The heart

The cardiovascular system acts as a transport system for the tissues and has the following functions:

• Supply of oxygen and removal of CO2

• Delivery of nutrients and removal of metabolic waste products

• Delivery of hormones and vasoactive substances to target cells

The heart is the driving force behind this system, and can be considered a transducer that converts chemical energy into mechanical energy. It consists of a right-sided low-pressure pump and a left-sided high-pressure pump. Each of these pumps is composed of an atrium and a ventricle. The atria prime the ventricles, which in turn eject the cardiac output (CO) into either the pulmonary or the systemic circulation.

Cardiac muscle

Cardiac muscle is striated, the striations being due to the structure of the contractile intracellular myofibrils. The myofibrils are composed of sarcomere units which are identical to those of skeletal muscle, composed of thick and thin filaments arranged to give the characteristic Z line, A band and I band striations. The thick filaments are composed of myosin molecules, whose tails are linked to form the filament leaving the actin binding ‘heads’ of the molecules free. Each thick filament is surrounded by six thin filaments composed of a double spiral of actin molecules in combination with tropomyosin and troponin. These thin filaments form a hexagonal tube around the thick myosin filament.