The heart is a hollow muscular, cone-shaped organ, lying between the lungs in a block of tissue called the mediastinum. It tilts obliquely, a little more to the left than the right and presents a circular base above and an apex below. It is usually located on the level of the fifth intercostals space about 9 cm from the midline. The heart measure about 10 cm (4 inch) long and is about the size of the owner’s first. It weights about 255 gm. (90Z) in women and is heavier in man
General Structure of the Heart
The heart is composed of three layers of tissues.
The pericardium is the outer part that covers the heart and consists of two layers or sacs – the outer layer or fibrous pericardium is securely attached to the diaphragm, the outer coat of the great vessels and the posterior surface of the sternum and therefore maintains the heart in its position. Its fibrous nature prevents over distension of the heart.
The inner layer the serous pericardium consists of two layers the outer or parietal layer lines the outer fibrous sac and the inner or visceral layer covers the heart muscle. The serous membrane secretes serous fluid into the space between the visceral and parietal layers that allows smooth movement between the layers, when the heart beats.
The myocardium is the middle layer of the heart and is composed of specialised muscle tissue called as the cardiac muscle, on which the circulation of blood depends. It varies in thickness, being thickest in the left ventricle, thinner in the right ventricle and thinnest in the artrium.
The endocardium forms the inner linging of the heart is a thin, smooth, glistening membrane consisting of flattened epithelial cells which is continuous with the valves and with the lining of the blood vessels.
Interior of the Heart
The heart is divided into a right and left side by a muscular partition called as the septum. The two sides of the heart have no communication with each other. Each side is subdivided into an upper and lower chamber. The upper chamber on each side is called the auricle or atrium and is the receiving chamber into which blood flows through veins. The lower chamber on each side is called the ventricle and is the discharging chamber from which the blood is driven into the arteries.
Each atrium communicates with the ventricles below it on the same side of the heart through an opening, guarded by a valve called as the artrio-ventricular valve. The valves ensure blood flow in one direction only- from the upper chamber or atrium to the lower chamber or ventricle. The valve separating the right atrium from the right ventricle is known as the right artrio- ventricular valve (tricuspid valve) and is made up of three flaps or cusps.
Similarly the valve separating the left atrium from the left ventricle is called as the left artrio-ventricular valve (mitral valve) and is composed of two flaps of cusps. The valve between the artria and the ventricles open and close as a result of changes in the pressure of blood within the chambers. Also the valves are prevented from opening upwards by the Chordate tendineae (tendinous cords), which originate in the papillary muscles in the walls of the ventricle.
Flow of blood through the heart
The heart acts as a pump which drives blood into and through the arteries, but the right and left side of the heart of function quiet separately from one another.
Blood from all parts of the body is collected and gets poured into the right atrium through the two largest veins of the body, the superior and inferior venacavae. When it gets filled, the right atrium contracts and drives the blood through the right artrio-ventricular valve into the right ventricle, which is turn contracts sending the blood through the pulmonary valve and into the pulmonary artery. The pulmonary artery passes through the walls of the heart and divides into left and right pulmonary artery. These arteries carry the venous blood to the lungs where the inter change of gases takes place, carbon dioxide is excreted and oxygen is absorbed.
The arterial or oxygenated blood is collected from each lung by four pulmonary veins, which empty their content into the left atrium of the heart. This oxygenated blood passes through the left artrio ventricular valve into the left ventricle from where it is pumped into the aorta, the main artery of the body to all parts of the body. Both the openings of the pulmonary artery and the aorta are also guarded by valves as pulmonary and aortic valves respectively.
These valves are formed by three half-moon shaped (semi lunar) cusps. It should however be remembered that both the atrium contract at the same time followed by the simultaneous contraction of both ventricles.
The heart is supplied with arterial blood by the right and left coronary arteries. On the contrary, its deoxygenated blood or the venous return is by the coronary sinus which empties into the right atrium.
The function of the heart is to maintain a constant circulation of blood throughout the body. It acts as a pump and its action is composed of a series of events known as the ‘Cardiac Cycle’. In human beings, when the heart is beating normally, the cardiac cycle occurs about 74 times per minute. Thus each cycle lasts about 0.8 of a second. The cardiac cycle consists of:
Contraction of the artia – Artial Systole
Contraction of the Ventricles – Ventricular Systole
Relaxation or rest period of – Complete cardiac diastole
The artria and ventricles.
One can hear two heart sounds for each heart beat or cardiac cycle. They are described in words as ‘lubb-dub’. The first sound, ‘lubb’, is fairly loud and is due to the contraction of the ventricular muscle and the closure of the artrio-ventricular valves. The second sound, ‘dup’ is softer and is due to the closure of semi lunar valves aortic and pulmonary valves.
The Circulation of the blood
The circulation of blood in the body is divided into three parts:
1. The Pulmonary Circulation
2. The Systemic Circulation,
3. The Portal Circulation
The Pulmonary Circulation:
The pulmonary circulation is concerned in carrying deoxygenated blood from the right side of the heart to the lungs and again bringing oxygenated blood from the lungs to the left part of the heart.
The pulmonary artery carrying deoxygenated or venous blood arises from the right ventricles. At the level of the fifth thoracic vertebra it divides into right and left pulmonary arteries, passing into the corresponding lungs. Within the lungs these arteries divide and subdivide into smaller arteries, subsequently becoming arteries and capillaries. It is between the capillaries and the lung tissue that the interchange of gases takes place. In each lung the capillaries carrying oxygenated blood join up and form two vein, so, two pulmonary veins from each lung, therefore becomes four pulmonary veins which return oxygenated blood to the left atrium of the heart. These are the only veins which carry oxygenated blood.
The Systemic Circulation:
The systemic or general circulation constitutes the circulation of blood from the left ventricle through the main artery, the aorta to all the parts of the body and is again returned to the right side of the heart by the superior and inferior Venacava. Oxygenated blood leaves the left ventricle through the aorta which branches and reaches every part of the body supplying 02 and nourishment to the body tissues. Similarly the deoxygenated blood is carried through the veins which eventually forms superior and inferior Venacava and gets poured into the right artrium of the heart.
Portal circulation includes all the veins which drain blood from the abdominal part of the digestive system from the spleen, pancreas and gall bladder via liver to the inferior Vena Cava. This indirect route, through the liver is followed, as there is no provision for blood entering directly to the heart. The blood from all these organs unites to form the portal vein and carried to the liver. In the liver substance, the portal vein breaks up into smaller veins and finally into capillaries. These capillaries again unite to form two or three large hepatic veins which leave the liver to join the inferior Vena Cava.
Thus, the portal veins unlike all other veins in the body both commence and end as capillaries. Hence, the digested food stuff in the alimentary canal are absorbed into the capillaries which go to make up the portal vein and are carried by it into the liver. In order that these materials can come in contact with the liver cells for metabolism, it is necessary for the blood to pass through a second set of capillaries. This is achieved in the portal circulation. In the portal system, the arrangement of blood vessels is:
Arteries -> Capillaries -> Veins -> Capillaries –> Veins