The double circulatory system
Mammals have a double circulatory system, which means blood passes through the heart twice for one full trip around the body. It is really two loops joined at the heart:
- The pulmonary circuit carries blood from the heart to the lungs and back, to pick up oxygen.
- The systemic circuit carries blood from the heart to the rest of the body and back, to deliver that oxygen.
The advantage is pressure. Blood loses pressure passing through the tiny lung capillaries, so it returns to the heart to be pumped out again at high pressure. That high pressure pushes blood quickly to the body, so oxygen and glucose reach respiring cells fast.
- Double circulatory system
- A system in which blood is pumped through the heart twice per complete circuit: once to the lungs (pulmonary) and once to the body (systemic). It keeps blood pressure high for efficient delivery.
The heart and the path of blood
The heart has four chambers: two upper atria that receive blood, and two lower ventricles that pump it out. The right side handles deoxygenated blood (to the lungs) and the left side handles oxygenated blood (to the body). Valves stop blood flowing backwards. The left ventricle wall is thicker than the right because it must pump blood all the way round the body, not just to the nearby lungs.
Trace one drop of blood from the body, through the heart and lungs, and back out to the body.
A trick for the vessels
Arteries carry blood Away from the heart. Both start with A, so if you remember that, veins must be the ones carrying blood back towards the heart.
Arteries, veins and capillaries
Blood travels in three kinds of vessel, and each is built for its job. Arteries carry high-pressure blood away from the heart, capillaries are where exchange happens, and veins return low-pressure blood back to the heart.
| Feature | Artery | Vein | Capillary |
|---|---|---|---|
| Direction | Away from the heart | Back to the heart | Links arteries to veins in tissues |
| Blood pressure | High | Low | Falling |
| Wall | Thick, muscular and elastic | Thin | One cell thick |
| Lumen (central space) | Narrow | Wide | Very narrow (one cell wide) |
| Valves? | No | Yes, to stop backflow | No |
| Main job | Withstand high pressure | Return blood at low pressure | Exchange of substances with cells |
The thin, one-cell-thick capillary wall is the key adaptation: it gives a short diffusion distance so oxygen and glucose can pass out to cells, and carbon dioxide can pass in, quickly.
Transport in plants: xylem and phloem
Plants also have two transport tissues, running through the roots, stem and leaves. They carry different things in different directions.
Xylem carries water and dissolved minerals in one direction only: up from the roots to the leaves. This flow is driven by transpiration, the evaporation of water from the leaves. As water evaporates from the leaf surface, it pulls up the whole column of water behind it, a bit like sucking through a straw. This is called the transpiration stream.
- Transpiration
- The loss of water vapour from a plant, mainly through small pores called stomata on the underside of leaves. It pulls water up the xylem from the roots.
Phloem carries dissolved sugars (mainly sucrose) made in the leaves by photosynthesis to the rest of the plant, for use or storage. This movement is called translocation, and unlike the xylem it can go in either direction, up or down, depending on where sugar is needed.
| Feature | Xylem | Phloem |
|---|---|---|
| Carries | Water and mineral ions | Dissolved sugars (sucrose) |
| Direction | Roots to leaves only (upwards) | Both directions (translocation) |
| Driven by | Transpiration from the leaves | Active transport, uses energy |
| Cells | Dead, hollow, strengthened tubes | Living cells with sieve plates |
Where this is assessed
Investigating how a factor such as temperature or wind affects the rate of transpiration is classic Criterion B (inquiring and designing) and Criterion C (processing and evaluating). Be ready to name the variable you would change, the one you would measure, and two you would keep the same.
Check yourself
1. Why is the human circulatory system called "double", and what is the advantage? +
Blood passes through the heart twice for each full circuit: once to the lungs and once to the body. The advantage is that blood is re-pressurised after slowing down in the lungs, so it reaches the body cells quickly and at high pressure.
2. Name the four chambers of the heart in the order a drop of deoxygenated blood from the body passes through them. +
Right atrium, then right ventricle (which sends it to the lungs), then left atrium, then left ventricle (which sends it to the body). The left ventricle has the thickest wall because it pumps blood all round the body.
3. State one difference between xylem and phloem in what they carry and the direction they move it. +
Xylem carries water and minerals upwards only, from roots to leaves. Phloem carries dissolved sugars in both directions (translocation) to wherever they are needed.
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