The Heart (Leaving Cert Biology): Revision Notes

The Heart

Introduction to the heart

The heart is a remarkable muscular organ that works as the body's central pump. Located between the two lungs in the chest cavity, it sits slightly to the left-hand side and is about the size of a clenched fist. This vital organ works tirelessly, contracting approximately 100,000 times each day and pumping between 5 and 20 litres of blood every minute to keep your body functioning properly.

Structure of the heart

The heart is constructed from a special type of muscle called cardiac muscle. This muscle tissue is unique because it can contract rhythmically without getting tired easily, unlike other muscles in your body. The entire heart is wrapped in a protective double membrane called the pericardium, which contains a small amount of fluid that helps reduce friction as the heart beats.

The heart is divided into four distinct chambers by a central wall called the septum. This separation is crucial because it keeps oxygenated and deoxygenated blood from mixing together. The septum ensures that oxygen-rich blood going to your body stays separate from oxygen-poor blood returning from your body.

Heart chambers and their functions

Atria - the upper chambers

The two upper chambers are called atria (singular: atrium). These chambers have relatively thin walls because they only need to pump blood a short distance down to the ventricles below them. The right atrium receives deoxygenated blood returning from the body through large veins called the vena cava. The left atrium receives oxygenated blood returning from the lungs through the pulmonary veins.

Ventricles - the lower chambers

The two lower chambers are called ventricles, and these are the real powerhouses of the heart. The ventricles have much thicker, stronger muscular walls because they must pump blood over much greater distances. The right ventricle pumps blood to the lungs through the pulmonary circuit, while the left ventricle pumps blood to the entire rest of the body through the systemic circuit.

Heart valves and blood flow

The heart contains several important valves that act like one-way doors, ensuring blood can only flow in the correct direction and preventing any backflow. These valves are held in place by tough cords called tendons, which are attached to small projecting muscles.

Types of valves

• Tricuspid valve: Located between the right atrium and right ventricle (has three flaps)
• Bicuspid valve: Located between the left atrium and left ventricle (has two flaps)
• Semilunar valves: Located at the exits from both ventricles (shaped like half-moons)

Blood flow pathway through the heart

Two-circuit circulatory system

The human circulatory system is organised into two separate circuits, which is why it's called a double circulation system. This sophisticated arrangement provides several key advantages over simpler circulatory systems found in other animals.

The pulmonary circuit (heart → lungs → heart)

This shorter circuit involves the right side of the heart. The right ventricle pumps deoxygenated blood to the lungs through the pulmonary artery. In the lungs, blood picks up oxygen and releases carbon dioxide. The now oxygenated blood returns to the left atrium through pulmonary veins. This circuit has relatively thin vessel walls because the distance is short and less pressure is needed.

The systemic circuit (heart → body → heart)

This longer circuit involves the left side of the heart. The left ventricle pumps oxygenated blood to all parts of the body through the aorta and its many branches. As blood travels through body tissues, it delivers oxygen and nutrients while collecting carbon dioxide and waste products. The deoxygenated blood then returns to the right atrium through the vena cava. This circuit requires higher pressure and thicker vessel walls because blood must reach every part of the body.

Cardiac blood supply

Even though the heart is constantly filled with blood, the heart muscle itself needs its own dedicated blood supply to function properly. This is because the blood inside the heart chambers cannot directly nourish the thick muscular walls.

Coronary arteries

The heart muscle receives oxygen and nutrients through a network of coronary arteries that branch from the aorta just as it leaves the heart. These arteries spread across the surface of the heart like a crown (which is what "coronary" means). The coronary arteries subdivide into numerous smaller vessels called cardiac capillaries that penetrate deep into the heart muscle.

Coronary veins

After delivering oxygen and nutrients, blood is collected by coronary veins that return the deoxygenated blood directly back to the right atrium through an opening called the coronary sinus.

The pacemaker and heartbeat control

Natural pacemaker

The heart has its own internal timing system called the pacemaker or sinoatrial (SA) node. This small bundle of specialised tissue is located at the top of the right atrium and acts like the heart's natural battery. Even if the heart were removed from the body and kept in nutrient-rich fluid, it would continue beating because of this pacemaker.

How the pacemaker works

External control of heart rate

While the pacemaker sets the basic rhythm, the brain can influence heart rate through the nervous system and hormones. Factors that increase heart rate include exercise, excitement, stress, and temperature increases. Factors that decrease heart rate include relaxation, sleep, and certain chemicals like alcohol.

Heart sounds

When you listen to a heartbeat, you hear the characteristic "lub-dub" sound. These sounds are caused by the heart valves closing, not by the heart muscle contracting as many people think.