Blood is pumped around the body by the Heart inside blood vessels, of which there are 3 types:
Arteries……carry blood under high pressure from the Heart to the capillaries Capillaries…carry blood at decreasing pressure to within rapid diffusible distance of cells Veins………carry blood at low pressure back towards the Heart.
In 1 COMPLETE circuit, blood flows through the Heart twice in mammals. Once to the lungs to give up CO2 and pick up Oxygen, and once to the rest of the body cells to give up the oxygen, and collect the CO2 formed from cell respiration. Therefore mammals have a DOUBLE CIRCULATORY SYSTEM.
Fish only have a single system, which is less efficient as the oxygenated blood going to the body cells flows slowly under low pressure. By pumping oxygenated blood to the body cells, much more blood reaches the cells, and so mammals are generally more active.
The pumping pressure to send blood around the body (on the Left side of the Heart) is > the pressure from the Right side, which only has to pump blood to the lungs next to the heart. So the heart muscle on the Left side is much thicker than on the Right side.
During exercise, blood is diverted to the active organs needing more oxygen and removal of CO2. These organs include the Brain, Heart (has its own blood supply called the coronary system), Lungs, Skeletal muscles, and Skin for temperature control (getting rid of more heat).
William Harvey in the 17th century, was the first person to put forward the idea (based on evidence of one way flow of blood in veins) that blood flows round the body in one direction, pumped by the heart. Prior to Harvey, people believed in Galen’s (1st century AD) view that blood flowed backwards and forwards like the tide. Harvey predicted the existence of capillaries linking arteries to veins.
In the Cardiac Cycle (Heartbeat), blood is received and pumped on both sides of the heart together: The Left side (Left Atrium) receives oxygenated blood from the lungs via the Pulmonary Vein. The LA then pumps the blood through the open Bicuspid Valve into the Left Ventricle, which pumps it out into the Aorta to go to the body via the open Semi-lunar valves, which prevent blood re-entering. The Right side (Right Atrium) receives de-oxygenated blood from the body via the Vena Cava vein. This blood is pumped into the Right Ventricle passing through the open Tricuspid Valve, which pumps it to the lungs in the Pulmonary Artery via the open Semi-lunar valves.
How do both sides of the heart work together? In the wall of the Right Atrium, is the Pacemaker, (Sino-Atrial Node).
