Unit 11: Human Circulatory System — Interactive Notes
Location: It is situated in the middle of chest cavity (between the lungs). Its back surface is near vertebral column while its front surface is behind sternum and rib cartilages.
Heart is enclosed in a sac called pericardium. Pericardium separates heart from surrounding organs. It is composed of:
1. Fibrous pericardium: Outer layer, made of strong connective tissue. It protects heart against external pressure and shocks. It also prevents excessive dilation of heart.
2. Serous pericardium: Inner layer, a sac made of two layers — (a) Parietal pericardium (beneath fibrous pericardium), (b) Visceral pericardium/epicardium (closely attached to heart).
Pericardial cavity: The space between parietal and visceral pericardium contains up to 50 mL pericardial fluid. It lubricates heart and protects it from infections.
The wall of heart is composed of three layers.
• Epicardium: The inner layer of pericardium makes the outer lining of heart wall.
• Myocardium: Beneath epicardium, there is the thickest layer of heart wall, made of cardiac muscles (cardiomyocytes).
• Endocardium: Present beneath myocardium. It is a single layer of epithelial cells and makes the inner linings of heart chambers.
There are four chambers of heart i.e., two upper thin-walled and comparatively smaller atria, and two lower thick-walled and comparatively larger ventricles. Atria receive blood from body and pass it to ventricles which distribute blood to body.
The chambers of human heart are completely separated by septa which are of three types.
• Atria and ventricles are separated by atrioventricular septum.
• The left and right atria are separated from each other by an interatrial septum.
• The left and right ventricles are separated from each other by an interventricular septum. It is much thicker than the interatrial septum.
At the entrance points of ventricles (in atrioventricular septum), there are two atrioventricular valves i.e., a tricuspid valve and a bicuspid valve.
• Tricuspid valve is made of three cusps and is present between right atrium and right ventricle.
• Bicuspid (mitral) valve is made of two cusps and is present between left atrium and left ventricle.
• When ventricles contract, tricuspid and bicuspid valves close and prevent the back flow of blood into atria.
At the exit points of ventricles, there are two semilunar valves (with shapes like half-moon). These are called pulmonary valve and aortic valve.
• Pulmonary valve is located at the base of pulmonary artery between right ventricle and pulmonary trunk.
• Aortic valve is present at the base of aorta between left ventricle and aorta.
• When ventricles relax, pulmonary and aortic valves close to prevent back flow of blood from pulmonary artery and aorta into ventricles.
Circulation of Blood through Heart: Human heart functions as a double pump. It carries out pulmonary circulation (supply of blood to lungs) and systemic circulation (supply of blood to all organs of body — except lungs). Complete separation of deoxygenated (right side) and oxygenated (left side) blood is maintained in heart.
Route of deoxygenated blood: The right atrium receives deoxygenated blood from body via two veins i.e., superior vena cava and inferior vena cava. Right atrium passes this blood to right ventricle via tricuspid valve. When right ventricle contracts, deoxygenated blood is passed to pulmonary trunk via semilunar pulmonary valve. The pulmonary trunk divides into left and right pulmonary arteries which carry deoxygenated blood to lungs for oxygenation.
Route of oxygenated blood: The oxygenated blood from lungs is brought to left atrium by four pulmonary veins. Left atrium passes this blood to left ventricle via bicuspid (or mitral) valve. When left ventricle contracts, oxygenated blood is passed to aorta via semilunar aortic valve. Aorta carries this blood to all parts of body (except lungs).
Definition: The sequence of events that occur during one complete heart beat are called cardiac cycle. Heart works in continuous cycles. Its chambers relax and are passively filled with blood from large veins. Then, its chambers contract and propel the blood throughout body. Its alternating relaxations and contractions are collectively called a cardiac cycle or one heartbeat.
(i) Cardiac diastole: In this phase atria are relaxed and being filled with blood, the ventricles are also relaxed. This relaxed period of heart chambers is called diastole.
(ii) Atrial systole: As blood accumulates in atria, their blood pressure rises, due to which both of them contract. It passes the blood through tricuspid and bicuspid valves into the two relaxed ventricles.
(iii) Ventricular systole: When ventricles are filled with blood, both of them contract. This pumps the blood to pulmonary and aortic valves which open.
There are two heartbeat sounds produced during heartbeat i.e. lubb and dubb which can be heard by using stethoscope.
1. Lubb sound: When both ventricles contract simultaneously to pump the blood to pulmonary arteries and aorta, the tricuspid and bicuspid valves close and "lubb" sound is produced.
2. Dubb sound: When ventricular systole ends and both ventricles relax simultaneously, the pulmonary and aortic semilunar valves close and "dubb" sound is produced.
3. Murmurs: If the valves are not closing fully, or if they open narrowly, turbulence is created within the heart. This turbulence can be heard as a heart murmur. A murmur sounds like a hiss.
The control system of heart is composed of the following structures: (i) SA-node (ii) AV-node (iii) Bundle of His (iv) Purkinje fibres (AV-bundle).
(i) Sinoatrial Node (SA-node): Consists of a small cluster of cardiac muscle cells. Embedded in the upper wall of right atrium. The pumping of heart is initiated by the SA node or pacemaker. Heartbeat starts when SA node sends electrical impulses to the walls of atria, causing both atria to contract simultaneously.
(ii) Atrioventricular node (AV-node): Made of small cluster of cardiac muscle cells. Lies at the lower portion of interatrial septum. Receives impulses from SA-node and propagates to ventricles through bundle of His.
(iii) Bundle of His/AV-Bundle: A group of specialized fibres that transmit electrical impulses from AV-node to the left and right AV-bundle branches. Located in interventricular septum.
(iv) Purkinje Fibres: Network of fibres formed by subdivisions of AV-bundles that spread throughout the ventricles, causing them to contract upon receiving nerve impulses.
Delay of conductance: There is a delay of about 0.15 second in conductance of impulses from the SA node to AV node, permitting atrial systole to be completed before ventricular systole begins.
Cure of blockage: In patients of weak SA node, artificial pacemaker is used. It is a battery-operated device that is surgically transplanted near the AV node. It emits electrical signals that trigger normal heartbeats.
The heart of an average adult beats about 70 times per minute. It pumps the entire blood volume (about 5 litres) every minute. Brain also exerts some influence on heart rate. During fever and exercise, the control centre in brain sends nerve signals to both the pacemaker and the AV node, making them increase the heart rate. In contrast, when we are asleep or at rest, the brain's control centre slows down the activity of pacemaker and AV node.
The recording of electrical potentials, generated by the currents of cardiac impulses, is known as electrocardiogram (ECG). When cardiac impulse passes over the surface of heart, a minute electrical current is generated. This current spreads into the tissues surrounding heart and travels to the surface of body, where these electrical potentials are recorded using an ECG machine (electrocardiograph). ECG helps to diagnose abnormalities in conduction system of heart.
1. P wave: Beginning of atrial depolarization, initiated by SA node; causes atrial contraction. Irregular/absent P waves may indicate arrhythmia.
2. PR segment: Shows completion of atrial depolarization; usually 0.12-0.20 seconds.
3. QRS complex: Beginning of depolarization of ventricles; abnormalities may indicate bundle branch block or ventricular tachycardia.
4. ST segment: Shows completion of depolarization of ventricles; lasts ~0.08 second; can be depressed in ischemia and elevated in myocardial infarction.
5. T wave: Represents beginning of repolarization of ventricles; abnormalities may indicate electrolyte disturbance.
6. QT interval: From beginning of QRS complex to end of T wave; normally ~0.40 seconds; deviations increase risk of arrhythmias.
1. Arteries: Carry blood away from heart to different parts of body. All arteries carry oxygenated blood, except pulmonary arteries. The central core of artery is the lumen.
Walls of arteries are made up of three layers: (i) Tunica externa/adventitia — outer layer of connective and elastic tissue. (ii) Tunica media — middle layer of thick muscular tissue and elastic fibres; can withstand higher blood pressure during ventricular systole. (iii) Tunica intima — inner layer of thin endothelial cells.
Arterioles are sub-divisions of arteries, leading to capillaries.
2. Capillaries: Formed by the division of arterioles. Capillaries join to form venules. Walls made of a single layer of endothelial cells; internal diameter ~8 micrometres. Function: sites where materials are exchanged between blood and body tissues by diffusion or active transport.
Interstitial fluid: The pressure within capillaries causes a continuous leakage of fluid from the blood plasma into tissues. This fluid consists of water with dissolved nutrients, hormones, gases, wastes and small proteins. Large proteins, RBCs and platelets remain within capillaries. But some WBCs can squeeze out through the intercellular spaces of capillary wall.
3. Veins: Carry blood from different parts of the body towards heart. All veins carry deoxygenated blood except pulmonary veins. Walls have the same three layers as arteries, but tunica media is relatively thinner because veins do not have to withstand high blood pressure. Semilunar valves are present in veins to prevent the back flow of blood. The pressure generated by the contraction of surrounding muscles presses veins and assists in the return of blood towards heart.
Relationship of arterioles, capillaries and venules: Smaller veins join to form larger veins and ultimately form vena cavae (inferior and superior), which pour blood into the right atrium of heart. Pulmonary veins from lungs empty into left atrium.
The amount of blood flowing in capillaries is controlled by constricting or dilating the capillaries. Capillaries do not have smooth muscles in their wall, so the diameter of a capillary is controlled by other factors such as nervous stimulation, as well as by chemicals. Nervous stimulation can constrict capillaries and certain chemicals such as histamine can dilate them.
Some capillaries are connected with arterioles and venules through loops of other capillaries. The entry of each loop is guarded by a ring of muscles called a pre-capillary sphincter. When the tissues suffer from hypoxia or need more nutrients, the diameter of the capillaries is increased by vasodilators — precapillary sphincters relax to increase blood flow. When the body is at rest or asleep, the precapillary sphincters constrict under nervous stimulation to reduce blood flow towards tissues.
Vasoconstriction in Arterioles: In the walls of arterioles, there are more circular muscles than elastic tissue. The contraction of the circular muscles of arterioles is under the control of nervous and endocrine systems. When these muscles contract, arterioles are constricted (vasoconstriction), reducing blood flow.
Vasodilation in Arterioles: When these muscles relax, arterioles are dilated (vasodilation), increasing blood flow.
Regulation and metabolic activity: When metabolic activity in a tissue rises, oxygen decreases and carbon dioxide increases in its interstitial fluid. In response, the circular muscles of the arterioles in that tissue relax (vasodilation). It increases blood flow in these arterioles and capillaries, supplying more oxygen and removing more carbon dioxide. Decreased metabolic activity causes vasoconstriction of arterioles.
Blood circulation from heart to lungs and from lungs back to heart is called pulmonary circulation. It supplies deoxygenated blood to lungs and returns oxygenated blood to heart.
At the level of heart: A big artery i.e. pulmonary trunk carries deoxygenated blood from the right ventricle of heart. Pulmonary trunk divides into right and left pulmonary arteries which carry deoxygenated blood to the right and left lungs.
At the level of lungs: Inside each lung, the pulmonary artery divides and makes pulmonary arterioles and lung capillaries. In lung capillaries, blood is oxygenated. Lung capillaries join to form pulmonary venules, which join to form pulmonary vein. Left and right pulmonary veins from lungs open in left atrium.
Definition: The systemic circulation supplies oxygenated blood to all the cells, tissues and organs of the body (except lungs) and returns deoxygenated blood to heart.
The heart walls are supplied with blood through a small portion of the systemic circulation. Two coronary arteries i.e., right and left coronary arteries, arise from aorta, near its origin. These arteries divide into many smaller arteries, arterioles and then into capillaries. After supplying oxygenated blood to heart muscles, the capillaries unite to form venules which make many coronary veins. The coronary veins join to form a coronary sinus which opens in right atrium. Small coronary veins drain directly into right atrium.
A portal system is a circulation in which veins end in capillaries. In hepatic portal system, a large hepatic portal vein collects blood from spleen and alimentary canal and takes it to liver. The blood from liver is taken to heart through hepatic veins. Hepatic portal system extends from the lower portion of oesophagus to the upper part of anal canal.
Purpose: The blood that comes from alimentary canal to liver contains substances absorbed from small intestine. These substances pass through liver before going to heart — liver removes harmful substances from blood and absorbs nutrients for storage before sending this blood to heart.
Right and left renal arteries carry oxygenated blood to the right and left kidneys. Inside the kidney, each renal artery divides repeatedly to make smaller arteries which branch into several afferent arterioles, and supply blood to nephrons. Each afferent arteriole divides to make the capillaries of glomeruli.
The capillaries of glomeruli unite to make efferent arteriole, which divides to make two sets of capillaries i.e., (i) peri-tubular capillaries (around nephron tubule in cortical portion of kidney), and (ii) vasa recta (around nephron tubule in the medulla of kidney). These capillaries unite to form venules that converge and make smaller veins, which unite to form a renal vein.
Blood pressure is the measure of force exerted by blood against the inner walls of blood vessels. This force keeps blood flowing from heart to the entire capillary network in body. Blood pressure is highest in aorta and then gradually reduces in systemic arteries. The walls of arteries are elastic. The flow of blood creates rhythmical throbbing of arteries, called pulse, which can be felt in wrist (radial pulse), thighs (femoral pulse), side of neck (carotid pulse).
Systolic pressure: The maximum pressure during ventricular systole or ventricular contraction. When ventricles contract, heart forces blood into pulmonary arteries and aorta, so pressure rises sharply. Systolic pressure in a normal young adult is 120 mm Hg.
Diastolic pressure: The lowest pressure that remains in arteries before the next ventricular contraction, or the blood pressure when the heart muscles are relaxed. Diastolic pressure in a normal young adult is 80 mm Hg.
Expression: Conventionally, blood pressure readings are expressed as 120/80. The instrument sphygmomanometer is used for the manual measurement of systolic and diastolic blood pressures.
Location: Pressure receptors, known as baroreceptors, are present in carotid arteries (arteries that supply blood to the head region and brain) and aortic arch. When blood pressure falls, baroreceptors activate sensory neurons that send information to brain. The control centre in brain reacts by increasing the rate and force of contraction of heart, and by causing vasoconstriction in arterioles. Both these changes restore blood pressure to normal.
The long-term regulation of blood pressure is done through hormones. Certain hormones regulate the volume of blood by affecting the reabsorption of water and salts in kidneys. When there is a decrease in blood volume and blood pressure, special receptors present in brain create thirst. They also stimulate posterior pituitary gland to secrete antidiuretic hormone (ADH). ADH stimulates kidneys to retain more water in blood, excreting less in urine. It restores the blood volume and ultimately blood pressure. ADH also constricts arterioles, which raises arterial blood pressure.
Role of heart: The walls of right atrium contain endocrine cells that secrete atrial natriuretic hormone (ANH). When there is stretching of the atrium by an increased blood volume, the right atrium secretes ANH. It speeds up the excretion of salts and water through urine, which lowers the blood volume and pressure.
1. Atherosclerosis means deposition of materials within arteries.
Causes: Various materials may accumulate in arteries e.g., fatty materials, abnormal amounts of smooth muscle cells, cholesterol, fibrin and cellular debris of various kinds. The accumulation of cholesterol is thought to be the prime contributor.
Effects: Can lead to heart attacks, because it causes the narrowing of arteries and increases the risk of thrombus formation.
2. Arteriosclerosis means hardening of arterial walls.
Causes: Occurs when calcium is deposited in arterial walls. The blood flow through these arteries is restricted and arteries cannot expand normally.
Effects: Forces the heart to work harder. Severe atherosclerosis usually leads to arteriosclerosis.
3. Thrombosis is the formation of thrombus — a solid mass or plug of blood constituents (clot) in a blood vessel. This mass may block (wholly or only in part) the vessel.
Causes: (i) irritation or infection of lining of blood vessels, (ii) reduced rate of blood flow due to long periods of inactivity, or (iii) pneumonia, tuberculosis, emphysema etc.
Effects: Blocks the blood flow to organs. A thrombosis in coronary arteries causes heart attack. A thrombus in the vessels of brain causes stroke. A thrombus may be dislodged and carried to some other location — called embolus (thromboembolism).
Various diagnostic tests are performed on cardiovascular patients to locate the exact problem and measure the severity of disease. The important tests are ECG and angiography.
Angiography: Coronary angiography is an X-ray examination of blood vessels or chambers of heart. A physician guides a small tube-like device called catheter through the large arteries of body. When the tip of catheter reaches the opening of coronary arteries, a special fluid (contrast medium or dye) is injected in catheter. This fluid is visible in X-ray machine. Pictures (angiograms) of fluid in coronary artery are obtained. If clots are present in the lumen of a coronary artery, the artery appears narrow. By changing the diagnostic catheter to a guiding catheter, physicians can also pass an instrument into coronary artery through the catheter — most commonly guide wires and balloon dilation catheters.
Angina pectoris: Blockage of coronary arteries is usually due to gradual build-up of lipids (especially cholesterol) in the inner wall of coronary artery. If such conditions persist, chest pain called angina pectoris can result during periods of stress or physical exertion. Angina indicates that oxygen demands are greater than its delivery and a heart attack may occur in future.
Myocardial infarction: Coronary arteries supply oxygen and nutrients to cardiac muscles. If blood flow is blocked in coronary arteries, it results in insufficient supply of blood to one or more parts of cardiac muscle. If heart muscles die due to insufficient supply of oxygen and nutrients, the condition is known as myocardial infarction (heart attack).
Open heart surgery: The open or beating-heart surgery is done when heart is still beating, which includes bypass surgery, valvular replacement, heart transplants etc.
1. Coronary Bypass: One of the most common and effective procedures to compensate the blockage of blood to cardiac muscles. In this surgery, surgeon takes a healthy blood vessel from leg, arm, chest or abdomen of the patient. He attaches the ends of blood vessel above and below the blocked coronary artery. So, blood is bypassed around the damaged or blocked area. Coronary bypass surgery doesn't cure the underlying disease process (atherosclerosis/coronary artery disease) — lifestyle changes especially smoking cessation are crucial to reduce the chance of future blockages.
2. Angioplasty: An invasive procedure that opens a blocked or narrowed artery. A small wire called catheter, under X-ray guidance, is passed through the narrowed coronary artery. A small sausage-shaped balloon is advanced over the wire into the narrowed section, then inflated to dilate the artery.
3. Stenting: May also be done during angioplasty. A stent is an expandable stainless steel mesh tube, mounted on a balloon catheter. When positioned within the narrowed artery, the balloon is inflated, expanding the stent and artery. The balloon is removed and the stent remains in place, supporting the artery walls and keeping it open and dilated.
(A) Hypertension: A medical condition in which a person suffers from a chronic consistent elevated level of blood pressure above 140/90 mmHg.
Causes: Any abnormality in nervous or hormonal mechanisms of blood pressure regulation. Other causes include stress, obesity, high salt intake, and smoking. There may also be hereditary reasons.
1. Brain haemorrhage: Whenever blood pressure is chronically elevated, there is an increased chance of rupture of blood vessels. When this occurs in brain, it damages the delicate structure of brain.
2. Congestive heart failure: Hypertension weakens cardiac muscles. If prolonged, heart is unable to pump effectively and blood may be retained in heart and lungs.
3. Damage to kidneys: Hypertension can damage the nephrons of kidneys, leading to further retention of salts and water, accelerating hypertension.
(B) Hypotension: Low blood pressure — a condition in which the force of blood against arterial walls falls below 90/60mmHg.
Causes: Medical conditions (bradycardia, heart failure, valve disease), endocrine disorders (hypothyroidism, hypoglycaemia), blood loss (trauma, surgery, internal bleeding), dehydration (vomiting, diarrhoea, insufficient fluid intake).
In humans, in addition to the blood circulatory system, there is another system responsible for the transport of materials — the lymphatic system. It also returns materials from tissues to blood.
Components: Lymph vessels, lymphoid masses, lymph nodes and lymph (the fluid which flows in the system).
Lymphatic system begins with small vessels called lymph capillaries which have blind endings in extracellular fluid (interstitial fluid). Pressure of the interstitial fluid forces it to enter lymph capillaries — once inside, it is called lymph. Lymph capillaries join to form larger lymphatic vessels. Lymph vessels join to form larger lymph ducts. There are two main lymph ducts i.e., right lymphatic duct and thoracic duct. These vessels open into right and left subclavian veins. The flow of lymph is always from body tissues towards thoracic duct.
Maintenance of lymph flow: Maintained by the activity of skeletal muscles, movement of viscera and breathing movements. Valves present in lymph vessels prevent the back flow of lymph.
At certain spots, the lymph vessels have masses of connective tissue where lymphocytes are present — these are lymph nodes. Several afferent lymph vessels enter a lymph node and the lymph is drained by a single efferent lymph vessel. Lymph nodes are present in the neck region, axilla and groin areas of man.
Lymphoid masses: In addition to lymph nodes, several lymphoid masses are present in different areas e.g., in the mucosa and submucosa of alimentary canal. The larger lymphoid masses are spleen, thymus, tonsils and adenoids. These produce lymphocytes (a type of WBCs).
• Returns the excess fluid and dissolved proteins and other substances to blood. In an average person, about three litres more fluid leaves blood capillaries daily, but it is absorbed by lymphatic capillaries and returned to bloodstream before the blood enters heart.
• Helps defend body against foreign invaders — lymph nodes filter lymph and have lymphocytes and macrophages that destroy bacteria and viruses.
• Spleen filters blood through its macrophages and lymphocytes that destroy foreign particles and aged RBCs; spleen also stores RBCs.
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