The Human Circulatory System

You have seen that a large animal cannot supply its cells by diffusion alone — its surface area to volume ratio is too small and its cells are too far from the surface. The solution in mammals is a transport system: the circulatory system, which carries blood — and the oxygen, nutrients and waste it contains — to every cell in the body. This lesson, part of Topic B2 of OCR Gateway Science A, covers why a transport system is needed, the structure of the heart, the three types of blood vessel, and the components of the blood, with a Higher-tier calculation of cardiac output.

By the end of this lesson you should be able to explain why mammals need a circulatory system, describe the structure of the heart and the double circulation, compare arteries, veins and capillaries, describe the components of blood, and (Higher) calculate cardiac output.

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Why a Transport System Is Needed

A human is a large, multicellular organism with a small surface area to volume ratio, and most cells lie deep inside the body, far from the surface. Diffusion alone is far too slow to supply them. The circulatory system solves this by carrying substances quickly around the body in the blood:

• it carries oxygen from the lungs and glucose from the gut to the cells;
• it carries waste such as carbon dioxide away from the cells (to the lungs to be breathed out);
• it links all the exchange surfaces (lungs, gut) to the cells that need the substances.

The heart pumps the blood, the blood vessels carry it, and together they form a transport system that reaches every cell.

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Double Circulation

Mammals have a double circulation — the blood passes through the heart twice for each complete circuit of the body. There are two linked loops:

1. The pulmonary circuit carries blood from the heart to the lungs (to pick up oxygen and drop off carbon dioxide) and back to the heart.
2. The systemic circuit carries blood from the heart to the rest of the body (to deliver oxygen) and back to the heart.

flowchart LR
  H["Heart"] --> L["Lungs<br/>(pick up oxygen)"]
  L --> H2["Heart"]
  H2 --> B["Body<br/>(deliver oxygen)"]
  B --> H

The big advantage of a double circulation is that the blood can be pumped at high pressure to the body after being topped up with oxygen at the lungs, so oxygen is delivered quickly and efficiently to active tissues.

Exam Tip: "Double circulation" means the blood goes through the heart twice per circuit — once to the lungs, once to the body. Its advantage is that blood is re-pressurised between the lungs and the body, so it reaches the body cells at high pressure and oxygen is delivered efficiently.

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The Structure of the Heart

The heart is a muscular pump with four chambers: two upper atria (singular: atrium) and two lower ventricles. The right side and left side are separated so that oxygenated and deoxygenated blood do not mix.

The path of blood through the heart is:

1. Deoxygenated blood from the body enters the right atrium through the vena cava.
2. It passes into the right ventricle, which pumps it through the pulmonary artery to the lungs.
3. Oxygenated blood returns from the lungs through the pulmonary vein into the left atrium.
4. It passes into the left ventricle, which pumps it out through the aorta to the whole body.

A few structural points the exam rewards:

• Valves between the atria and ventricles (and in the vessels leaving the heart) stop blood flowing backwards, keeping it moving one way.
• The left ventricle has a thicker, more muscular wall than the right, because it must pump blood at high pressure all the way round the whole body; the right ventricle only pumps to the nearby lungs.
• The heart muscle itself is supplied with oxygenated blood by the coronary arteries.

Exam Tip: A favourite question asks why the left ventricle wall is thicker than the right. The answer: the left ventricle pumps blood to the whole body (against high resistance/over a long distance), so it needs more muscle to generate higher pressure; the right ventricle only pumps to the lungs, which are nearby. Remember the order of the four chambers and the four great vessels (vena cava, pulmonary artery, pulmonary vein, aorta).

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The Three Blood Vessels

There are three types of blood vessel, each adapted to its job.

	Artery	Vein	Capillary
Carries blood	Away from the heart	Towards the heart	Through tissues, linking arteries to veins
Pressure of blood	High	Low	Falling
Wall	Thick, muscular and elastic	Thinner wall	One cell thick
Lumen (central space)	Narrow	Wide	Tiny (just wide enough for a red blood cell)
Valves?	No	Yes (to stop backflow)	No

• Arteries carry blood away from the heart at high pressure, so they have thick, elastic, muscular walls to withstand and smooth out the pressure.
• Veins carry blood back to the heart at low pressure, so they have thinner walls and a wide lumen; they contain valves to stop the low-pressure blood flowing backwards.
• Capillaries are the tiny vessels where exchange happens. Their walls are only one cell thick, giving a short diffusion distance so that oxygen, glucose and waste can pass quickly between the blood and the cells.

Exam Tip: Remember the direction with a memory aid: Arteries carry blood Away from the heart. The capillary's "one-cell-thick wall" is the key adaptation — it gives a short diffusion distance for exchange, linking straight back to the diffusion factors in B2.

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The Components of Blood

Blood is a tissue made of four main components, each with its own job.

Component	What it is	Function
Red blood cells	Biconcave cells, no nucleus, full of haemoglobin	Carry oxygen from the lungs to the cells
White blood cells	Larger cells with a nucleus	Defend the body against pathogens (part of the immune system)
Platelets	Tiny cell fragments	Help the blood to clot at a wound
Plasma	Pale yellow liquid	Carries blood cells, dissolved glucose, carbon dioxide, urea, hormones and heat around the body

• Red blood cells are adapted (as you saw earlier in B2) with a biconcave shape for a large surface area and no nucleus for more haemoglobin, which binds oxygen.
• White blood cells fight infection — some engulf pathogens, others produce antibodies.
• Platelets trigger clotting, which seals wounds and stops blood loss and the entry of pathogens.
• Plasma is the liquid that transports everything else, including dissolved carbon dioxide, the waste product urea, glucose, hormones and heat.

Exam Tip: Do not confuse the jobs: red cells carry oxygen, white cells fight pathogens, platelets clot the blood, and plasma transports dissolved substances. A common slip is saying plasma carries oxygen — most oxygen is carried by red blood cells (haemoglobin); plasma mainly carries dissolved CO₂, glucose, urea and hormones.

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