Moreover, the heart is covered by a two- layered sac known as pericardium. The inner layer, called visceral layer of pericardium invests the heart very closely. And an outer layer is pericardium. In-between these two layers of pericardium is present a narrow space known as pericardial cavity. It is full of a self-secreted fluid called pericardial fluid.
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Pericardial fluid performs the following three functions :
- It protects the heart from any kind of mechanical injury and shock.
- It keeps the tissue of heart moist for proper functioning.
- It acts as lubricant and reduces friction for the beating of human heart.
Structure of human heart
The heart is hollow, highly muscular, highly specialized blood vessel. It undergoes rhythmic beating. The heart consists of four chambers which are as follows :
- Two atria or auricles
- Two ventricles
1. Atria (singular atrium) or auricles :
The atria are a pair of thin-walled chambers present towards the base of the human heart. They are known as right and left atrium or auricle according to their position. The auricles are very thin walled chambers as they are to push the blood into ventricles present just below them.
The ventricles are a pair of very thick walled chambers. They are present towards the apex and are known as right and left ventricles. The ventricles are broad at the upper end but taper towards apex. From the right upper corner of left ventricle starts a large tubular aorta that supplies blood to all the body parts.
From the left upper corner of right ventricle starts a large tubular pulmonary artery. It supplies impure blood to lungs. The ventricles are very thick walled chambers. The left ventricle has thickest walls as it has to push the blood to the remotest parts of the body such as toes of the feet.
Blood vessels entering and leaving the human heart, their apertures and valves
Various blood vessels that enter or leave the heart are called great blood vessels. The apertures by which these great blood vessels enter or leave the heart are guarded by valves. They allow only unidirectional flow of blood through the human heart. These blood vessels are :
A. Blood vessels entering the human heart
(a) In the right atrium :
Deoxygenated blood from all the body parts is returned into the right atrium of the heart by way of two main blood vessels, i.e.
(i) Superior (Anterior) vena cava : I brings deoxygenated blood from anterior or upper parts including the head, neck, chest and arms and pours it into the right auricle through an aperture which is not guarded by any valve. It is also called Precaval.
(ii) Inferior (Posterior) vena cava : It brings deoxygenated blood from posterior or lower parts including abdomen and legs, into the right auricle. Its opening is guarded by Eustachius valve. The deoxygenated blood collected into the right auricle is pumped into the right ventricle through right auriculo-ventricular aperture.
This aperture is guarded by three leaf-like flaps (cusps) called tricuspid valve. One margin of this valve is fused with the aperture. Whereas, free margins are attached with whitish thread like chordae tendinae. These chorda tendinae arise from muscular projections called papillary muscles from the ventricular wall.
(b) In the left atrium :
Oxygenated blood from the lungs collected by four pulmonary veins (two from each lung). They open into the left atrium h separate apertures. These apertures are without any valves. Oxygenated blood collected into the lef atrium is pumped into the left ventricle through left auriculo-ventricular aperture.
This aperture is guarded by two leaf-like flaps (cusps) called bicuspid or mitral valve, Free margins of the bicuspid valve are attached with the chordae tendinae. They arise from papillary muscles which are muscular projections from the walls of left ventricle.
B. Blood vessels leaving the human heart
(i) Pulmonary artery : It carries deoxygenated blood from the right ventricle to the lungs for oxygenation. Opening of pulmonary artery is guarded by three pocket shaped or half moon- shaped valves called pulmonary semilunar valves.
(ii) Aorta : It arises from the left ventricle. It supplies oxygenated blood to all the body parts by way of its various branches. Its opening is also guarded by three pocket shaped valves called aortic semilunar valves.
A pair of coronary arteries arises from the aorta, near its base. They supply blood to the muscles of human heart itself. Blockage of any coronary artery or its branches causes deadening of corresponding area of human heart muscles.
It is called myocardial infarction or a heart attack in popular language. It causes pain in the chest and is called angina pectoris. The cardiac veins collect blood from walls of human heart and pour it into the right auricle.
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