CARDIOVASCULAR SYSTEM

It is a hollow, cone-shaped muscle functioning as a pump that maintains the circulation of the blood going on (Fig. 1). The heart is seen in the Middle Mediastinum, a site in the thoracic cavity between the two pleural sacs. It is also surrounded by Pericardium, a double sac of serous membrane (Fig. 2). The heart is made up of three layers: the endocardium (a smooth inner lining), the myocardium (a muscular middle layer), and the epicardium (the outer layer) (Fig. 3). The cardiac muscle is the main component of the heart. It is sited between the lungs within the thoracic cavity and behind the sternum. Also, it has apex and base with diaphragmatic and sternocostal surfaces. The heart is enclosed by right, left and inferior borders. 2/3 of the heart lies to the left of median plane (Fig. 4). 

The heart is divided into four champers:  two upper chambers (the right and left atria), and two lower chambers (the right and left ventricles). This division into the right and left sides is done through a thick muscle known as the septum (Fig. 5). The left and right atria represent the receiving chambers. While the right atrium receives the venous blood coming from the different parts of the body to the heart, the left atrium receives arterial blood coming from the lungs to the heart. Both atria have thin walls, and the auricle is the upper part of each atrium (Fig. 6). However, the left and right ventricles exist below the two atria, and they represent the pumping or discharging chambers which contract to push blood out of the heart into the circulation (Fig. 6).

There are two Atrioventricular Valves: right AVV (Tricuspid) and left AVV (Bicuspid). They function as a pathway for the blood from the atria to the ventricles allowing the deoxygenated blood to flow smoothly in one direction. Also, there are two semilunar Valves: Aortic and Pulmonary. They allow the blood to move from the right and left ventricles to the great arteries leaving the heart (Fig. 7).

The cardiovascular system works through what is called the cardiac cycle. It is a continuous cycle of oxygenation, blood pumping to body cells and return of deoxygenated blood. This process starts in the right atrium which receives the deoxygenated blood from the upper part and the lower part of the body through the superior vena cava and the inferior vena cava, respectively (Fig. 8). Also, the coronary sinus acts as a canal taking the blood from the heart itself to the right atrium (Fig. 9). The right atrioventricular valve opens the way to the deoxygenated blood to flow into the right ventricle. For a successful process of blood transferring, the blood should flow in only one direction, and this is the function of the heart valves. The cardiovascular system is closely connected to the respiratory system. The deoxygenated blood from the right ventricle is pumped through the pulmonary valve and it comes into the lungs via the right and left pulmonary arteries which are very matchless for being the only arteries able to carry the deoxygenated blood and feed it with oxygen. The cardiac cycle is completed through the arrival of the blood full of oxygen to the left ventricle and then pumping it to all the body cells allowing them to work optimally (Fig. 10). 

The main organs of this cycle are the left and right atria and ventricles which undergo through what is called systole and diastole; they refer to the continuous contraction and relaxation of the two atria and the two ventricles by sequence (Fig. 11).  

When there is a need to additional oxygen in body cells, as in times of stress or exercise, the sinoatrial node receives messages from the sympathetic division of the autonomic nervous system to raise the heart rate (Fig. 12). In contrast, the heart rate becomes slow through an instruction from the parasympathetic division. When the arteries contract, the heart beats, and it is called pulse which is measured by feeling a superficial vessel against a bone, most commonly the radial artery in the wrist. Heart health, body weight, level of fitness, activity or relaxation highly affect heart rate. However, a normal resting heart rate is between 60 and 80 beats per minute. When the heart beats fast, it is called tachycardia. By contrast, bradycardia indicates the slow heart rate. 

Cardiac muscle cells, unlike skeletal muscle cells, are unique for being able to perform independent and automatic contractions in a regular and constant way even with the absence of nervous connections (Fig. 13). Cardiac rhythms vary according to the type of cells in the heart. While ventricular cells contract 20–40 times per minute, atrial cells beat faster about 60 times per minute. Therefore, for having a way to unify the control system, the heart works as an efficient and coordinated pump.

Blood pressure varies from one person to another. Also, it varies in the same individual from time to time according to the different demands of the body. However normal blood pressure is around 120/80 mmHg. One of the main factors which controls blood pressure is the sinoatrial node sited in the right atrium (Fig. 12). It is responsible for generating impulses causing the contraction of the heart muscle and consequently the contraction of other heart cells in an orderly sequence. As a result, blood is pumped around the heart. Sinus rhythm refers to the normal heart rhythm and it is highly dominated by the sinoatrial node. 

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Blood vessels refer to the channels that carry blood throughout the body. They are divided into arteries, veins and capillaries. 

The oxygenated blood is taken away from the heart to the rest of the body through elastic vessels called arteries. Pulmonary arteries and systemic arteries are the two main types of arteries. Pulmonary arteries are unique for being the only vessels carrying deoxygenated blood (Fig. 14). They take blood, devoid of oxygen, from the heart to the lungs to collect oxygen. On the other hand, the pulmonary veins carry the blood full of oxygen back to the heart (Fig. 14). They are matchless for being the only veins in the body able to carry blood rich with oxygen. In regard to the systemic arteries, they are responsible for feeding the other parts of the body with blood. The aorta is the largest. 

The aorta arises in the top of the heart (Fig. 15). Then it is split into smaller vessels to supply the head, the heart itself, the upper limbs and the lower body including lower limbs. The first part of aorta is called ascending aorta that is giving only two branches named coronary arteries to provide blood to the entire heart (Fig. 16). The second part of aorta is called the arch of aorta where three branches are giving of to provide blood to head, neck and upper limbs. Brachiocephalic trunk is one of those three branches which splits intro right subclavian artery to supply the right upper limb and right common carotid artery to supply head and neck. Left common carotid and left subclavian are the remaining branches of ascending aorta to provide blood to the left side of head and neck, and to the left upper limb, respectively (Fig. 15). The third part of aorta is called descending aorta where it named thoracic in the area below the heart and above diaphragm, while it named abdominal aorta below the diaphragm in abdominal region. The descending aorta ends up with two terminal branches; right and left common iliac arteries to supply pelvic region and lower limbs (Fig. 17). 

With the branching of arteries, arterioles are produced, and they are the smallest branches of arteries (Fig. 18). They are involved in the circulation of blood to the body tissues. On the other hand, the smallest branches of the veins are called venules (Fig. 18).

Pulmonary veins, systemic veins and portal veins are the three main divisions of veins. While blood rich with oxygen is carried from the lungs to the heart in the left atrium via the pulmonary veins (Fig. 8), the deoxygenated blood returns from the tissues back to the heart via the systemic veins (Fig. 8). The gastrointestinal tract is drained through the portal veins which transport nutriments to the liver (Fig. 19). There are superficial veins which are sited near to the surface of the body and deep veins which arise in deep muscle with a corresponding artery. The largest systemic veins are called venae cavae, and they refer collectively to the inferior and superior venae cava (Fig. 14).

The wall of arteries and veins is made of three layers. The middle layer in the arteries is thicker than it is in the veins to maintain the high pressure because of the massive contraction to pump the blood away from the heart to reach the lungs and the entire body. Tunica Adventitia is the outermost layer, and it is made of connective tissue. This layer is considered the thickest in veins. Tunica Media which is the thickest layer in arteries, and it is composed of smooth muscle and elastic tissue. Tunica Intima is thin with a single layer of simple squamous endothelial cells. veins have valve to ensure one way of blood flow since the pressure in veins is passive, while arteries missing valves because of the high pressure that prevent the return of blood in the direction (Fig. 20).

Capillaries are the smallest blood vessel compared to arteries and veins. They are so tiny that they receive red blood cells in a single file only (Fig. 21). The arteries and veins are connected through a network of capillaries which act as channels through which oxygen, carbon dioxide, nutrients and wastes can pass. Also, the regulation of body temperature depends in some way on capillaries. With the increase of body temperature, the blood temperature also rises. The capillaries carry the heated blood to the body tissues to shoot the heat. This causes the flushed look on the faces of people who have fever.

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When the heart receives insufficient blood, a temporary chest pain occurs, and it is called Angina Pectoris. Its presentations include a constriction around the heart, a severe steady pain in the chest and the left shoulder as well as down the left arm, very pale skin, dyspnea and a variable raised blood pressure. The duration of angina pectoris attack can last for a few seconds or a few minutes. The patient is treated by removing the stressor and the medication may include sublingual nitroglycerin (Fig.22).

When the different parts of the body receive insufficient blood and oxygen from the heart, it is called Congestive Cardiac Failure. In this condition, the heart does not stop working, but its performance is not as efficient as it should be. Coronary artery diseases such as hypertension, stenosis, damaged heart valves, scarring from a previous myocardial infarction, endocarditis or cardiomyopathy can lead to CCF. People with CCF suffer from short of breath and fatigue, hence they cannot exert even simple efforts. Short of breath, especially in the position of lying down, happens as a result of the collection of fluid in the lungs and their interference with breathing. Clinical examination, blood tests, echocardiogram and electrocardiogram can easily diagnose this condition which can be treated through fluid intake, reduction of salt and a collection of cardiac specific medications (Fig.23).

When the atherosclerosis occurs, coronary artery disease results. The former is a condition affecting the inner walls of the arteries causing an accumulation of cholesterol-rich plaque. Over time, the coronary arteries become narrowed and hardened; this obstacle and reduces the blood supply to the heart muscle and causes CAD or coronary heart disease (Fig. 24). The terms of ischemia and angina are relevant to this disease. Ischemia is defined as the reduced blood supply to the heart muscle while angina refers to the chest pain resulting from the insufficient blood pumped to the heart muscle. With the progress of the disease, ischemic heart disease takes place. Angina is the most common symptom of CAD and myocardial infarction is a probable consequence.

When the wall of an artery weakens, a localized swelling in the wall of the blood vessel occurs and it is called aneurysm. This condition can be treated through a surgery to repair the defect and insert stents or coils put in through other arteries of the neck or body and directed to the defected vessel. The most common blood vessels prone to such condition are the aorta, mesenteric and cerebral arteries. In the case of the aneurysm laceration, a life-threatening emergency arises (Fig. 25).

It is defined as hardening of the arteries. Arteriosclerosis is a medical condition taking place when the arteries walls become thick and hard losing their elasticity and causing the reduction of blood flow to the heart. The occurrence of Arteriosclerosis is more probable in the coronary arteries, resulting in a myocardial infarction. Atherosclerosis is a subdivision of arteriosclerosis taking place as a result of the aggregation of fatty plaques, cholesterol, and some other substances in and on the artery walls (Fig. 26). 

High blood pressure can describe this medical term. There are primary hypertension and secondary hypertension. There is no specific cause for the primary one which account for about 90% of all cases. Hypernatremia, Hypercholesterolemia and obesity are the risk factors. On the other hand, the secondary hypertension takes place as a result of another disease such as kidney disease or a complication of pregnancy. When the original condition is treated, the hypertension is spontaneously reduced to normal levels (Fig. 27).

When the circulatory system is breached, for example, by a central venous line, drug injections, recent dental work, or previous cardiac surgery, the occurrence of endocarditis is more likely. Endocarditis is described as a bacterial infection of the endocardium causing valvular deformity. This condition is treated through long-term intravenous antibiotics, and in some cases replacement of heart valves is required (Fig. 28).

They are dark rope-shaped veins with enlarged and twisted appearance. The occurrence of varicose is higher in the veins of the lower legs and thighs. The existence of leaking valves in the leg veins can explain as the backflow of blood is allowed and consequently the venous return to the heart is weak. Other conditions such as obesity, long periods of standing, advanced age, pregnancy, and chronic constipation increase the probability of varicose and aggravate the problem. Hemorrhoids is regarded as varicose veins in the anorectal area (Fig. 29).

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  a. Myocardium

  b. Pericardium

  c. Endocardium

  d. Epicardium

  a. Aortic semilunar valve

  b. Bicuspid

  c. Pulmonary semilunar valve

  d. Tricuspid

  a. Hepatic vein

  b. Pulmonary vein 

  c. Portal vein

  d. Vena cava

  a. Bradycardia 

  b. Endocardia

  c. Pericardia

  d. Tachycardia

  a. Angina pectoris 

  b. Endocarditis 

  c. Aneurysm

  d. Atherosclerosis