The heart is a muscular organ located behind and to the left of the breastbone (or sternum) and between the lungs. It is shaped like an upside-down pear and is about the size of your fist. Contained within a sack (called the pericardium), it sits on top of the diaphragmatic muscle and is surrounded by the rib-cage. These structures all serve to protect the heart.
The heart is made up of four chambers: two small chambers called atria and two larger chambers called ventricles. The left ventricle is the largest and strongest chamber in the heart. As it is required to pump blood to the rest of the body it needs to have more muscle than the other chambers. The heart muscle itself is called the myocardium.
Coordination of blood flow through the heart is maintained by electrical activity and four heart valves. These valves open and close to allow blood to flow in the correct direction only. The four heart valves are:
- The tricuspid valve which controls blood flow between the right atrium and right ventricle.
- The pulmonary valve which lies between the right ventricle and pulmonary artery. The pulmonary artery takes deoxygenated blood from the heart to the lungs.
- The mitral valve which allows oxygen-rich blood from the lungs to flow between the left atrium and left ventricle.
- The aortic valve which controls blood flow between the left ventricle and the main blood vessel leaving the heart called the aorta.
The rhythmic contraction of the heart is governed by an electrical impulse.
How the blood flows in the heart
Beating on average 70 times per minute, each day the heart beats about 100,000 times. In the course of a 70-year lifetime it will beat more than 2.5 billion times without ever pausing to rest. The cycle of blood flow is as follows:
- The right atrium receives blood from the great veins - the superior and inferior vena cava.
- Blood passes from the right atrium through the tricuspid valve to the right ventricle.
- From the right ventricle blood is pumped through the pulmonary valve and pulmonary arteries to the lungs.
- In the lungs carbon dioxide is removed from the blood and oxygen is picked up.
- Through the pulmonary veins oxygenated blood returns to the left atrium.
- From the left atrium blood passes through the mitral valve to the left ventricle.
- As the left ventricle contracts, the pressure closes the mitral valve. The aortic valve opens allowing blood to leave the heart via the main blood vessel called the aorta. When this cycle of pumping is complete the aortic valve closes to prevent blood from dropping back into the heart.
The heart is two separate pumps that work together. Oxygenated and nutrient-rich blood is pumped around the body. Deoxygenated blood, which contains waste products, is returned to the lungs to allow the entire cycle to begin again.
The electrical activity of the heart
The rhythmic contraction of the heart is governed by an electrical impulse. A chamber of the heart beats when an electrical impulse moves across it causing the muscle to contract. These electrical impulses originate rhythmically in a special area of nerve tissue situated in the right atrium. This nerve tissue is called the sinoatrial node and is sometimes referred to as the heart's own pacemaker.
In an average person at rest the sinoatrial node will discharge 70 times per minute. This in effect means that the heart will beat 70 times per minute. The heartbeat itself consists of a two-part cycle of relaxation (called diastole) and contraction (called systole). In total this usually takes about one second.
The electrical impulses travel through a series of special nerve fibres causing a coordinated wave of muscle contraction in the atria, which helps to move blood through the tricuspid and mitral valves and into the ventricles. The electrical impulses pause briefly in the atrioventricular node (AV node). This allows atrial contraction to fill the ventricles with blood before the ventricles are then made to contract. This part of the cardiac cycle is known as diastole.
When the ventricles are full of blood the second part of the cardiac cycle known as systole begins. This is initiated by the electrical impulse passing from the AV node rapidly through a series of nerves and causing the ventricles to contract.
This sequence of electrical activity allows contraction of the heart muscle to propel blood through the heart in a controlled and rhythmic fashion. The heart does not work in isolation and the heart rate is influenced by many factors including exercise, stress, temperature and nervous or hormonal influences. This allows the circulatory system to be very flexible in meeting the body's needs, which can vary considerably from time to time.
The coronary (heart) arteries
The heart muscle, like every other muscle in the body needs its own supply of oxygen in order to survive and function properly. Although the heart chambers contain blood, the heart muscle derives no nourishment from this blood. The heart muscle receives its blood supply from its own arterial system. This blood supply is known as the coronary arteries. The coronary arteries arise at the beginning of the aorta, the main blood vessel leaving the heart.
The two main coronary arteries, the left main coronary artery and the right coronary artery branch out like a tree. They divide many times into smaller arteries covering the surface of the heart muscle. They supply the muscle with oxygenated and nutrient-rich blood.
The left main coronary artery quickly divides into the left anterior descending artery (LAD), which travels across the front of the heart, and the circumflex artery, which runs round the back of the heart. These together with the right coronary artery, form the main heart arteries. The size and shape of the coronary arteries differ from one person to another.