III. 1. Heart Form


Roughly the size of a fist, the human heart is a vital organ composed primary of cardiac musclea strong, involuntary type of muscleand is responsible for the pumping of blood throughout the body. The heart is divided into two main halves, the right side which pumps deoxygenated blood to the heart and the left side which receives oxygenated blood and pumps it to the rest of the body. The two are located together in one heart for the purposes of easy pacekeeping, which will be discussed later.

As organisms evolved hearts developed more chambers. Fish have a two-chambered heart, amphibians evolved from fish and have a three-chambered heart, and reptiles and their descendants evolved from amphibians and have a four chambered heart. This section provides an overview of the four-chambered mammalian heart.

The four chambers of the heart are divided into two atria (singular, atrium) and two ventricles, one in each half of the heart. The atria receive blood from veins and then transport them to the ventricles, which in turn pump blood to its next destination: either the lung capillaries or the systemic capillaries. Due to this greater demand for pressure, ventricle walls are much thicker and stronger than atrium walls. Chambers of the heart are separated by flaps of connective tissue called valves. Atria are separated from ventricles by atriovenicular (AV) valves and each ventricle is separated from the artery it connects to by semilunar valves.

III. 2. Heart Function

The heart pumps in a rhythmic fashion to keep blood flow constant and too avoid too accumulating too much pressure in any one area of the circulatory system. A cardiac cycle is defined as one complete sequence of pumping and filling of the heart and takes about 0.8 seconds. The contraction (pumping) phase is referred to as systole and the complementary relaxation (filling) phase is referred to as diastole. The amount of blood pumped by per minute by each ventricle is referred to as cardiac output. The average cardiac output is about 5 L/min, (roughly all of the blood in the human body!) meaning it takes blood only about a minute from leaving the heart to return to it.

How is the cardiac cycle regulated? While in a lab cardiac cells have been shown to contract on their own, what keeps all of the cells in the correct rhythm? The answer lies in a group of cells located near the right atrium that form the sinoatrial (SA) node, also known as the pacemaker. The SA node generates electrical impulses that mimics nerve function and causes cells to contract. The SA node quickly makes both atrium contract in unison. The electrical impulses eventually reach another node, the atrioventricular (AV) node, which delays the impulse for 0.1 seconds before causing the walls of the ventricle to contract. Together, these two sets of impulses control the beating steadily for the one billion heartbeats a person is expected to have in their lifetime.