S-Cool Revision Summary

S-Cool Revision Summary

Structure of the Heart

The adult human heart weighs about a kilogram and has four cavities inside it. These 'chambers' are divided into two at the top called the atria (each is called an atrium) and two at the bottom, called the ventricles.

Structure of the heart

The diagram shows that the heart is in fact divided neatly in two down its middle so that each side has one atrium and one ventricle. Both the left and right sides of the heart pump blood. The only difference between them is where the blood arrives from and to where it is pumped.

The muscle that makes up the heart is unusual as it doesn't get tired while it pumps. But it can be damaged by drugs and poor diet leading to heart disease.

Blood Vessels

The heart pumps the blood along a series of tubes that are collectively called blood vessels. But they are more than just simple pipes.


Arteries carry blood away from the heart.

Each time the heart beats it fires blood into the arteries at a high pressure, so they need to be tough so that they don't burst. They are also quite thick with only a small space, known as the lumen, down the centre.

Fortunately arteries are provided with a tough outer layer and another layer inside this that can cope with the stretching the pulses of blood. This elastic layer is made up of elastic fibres and smooth muscle which contracts and helps to keep the blood moving along.

The third layer that lines the lumen of the artery is called the endothelium. This is made up of special endothelial lining cells which give a smooth surface to the lumen.


Veins carry blood back to the heart.

The blood returning from the body is at a much lower pressure than that fired out by the beating heart. So veins do not need to be as strong as arteries.Veins have a cross-sectional structure that is very similar to arteries.

One of the obvious differences is that they have a much wider lumen and thinner walls.The other main difference is that veins have valves inside them, while arteries do not.

The valves occur occasionally along their length and ensure that blood can only travel in one direction.

Having valves in arteries would not be much use as it would slow the blood down and stop it reaching those important bits of you, like your head.


Capillaries are tiny, thin-walled vessels. They carry blood close to all the body's cells in its tissues and organs. They may not be the most glamorous of the vessels but they are perhaps the most important.

Capillaries are made up of a single layer of endothelial cells around a very small lumen.

Molecules can easily move into and out of the capillaries by diffusion. This allows food, gas and waste molecules to be taken to and from every cell in the body.

The Circulatory System

The heart and blood vessels carry out a transport function. They carry food molecules, water and oxygen to cells and remove waste products such as carbon dioxide. They form the circulatory system.

A double circulation

Instead of just being a single loop the circulation has two interconnected loops, in a sort of figure of eight.

Blood returns from the body to the right atrium. The blood has lost most of the oxygen it carries and is now deoxygenated.

The right ventricle pumps the blood along the pulmonary artery to the lungs where it picks up fresh oxygen. It is now oxygenated.

The oxygenated blood enters the left side of the heart and is pumped out through the aorta to the body.

Once it reaches the capillaries around the body, oxygen diffuses out to the surrounding cells.

The deoxygenated blood is carried back towards the heart in the veins. These join up to form the vena cava which is the largest vein.

Useful tip: One sneaky exam fact is that veins only carry deoxygenated blood except for the pulmonary vein. This is the only one that carries oxygenated blood because it takes blood from the lungs to the left heart ready to get pumped round the body.

The hepatic circulation

This is a special part of the circulation system.

Normally the circulation system takes blood straight back from the capillaries in each organ or tissue.

But the blood from the digestive system carries all sorts of molecules that have been absorbed there. In order to stop the rest of the blood system getting clogged up there is a special detour. It is called the hepatic portal system.

Hepatic means to do with the liver. The liver is the factory organ of the body. It deals with all sorts of chemicals, breaking them down and rearranging them.

The hepatic portal vein carries blood to the liver. Then the blood can leave the liver for the heart.

The Blood

Blood isn't just a red liquid. It is five litres of a careful mixture of plasma and blood cells. These cells come in three varieties: red, white and platelets.


The plasma makes up most of the blood. It is mainly water but carries lots of other essential ingredients.

The following substances are carried in the plasma:

  1. Dissolved carbon dioxide: This is the waste gas produced by respiration in cells
  2. Dissolved glucose and amino acids: Food molecules for respiration, building and repairing cells
  3. Urea: Waste product of digestion, this is lost from the kidney.
  4. Antibodies and antitoxins: Chemicals that protect us from disease and poisons
  5. Hormones: Chemicals that control some of our body functions

Plasma has a yellowish appearance. It sometimes oozes out of blisters. Nice!

Red Blood Cells

The best known of the cells are the red blood cells, correctly called erythrocytes.

Erythrocytes contain the oxygen carrying molecule haemoglobin; this is a special pigment that gives blood its red colour. Iron is needed in the production of haemoglobin; if your diet lacks this mineral you can develop the condition anaemia.

Red blood cells are unlike other cells in that they do not contain a nucleus. They are really just a bag containing the haemoglobin. The cells have a doughnut-shape with a flattened centre instead of a hole.

White Blood Cells

When blood picks up oxygen we say that it has been oxygenated. This happens because haemoglobin molecules form weak bonds with oxygen to make a new complex molecule called oxyhaemoglobin.


Platelets are fragments of larger cells. Their job is to form part of a clot so that they plug a wound and stop too much blood being lost.