Cardiovascular system introduction.
[1]. Cardiovascular system includes heart and blood vessels. Heart pumps blood into the blood vessels. Blood vessels circulate the blood throughout the body.
[2]. Blood transports nutrients and oxygen to the tissues and removes carbon dioxide and waste products from the tissues.
Heart.
[1]. Heart is a muscular organ that pumps blood throughout the circulatory system. It is situated in between two lungs in the mediastinum. It is made up of four chambers, two atria and two ventricles.
[2]. The musculature of ventricles is thicker than that of atria. Force of contraction of heart depends upon the muscles.
Right side of the heart.
[1]. Right side of the heart has two chambers, right atrium and right ventricle. Right atrium is a thin walled and low pressure chamber.
[2]. It has got the pacemaker known as sinoatrial node that produces cardiac impulses and atrioventricular node that conducts the impulses to the ventricles.
[3]. Right atrium receives venous (deoxygenated) blood via two large veins:
1. Superior vena cava that returns venous blood from the head, neck and upper limbs.
2. Inferior vena cava that returns venous blood from lower parts of the body .
[4]. Right atrium communicates with right ventricle through tricuspid valve. Wall of right ventricle is thick. Venous blood from the right atrium enters the right ventricle through this valve.
[5]. From the right ventricle, pulmonary artery arises. It carries the venous blood from right ventricle to lungs. In the lungs, the deoxygenated blood is oxygenated.
Left side of the heart
[1]. Left side of the heart has two chambers, left atrium and left ventricle. Left atrium is a thin walled and low pressure chamber. It receives oxygenated blood from the lungs through pulmonary veins. This is the only exception in the body, where an artery carries venous blood and vein carries the arterial blood.
[2]. Blood from left atrium enters the left ventricle through mitral valve (bicuspid valve). Wall of the left ventricle is very thick. Left ventricle pumps the arterial blood to different parts of the body through systemic aorta.
Septa of the heart.
[1]. Right and left atria are separated from one another by a fibrous septum called interatrial septum. Right and left ventricles are separated from one another by interventricular septum.
[2]. The upper part of this septum is a membranous structure, whereas the lower part of it is muscular in nature.
Layers of Wall of the heart.
Heart is made up of three layers of tissues:
1. Outer pericardium.
2. Middle myocardium.
3. Inner endocardium.
Pericardium.
Pericardium is the outer covering of the heart. It is made up of two layers:
a. Outer parietal pericardium.
b. Inner visceral pericardium.
The space between the two layers is called pericardial cavity or pericardial space and it contains a thin film of fluid.
a. Outer Parietal Pericardium.
[1]. Parietal pericardium forms a strong protective sac for the heart. It helps also to anchor the heart within the mediastinum.
[2]. Parietal pericardium is made up two layers:
a. Outer fibrous layer.
b. Inner serous layer.
Fibrous layer.
[1]. Fibrous layer of the parietal pericardium is formed by thick fibrous connective tissue.
[2]. It is attached to the diaphragm and it is continuous with tunica adventitia (outer wall) of the blood vessels, entering and leaving the heart.
[3]. It is attached with diaphragm below. Because of the fibrous nature, it protects the heart from over stretching.
Serous layer.
[1].Serous layer is formed by mesothelium, together with a small amount of connective tissue. Mesothelium contains squamous epithelial cells which secrete a small amount of fluid, which lines the pericardial space.
[2]. This fluid prevents friction and allows free movement of heart within pericardium, when it contracts and relaxes. The total volume of this fluid is only about 25 to 35 mL.
b. Inner Visceral Pericardium.
Inner visceral pericardium lines the surface of myocardium. It is made up of flattened epithelial cells. This layer is also known as epicardium.
Myocardium.
[1]. Myocardium is the middle layer of wall of the heart and it is formed by cardiac muscle fibers or cardiac myocytes.
[2]. Myocardium forms the bulk of the heart and it is responsible for pumping action of the heart. Unlike skeletal muscle fibers, the cardiac muscle fibers are involuntary in nature.
[3]. Myocardium has three types of muscle fibers:
a. Muscle fibers which form contractile unit of heart.
b. Muscle fibers which form pacemaker.
c. Muscle fibers which form conductive system.
a. Muscle Fibers which Form Contractile Unit of Heart.
[1]. These cardiac muscle fibers are striated and resemble the skeletal muscle fibers in structure. Cardiac muscle fiber is bound by sarcolemma. It has a centrally placed nucleus. Myofibrils are embedded in the sarcoplasm.
[2]. Sarcomere of the cardiac muscle has all the contractile proteins, namely actin, myosin, troponin and tropomyosin. Sarcotubular system in cardiac muscle is similar to that of skeletal muscle.
[3]. Important difference between skeletal muscle and cardiac muscle is that the cardiac muscle fiber is branched and the skeletal muscle is not branched. Intercalated disk is a tough double membranous structure, situated at the junction between the branches of neighboring cardiac muscle fibers.
[4]. It is formed by the fusion of the membrane of the cardiac muscle branches. Intercalated disks form adherens junctions, which
play an important role in the contraction of cardiac muscle as a single unit .
[5]. Syncytium means tissue with cytoplasmic continuity between adjacent cells. However, cardiac muscle is like a physiological syncytium, since there is no continuity of the cytoplasm and the muscle fibers are separated from each other by cell membrane. At the sides, the membranes of the adjacent muscle fibers fuse together to form gap junctions.
[6]. Gap junction is permeable to ions and it facilitates the rapid conduction of action potential from one fiber to another. Because
of this, all the cardiac muscle fibers act like a single unit, which is referred as syncytium.
[7]. Syncytium in human heart has two portions, syncytium of atria and the syncytium of ventricles. Both the portions of syncytium are connected by a thick nonconducting fibrous ring called the atrioventricular ring.
b. Muscle Fibers which Form the Pacemaker.
[1]. Some of the muscle fibers of heart are modified into a specialized structure known as pacemaker. These muscle fibers forming the pacemaker have less striation.
[2]. Pacemaker is structure in the heart that generates the impulses for heart beat. It is formed by pacemaker cells called P cells. Sinoatrial (SA) node forms the pacemaker in human heart.
c. Muscle Fibers which Form Conductive System.
Conductive system of the heart is formed by modified cardiac muscle fibers. Impulses from SA node are transmitted to the atria directly. However, the impulses are transmitted to ventricles through various components of conducting system,
Endocardium.
Endocardium is the inner most layer of heart wall. It is a thin, smooth and glistening membrane. It is formed by a single layer of endothelial cells, lining the inner surface of the heart. Endocardium continues as endothelium of the blood vessels.
Valves of the heart.
[1]. There are four valves in human heart. Two valves are in between atria and the ventricles called atrioventricular valves.
[2]. Other two are the semilunar valves, placed at the opening of blood vessels arising from ventricles, namely systemic aorta and pulmonary artery. Valves of the heart permit the flow of blood through heart in only one direction.
Atrioventricular Valves .
[1]. Left atrioventricular valve is otherwise known as mitral valve or bicuspid valve. It is formed by two valvular cusps or flaps . Right atrioventricular valve is known as tricuspid valve and it is formed by three cusps. Brim of the atrioventricular valves is attached to
atrioventricular ring, which is the fibrous connection between the atria and ventricles.
[2]. Cusps of the valves are attached to papillary muscles by means of chordae tendineae. Papillary muscles arise from inner surface of the ventricles. Papillary muscles play an important role in closure of the cusps and in preventing the back flow of blood from ventricle to atria during ventricular contraction. Atrioventricular valves open only towards ventricles and prevent the backflow of blood into atria.
Semilunar Valves.
[1]. Semilunar valves are present at the openings of systemic aorta and12 pulmonary artery and are known as aortic valve and pulmonary valve respectively. Because of the half moon shape, these two valves are called semilunar valves.
[2]. Semilunar valves are made up of three flaps. Semilunar valves open only towards the aorta and pulmonary artery and prevent the backflow of blood into the ventricles.
Actions of the heart.
Actions of the heart are classified into four types:
1. Chronotropic action.
2. Inotropic action.
3. Dromotropic action.
4. Bathmotropic action.
1. Chronotropic action.
Chronotropic action is the frequency of heartbeat or heart rate. It is of two types:
a. Tachycardia or increase in heart rate.
b. Bradycardia or decrease in heart rate.
2. Inotropic action.
Force of contraction of heart is called inotropic action. It is of two types:
a. Positive inotropic action or increase in the force of contraction.
b. Negative inotropic action or decrease in the force of contraction.
3. Dromotropic action.
Dromotropic action is the conduction of impulse through heart. It is of two types:
a. Positive dromotropic action or increase in the velocity of conduction.
b. Negative dromotropic action or decrease in the velocity of conduction.
4. Bathmotropic action.
Bathmotropic action is the excitability of cardiac muscle. It is also of two types:
a. Positive bathmotropic action or increase in the excitability of cardiac muscle.
b. Negative bathmotropic action or decrease in the excitability of cardiac muscle.
Regulation of Actions of Heart.
All the actions of heart are continuously regulated. It is essential for the heart to cope up with the needs of the body. All the actions are altered by stimulation of nerves supplying the heart or some hormones or hormonal substances secreted in the body.
Blood Vessels.
Vessels of circulatory system are the aorta, arteries, arterioles, capillaries, venules, veins and venae cavae.
Arterial system .
Arterial system comprises the aorta, arteries and arterioles. Walls of the aorta and arteries are formed by three layers:
1. Outer tunica adventitia, which is made up of connective tissue layer. It is the continuation of fibrous layer of parietal pericardium.
2. Middle tunica media, which is formed by smooth muscles
3. Inner tunica intima, which is made up of endothelium.
It is the continuation of endocardium. Aorta, arteries and arterioles have two laminae of elastic tissues:
a. External elastic lamina between tunica adventitia and tunica media.
b. Internal elastic lamina between tunica media and tunica intima.
[1]. Aorta and arteries have more elastic tissues and the arterioles have more smooth muscles. Arterial branches become narrower and their walls become thinner while reaching the periphery.
[2]. Aorta has got the maximum diameter of about 25 mm. Diameter of the arteries is gradually decreased and at the end arteries, it is about 4 mm. It further decreases to 30 µ in the arterioles and ends up with 10 µ in the terminal arterioles. Resistance (peripheral resistance) is offered to blood flow in the arterioles and so these vessels are called resistant vessels.
[3]. Arterioles are continued as capillaries, which are small, thin walled vessels having a diameter of about 5 to 8 µ. Capillaries are functionally very important because, the exchange of materials between the blood and the tissues occurs through these vessels.
Venous System.
[1]. From the capillaries, venous system starts and it includes venules, veins and venae cavae. Capillaries end in venules, which are the smaller vessels with thin muscular wall than the arterioles. Diameter of the venules is about 20 µ. At a time, a large quantity of blood is held in venules and hence the venules are called capacitance vessels.
[2]. Venules are continued as veins, which have the diameter of 5 mm. Veins form superior and inferior venae cavae, which have a diameter of about 30 mm.
[3]. Walls of the veins and venae cavae are made up of inner endothelium, elastic tissues, smooth muscles and outer connective tissue layer. In the veins and venae cavae, the elastic tissue is less but the smooth muscle fibers are more.
Complication in Blood Vessels.
Aorta and Arteries.
[1]. Arterial blood vessels are highly susceptible for arteriosclerosis and atherosclerosis. Arteriosclerosis is the disease of the arteries, associated with hardening, thickening and loss of elasticity in the wall of the vessels.
[2]. Atherosclerosis is the disease marked by the narrowing of lumen of arterial vessel due to deposition of cholesterol.
Arterioles.
When the tone of the smooth muscles in the arterioles increases, hypertension occurs.
Capillaries.
Permeability of the capillary membrane may increase resulting in shock or edema due to leakage of fluid, proteins and other substances from blood.
Veins.
[1]. Inflammation of the wall of veins leads to the formation of intravascular clot called thrombosis. The clot gets dislodged, as thrombus. The thrombus travels through blood and causes embolism.
[2]. Embolism obstructs the blood flow to vital organs such as brain, heart and lungs, leading to many complications.
Division of Circulation.
Blood flows through two divisions of circulatory system:
1. Systemic circulation.
2. Pulmonary circulation.
1. Systemic circulation.
[1]. Systemic circulation is otherwise known as greater circulation . Blood pumped from left ventricle passes through a series of blood vessels, arterial system and reaches the tissues. Exchange of various substances between blood and the tissues occurs at the capillaries.
[2]. After exchange of materials, blood enters the venous system and returns to right atrium of the heart. From right atrium, blood enters the right ventricle. Thus, through systemic circulation, oxygenated blood is supplied from heart to the tissues and venous blood returns to the heart from tissues.
2. Pulmonary circulation.
[1]. Pulmonary circulation is otherwise called lesser circulation. Blood is pumped from right ventricle to lungs through pulmonary artery. Exchange of gases occurs between blood and alveoli of the lungs at pulmonary capillaries.
[2]. Oxygenated blood returns to left atrium through the pulmonary veins. Thus, left side of the heart contains oxygenated or arterial blood and the right side of the heart contains deoxygenated or venous blood.
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