Circulatory System in Animals

Lessons

Senior 6 · Biology

Circulatory System in Animals

5 lessons

Lesson 2 of 5

The mammalian heart: structure and function

25 min

A pump that never rests

Place two fingers on the side of your neck, beside the windpipe, and count the beats for 15 seconds. Multiply by four: at rest, most teenagers have a heart rate of about 60 to 100 beats per minute. At 70 beats a minute, your heart beats about 100 000 times a day and more than two and a half thousand million times in a lifetime of 70 years, without a single rest. This lesson looks at how it is built to do that: two pumps side by side, four chambers, four valves and walls of a muscle found nowhere else in the body.

Definition

Atrium (plural atria): one of the two thin-walled upper chambers of the heart, which receive blood from the veins and pass it to the ventricles.

Ventricle: one of the two thick-walled lower chambers, which pump blood out of the heart into the arteries.

Atrioventricular (AV) valve: a valve between an atrium and a ventricle: the tricuspid valve on the right, the bicuspid (mitral) valve on the left.

Semilunar valve: a valve of three pocket-shaped cusps at the base of the pulmonary artery (pulmonary valve) or the aorta (aortic valve), which stops blood flowing back into the ventricles.

Cardiac muscle: the muscle of the heart wall; its branched cells are joined by intercalated discs, it contracts without being stimulated by nerves (it is myogenic) and it does not fatigue.

The structure of the heart

Figure 1: The human heart in frontal section, seen from the front: the heart's right side is on your left. Blue: deoxygenated blood. Red: oxygenated blood. The pulmonary artery runs behind the superior vena cava and the aorta in this view.

Follow the blue arrows first. Deoxygenated blood from the head and arms arrives in the superior vena cava, and from the rest of the body in the inferior vena cava. Both empty into the right atrium. The blood passes through the tricuspid valve into the right ventricle, which pumps it through the pulmonary valve into the pulmonary artery and on to the lungs.

Now the red arrows. Oxygenated blood returns from the lungs in the four pulmonary veins to the left atrium, passes through the bicuspid (mitral) valve into the left ventricle, and is pumped through the aortic valve into the aorta, which carries it to every part of the body.

The heart muscle itself is supplied by the coronary arteries, the first branches of the aorta, which run over its surface. A blockage in one of them starves part of the muscle of oxygen: a heart attack (Lesson 4).

Structure fits function

Feature	How it suits the heart's job
Atria have thin walls	They only push blood a few centimetres, into the ventricles below, so they need little muscle
Left ventricle wall about three times as thick as the right	The left ventricle pumps blood all round the body against high resistance (about 16 kPa, 120 mmHg, at each beat); the right ventricle pumps only to the nearby lungs, at about 3 kPa (25 mmHg), which protects the delicate lung capillaries
Septum	Keeps oxygenated and deoxygenated blood completely separate
AV valves with chordae tendineae and papillary muscles	The flaps close when the ventricles contract; the tendons, tightened by the papillary muscles, stop the flaps turning inside out into the atria
Semilunar valves	Their cusps fill with blood and close when ventricular pressure falls below arterial pressure, so blood cannot flow back
Coronary arteries	Supply the heart muscle, which respires aerobically all the time, with oxygen and glucose

Valves open and close passively, only because of differences in blood pressure on their two sides: a valve opens when the pressure behind it is higher than the pressure in front, and closes when the pressure in front becomes higher. Lesson 3 uses this rule to explain the cardiac cycle.

Worked example 1: pressures in the two ventricles

Doctors measure blood pressure in millimetres of mercury (mmHg); physiologists often use kilopascals, where $1\,\mathrm{mmHg} = 0.133\,\mathrm{kPa}$ . The peak pressure is about $120\,\mathrm{mmHg}$ in the left ventricle and $25\,\mathrm{mmHg}$ in the right. (a) Convert both to kPa. (b) Calculate the ratio. (c) Relate it to the walls.

(a) Left: $120 \times 0.133 = 16.0\,\mathrm{kPa}$ . Right: $25 \times 0.133 = 3.3\,\mathrm{kPa}$ (2 s.f.).
(b) $\frac{120}{25} = 4.8$ : the left ventricle generates almost five times the pressure of the right.
(c) A higher pressure needs more force from more muscle, so the left ventricle wall is much thicker. Both ventricles still pump the same volume of blood at each beat: whatever enters the lungs must leave them.

Practical: dissecting a heart

A goat, sheep or cow heart from a licensed butcher shows every structure in Figure 1. Ask for a heart with the vessels left long.

You need: a fresh heart, dissecting board or tray, scalpel or sharp knife, scissors, a blunt probe or glass rod, disposable gloves, a hand lens, paper towels.

Outside. Find the front of the heart: the coronary arteries run diagonally across it, marking the septum. Press the ventricles: the left one feels firm and thick, the right one soft. Identify the aorta (thick, pale, elastic), the pulmonary artery, the venae cavae and the pulmonary veins (thin, floppy walls).
Test the valves. Push a blunt probe into the aorta towards the heart. It meets the closed aortic valve. From inside the ventricle, a probe passes out into the aorta easily: the valve lets blood go one way only.
Open the right side. Cut from the pulmonary artery down into the right ventricle. Find the tricuspid valve, its chordae tendineae and the papillary muscles.
Open the left side the same way, from the aorta into the left ventricle. Compare the thickness of the two ventricle walls, and find the three cusps of the aortic valve, and the openings of the coronary arteries just above them.
Record. Draw a labelled diagram, and measure the thickness of each ventricle wall with a ruler, to the nearest millimetre.

Warning

Safety

Use only fresh material from a licensed butcher or abattoir, kept cold, and dissect on the day.
Wear disposable gloves and cover any cut on your hands. Never eat or drink in the laboratory.
Cut away from yourself and from others, on a board; carry scalpels blade down and never leave them hidden under tissue.
Afterwards, wrap the tissue for disposal as your teacher directs, disinfect the board and tools, and wash your hands with soap and water.

Worked example 2: analysing dissection results

A class measured the ventricle walls of five goat hearts (illustrative data). Left ventricle: 12, 14, 11, 13, 15 mm. Right ventricle: 4, 5, 4, 3, 4 mm. Calculate the mean for each, the ratio, and comment on the reliability.

Left: $\bar{x} = \frac{12 + 14 + 11 + 13 + 15}{5} = \frac{65}{5} = 13\,\mathrm{mm}$ .
Right: $\bar{x} = \frac{4 + 5 + 4 + 3 + 4}{5} = \frac{20}{5} = 4\,\mathrm{mm}$ .
Ratio $= \frac{13}{4} = 3.25$ : the left wall is about three times as thick.
Reliability: the ranges (11 to 15 mm and 3 to 5 mm) do not overlap, so the difference is clear. But a ruler reading to 1 mm on a 4 mm wall has a large percentage uncertainty ( $\frac{0.5}{4} \times 100 = 12.5\,\%$ ), walls vary in thickness along their length, and hearts differ in size. Measuring at the same marked level on each heart, and using calipers, would improve it.

CrashCourse: "The Heart, Part 1 - Under Pressure" (Crash Course Anatomy & Physiology #25). Watch for the path of blood through the four chambers and the role of each valve, and check it against Figure 1.

Tip

Memory hook: "tri before you bi": blood passes the tricuspid valve (right side) before it reaches the lungs, and the bicuspid valve (left side) after. And "Arteries carry blood Away from the heart": the pulmonary artery is an artery even though its blood is deoxygenated.

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Practice: the mammalian heart

Name the blood vessel that (a) carries oxygenated blood from the lungs to the heart, (b) carries deoxygenated blood from the heart, (c) supplies the heart muscle.
List, in order, the chambers, valves and vessels a red blood cell passes through from the inferior vena cava to the aorta.
Explain why the wall of the left ventricle is thicker than that of the right ventricle.
Explain the role of the chordae tendineae.
Convert a pressure of 90 mmHg to kPa ( $1\,\mathrm{mmHg} = 0.133\,\mathrm{kPa}$ ).
A baby is born with a hole in the septum between the ventricles. Suggest two effects on the blood reaching the body, and explain why.
In a dissection, a student pushes a probe from the pulmonary artery towards the right ventricle and it is stopped. Name the structure and explain what this shows.

Show answersHide answers

(a) Pulmonary vein. (b) Pulmonary artery. (c) Coronary artery.
Inferior vena cava → right atrium → tricuspid valve → right ventricle → pulmonary valve → pulmonary artery → lungs → pulmonary vein → left atrium → bicuspid valve → left ventricle → aortic valve → aorta.
The left ventricle pumps blood all round the body, against a high resistance, so it must generate a high pressure (about 16 kPa); the right ventricle pumps only to the lungs at low pressure (about 3 kPa), which also protects the lung capillaries. More pressure needs more muscle.
They are tendons that attach the AV valve flaps to the papillary muscles; when the ventricles contract, they stop the flaps being pushed back into the atria, so the valves close without turning inside out and blood does not flow back.
$90 \times 0.133 = 12.0\,\mathrm{kPa}$ (3 s.f.).
Some blood from the high-pressure left ventricle passes through the hole to the right, so less blood, at a lower pressure, reaches the body in the aorta, while the right ventricle and lungs are overloaded (the baby may tire easily and grow slowly); and oxygenated and deoxygenated blood mix, so if the hole is large enough for some deoxygenated blood to reach the aorta, the blood reaching the body carries less oxygen.
The pulmonary (semilunar) valve. It shows that the valve allows blood to flow only from the ventricle into the artery, not back.

Key points

Right side: vena cava → right atrium → tricuspid → right ventricle → pulmonary valve → pulmonary artery. Left side: pulmonary vein → left atrium → bicuspid → left ventricle → aortic valve → aorta.
The left ventricle wall is about three times as thick as the right, because it pumps at about five times the pressure.
Valves open and close passively, by pressure differences; chordae tendineae stop AV valves inverting.

Sources and credits

7.8.4: Mammalian Heart and Blood Vessels, from Biology 2e by OpenStax (Mary Ann Clark, Matthew Douglas, Jung Choi), via LibreTexts Biology, licensed under CC BY 4.0. Used for the structure of the heart, its valves and the coronary arteries. Adapted by Ganzaa.
Photo: Sheep's heart ready for dissection, brittgow, CC BY 2.0, via Wikimedia Commons.
Video: The Heart, Part 1 - Under Pressure: Crash Course Anatomy & Physiology #25, CrashCourse, embedded from YouTube.

Figure 1, the table, the dissection, the worked examples and the practice questions are written by Ganzaa. The dissection data are illustrative.

1A pump that never rests

2The structure of the heart

3Structure fits function

4Practical: dissecting a heart

A pump that never rests

The structure of the heart

Structure fits function

Practical: dissecting a heart