ABSTRACT
The constant need of cells for a supply of oxygen for metabolism and for the removal of metabolic products (such as CO2) requires a mass transport system. The network of capillaries in the human body is such that no cell is more than 20 μm-30 μm from blood vessel. In simpler organisms, as soon as the number of cells exceeds around a few thousand, there is a rudimentary circulation in place, which can be either open or closed. The earthworm, for example, has a closed circulation, consisting of two vessels, one of which contains a contractile region and a series of valves. Fish have a single circuit with a two-chambered heart. Crocodiles and amphibia, on the other hand, have two circuits, with a three-chambered heart (blood from the two circuits mix to a certain extent). Birds and mammals have fourchambered hearts, with two completely separate circuits. Hence, essentially, the human heart consists of two pumps side by side, each consisting of two chambers, an atrium (or auricle) and a ventricle. The atrium (which is now a common architectural term) refers to a receiving space, and the ventricle comes from a word meaning “a small belly.” Although the heart is two independent pumps in parallel, they share a common electrical activation
CONTENTS
8.1 Introduction ........................................................................................................................ 199 8.2 Mechanical Events: The Heart as a Pump ......................................................................200
