Cardiovascular System: The Beating Heart
The cardiovascular system is a key element in transporting and exchanging substances (such as nutrients, wastes, oxygen, and carbon dioxide) between the environment and the cells that function in tissues. Cardio is from the Greek word kardia, meaning heart, and vascular is from the Latin word vasculum, meaning vessel. Below we look at the operation of the mammalian heart, using the human heart as an example. It is the heart that provides the motive force to move blood through the circulatory system so that metabolic fuels and waste products can be transported to and from the body tissues. We can also see how the vascular system and the exchange of substances between blood and body tissues via the capillaries happens.
Concept 1—Heart Anatomy
The human heart is a cone-shaped muscular pump located in the mediastinal cavity of the thorax between the lungs and beneath the sternum.
The cone-shaped heart lies on its side on the diaphragm, with its base (the widest part) upward and leaning toward the right shoulder, and its apex pointing down and to the left.
In this biological pump, the muscles of the chamber walls provide the motive force, the valves direct the flow, and the great vessels convey blood to and from body tissues. Most of the great vessels (pulmonary trunk, aorta, and superior vena cava) emerge upward from the base of the heart.
Concept 2—The Cardiac Cycle
The chambers of the heart alternately contract and relax in a rhythmic cycle.
- During the period of contraction (systole), the heart pumps blood out through the arteries; during the period of relaxation (diastole), the heart fills with blood.
- One complete sequence of filling and pumping blood is called a cardiac cycle, or heartbeat.
Concept 3—Electrical Activity of the Heart
The heart’s rhythm of contraction is controlled by the sinoatrial node (SA node), often called the pacemaker. This node is part of the heart’s intrinsic conduction system, which is made up of specialized myocardial cells called nodal cells.
- The heart will beat without input from the nervous system and will continue to beat, even outside the body, as long as its cells are alive.
- The automatic nature of the heartbeat is referred to as automaticity.
- Automaticity is due to the spontaneous electrical activity of the SA node. Electrical impulses generated from the SA node spread through the heart via a nodal tissue pathway that coordinates the events of the cardiac cycle.
Concept 4—Heart Pump Operation
The activity of the conduction system, muscles, and valves of the heart are synchronized so that the heart can operate as a pump.
- The conduction system initiates and coordinates the muscular activity of the heart.
- Pressure differentials that result from muscle activity actuate the opening and closing of valves.
- The opening and closing of valves directs the flow of blood through the heart.