The cardiovascular system comprises:

* A pump: the heart, which comprises two atria (reservoirs for blood and booster pumps to augment ventricular filling) and two ventricles (pumps). It consists of two pumps in parallel, with synchronised actions.

* A high pressure distribution circuit: the elastic arteries, i.e.the aorta and its major branches, serve a transport function. The muscular arteries serve a distributive function.

* Exchange vessels: the capillaries, which are 8-10 m in diameter. Capillaries can possess either continuous endothelium (muscle, heart, liver), fenestrated endothelium (gastrointestinal tract, renal glomeruli), or discontinuous endothelium (liver, spleen).

* A low pressure collection and return circuit.

The system allows rapid transport of oxygen, nutrients, hormones and waste products throughout the body. The structure of the components is related to function across the vascular tree.

This has the following characteristics:

* The right and left ventricles function in parallel;

* The placenta provides for gas and metabolite exchange;

* The parallel circulation is maintained by shunts at the ductus venosus, foramen ovale and the ductus arteriosus.

* Oxygenated blood flows from the placenta via the ductus venosus and inferior vena cava into the right atrium, where it is deflected by the crista dividens and the eustachian valve across the foramen ovale into the left atrium and thence into the left ventricle. It is then ejected into the ascending aorta.

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