So far in this series we’ve looked at how substances are exchanged across membranes, particularly oxygen and carbon dioxide in gas exchange systems. Today, we’re going to start looking at how substances get transported around organisms, starting off by looking at the structure of the human heart.
The Human Heart
The heart is the key organ belonging to the circulatory system. It keeps blood flowing around the body. Blood transports many essential substances including glucose and oxygen for respiration. (Remember that in the lungs, having the blood constantly flowing helps to maintain high concentration gradients of oxygen and carbon dioxide between the blood and alveolar space).
The circulatory system is made up of two circuits: one loop goes through the lungs and heart, the other loop goes through the heart and the rest of the body. We call it a double circulatory system. You need to know the names of the blood vessels that enter and leave the heart, the chambers of the heart, and the valves. The below diagram and table show and describe them.
| Vena cava | Vein carrying deoxygenated blood from the body to the heart. The upper branch is called the superior vena cava and the lower branch is called the inferior vena cava. |
| Pulmonary artery | Artery carrying deoxygenated blood from the heart to the lungs. The only artery to carry deoxygenated blood. |
| Aorta | Artery carrying oxygenated blood from the heart to the body. |
| Pulmonary vein | Vein carrying oxygenated blood from the lungs to the heart. The only vein to carry oxygenated blood. |
| Left and right atria | The two upper chambers of the heart where blood enters. |
| Left and right ventricles | The two lower chambers of the heart where blood is pumped out to the body and lungs. |
| Atrioventricular valves | Valves between the atria and ventricles which prevent blood flowing back into the atria when the ventricles contract. |
| Semi-lunar valves | Prevent backflow into the heart from the aorta and pulmonary artery. |
| Valve tendons (cords) | Prevent the atrioventricular valves being forced back into the atria when the ventricles contract. |
The other blood vessels you need to know about are the arteries that provide oxygenated blood to the heart muscle tissue itself. These are the coronary arteries, and can be seen around the outside of the heart. Blockage of a coronary artery can result in a heart attack. Will we look at this in more detail when we cover cardiovascular disease.
The ventricles have thicker muscular walls than the atria. This is because when the ventricles contract, they have to contract with enough force to squeeze the blood out of the heart to its destination in the body, whereas the atria only have to squeeze the blood into the ventricles next door. For a similar reason, the wall of the left ventricle is thicker than the wall of the right ventricle. The left ventricle has to contract with enough force to squeeze the blood all the way around the body, whereas the right ventricle only has to get the blood as far as the lungs which are quite close by. If you looked at the heart of a giraffe, you would see that the wall of the left ventricle is exceptionally thick because it has to get the blood all the way up that long neck!
Summary
- Humans have a double circulatory system.
- The human heart has four chambers – two atria and two ventricles.
- The vena cava and pulmonary artery carry deoxygenated blood.
- The aorta and pulmonary vein carry oxygenated blood.
- Valves prevent backflow.
In the next article we will look at the cardiac cycle and how the heart carries out its function. Any questions? Leave them below.
