HUMAN HEART

BY

DR MUBASHAR IQBAL

INTRODUCTION to HEARTThe heart is a muscular organ in both humans

and other animals, which pumps blood through the blood vessels of the circulatory system. Blood provides the body with oxygen and nutrients, and also assists in the removal of metabolic wastes. The heart is located in the middle compartment of the mediastinum in the chest

HUMAN HEART FACTS

The heart is one of the most important organs in the

human body, continuously pumping blood around our body through blood vessels.

Your heart is located in your chest and is well protected

by your rib cage.

The study of the human heart and its various disorders is known as

cardiology.

The heart is made up of four chambers, the left atrium, right atrium, left ventricle

and right ventricle.

There are four valves in the human heart, they ensure that blood only goes one

way, either in or out.

Blood that leaves the heart is carried through arteries. The main artery leaving the left ventricle is the aorta while the main artery leaving the

right ventricle is the pulmonary artery.

STRUCTURE OF HEART

The heart consists of four chambers, right and left atria above right and left ventricles. The functions of each part are as follows.

• The right atrium links to the right ventricle by the tricuspid valve. This valve prevents backflow of the blood into the atrium above, when the ventricle contracts.• The left atrium links to the left ventricle by the bicuspid valve (mitral valve). This also prevents backflow of the blood into the atrium above.• The chordae tendonae attach each ventricle to its atrioventricular valve. Contractions of the ventricles have a tendency to force these valves up into the atria. Backflow of blood would be dangerous, so the chordae tendonae hold each valve firmly to prevent this from occurring.

Semi-lunar (pocket) valves are found in the blood vessels leaving the heart (pulmonary artery and aorta). They only allow exit of blood from the heart through these vessels following ventricular contractions. Elastic recoil of these arteries and relaxation of the ventricles closes each semi-lunar valve.• Ventricles have thicker muscular walls than atria. When each atrium contracts it only needs to propel the blood a short distance into each ventricle.• The left ventricle has even thicker muscular walls than the right ventricle. The left ventricle needs a more powerful contraction to propel blood to the systemic circulation (all of the body apart from the lungs). The right ventricle propels blood to the nearby lungs. The contraction does not need to be so powerful.

STRUCTURE OF HEART

INTERNAL ANATOMY OF HEART

CARDIAC CYCLE

Stage 1 - Ventricular diastole, atrial systole - Both ventricles relax simultaneously. This results in lower pressure in each ventricle compared to each atrium above.

The atrioventricular valves open partially. This is followed by the atria contracting which forces blood through the atrioventricular valves. It also closes the valves in

the vena cava and pulmonary vein. This prevents backflow of blood.

Blood must continuously be moved around the body, collecting and supplying vital substances to cells as well as removing waste from them. The heart acts as a pump using a combination of systole (contractions) and diastole (relaxation) of

the chambers. The cycle takes place in the following sequence.

Stage 2 - Ventricular systole, atrial diastole - Both atria then relax. Both ventricles contract simultaneously. This results in higher pressure in the ventricles compared to

the atria above. The difference in pressure closes each atrioventricular valve. This prevents backflow

of blood into each atrium. Higher pressure in the ventricles compared to the aorta and pulmonary

artery opens the semi-lunar valves and blood is ejected into these arteries. So blood flows through the systemic

circulatory system via the aorta and vena cava and through the lungs via the pulmonary vessels. -

Stage 3 - Ventricular diastole, atrial diastole - Immediately following ventricular systole, both ventricles and atria relax for a short time. Higher pressure in the aorta and pulmonary artery than in the ventricles closes the semi-lunar valves. This prevents the backflow of blood. Higher pressure in the vena cava and pulmonary vein than in the atria results in the refilling of the atria.

HOW is The Heart RATE Control?

It has already been stated that the cardiac muscle cells have their own inherent rhythm. Even an

individual cardiac muscle cell will contract and relax on a microscope slide under suitable conditions. An

orchestra would not be able to play music in a coordinated way without a conductor. The cardiac

muscle cells must be similarly coordinated, by a pacemaker area in the heart. Electrical stimulation

from the brain can alter the activity of the pacemaker and therefore change the rate and strength of the

heartbeat. -

The heart control centre in the brain is in the medulla oblongata.

• The sympathetic nerve stimulates an increase in heart rate.

• The vagus nerve stimulates a decrease in heart rate.

• These nerves link to a pacemaker structure in the wall of the right atrium, the sinoatrial node (SAN).

• A wave of electrical excitation moves across both atria.

• They respond by contracting (the right one slightly before the left).

• The wave of electrical activity reaches a second pacemaker, the atrioventricular node (AVN), which conducts the electrical activity through the Purkinje fibres. - All of the Purkinje fibres together are known as the Bundle of His.

• These Purkinje fibres pass through the septum of the heart deep into the walls of the left and right ventricles.

• The ventricle walls begin to contract from the apex (base) upwards.

•]• This ensures that blood is ejected efficiently from the ventricles.