Introduction To Cardiovascular System Notes

Introduction To Cardiovascular System Cardiovascular System

• The cardiovascular system is the system of the heart and blood vessels that circulates blood throughout the body.
• The blood circulating the body transports nutrients and oxygen to the tissues and removes carbon dioxide and waste products from the tissues.
• The heart is the central pump and the blood vessels are the series of distributing and collecting tubes.
• The thinnest blood vessels are in the form of extensive thin-walled vascular channels called capillaries through which interchange between the cardiovascular system and tissues occurs.
• The cardiovascular system constitutes one of the major coordinating and integrating systems of the body.
• The other two systems with such actions are the nervous system and the endocrine system.

Read And Learn More: Medical Physiology Notes

Heart

• The heart is a muscular organ that pumps blood throughout the circulatory system.
• It is situated between the two lungs in the mediastinum. It is made up of four chambers – two atria and two ventricles.
• The musculature is more and thick in the ventricles than in the atria. The force of contraction of the heart depends upon the muscles.

Right Side Of The Heart

• The right side of the heart has two chambers, the upper right atrium, and the lower right ventricle.
• The right atrium is a thin-walled and low-pressure chamber. The right atrium has got the pacemaker known as a sinoatrial node that produces cardiac impulses and the atrioventricular node that conducts the impulses to the ventricles.

 

It receives venous (deoxygenated) blood via two large veins:

• Superior vena cava that returns the venous blood from the head, neck, and upper limbs
• The inferior vena cava returns the venous blood from the lower parts of the body.
• The right atrium communicates with the right ventricle through the tricuspid valve.
• The wall of the right ventricle is thick. Venous blood from the right atrium enters the right ventricle through this valve.
• From the right ventricle, the pulmonary artery arises. It carries the venous blood from the right ventricle to the lungs. In the lungs, the deoxygenated blood is oxygenated.

Left Side Of The Heart

• The left side of the heart has two chambers, the upper left atrium, and the lower left ventricle.
• The left atrium is a thin-walled and low-pressure chamber. The left atrium receives oxygenated blood from the lungs through pulmonary veins.
• This is the only exception in the body where an artery carries venous blood and a vein carries the arterial blood.
• Blood from the left atrium enters the left ventricle through the mitral valve (bicuspid valve).
• The wall of the left ventricle is very thick. The left ventricle pumps arterial blood to different parts of the body through the systemic aorta.

Layers of Wall of the Heart

The heart is made up of three layers of tissues:

1. Outer pericardium
2. Middle myocardium
3. Inner endocardium.

Pericardium

• The pericardium is the outer covering of the heart. It is made up of two layers which are separated by a space called a pericardial cavity.
• The pericardial space is only a potential space. In healthy conditions, the two layers of pericardium lie in close approximation separated by a thin film of fluid.

The two layers of the pericardium are:

1. Outer parietal pericardium
2. Inner visceral pericardium.

1. Outer Parietal Pericardium

The parietal pericardium forms a strong protective sac for the heart. It helps also to anchor the heart within the mediastinum.

It is made up of two layers:

1. Outer fibrous layer
2. Inner serous layer.

Fibrous layer

• The fibrous layer of the parietal pericardium is formed by thick fibrous connective tissue.
• It is attached to the diaphragm and it is continuous with tunica adventitia (outer wall) of the great blood vessels entering and leaving the heart. It is attached with the diaphragm below.
• Because of its fibrous nature, it protects the heart from over-stretching.

Serous layer

• The 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 that lines the pericardial space.
• This fluid prevents friction and allows free movement of the heart within the pericardium when it contracts and relaxes.
• The total volume of this fluid is only about 25-35 mL.

2. Inner Visceral Pericardium

• The inner visceral pericardium closely lines the surface of the myocardium.
• It is made up of flattened epithelial cells. This layer is also known as the epicardium

Myocardium

• Myocardium is the middle layer of the wall of the heart and it is formed by cardiac muscle fibers or cardiac myocytes.
• The myocardium is responsible for the pumping action of the heart. It forms the bulk of the heart for features of cardiac muscles.

Three types of cardiac muscle fibers are present in the myocardium:

1. Muscle fibers which form the contractile unit of the heart
2. Muscle fibers that form pacemaker
3. Muscle fibers form the conductive system.

1. Muscle Fibers which Form the Contractile Unit of the Heart

• The cardiac muscle fibers, which form the contractile unit of the heart are striated and are more or less similar to the skeletal muscles in structure. But, unlike the skeletal muscle fibers, the cardiac muscle fibers are involuntary in nature.
• The cardiac muscle fiber is bound by sarcolemma. It has a nucleus, which lies in the center of the cell.

• The myofibrils are embedded in the sarcoplasm. The sarcomere of the cardiac muscle has structures similar to those in skeletal muscle. It has all the muscle proteins namely, actin, myosin, troponin, and tropomyosin.
• The cardiac muscles also have sarcotubular system like that of skeletal muscle.
• The 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

• intercalated disk is a tough double membranous structure situated at the junction between the branches of neighboring cardiac muscle fibers.
• It is formed by the fusion of the membrane of the cardiac muscle branches.
• The intercalated disks form adherens junctions which play an important role in the contraction of the muscle by pulling the muscle fibers with one another.

Syncytium

• The word syncytium refers to the tissue in which there is cytoplasmic continuity between the adjacent cells.
• However, the cardiac muscle acts like only 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.
• The gap junction is permeable to ions. It facilitates the rapid conduction of electrical activity from one fiber to another.
• Thus, because of the gap junctions, all cardiac muscle fibers act like a single unit which is referred to as syncytium.
• The syncytium in the human heart has two portions, the syncytium of the atria and the syncytium of the ventricles.

Both portions of syncytium are connected by a thick nonconducting fibrous ring called the atrioventricular ring.

2. Muscle Fibers which Form the Pacemaker

• Some of the muscle fibers of the heart are modified into a specialized structure known as a pacemaker.
• The muscle fibers forming the pacemaker have less striation.

Pacemaker

• A pacemaker is a structure in the heart that generates the impulses for a heartbeat.
• It is formed by the pacemaker cells called P cells. The Sinoatrial (SA) node forms the pacemaker in the human heart.

3. Muscle Fibers which Form the Conductive System

• The conductive system of the heart is formed by the modified cardiac muscle fibers.
• The impulses from the SA node are transmitted to the atria directly.
• However, the impulses are transmitted to the ventricles, through various components of the conducting system namely internodal fibers, AV node, bundle of His, branches of bundle of His, and Purkinje fibers.
• The details of the conductive system are given in the next chapter.

Endocardium

• The endocardium is the innermost layer of the 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.

Septa Of The Heart

• The Atria of the heart are separated from one another by a fibrous septum called the interatrial septum.
• The ventricles are separated from one another by another septum called interventricular septum.
• The upper part of this septum is a membranous structure, whereas, the lower part of it is muscular in nature.

Valves Of The Heart

• There are four valves in the human heart. Two of the valves are in between the atria and the ventricles called atrioventricular valves.
• The other two are the semilunar valves, placed at the opening of the blood vessels arising from the ventricles, i.e. systemic aorta and pulmonary artery.
• The valves of the heart permit the flow of blood through the heart in only one direction.

Atrioventricular Valves

• The left atrioventricular valve is otherwise known as the mitral valve or bicuspid valve. It is formed by two valvular cusps or flaps.
• The right atrioventricular valve is known as the tricuspid valve and it is formed by three cusps.
• The brim of the atrioventricular valves is attached to the atrioventricular ring, which is the fibrous connection between the atria and ventricles.

• The cusps of the valves are attached to the papillary muscles by means of chord ao tendinae.
• The papillary muscles arise from the Inik’u surface of the ventricles.
• The papillary muscles play, an important role in the closure of the cusps and in preventing the backflow of blood from the ventricle to the atria during ventricular contraction.
• Atrioventricular valves open only towards ventricles and prevent the backflow of blood into the atria.

Semilunar Valves

• The semilunar valves are present at the openings of the systemic aorta and pulmonary artery and are known as the aortic valve and pulmonary valve respectively.
• Because of the half-moon shape, these two valves are called semilunar valves.
• Both the semilunar valves are similar in structure and each one has three flaps.
• The semilunar valves open only towards the aorta and pulmonary artery and prevent the backflow of blood into the ventricles.

Actions Of The Heart

The actions of the heart are classified into four types:

1. Chronotropic action
2. Inotropic action
3. Dromotropic action
4. Bathmotropic action.

Chronotropic Action

Chronotropic action is the frequency of heartbeat or heart rate.

It is of two types:

1. Tachycardia or increase in heart rate
2. Bradycardia or decrease in the heart rate.

Inotropic Action: A force of contraction of the heart is called inotropic action.

It is of two types:

1. Positive inotropic action or increase in the force of contraction
2. Negative inotropic action or decrease in the force of contraction.

Dromotropic Action: Dromotropic action is the conduction of impulses through the heart.

It is of two types:

1. Positive dromotropic action or increase in the velocity of conduction
2. Negative dromotropic action or decrease in the velocity of conduction

Bathmotropic Action: Bathmotropic action is the excitability of cardiac muscle.

It is also of two types:

1. Positive bathmotropic action or increase in the excitability of cardiac muscle
2. Negative bathmotropic action or the decrease m the excitability of cardiac muscle.

Regulation of Actions of the Heart

• All the actions of the 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 the stimulation of nerves supplying the heart or some hormones or hormonal substances secreted in the body.

Blood Vessels

• The vessels of the circulatory system are the aorta, arteries, arterioles, capillaries, venules, veins, and vena cava.
• The structural differences between different blood vessels are given in Table 89-1.

Arterial System

1. The arterial system comprises the aorta, arteries, and arterioles. The walls of the aorta and arteries are formed by three layers.
2. Outer tunica adventitia, which is made up of a connective tissue layer. It is the continuation of the fibrous layer of the parietal pericardium
3. Middle tunica media, which is formed by smooth muscles
4. Inner tunica intima, which is made up of endothelium. It is the continuation of the endocardium.

The aorta, arteries, and arterioles have two laminae of elastic tissues:

• External elastic lamina between tunica adventitia and tunica media internal elastic lamina between tunica media and tunica intima.
• The aorta and arteries have more elastic tissues and the arterioles have more smooth muscles.
• The arterial branches become narrower and their wails become thinner while reaching the periphery.
• The aorta has got a maximum diameter of about 25 mm. The diameter of the arteries is gradually decreased and at the end arteries, it is about 4 mm.
• It further decreases to 30 p in the arterioles and ends up with 10 p in the terminal arterioles.
• The resistance (peripheral resistance) is offered to the blood flow in the arterioles and so these vessels are called resistant vessels.
• The arterioles are continued as capillaries which are small, thin-walled vessels having a diameter of about 5-8 p.
• The capillaries are functionally very important because the exchange of materials between the blood and the tissues occurs through these vessels.

Venous System

• From the capillaries venous system starts and it includes the venules, and veins arid vena cavae.
• The capillaries end in the venules. The venules are smaller vessels with thin muscular walls than the arterioles.
• The diameter of the venules is about 20 p. At a given time, a large quantity of blood is held in venules and so the venules are called capacitance vessels.
• The venules are continued as veins, which have a diameter of 5 mm. The veins form superior and inferior vena cavae which have a diameter of about 30 mm.
• The walls of the veins and vena cavae are made up of inner endothelium, elastic tissues, smooth muscles, and an outer connective tissue layer.
• In the veins and vena cavae, the elastic tissue is less but the smooth muscle fibers are more.

Complications In Blood Vessels

Aorta and Arteries

• The arterial blood vessels are highly susceptible to arteriosclerosis and atherosclerosis.
• Arteriosclerosis is the disease of the arteries associated with hardening, thickening, and loss of elasticity in the wall of the vessels.
• Atherosclerosis is a disease marked by the narrowing of the lumen of arterial vessels due to the deposition of cholesterol.

Arterioles: When the tone of the smooth muscles in the arterioles increases hypertension occurs.

Capillaries: The permeability of the capillary membrane may increase resulting in shock or edema due to leakage of fluid, proteins, and other substances from blood.

 

 

Veins

• Inflammation of the wall of veins leads to the formation of the intro muscular clot called thrombosis.
• The clot gets d dodged as a thrombus. The thrombus travels in the blood a not cause an embolism.
• It obstructs the blood flow to vital organs like the brain, heart, and lungs leading to many complications.

Divisions Of Circulation

The blood flows through two divisions of the circulatory system:

1. Systemic circulation
2. Pulmonary circulation.

Systemic Circulation

• It is otherwise known as greater circulation. The blood pumped from the left ventricle passes through a series of blood vessels of an arterial tree or arterial system and reaches the tissues.
• The blood vessels of the arterial system are the aorta, larger arteries, smaller arteries, and arterioles.
• The arterioles branch into the capillaries. The capillaries are responsible for the exchange of various substances between the blood and the tissues.
• It is because the wall of the capillaries is permeable to various substances.
• After an exchange of materials at the capillaries, the blood enters the venous system and returns to the right atrium of the heart.
• The blood vessels of the venous tree or venous system are the venules, smaller veins, larger veins, and vena cava.
• From the right atrium, blood enters the right ventricle. Thus, through systemic circulation, the oxygenated blood or arterial blood is supplied from the heart to the tissues, and the venous blood returns to the heart from the tissues.

Pulmonary Circulation

• It is otherwise called lesser circulation. Blood is pumped from the right ventricle to the lungs through the pulmonary artery.
• The exchange of gases occurs between blood and the alveoli of the lungs through the pulmonary capillary membrane.
• The oxygenated blood returns to the left atrium through the pulmonary veins.
• Thus, the left side of the heart contains oxygenated or arterial blood and the right side of the heart contains the venous blood.