TRANSPORT IN ANIMALS
The Circulatory System
- Large and complex animals have circulatory systems that consist of tubes, a transport fluid and a means of pumping the fluid.
- Blood is the transport fluid which contains dissolved substances and cells.
- The tubes are blood vessels through which dissolved substances are circulated around the body.
- The heart is the pumping organ which keeps the blood in circulation.
The types of circulatory system exist in animals: open and closed.
In an open circulatory system;
- The heart pumps blood into vessels which open into body spaces known as haemocoel.
- Blood comes into contact with tissues.
A closed circulatory system;
- Found in vertebrates and annelids where the blood is confined within blood vessels and does not come into direct contact with tissues.
Transport in Insects
- In an insect, there is a tubular heart just above the alimentary canal.
- This heart is suspended in a pericardial cavity by ligaments.
- The heart has five chambers and extends along the thorax and abdomen .
- Blood is pumped forwards into the aorta by waves of contractions in the heart.
- It enters the haemocoel and flows towards the posterior.
- The blood flows back into the heart through openings in each chamber called ostia.
- The ostia have valves which prevent the backflow of blood.
- Blood is not used as a medium for transport of oxygen in insects.
- This is because oxygen is supplied directly to the tissues by the tracheal system.
- The main functions of blood in an insect are to transport nutrients, excretory products and hormones.
Mammalian Circulatory System
- Mammals have a closed circulatory system where a powerful heart pumps blood into arteries.
- The arteries divide into smaller vessels called arterioles.
- Each arteriole divides to form a network of capillaries inside the tissues.
- The capillaries eventually re-unite to form venules, which form larger vessels called veins.
- The veins take the blood back to the heart.
- Blood from the heart goes through the pulmonary artery to the lungs and then back to the heart through pulmonary vein.
- This circulation is called pulmonary circulation.
- Oxygenated blood leaves the heart through the aorta and goes to all the tissues of the body.
- From the tissues, deoxygenated blood flows back to the heart through the vena cava.
- This circulation is called systemic circulation.
- In each complete circulation, the blood flows into the heart twice.
- This is called double circulation.
- Some other animals like fish have a single circulation.
- Blood flows only once through the heart for every complete circuit.