Open and closed circulatory systems
Animals are very active and require large quantities of oxygen and food - far more than could be delivered by diffusion from those exchange surfaces. as the distances are too great for diffusion alone, multicellular animals have a transport system consisting of a fluid that travels around the whole body, and some sort of pump to move it.
Insects have an Open circulatory system, s their blood is not enclosed in vessels but circulates in body spaces. Their cells are surrounded by blood. A long tubular heart circulates the blood through the body spaces. Insect blood has no haemoglobin and no red blood cells. This is because the gas exchange system delivers O2 and CO2 to the tissues in tiny, air-filled tubes.
The transport systems in fish and mammals are Closed circulatory systems, as the blood flows inside vessels such as arteries, veins and capillaries. Their cells are surrounded not by blood, but by tissue fluid.
Single and double circulatory systems
Fish have single circulatory systems. Blood flows through the heart once during one complete circuit of the body. Blood flows from the heart to the fills and then to the rest of the body before returning to the heart.
- Heart - Body - Gills - Heart
Mammals have a double circulatory system - blood flows through two circuits:
- Pulmonary circuit - from the heart to the lungs and back.
- Systemic circuit - from the heart to the rest of the body and back.
When the heart pumps blood, it creates pressure that forces the blood through blood vessels. It is difficult to push blood through the blood vessels because they exert a resistance to the flow of blood. To overcome this resistance, the heart gives the blood considerable pressure. Mammalian heart is adapted to form 2 pumps - one for each circulation. Blood flows through the heart twice for each circulation of the body.
- Heart - Body - Heart - Lungs - Heart
Advantages of a double circulation (Comparison)
An efficient circulatory system will deliver oxygen and nutrients quickly to the parts of the body where they are needed. The blood can be made to flow more quickly by increasing the blood pressure created by the heart.
By comparing the transport systems of fish and mammals, we shall see the advantages of a double circulatory system.
In the fish single circulatory system:
- Blood pressure is reduced as blood passes through the tiny capillaries of the gills.
- This means it will not flow very quickly to the rest of the body.
- This limits the rate at which oxygen and nutrients are delivered to respiring tissue.
Fish are not as active as mammals and they do not maintain their body temperature, so they need less energy. Their single circulatory system delivers oxygen and nutrients quickly enough for their needs, so for them it is efficient.
In the mammal double circulatory system:
- The heart can increase the pressure of the blood after it has passed through the lungs, so blood flows more quickly to the body tissues.
- The systematic circulation can carry blood at the higher pressure than the pulmonary circulation.
- The blood pressure must not be too high in the pulmonary circulation, otherwise it may damage the delicate capillaries in the lungs.
Mammals are active animals and maintain their body temperature. Both the energy for activity and the heat needed to keep the body warm require energy from food. The energy is released from food in the process of respiration. To release a lot of energy, the cells need good supplies of both nutrients and oxygen.
Substances such as O2, CO2 and glucose are exchanged between blood and tissue through the walls of capillaries. Blood flows from the heart through arteries to reach capillaries and then returns to the heart inside veins. All blood vessels are lined by squamous epithelial cells forming a layer known as endothelium. These endothelial cells help to maintain the uninterrupted flow of blood through the circulation.
Various types of blood vessels:
- Walls are thick and strong to withstand high blood pressure.
- Elastic fibres stretch when the heart pumps blood into an artery.
- Elastic fibres recoil to push blood on its way towards capillaries.
- Smooth muscle in smaller arteries controls diameter to alter blood flow.
- Lined by layer of squamous epithelial known as endothelium.
Blood vessels cont.
- Walls are thin, as blood pressure is low.
- Walls distend (stretch) to accommodate larger volumes of blood.
- Veins have semilunar valves to ensure blood travels towards the heart.
- Lined by endothelial cells.
Actions of semilunar valves in veins -
- Pressure of blood forces valve open and back flow of blood closes the valve.
- Muscles of the body contract e.g. during walking and squeeze veins, so push blood back towards the heart.
- Wall is made of one layer of endothelial cells, so diffusion distance is short.
- Tiny holes in endothelial cells allow water and some solutes to leave the blood.
- Very small, so many capillaries in a small space give a large surface area for diffusion.