Good day everyone, you are welcome to my blog. Today we are discussing about the heart:
After studying this section, you should be able to:
- describe the structure of the heart and its position within the thorax
- trace the circulation of the blood through the heart and the blood vessels of the body
- outline the conducting system of the heart relate the electrical activity of the cardiac conduction system to the cardiac cycle
- describe the main factors determining heart rate and cardiac output.
The heart is a roughly cone-shaped hollow muscular about 10 cm long and is about the size of the owner’s fist . It weighs about 225 g in women and is heavier in men (about 310 g).
The heart lies in the thoracic cavity in the mediastinum (the space between the lungs). It lies obliquely, a little more to the left than the right, and presents a base above, and an apex below. The apex is about 9 cm to the left of the midline at the level of the 5th intercostal space, i.e. a little below the nipple and slightly nearer the midline. The base extends to the level of the 2nd rib.
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Organs associated with the heart
- Inferiorly – the apex rests on the central tendon of the diaphragm
- Superiorly – the great blood vessels, i.e. the aorta, superior vena cava, pulmonary artery and pulmonary veins
- Posteriorly- the oesophagus, trachea, left and right bronchus, descending aorta, inferior vena cava and thoracic vertebrae
- Laterally-the lungs – the left lung overlaps the left side of the heart
- Anteriorly- the sternum, ribs and intercostal muscles.
The heart wall
The heart wall is composed of three layers of tissue: pericardium, myocardium and endocardium.
The pericardium is the outermost layer and is made up of two sacs. The outer sac (the fibrous pericardium) consists of fibrous tissue and the inner (the serous pericardium) of a continuous double layer of serous membrane.
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The fibrous pericardium is continuous with the tunica adventitia of the great blood vessels above and is adherent to the diaphragm below. Its inelastic, fibrous nature prevents overdistension of the heart.
The outer layer of the serous pericardium, the parietal pericardium, lines the fibrous pericardium. The inner layer, the visceral pericardium, which is continuous with the parietal pericardium, is adherent to the heart muscle.
A similar arrangement of a double membrane forming a closed space is seen also with the pleura, the membrane enclosing the lungs.
The serous membrane consists of flattened epithelial cells. It secretes serous fluid, called pericardial fluid, into the space between the visceral and parietal layers, which allows smooth movement between them when the heart beats. The space between the parietal and visceral pericardium is only a potential space. In health the two layers lie closely together, with only the thin film of pericardial fluid between them.
This lines the chambers and valves of heart. It is a thin, smooth membrane to ensure smooth flow of blood through the heart. It consists of flattened epithelial cells, and it is continuous with the endothelium lining the blood vessels.
Interior of the heart
The heart is divided into a right and left side by the septum , a partition consisting of myocardium covered by endocardium. After birth, blood cannot cross the septum from one side to the other. Each side is divided by an atrioventricular valve into the upper atrium and the ventricle below. The atrioventricular valves are formed by double folds of endocardium strengthened by a little fibrous tissue. The right atrioventricular valve (tricuspid valve) has three flaps or cusps and the left atrioventricular valve has two cusps. Flow of blood in heart is one way; blood enters the heart via the atria and passes into the ventricles below.
The valves between the atria and ventricles close passively according to changes in pressure in the open and chambers. They open when the pres sure in the atria is greater than that in the ventricles During ventricular systole (contraction) the pressure in the ventricles rises above that in the atria and the valves snap shut, preventing backward flow of blood.