By the end of this lesson you should be able to
- Describe the structure of the heart and the path of blood through it
- Explain double circulation
- Compare arteries, veins and capillaries
- State the components of blood and their functions
Why we need a transport system
In a large, active animal, substances cannot reach every cell by diffusion alone because the distances are too long. The circulatory system carries oxygen, glucose and other nutrients to cells, and removes carbon dioxide and other wastes. It also carries hormones and heat, and protects the body against disease.
Key ideas: the heart
The heart is a muscular pump made of cardiac muscle. It has four chambers: two upper atria (singular: atrium) that receive blood, and two lower ventricles that pump blood out.
| Part | Function |
|---|---|
| Right atrium | Receives deoxygenated blood from the body through the vena cava |
| Right ventricle | Pumps deoxygenated blood to the lungs through the pulmonary artery |
| Left atrium | Receives oxygenated blood from the lungs through the pulmonary vein |
| Left ventricle | Pumps oxygenated blood to the whole body through the aorta |
| Valves | Stop blood flowing backwards |
| Septum | Separates the left and right sides, so oxygenated and deoxygenated blood do not mix |
The wall of the left ventricle is thicker than that of the right ventricle, because it must pump blood at high pressure around the whole body, while the right ventricle pumps blood only to the nearby lungs.
The path of blood through the heart:
body → vena cava → right atrium → right ventricle → pulmonary artery → lungs → pulmonary vein → left atrium → left ventricle → aorta → body
Double circulation
Blood passes through the heart twice on each complete circuit of the body. This is called double circulation.
- The pulmonary circulation carries blood from the heart to the lungs and back. In the lungs, blood picks up oxygen and gives up carbon dioxide.
- The systemic circulation carries blood from the heart to the rest of the body and back.
The advantage is that blood returns to the heart after passing through the lungs, so it is pumped again at high pressure to the body. This delivers oxygen to the tissues quickly.
Blood vessels
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Direction of blood | Away from the heart | Towards the heart | Between arteries and veins |
| Wall | Thick, muscular and elastic | Thin | One cell thick |
| Lumen (space inside) | Narrow | Wide | Very narrow |
| Blood pressure | High | Low | Falls as blood passes through |
| Valves | No (except at the heart) | Yes | No |
| Function | Carry blood at high pressure to organs | Return blood to the heart | Exchange of substances with cells |
Most arteries carry oxygenated blood and most veins carry deoxygenated blood. The pulmonary artery and pulmonary vein are exceptions. The pulmonary artery carries deoxygenated blood, and the pulmonary vein carries oxygenated blood.
Why capillaries are suited to exchange: they have walls that are one cell thick (short diffusion distance) and they form a large network with a large surface area.
Blood
Blood is made of a liquid called plasma and several types of cell.
| Component | Description | Function |
|---|---|---|
| Plasma | Pale yellow liquid, mostly water | Carries dissolved substances (glucose, amino acids, urea, hormones, carbon dioxide) and heat |
| Red blood cells | Biconcave discs with no nucleus, full of haemoglobin | Carry oxygen as oxyhaemoglobin |
| White blood cells | Have a nucleus, and are larger than red cells | Defend the body: some engulf microbes (phagocytes), others make antibodies (lymphocytes) |
| Platelets | Small cell fragments | Help blood to clot at a wound |
Worked examples
Example 1: tracing the blood. Describe the path of blood from the left ventricle to the kidneys and back to the heart.
Blood leaves the left ventricle through the aorta, travels in an artery to the kidneys, passes through capillaries in the kidneys, and returns in a vein (the renal vein) to the vena cava, which brings it to the right atrium.
Example 2: reading heart-rate data. A resting heart pumps about 70 mL of blood with each beat, and beats 72 times a minute.
That is about 5 litres per minute, roughly the total volume of blood in the body, so all the blood passes through the heart about once every minute.
Common mistakes
Saying that all arteries carry oxygenated blood. The pulmonary artery carries deoxygenated blood to the lungs.
Mixing up left and right. In diagrams, the left side of the heart is on the reader’s right, because the diagram is drawn as if you are facing the person.
Writing that blood is “made of red blood cells”. Plasma makes up more than half of blood by volume.
Saying red blood cells have a nucleus. Human mature red blood cells do not have a nucleus, which leaves more room for haemoglobin.
Practice questions
- Name the four chambers of the heart.
- State the function of the valves in the heart and veins.
- Explain why the wall of the left ventricle is thicker than that of the right ventricle.
- What is meant by double circulation?
- State three differences between arteries and veins.
- Explain how a capillary is adapted for exchange of substances.
- Name the blood component that (a) carries oxygen, (b) makes antibodies, (c) helps blood to clot.
- A heart has a stroke volume of 60 mL and beats 80 times a minute. Calculate the cardiac output in litres per minute.
Show answers
- Right atrium, right ventricle, left atrium and left ventricle.
- They prevent blood flowing backwards.
- The left ventricle pumps blood at high pressure around the whole body, while the right ventricle pumps blood only to the lungs, so the left ventricle needs a thicker muscular wall.
- Blood passes through the heart twice on each complete circuit: from the heart to the lungs and back, then from the heart to the body and back.
- Any three of: arteries carry blood away from the heart and veins carry blood towards it; arteries have thicker, more muscular walls; veins have valves and arteries do not; arteries have a narrower lumen; blood pressure is higher in arteries.
- Its wall is one cell thick, so the diffusion distance is short, and capillaries form a large network with a large surface area.
- (a) Red blood cells (haemoglobin). (b) White blood cells (lymphocytes). (c) Platelets.
- mL per minute, which is 4.8 litres per minute.
Summary
- The heart has four chambers. The right side pumps blood to the lungs, and the left side pumps blood to the body.
- Double circulation gives high-pressure blood flow to the tissues.
- Arteries carry blood at high pressure away from the heart, veins return it with the help of valves, and capillaries exchange substances.
- Blood has plasma, red blood cells, white blood cells and platelets, each with a function.
Now practise this topic
Real questions show you what still needs work. Try a past paper under timed conditions, or take a short quiz.