THE STRUCTURE OF THE HEARTTHE FUNCTION OF THE HEART

The external structure of the heart Discuss with

colleagues and describe the external structure of the heart in your own words.

Internal structure of the heart showing the pathway for deoxygenated and oxygenated blood

The human heart is the size of the individual’s clenched fist and weighs 250 - 350 grams.

It is surrounded by the pericardium which is a double walled sac .

The pericardium

The pericardium is made up of the outer epicardium which is fibrous and protective in function, the myocardium which is the middle layer which is made up of cardiac muscle and the inner endocardium which is lined with endothelium.

The 4 chambers of the heart The has four

chambers, two superior atria and two inferior ventricles. The heart is divided into the left and right sides by the septum.

Its main blood vessels are the inferior and superior vena cavae, the pulmonary veins and arteries and the aorta.

Atria and Ventricles

The walls of the ventricles are thicker than the walls of the atria. Why?

The wall of the left ventricle is thicker than the wall of the right ventricle. Why?

The function of the heart

Deoxygenated blood flows is brought to the right atrium of the heart by the superior vena cava. From here the blood is pumped into the right ventricle through the tricuspid valve.

Tricuspid valve?

Function of heart valves

From the right ventricle the blood is pumped to the lungs via the pulmonary arteries. Backflow of blood from the pulmonary arteries into the right ventricle is prevented by the pulmonary valve.

Blood circulation through the heart Oxygenated blood from

the lungs comes back to the left atrium of the heart through the pulmonary veins. From here it is pumped through the mitral or bicuspid valve into the left ventricle. From the left ventricle, blood is pumped through the aortic valve into the aorta into the systemic circulation.

Double circulation diagram

Functions of the heart

Pumping blood around the body is the main function of the heart.

Sending deoxygenated blood to the lungs to be oxygenated

Sending oxygenated blood to the whole body

Ensuring that oxygenated and deoxygenated blood do not mix (septum)

Preventing back flow of blood (valves)

The functioning of the heart

Cardiac muscle is myogenic ie the muscle cells contracting without any signal from the nervous system. Each of these cells have their own intrinsic contraction rhythm. The sino-atrial (SA) node also known as the pacemaker, sets the rate at which all cardiac muscle cells contract. The SA node generates electrical impulses similar to those produced by nerve cells. Since cardiac muscle cells have intercalated disks between adjacent cells impulses from the SA node spread quickly through the atrial walls enabling both atria to contract simultaneously.

The functioning of the heart

The electrical impulses from the SA node eventually reach the atrio-ventricular node which is another area of specialized cardiac muscle tissue which is situated in the wall between the right atrium and the right ventricle. Purkinje fibres are specialized muscle fibres which conduct the electrical signals to the apex of the heart via the ventricular walls.

The cardiac cycle

This entire cycle, a single heart beat, lasts about 0.8 seconds. The impulses generated during the heart cycle produce electrical currents, which are conducted through body fluids to the skin, where they can be detected by electrodes and recorded as an electrocardiogram (ECG) . The events related to the flow or blood pressure which occurs from the beginning of one heartbeat to the beginning of the next is known as the cardiac cycle.

Cardiac Cycle

Just before atrial systole blood from the lower part of the body and from the pulmonary veins enter the atria. The SAN (Sino- atrial node – a specialised area of the right atrium generates an electrical impulse which spreads across both atria causing them to contract. This pushes blood out of the atria into the ventricles.

This is followed by atrial diastole

The Cardiac Cycle

A little later, ventricular systole begins. The ventricles have been filled with blood and the AVN (atrio-ventricular node) sends an electrical impulse (through the bundle of His)down the ventricular septum through the Purkinje fibres causing the ventricles to contract. This pumps blood into the aorta and the pulmonary arteries.

During ventricular diastole the ventricles relax so that blood flows into them from the atria.

Coronary Circulation

Coronary arteries supply oxygenated blood to the heart muscle (myocardium). The cardiac veins are responsible for removing deoxygenated blood from the heart. (See next slide)

Anterior Coronary Circulation The superficial

coronary arteries are known as the epicardial coronary arteries. Why?

The coronary arteries that supply the myocardium with blood are called the subendocardial coronary arteries.

Posterior Coronary Circulation