IMAT Biology · Human Anatomy and Physiology

The Heart, the Cardiac Cycle and Blood Vessels

Two pumps side by side send blood through the lungs and round the body; valves open and close only because of pressure differences.

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Why this matters

The cardiac cycle was a favourite of the Cambridge papers (2014, 2015, 2017, 2018, 2021, 2022), usually as which valve is open or which pressure is higher. They also asked which vessel carries the least carbon dioxide or urea (2012) and about the hole in the fetal heart (2020).

Concept 1 of 4: Double circulation and the main blood vessels

Blood passes through the heart twice on every full circuit: once to be pumped to the lungs and once to be pumped to the body. Each organ changes the blood that leaves it, so every named vessel has a known content: the lungs add oxygen, the kidneys remove urea, the gut adds digested food.

Definition

Double circulation:

Pulmonary circuit: right ventricle, pulmonary artery, lungs, pulmonary veins, left atrium.
Systemic circuit: left ventricle, aorta, body organs, venae cavae, right atrium.
Arteries carry blood away from the heart; veins carry it towards the heart. This is about direction, not oxygen.
Arteries: thick muscular and elastic walls, narrow lumen, high pressure, no valves along their length.
Veins: thin walls, wide lumen, low pressure, valves to stop backflow; contracting skeletal muscles squeeze blood back to the heart.
Capillaries: walls one endothelial cell thick, where exchange with the tissues happens.
The hepatic portal vein takes blood from the gut to the liver before it reaches the heart.

Vessel	From and to	Oxygen content	Notable feature
Aorta	Left ventricle to the body	High	Highest blood pressure
Pulmonary artery	Right ventricle to the lungs	Low	The only artery with deoxygenated blood; high $\mathrm{CO_2}$
Pulmonary vein	Lungs to the left atrium	High	The only vein with oxygenated blood; lowest $\mathrm{CO_2}$
Venae cavae	Body to the right atrium	Low	Lowest blood pressure
Hepatic portal vein	Gut to the liver	Low	Highest glucose and amino acids after a meal
Hepatic vein	Liver to the vena cava	Low	Carries the urea the liver has just made
Renal artery	Aorta to the kidney	High	Still carries urea to be removed
Renal vein	Kidney to the vena cava	Low	Lowest urea concentration in the body

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Artery does not mean oxygenated

An artery is any vessel carrying blood away from the heart. The pulmonary artery carries blood low in

\mathrm{O_2}

to the lungs, and the pulmonary vein carries blood rich in

\mathrm{O_2}

back to the heart.

Concept 2 of 4: Heart structure, the pacemaker and the control of heart rate

Heart muscle starts its own beat: it is myogenic, so a heart keeps beating even with its nerves cut. A small patch of tissue, the pacemaker, fires first, and a delay station makes sure the atria finish emptying before the ventricles squeeze. The brain only speeds the rhythm up or slows it down.

Definition

Chambers: two atria on top receive blood; two ventricles below pump it out. The left ventricle has the thickest wall because it pumps to the whole body.

The septum separates the left and right sides.
Atrioventricular (AV) valves: tricuspid on the right, bicuspid (mitral) on the left, held by tendinous cords.
Semilunar valves: the pulmonary valve and the aortic valve, at the base of the two arteries.
Heart rate is set by the sinoatrial node and changed by the cardiovascular centre in the medulla oblongata: sympathetic nerves speed it up, the parasympathetic vagus nerve slows it down. Adrenaline in the blood also speeds it up.
Fetal heart: a hole in the wall between the atria (the foramen ovale) and a short vessel from the pulmonary artery to the aorta (the ductus arteriosus) let blood bypass the lungs, which do not work before birth. Both close soon after birth.

Structure	Where	Job
Sinoatrial node (SAN)	Wall of the right atrium	Pacemaker: starts each wave of excitation, which spreads over both atria
Atrioventricular node (AVN)	Between the atria and the ventricles	Delays the wave by about 0.1 s so the atria empty first
Bundle of His and Purkinje fibres	In the septum and the ventricle walls	Carry the wave to the apex, so the ventricles contract from the bottom up
AV valves	Between each atrium and ventricle	Stop backflow into the atria when the ventricles contract
Semilunar valves	At the base of the aorta and the pulmonary artery	Stop backflow into the ventricles when they relax
Cardiovascular centre	Medulla oblongata	Adjusts heart rate through sympathetic and parasympathetic nerves

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The brain changes the heart rate but does not start the beat

Each beat starts at the sinoatrial node. The medulla only adjusts the rate through autonomic nerves. Pick the medulla oblongata, not the cerebrum or cerebellum, for the nervous control of heart rate.

Concept 3 of 4: The cardiac cycle: pressure changes and valve positions

Valves are passive flaps. A valve is pushed open when the pressure behind it is higher than the pressure in front, and slams shut when the pressure in front becomes higher. So if you know which chamber is squeezing, you can work out every valve.

Definition

One cycle takes about 0.8 s at 75 beats per minute. Left and right sides do the same thing at the same time.

Systole means contraction; diastole means relaxation.
Valve rule: a valve opens when the pressure upstream is higher than downstream, and closes when it is lower.
Most ventricular filling (about 70%) is passive, before the atria contract.
The atria fill while the ventricles are contracting, so when ventricular pressure is highest the atria are filling.
First heart sound (lub): AV valves closing. Second heart sound (dub): semilunar valves closing.
Peak pressure is about 120 mmHg in the left ventricle and about 25 mmHg in the right.

Phase	Atria	Ventricles	AV valves	Semilunar valves
Atrial systole	Contract: volume falls, pressure rises	Relaxed, receiving the last blood	Open	Closed
Start of ventricular systole	Relaxed, begin to fill	Contract; pressure rises above the atria but not yet above the arteries	Closed (first sound)	Closed
Ventricular ejection	Relaxed, filling	Pressure above the aorta and pulmonary artery: blood leaves	Closed	Open
Start of ventricular diastole	Relaxed, filling	Relax; pressure falls below the arteries	Closed	Close (second sound)
Passive filling	Relaxed; blood flows through	Relaxed; pressure below the atria	Open	Closed

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Valves follow pressure, not timing words

Do not learn the cycle as a list of times. Compare the pressure on each side of a valve: higher upstream means open, higher downstream means closed. An option saying an AV valve is open while its ventricle contracts is always wrong.

Left and right contract together

Both atria contract together, then both ventricles. The right atrium does not contract at the same moment as the left ventricle, and one AV valve does not open as the other closes.

Concept 4 of 4: Cardiac output, stroke volume and heart rate

The volume of blood the heart pumps each minute is the volume per beat times the number of beats. A trained athlete has a bigger stroke volume, so at rest the same output needs fewer beats.

Definition

Stroke volume (SV): the volume pumped by one ventricle in one beat, about 70 mL at rest.
Heart rate (HR): beats per minute.
Cardiac output (CO): the volume pumped by one ventricle per minute, about 5 L/min at rest and up to about 25 L/min in hard exercise.
Stroke volume is the difference between the volume in the ventricle at the end of filling and at the end of contraction. The ejection fraction is the share of the filled volume that is pumped out, normally about 55% to 70%.

Cardiac output

CO = SV \times HR \qquad SV = EDV - ESV

$CO$ cardiac output, in mL/min or L/min
$SV$ stroke volume, in mL per beat
$HR$ heart rate, in beats per minute
$EDV, ESV$ volume in the ventricle at the end of diastole and at the end of systole, in mL

Worked example

A person's left ventricle holds 130 mL at the end of filling and 60 mL at the end of contraction. The heart rate is 70 beats per minute. Find the stroke volume, the cardiac output and the ejection fraction.

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Convert litres to millilitres before dividing

Cardiac output is usually given in L/min and stroke volume in mL. Dividing litres by beats per minute gives litres per beat, so 20 L/min at 160 per minute is 0.125 L, which is 125 mL, not 0.125 mL.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Cardiac output, stroke volume and heart rate
Cardiac output
$CO = SV \times HR \qquad SV = EDV - ESV$

Reference tables (3)

Double circulation and the main blood vessels8 rows

Vessel	From and to	Oxygen content	Notable feature
Aorta	Left ventricle to the body	High	Highest blood pressure
Pulmonary artery	Right ventricle to the lungs	Low	The only artery with deoxygenated blood; high $\mathrm{CO_2}$
Pulmonary vein	Lungs to the left atrium	High	The only vein with oxygenated blood; lowest $\mathrm{CO_2}$
Venae cavae	Body to the right atrium	Low	Lowest blood pressure
Hepatic portal vein	Gut to the liver	Low	Highest glucose and amino acids after a meal
Hepatic vein	Liver to the vena cava	Low	Carries the urea the liver has just made
Renal artery	Aorta to the kidney	High	Still carries urea to be removed
Renal vein	Kidney to the vena cava	Low	Lowest urea concentration in the body

Heart structure, the pacemaker and the control of heart rate6 rows

Structure	Where	Job
Sinoatrial node (SAN)	Wall of the right atrium	Pacemaker: starts each wave of excitation, which spreads over both atria
Atrioventricular node (AVN)	Between the atria and the ventricles	Delays the wave by about 0.1 s so the atria empty first
Bundle of His and Purkinje fibres	In the septum and the ventricle walls	Carry the wave to the apex, so the ventricles contract from the bottom up
AV valves	Between each atrium and ventricle	Stop backflow into the atria when the ventricles contract
Semilunar valves	At the base of the aorta and the pulmonary artery	Stop backflow into the ventricles when they relax
Cardiovascular centre	Medulla oblongata	Adjusts heart rate through sympathetic and parasympathetic nerves

The cardiac cycle: pressure changes and valve positions5 rows

Phase	Atria	Ventricles	AV valves	Semilunar valves
Atrial systole	Contract: volume falls, pressure rises	Relaxed, receiving the last blood	Open	Closed
Start of ventricular systole	Relaxed, begin to fill	Contract; pressure rises above the atria but not yet above the arteries	Closed (first sound)	Closed
Ventricular ejection	Relaxed, filling	Pressure above the aorta and pulmonary artery: blood leaves	Closed	Open
Start of ventricular diastole	Relaxed, filling	Relax; pressure falls below the arteries	Closed	Close (second sound)
Passive filling	Relaxed; blood flows through	Relaxed; pressure below the atria	Open	Closed

Watch out for (5)

Artery does not mean oxygenated
→ Double circulation and the main blood vessels
The brain changes the heart rate but does not start the beat
→ Heart structure, the pacemaker and the control of heart rate
Valves follow pressure, not timing words
→ The cardiac cycle: pressure changes and valve positions
Left and right contract together
→ The cardiac cycle: pressure changes and valve positions
Convert litres to millilitres before dividing
→ Cardiac output, stroke volume and heart rate

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