Circulatory systems

Very small organisms, like amoeba or flatworms, 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 large surface area compared to their volume (a high surface area to volume ratio). The diffusion path from the outside to their innermost cells is very short. However, large, complex organisms like humans have a small surface area to volume ratio. Our cells, especially those deep inside the body, are too far from the surface for diffusion to be fast enough to keep them alive. To solve this, we have a specialised transport system, the circulatory system, which acts like a delivery network. It's a 'mass flow' system that rapidly moves large volumes of blood, carrying essential substances, to all parts of the body.

Humans have a double circulatory system. This means that for one complete circuit of the body, blood passes through the heart twice. This setup is highly efficient and consists of two distinct circuits:

1. The Pulmonary Circuit: The right side of the heart pumps deoxygenated blood to the lungs to pick up oxygen and release carbon dioxide. This oxygenated blood then returns to the left side of the heart.
2. The Systemic Circuit: The left side of the heart pumps this newly oxygenated blood at high pressure to the rest of the body's organs and tissues. After delivering oxygen and nutrients, the deoxygenated blood returns to the right side of the heart to begin the cycle again. The main advantage of this system is that the heart can give the blood an extra pressure 'boost' after it has been through the lungs. This ensures that blood flows quickly and at high pressure to all body tissues, allowing for a high metabolic rate.

Blood travels around the body in three main types of blood vessels: arteries, veins, and capillaries. Each is structurally adapted for its specific role.

• Arteries: Carry blood AWAY from the heart. The blood is at high pressure, so arteries have thick, strong, muscular, and elastic walls to withstand and maintain this pressure. The elastic fibres allow the artery to stretch and recoil, which helps to smooth out the pulses of blood flow from the heart. They have a relatively narrow central tube, or lumen.
• Veins: Carry blood TOWARDS the heart. The blood is at low pressure, so veins have thinner walls with less muscle and elastic tissue than arteries. They have a wider lumen to reduce resistance to blood flow. To prevent the low-pressure blood from flowing backwards, especially in the limbs, veins have valves.
• Capillaries: These are the smallest blood vessels, linking arteries and veins. They form vast networks (capillary beds) throughout the body's tissues. Their function is the exchange of materials between the blood and the body's cells. Their structure is perfectly adapted for this: their walls are only one cell thick, providing a very short diffusion distance, and they have a very narrow lumen, which slows blood flow and forces red blood cells to pass in single file, maximising time for exchange.

The heart is a muscular organ that acts as the pump for the circulatory system. It has four chambers. The two upper chambers are called atria (singular: atrium), and they receive blood returning to the heart. The two lower, more muscular chambers are called ventricles, and they pump blood out of the heart. A muscular wall called the septum separates the right side of the heart from the left side, preventing oxygenated and deoxygenated blood from mixing.

Path of Blood: Deoxygenated blood from the body enters the right atrium via the vena cava. It passes to the right ventricle, which pumps it to the lungs via the pulmonary artery. Oxygenated blood from the lungs returns to the left atrium via the pulmonary vein. It passes to the left ventricle, which pumps it to the rest of the body via the aorta, the body's largest artery. The left ventricle wall is much thicker and more muscular than the right because it has to pump blood to the entire body, whereas the right ventricle only pumps to the lungs.

Major Vessels:

• To/From Heart: Vena Cava (to right atrium), Aorta (from left ventricle), Pulmonary Artery (from right ventricle), Pulmonary Vein (to left atrium).
• To/From Organs: Renal Artery/Vein (to/from kidneys), Hepatic Artery/Vein (to/from liver).
• Special Vessel: The Hepatic Portal Vein carries blood rich in digested food from the gut to the liver for processing.

Capillaries are the crucial link between the arterial and venous systems. They form extensive networks, called capillary beds, that permeate every tissue, ensuring no cell is far from the blood supply. Their primary role is the exchange of substances. At the arterial end of a capillary bed, the blood is under high pressure. This pressure forces some of the watery plasma out of the capillaries through tiny gaps in their walls. This fluid, now called tissue fluid, surrounds and bathes the body cells. It is similar in composition to plasma but lacks large proteins and red blood cells, which are too big to pass through the capillary walls. This tissue fluid allows for efficient exchange: oxygen and nutrients like glucose diffuse from the high concentration in the fluid into the cells, while carbon dioxide and other waste products diffuse from the cells into the fluid. Towards the venous end of the capillary bed, the blood pressure is lower. Most of the tissue fluid moves back into the capillaries by osmosis. The small amount that remains is collected by the lymphatic system and eventually returned to the blood.