Circulatory System in Humans
Senior 3 · Biology
Circulatory System in Humans
Lesson 2 of 5
25 min
At rest, an adult heart beats about 70 times a minute and pumps about 5 litres of blood a minute, roughly all the blood in the body. That is about 100 000 beats a day, without a rest, for a whole lifetime. The heart is a pump made of muscle, about the size of your fist, lying in the chest between the lungs, slightly to the left. To do its job it must push blood in one direction only, and that depends on its valves.
Definition
Atrium (plural atria): an upper, thin-walled chamber of the heart that receives blood from the veins.
Ventricle: a lower, thick-walled chamber that pumps blood out into an artery.
Valve: a flap of tissue that lets blood flow one way only.
Septum: the wall of muscle that separates the right side of the heart from the left.
Heart rate: the number of heartbeats per minute. Each beat sends a surge of blood along the arteries, felt as the pulse.
Figure 1: A vertical section through the human heart. It is drawn as if you are facing the person, so the heart's right side is on your left. Blue: deoxygenated blood; red: oxygenated blood. Arrows show the direction of flow.
| Part | What it does |
|---|---|
| Vena cava (two: from the upper and the lower body) | Brings deoxygenated blood from the body to the right atrium |
| Right atrium | Receives blood from the body and pushes it into the right ventricle |
| Tricuspid valve (three flaps) | Stops blood flowing back from the right ventricle into the right atrium |
| Right ventricle | Pumps deoxygenated blood to the lungs |
| Pulmonary artery | Carries deoxygenated blood to the lungs |
| Pulmonary veins | Bring oxygenated blood from the lungs to the left atrium |
| Bicuspid (mitral) valve (two flaps) | Stops blood flowing back from the left ventricle into the left atrium |
| Left ventricle | Pumps oxygenated blood to the whole body |
| Aorta | The largest artery: carries oxygenated blood to the body |
| Semilunar valves | At the base of the pulmonary artery and aorta; stop blood flowing back into the ventricles |
| Tendons | Hold the valve flaps so they cannot turn inside out when the ventricles contract |
Two features are tested again and again. The left ventricle's wall is much thicker than the right's, because it must pump blood all round the body, while the right ventricle pumps only to the nearby lungs. The atria have thin walls, because they push blood only a short way, down into the ventricles. The heart muscle itself is supplied with blood by the coronary arteries, which branch from the aorta just above the heart.
Both sides of the heart work at the same time. Each beat has three stages:
| Stage | What contracts | Valves | Where the blood goes |
|---|---|---|---|
| 1. Atria contract | Both atria | Tricuspid and bicuspid open | From the atria into the ventricles |
| 2. Ventricles contract | Both ventricles | Tricuspid and bicuspid close; semilunar valves open | Into the pulmonary artery and aorta |
| 3. Heart relaxes | Nothing | Semilunar valves close | From the veins into the atria, refilling the heart |
The closing valves make the two heart sounds a health worker hears through a stethoscope: the first as the tricuspid and bicuspid valves shut, the second as the semilunar valves shut.
Worked example 1: a lifetime of beats
A heart beats on average 70 times a minute, pumping 70 cm³ of blood from the left ventricle each beat. (a) How many times does it beat in one day? (b) In 70 years? (c) What volume of blood does the left ventricle pump per minute, in litres?
You can measure your heart rate by counting your pulse. Rest your arm, place two fingertips (not your thumb) on the thumb side of your wrist, and count the beats for 30 seconds, then multiply by 2.
Investigation. Measure your resting pulse three times and take the mean. Step up and down on a low bench, or run on the spot, for 3 minutes. Then, sitting down, count your pulse at the end of every minute until it is back to the resting rate. The time this takes is your recovery time.
Figure 2: Heart rate before, during and after 3 minutes of exercise (illustrative data). The shaded band is the exercise period.
Why the heart speeds up: working muscles respire faster, so they need more oxygen and glucose and make more carbon dioxide. The brain detects the extra carbon dioxide in the blood and sends nerve impulses that make the heart beat faster and more strongly, while breathing also speeds up. More blood reaches the muscles each minute, bringing oxygen and glucose and removing carbon dioxide and heat. After exercise the rate stays high for a while, until the extra carbon dioxide (and the lactic acid you will meet in the next unit) has been removed.
Regular exercise makes the heart muscle stronger, so it pumps more blood with each beat. A fit person therefore has a lower resting heart rate, a smaller rise during exercise and a shorter recovery time.
Worked example 2: reading Figure 2
(a) For Student A, calculate the rise in heart rate, and the rise as a percentage of the resting rate. (b) Find each student's recovery time. (c) Which student is fitter? Give two pieces of evidence.
Warning
Common mistakes and safety. (1) Mixing up sides: in a diagram the heart's left side is on your right. (2) Saying the left ventricle is thicker "because it holds more blood": both ventricles hold the same volume; the left is thicker because it pumps at a higher pressure, all round the body. (3) Saying the valves pump blood: the muscle pumps; valves only stop backflow. Safety: anyone with asthma, a heart condition or an injury should not do the exercise investigation without their teacher's agreement; stop at once if you feel dizzy, faint or have chest pain.
Circulatory System and Pathway of Blood Through the Heart, Amoeba Sisters. Pause at each chamber and say aloud where the blood goes next; check yourself against Figure 1.
Tip
Memory hook: "Try before you buy." The tricuspid is on the right, the bicuspid on the left, and you meet the tricuspid first. Blood order: vena cava, RA, RV, pulmonary artery, lungs, pulmonary veins, LA, LV, aorta.
Practice: the heart and heart rate
Key points
Sources and credits
Written by Ganzaa: the text, Figures 1 and 2 (Figure 2 from illustrative data), the tables, the worked examples and the practice questions.
