Double circulatory system

The double circulatory system of blood flow references two distinct and separate systems. This distinction is shared by most vertebrates to varying degrees to include amphibians, birds and mammals (including humans.) In contrast, fish have a single circulation system because they lack lungs. Most animals living above the water require a double circulatory system to allow the added benefit of direct oxygenation from a developed pulmonary circuit. Embryology of the human circulatory system is an advanced study of the double circulatory system as the distinction between the right and left portions of the heart is founded.

For example, the adult human heart consists of two separated pumps or flow circuits. One side is dedicated to the right atrium and right ventricle (which pumpsdeoxygenated blood into the pulmonary circulation). This circuit is very low in pressure incumbent upon systemic resistance. The left circuit inclusive of the left atrium and ventricle (which pumps oxygenated blood into the systemic circulation) is better gated to high pressure. Blood in one circuit must go through the heart to enter the other circuit.

Regulation of circulation

Throughout the circulatory system there are many valves. There are 3 main types. The first is the bicuspid valve (mitral valve) which is found between the left atrium (auricle) and the left ventricle. This valve allows oxygenated blood to flow from the left atrium into the left ventricle during ventricular relaxation diastole. The bicuspid valve also prevents the backflow of blood into the left atrium during ventricular contraction (systole). Then there is the tricuspid valve which is found between the right atrium and the right ventricle. This valve allows deoxygenated blood to flow from the right atrium into the right ventricle during ventricular relaxation. It prevents the backflow of blood into the right atrium during ventricular contraction. Finally there are the two semilunar valves, found at the beginning of the arteries leaving the heart. It is called a double circulatory system because it has two loops, one from the heart to the lungs and one from the heart to the rest of the body. In a first order circulatory circuit, blood is pumped to the lungs, thus acquiring oxygen while simultaneously releasing carbon dioxide. Fully oxygenated blood then enters the second order circuit, going to the brain and body. The second order systemic output blood is selectively deoxygenated by the needs of the peripheral organs, returning once again to the heart.

The advantage of a double circulatory system is that blood can be pumped simultaneously into low pressure/compliance (pulmonary/right) and high pressure/compliance (systemic/left) loops.