1. What is double circulation?
The heart is the centre of the circulatory system. It is described as a double pump.
One side of the heart (the right) pumps blood to lungs in order to receive inhaled oxygen and to release carbon dioxide.
The left side of the heart pumps blood to the body cells to supply oxygen for respiration and to remove carbon dioxide.
The blood on each side of the heart is kept separate so oxygenated and deoxygenated blood do not mix.
The septum separates the two sides.
Each side of the heart contains an atrium (where blood arrives) and a ventricle (where blood leaves the heart)
The atria and ventricles are separated by the bicuspid and tricuspid valves.

3. The circulatory system
Blood is pumped around the body when the heart mucles contract.
Blood leaves the ventricles through the arteries at high pressure
It returns to the atria through veins at lower pressure.
The highest pressure is generated in the aorta when blood leaves the left ventricle to travel to body cells.
The left side of the heart is thicker than the right.
It needs to generate more pressure so blood reaches cells further away.
The lowest pressure is found in the vena cava.

4. How does the heart work?
When blood arrives in the heart it fill each atrium from the veins.
The atria muscles are relaxed and the heart is filling with blood. We say it is in diastole.
Once full atria contract to squeeze/push blood into the ventricles.
This is called atrial systole and it causes ventricles to fill with blood.
The ventricles contract to push blood into arteries. This is ventricle systole.
The main blood vessels are the aorta and vena cava, pulmonary artery and pulmonary vein.

5. Tricuspid valve
bicuspid valve
Mitral valve

6. Circulation
The main artery (aorta) branches off to take oxygenated blood all over the body.
The hepatic artery carries blood to the liver and the hepatic vein carries blood away from the liver.
Note* the hepatic portal vein carries blood from the intestine to the liver.
The renal artery carries blood to the kidney and the renal vein carries blood from the kidney.
Other arteries & veins carry blood to and from the head, limbs and other tissues.
Blood flows in one direction only in the digestive system. How is this ensured?
Pressure from contractions pushes blood forward in arteries and valves prevent backflow in veins.