Cardiovascular System

 Chapter 9

Cardiovascular System

The Heart 

• The heart is a roughly cone-shaped hollow muscular organ. It is about 10 cm long and is of the size of a person's fist. It weighs about 300 grams in an adult.

• It is located in the thoracic cavity in the mediastinum between the lungs. It lies obliquely, a little more to the left than the right, and presents a base above, and an apex below. It is about 12 cm long, 09 cm wide at its broadest point, and 06 cm thick

• The apex is about 09 cm to the left of the midline at the level of the fifth intercostal space; the base extends to the level of second rib.

Structure

• The heart is composed of three layers of tissue: pericardium, myocardium and endocardium.

• (a) Pericardium

• It is made up of two sacs. The outer sac consists of fibrous tissue and the inner of a double layer of serous membrane. The outer fibrous sac is continuous with the tunica adventitia of the great blood vessels above and is adherent to the diaphragm below. Its fibrous nature prevents over distension of the heart. The serous membrane consists of flattened epithelial cells. It secretes serous 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.

• (b)      Myocardium

• It is composed of specialized cardiac muscle tissue found only in the heart.

• (c) Endocardium

• It forms the lining of the myocardium and the heart valves. It is thin, smooth, glistening membrane which permits smooth flow of blood inside the heart. It consists of epithelial cells, continuous with the endothelium that lines the blood vessels. 

Flow of the Blood through the Heart 

• The superior and inferior venae cave are the two largest veins in the body. They empty their contents into the right atrium. This blood passes through the right atrioventricular valve into the right ventricle, and from there it is pumped into the pulmonary artery, which carries deoxygenated blood to the lungs. The opening of pulmonary artery is controlled by the pulmonary valve, formed by three semi lunar cusps. This valve prevents the back flow of blood into the right ventricle when the ventricle muscle relaxes. The pulmonary artery then divides into left and right pulmonary arteries which carry the venous blood to the lungs where it gets purified.

• Two pulmonary veins from each lung carry oxygenated blood to the left atrium. Then it passes through the left atrioventricular valve into the left ventricle, from where it is pumped into the aorta for general circulation. The opening of the aorta is controlled by the aortic valve, formed by the three semilunar cusps.

Blood supplied to the Heart

Arterial circulation

• The heart is supplied with the right and left coronary arteries which branch from the aorta next to the aortic valve. The coronary arteries receive about 05 per cent of the blood from the heart. The coronary arteries traverse the heart, eventually by forming a large network of capillaries.

Venous circulation

• Most of the venous blood is collected into several small veins that join to form coronary sinus which opens into the right atrium. The remainder passes directly into the heart chambers through little venous channels.

Conducfing System of the Heart

• The heart has an intrinsic system whereby the cardiac muscle is automatically stimulated to contract; however, it can be stimulated or depressed by the nerve impulses initiated in the brain and by the circulating neurotransmitters or hormones. There are small groups of specialized neuromuscular cells in the myocardium which initiate and conduct impulses causing coordinated and synchronized contraction of the heart muscle.

Circulation of Blood

• Liver, kidneys and brain are the organs which get constant blood supply. There are three types of blood circulations in the human body, viz.,

(i) Greater circulation or systemic circulation;

(ii) Pulmonary circulation or lesser circulation; and 

(iii)Portal circulation.

The Blood Vessels

• Arteries are the blood vessels which transport blood away from the heart. They consist of three layers of tissue (Fig. 9.13). Outer layer consists of fibrous tissue called tunica adventitia. The middle layer consists of smooth muscle and elastic tissue called tunica media. The inner lining consists of squamous epithelium called endothelium. In the larger arteries, the tunica media consists of more elastic tissue and less muscle. In the smaller arteries, the tunica media consists almost entirely of smooth muscle.

• The small arteries break up into minute vessels called capillaries. The wall of the capillary is composed of a single layer of endothelial cells which is very thin and permits the passage of water and other small molecules. Blood cells and large molecular weight substances do not normally pass through capillary walls. Capillaries form a vast network and link arterioles to venules.

Arteriol and Venous Systems

• The aorta after passing upwards for a short distance arch backwards and to the left. Two common carotid arteries, viz., left and right arise from the aorta. The left common carotid artery arises directly from the arch of aorta and the right one arises as a branch of brachiocephalic artery. There are two subclavian arteries out of which the right subclavian artery arises from the brachiocephalic artery and directly from the arch of the aorta. The axillary artery is a continuation of the subclavian artery and lies in the axilla. The first part is deep and then it runs superficially to become the brachial artery. It runs down the medial aspect of the upper arm and extends below the elbow joint where it divides into a radial and an ulnar artery. Together, the radial and ulnar arteries form palmar arches in the hand. Branches from the auxiliary, brachial, radial and ulnar arteries supply to all the structures in the upper limb.

Venous System

• The external jugular vein begins in the neck and finally passes into the subclavian vein. The internal jugular vein collects blood from various regions of the brain and unites with the subclavian veins to form the brachiocephalic veins. Two brachiocephalic veins join together to form superior Vena cava. The superior Vena cava drains all the venous blood from the head, neck and upper limbs and empties its content into the right atria.

Blood Pressure

• It is defined as the force or pressure which the blood exerts on the walls of the blood vessels. When the left ventricle contracts and pushes blood into the aorta, the pressure produced is called the systolic pressure. In adults it is about 120 mm Hg (millimeters of mercury). When complete cardiac diastole occurs and the heart is resting, following the ejection of the blood, the pressure within the arteries is called diastolic blood pressure. In an adult it is about 80 mm Hg. These figures vary according to the time of day, the posture, gender, and age of the individual. Arterial blood pressure is measured by use of a sphygmomanometer.

Control of Blood Pressure

• Following mechanisms are involved in the maintenance of blood pressure. The cardiovascular canter is located in the brain and is situated in the medulla and pons. The canter receives, integrates and co-ordinates inputs from baroreceptors, chemoreceptors and Following mechanisms are involved in the maintenance of blood pressure. The cardiovascular canter is located in the brain and is situated in the medulla and pons. The canter receives, integrates and co-ordinates inputs from baroreceptors, chemoreceptors and.

Pulse

• It is described as a wave of distension and elongation felt in an artery wall due to the contraction of the left ventricle forcing about 60-80 ml. of blood into the already full aorta. When the aorta is distended, a wave passes along the walls of the arteries and can be felt at any point where an artery can be pressed gently against a bone. An average of 60-80 beats/ min is common at rest. Information that can be obtained from the pulse includes.

• The rate at which the heart is beating.
• The regularity with which the heartbeats occur. 
• The volume or strength of the beat.
• The tension: the artery wall should feel soft under the fingers.

Hypertension

• Hypertension describes blood pressure which is sustained at a level higher than the generally accepted normal maximum level for a particular age group.

• At 20 years—140/90 mm Hg.

• At 50 years—160/95 mm Hg.

• At 75 years-170/105 mm Hg.

Heal Diseases

Following heart diseases are indicated below:

(i) Cardiac failure

(ii) Disorders of heart valves

(iii) Ischemic heart disease

(iv) Rheumatic Heart Disease

(v) Infective endocarditic

(vi) Cardiac Arrhythmias

(viii) Congenital abnormalities