Circulatory systems

Single-celled organisms, like an amoeba, can get all the substances they need (like oxygen and glucose) and get rid of waste products (like carbon dioxide) simply by diffusion. This is because they have a very large surface area compared to their volume. This is known as a high surface area to volume ratio (SA:V ratio). For them, the diffusion distance from the outside to the centre of the cell is tiny. However, as organisms get larger and more complex, like humans, their volume increases much faster than their surface area. This leads to a small SA:V ratio. Cells deep inside the body are too far from the surface for diffusion to be fast enough to supply their needs. To solve this, large organisms have a circulatory system, which acts as a mass transport system to move substances quickly over long distances.

Humans have a double circulatory system. This means that for one complete circuit of the body, blood passes through the heart twice. This system is divided into two distinct circuits. The first is the pulmonary circuit, which carries deoxygenated blood from the right side of the heart to the lungs, and then returns oxygenated blood to the left side of the heart. The second is the systemic circuit, which carries this oxygenated blood from the left side of the heart to all other parts of the body, and then returns deoxygenated blood to the right side of the heart. The main advantage of this system is that the heart can give the blood an extra 'boost' of pressure after it has been to the lungs. This allows blood to be sent to the body at high pressure for rapid delivery of oxygen, while also being sent to the delicate lungs at a lower pressure to prevent damage to the capillaries.

The heart is a muscular organ that pumps blood around the body. It is divided into four chambers: two atria at the top and two ventricles at the bottom. The right side of the heart deals with deoxygenated blood, while the left side deals with oxygenated blood. A muscular wall called the septum completely separates these two sides, preventing the blood from mixing. Blood flows from the atria to the ventricles through atrioventricular valves. When the ventricles contract, these valves shut to prevent backflow into the atria. Blood is then forced out of the heart into the main arteries through semilunar valves, which also prevent backflow into the ventricles once they relax. The wall of the left ventricle is significantly thicker and more muscular than the right ventricle because it needs to generate much higher pressure to pump blood all around the body, whereas the right ventricle only pumps blood to the nearby lungs.

Blood is transported around the body in three types of blood vessels. Arteries carry high-pressure blood away from the heart. To withstand this pressure, they have thick, muscular, and elastic walls. The elastic walls stretch and recoil with each heartbeat, helping to smooth blood flow, and the thick muscular wall helps maintain pressure. They have a relatively narrow central tube, or lumen. Veins carry low-pressure blood towards the heart. Their walls are much thinner and less muscular than arteries, and they have a wider lumen to reduce resistance to blood flow. Because the pressure is low, veins have valves along their length to prevent the backflow of blood. Capillaries are the site of exchange between blood and body cells. They form vast networks that link arteries and veins. Their walls are only one cell thick, providing a very short diffusion distance for substances like oxygen, glucose, and carbon dioxide to move between the blood and the tissues.

To understand blood circulation, you must know the main blood vessels. The Aorta is the largest artery, carrying oxygenated blood from the left ventricle to the whole body. The Vena Cava is the largest vein, bringing deoxygenated blood from the body back to the right atrium. The Pulmonary Artery is unique as it's an artery that carries deoxygenated blood, from the right ventricle to the lungs. The Pulmonary Vein is also unique as it's a vein that carries oxygenated blood, from the lungs to the left atrium. Other key vessels include the Renal Artery (carries blood to the kidneys) and Renal Vein (carries blood away from the kidneys), and the vessels supplying the liver. The Hepatic Artery supplies the liver with oxygenated blood. Uniquely, the Hepatic Portal Vein carries blood rich in digested food from the intestines to the liver for processing, before it enters the main circulation.