2 and H+ within active skeletal muscle primarily cause what?
A: Local vasodilation (active hyperaemia). These metabolic by-products relax arteriolar smooth muscle locally.
Q: Which variable is primarily regulated by extrinsic (sympathetic) control of arterioles?
A: Mean arterial pressure. Sympathetic vasoconstriction adjusts TPR to maintain MAP.
Q: Which equation correctly relates cardiovascular variables?
A: MAP = CO × TPR.
Q: Arterial baroreceptors are primarily located where?
A: The carotid sinus and aortic arch.
Q: A sudden decrease in mean arterial pressure will initially cause what?
A: Increased sympathetic activity (and decreased parasympathetic activity) via the baroreceptor reflex.
Q: High blood pressure activates the baroreceptor reflex. What is the expected response?
A: Increased parasympathetic stimulation to the heart (decreased HR) and decreased sympathetic output (vasodilation), bringing MAP back down.
Q: A patient suddenly stands up. What change occurs first?
A: Decreased venous return, because gravity pools blood in the lower limbs.
Q: A decrease in vessel radius will do what to resistance?
A: Greatly increase resistance (resistance is proportional to 1/r⁴).
Q: Which factor would increase venous return during exercise?
A: The skeletal muscle pump. Rhythmic contraction of leg muscles compresses veins and pushes blood towards the heart.
Q: Which Starling force primarily opposes filtration?
A: Plasma colloid osmotic pressure (oncotic pressure).
Q: A patient has severe hypoproteinaemia. Which complication is most likely?
A: Oedema. Low plasma protein means reduced oncotic pressure, so filtration exceeds reabsorption and fluid accumulates in the interstitium.
Q: Blood flow through an organ doubles while the pressure gradient stays constant. What has happened to resistance?
A: Resistance has decreased by approximately one-half (from Flow = ΔP / R).
Q: Which vessel type contains the greatest percentage of total blood volume?
A: Veins (roughly 60-70% of total blood volume).
Q: During dynamic exercise, which combination of changes is most likely?
A: Increased CO and decreased TPR. Massive vasodilation in working muscles lowers TPR, while increased HR and SV raise CO.
Q: A patient receives a drug causing generalised arteriolar vasodilation. Assuming CO does not change, what happens to MAP?
A: MAP decreases, because MAP = CO × TPR and TPR has fallen.
Q: A drug causes generalised venoconstriction. Which variable increases most directly?
A: Venous return. Venoconstriction shifts blood from the venous reservoir back towards the heart.
This material connects back to cardiac output (Part 2) through the MAP equation: MAP = CO × TPR ties the heart's pump function directly to the vasculature. Capillary exchange and Starling forces connect to renal physiology (how the kidneys filter blood) and to the lymphatic system (which drains excess interstitial fluid). The baroreceptor reflex and autonomic nervous system concepts here overlap with the neuroscience material on the sympathetic and parasympathetic divisions. Exercise physiology integrates all three parts of these notes: electrophysiology (HR control), pump mechanics (SV changes), and vascular regulation (TPR changes and local vasodilation).
Mean arterial pressure, MAP, total peripheral resistance, TPR, systemic vascular resistance, SVR, arterioles, resistance vessels, capillaries, exchange vessels, veins, capacitance vessels, blood volume distribution, baroreceptor reflex, carotid sinus, aortic arch, sympathetic vasoconstriction, parasympathetic, vagal tone, orthostatic hypotension, standing up, haemorrhage response, blood loss, Poiseuille's law, vessel radius, blood viscosity, blood flow, pressure gradient, active hyperaemia, reactive hyperaemia, metabolic vasodilators, CO2, adenosine, local blood flow regulation, extrinsic control, intrinsic control, Starling forces, capillary hydrostatic pressure, plasma colloid osmotic pressure, oncotic pressure, filtration, reabsorption, oedema, edema, hypoproteinaemia, liver failure, skeletal muscle pump, respiratory pump, venous return, venoconstriction, exercise cardiovascular response, dynamic exercise, APK2105, UF anatomy and physiology, Chapter 13, Chapter 14