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The circulatory system

The circulatory system transports useful substances to cells and removes waste from cells.

Cells require a constant supply of glucose and oxygen. They also produce toxic substances such as carbon dioxide and urea that must be removed.

These substances are transported through the circulatory system in the blood.

Some small animals like flatworms do not need a circulatory system!

The human circulatory system has three components:

  • Heart: pumps blood to the lungs and around the body.
  • Blood: exchanges material with cells, protects the body.
  • Blood vessels: carry blood around the body

Humans have a closed circulatory system. This means the blood never leaves the blood vessels.

Some animals, such as spiders, have an open circulatory system. In these animals, the blood bathes all the cells directly.

The ancient Egyptians recognised how important the heart was for life.
The ancient Egyptians recognised how important the heart was for life.

Humans have a double circulatory system. This means that there are two separate pathways for blood from the heart. One is to the lungs and the other is to the rest of the body.

Fish have a single circulatory system. Blood absorbs oxygen (through gills) on its way around the body.

Blood moves to and from the heart through blood vessels:

  1. The heart pumps deoxygenated blood to the lungs via the pulmonary artery .
  2. The pulmonary vein carries oxygenated blood back to the heart from the lungs.
  3. The heart pumps oxygenated blood around the body via the aorta .
  4. The vena cava carries deoxygenated blood back to the heart.

In a single journey around the body, blood must go through the heart twice. This system ensures the blood delivered to all tissues is well oxygenated.

Both circulatory paths carry deoxygenated and oxygenated blood.
Both circulatory paths carry deoxygenated and oxygenated blood.

Arteries carry blood away from the heart to other parts of the body.

The heart pumps blood into arteries at high pressure. This forces blood to move through arteries. Arterial structure is adapted to cope with high blood pressure:

  • Muscular walls prevent damage from high pressure.
  • Elastic fibres in the walls allow arteries to stretch under high pressure and recoil during low pressure.
  • Small lumen (space inside the vessel) ensures blood pressure stays high for transport to the whole body.

Almost all arteries carry oxygenated blood.

The pulmonary artery is the only artery that does not carry oxygenated blood. It carries deoxygenated blood from the heart to the lungs.

Arteries are the largest blood vessel (left), followed by veins (centre) and capillaries (right).
Arteries are the largest blood vessel (left), followed by veins (centre) and capillaries (right).

Veins carry deoxygenated blood towards the heart from other parts of the body.

The pulmonary vein is an exception. It carries oxygenated blood from the lungs back to the heart.

Veins carry blood at low pressure. They are adapted for this:

  • Thin muscular layer : provides tension that pushes blood along the vessel.
  • Valves : prevent the blood flowing away from the heart (backflow).

Blood is pushed through veins by the contraction of muscles next to veins, such as the leg muscles. Reduced pressure in the right side of the heart also helps to suck blood up from veins.

The blood pressure in veins is too low to force the blood to flow in one direction.
The blood pressure in veins is too low to force the blood to flow in one direction.

The deoxygenated blood carried in veins is dark red (arterial blood is bright red).

The blue-green colour that veins appear through the skin is due to fat below the skin, not due to veins carrying blue blood.

A capillary is a tiny blood vessel that connects arteries to veins.

At the end of the arteries, blood vessels branch out into a network of capillaries. Capillaries are small enough to penetrate any tissue and exchange substances with surrounding cells:

  • The capillary wall is only one cell thick, creating short pathways for diffusion.
  • The capillary wall is leaky, allowing blood plasma to leave the capillary vessel. This allows nutrients to move from capillaries to cells.
  • Lumen is only wide enough for one red blood cell, increasing surface area for diffusion.

Blood in the capillary network is pooled into veins and returned to the heart.

Tissue fluid is formed from the blood plasma and dissolved substances that leave capillary vessels.

The relatively high blood pressure in capillaries forces the fluid in blood (called blood plasma) to leak into the surrounding tissue. Not all substances in the blood can pass across the leaky capillary wall:

  • Small molecules like oxygen can move through the capillary walls into the tissue fluid.
  • Larger objects like red blood cells and proteins cannot fit through the capillary walls.

Despite being larger than red blood cells, white blood cells squeeze through the capillary wall to reach pathogens in the tissue fluid!

Waste substances such as carbon dioxide and urea diffuse into capillaries from cells. These substances are then removed from the body.

Once in the tissue fluid, substances can easily diffuse into nearby cells.
Once in the tissue fluid, substances can easily diffuse into nearby cells.

Below is a table illustrating the differences in structure and function between arteries, veins and capillaries.

Features Artery Capillary Vein
Walls Thick, muscular and elastic One cell thick Thin layer of muscle
Lumen Relatively small Very small (blood cells travel in single-file) Relatively large
Valves No No Yes
Pressure High Medium Low
Blood content Oxygenated Both Deoxygenated
Direction Away from heart Within tissue Towards heart
Arteries branch into capillaries, which combine to form veins.
Arteries branch into capillaries, which combine to form veins.

The aorta splits into multiple arteries that supply blood to specific organs and limbs. Blood is then pooled into the vena cava to return to the heart.

Below is a diagram showing some of the important blood vessels that deliver blood to and from the organs.

The arteries and veins supplying blood to specific organs have different names.
The arteries and veins supplying blood to specific organs have different names.