You actually have two transport systems. Humans have a double circulation. One carries blood from your heart to your lungs and back again to exchange exygen and carbon dioxide with the air. The other carries blood all around the rest of your body and back again.
A double circulation like this is very important in warm blooded, active animals like ourselves. It makes our circulatory system very efficient. Fully oxygenated blood returns to the heart and can then be sent off to the different parts of the body. This means more areas of your body can receive fully oxygenated blood quickly.
Q. Why do we need a blood circulation system?
A. So blood can be carried from your heart to your lungs and back again to exchange oxygen around the rest of your body and back again.
The Blood Vessels
Distinctive features of:
Q.Substances can only enter and leave the blood in the capillaries. Why is this?
The Blood Vessels
Your arteries carry blood away from your heart to the organs of your body. This is usually oxygentated blood so it is bright red. They stretch as the blood is forced through themand go back into shape afterwards. You can feel this as a pulse where the arteries run close to the surface (like your wrist).
The veins carry blood towards your heart - it is usually low in oxygen and so is a deep purply-red colour. They do not have a pulse, but they have valves to prevent the back flow of blood as it moves from various parts of the body back to the heart.
You blood flows through a huge netwrok of cappilaries. Cappilaries are narrow with vey thin walls so the substances needed by your body cells (oxygen and glucose) can easily pass out of your blood and into your cells by diffusion.
A. Because the walls are thin enough.