Human Circulatory System

Heart has four chambers, tow relatively small upper chambers called atria and two larger lower chambers called ventricles.

A thin, muscular wall called the interatrial septum separates the right and the left atria, whereas a thick-walled, the inter-ventricular septum, separates the left and the right ventricles

The atrium and the ventricle of the same side are also separated by a thick fibrous tissue called the atrio-ventricular septum. However, each of these septa are provided with an opening through which the two chambers of the same side· are connected. The opening between the right atrium and the right ventricle is guarded by a valve formed of three muscular flaps or cusps, the tricuspid valve, whereas a bicuspid or mitral valve guards the opening between the left atrium and the left ventricle.

The openings of the right and the left ventricles into the pulmonary artery and the aorta respectively are provided with the semilunar valves.

The valves in the heart allows the flow of blood only in one direction, i.e., from the atria to the ventricles and from the ventricles to the pulmonary artery or aorta. These valves prevent any backward flow.

The walls of ventricles are much thicker than that of the atria.

A specialised cardiac musculature called the nodal tissue is also distributed in the heart. A patch of this tissue is present in the right upper corner of the right atrium called the sino-atrial node (SAN). Another mass of this tissue is seen in the lower left corner of the right atrium close to the ventricular septum called the atrio-ventricular (AVN). A bundle of nodal fibres, atrioventricular bundle (AV bundle) continues from the AVN which passes through the atrio-ventricular septa to emerge on the top of the interventricular septum and immediately divides into a right an bundle.

These branches give rise to minute fibres throughout the ventricular musculature, respective sides and are called purkinje fibres. These fibres along with right and left bundles are known as bundle of HIS. 

The nodal musculature has the ability to generate action potentials without any external stimuli, i.e., it is autoexcitable. However, the number of action potentials that could be generated in a minute vary at different parts of the nodal system. The SAN can generate the maximum number of action potentials, i.e., 70-75 min^-1, and is responsible for initiating and maintaining the rhythmic contractile activity of the heart. Therefore, it is called the pacemaker.

Our heart normally beats 70-75 times in a minute (average 72 beats min^-1).

Less number of heart beat than normal is called bradycardia.

More rate of heart beat than normal is called tachycardia.

Rate of heart beat increases:

(i) After taking food

(ii) Exercise

(iii Decrease blood pH

(iv) Increase acidity and CO₂ concentration.

(v) Increase in temperature

(vi) Tension shock

(vii) In high /B.P. 

Regulation of heart beat: The centre controlling the heart rate (cardiac centre) is present in medulla oblongata of brain and possess chemoreceptors sensitive for CO₂, O₂ and also for blood pressure. This centre is under the influence of hypothalamus which is the controller of autonomic activities.

(1) Nervous control: Brain receives two sets of nerve fibres: Sympathetic and para sympathetic or vagal.When there is increase in blood CO₂, the sympathetic nerve fibres stimulate S.A. node by producing sympathin (adrenaline + noradrenaline). This compound induces impulse generation by inducing entry of Ca²⁺ into cardiac muscles. So, heart beat and force of contraction increase (Tachycardia). After action, sympathin is destroyed by sympathenase, COMT (catechol orthomethyl transferase) and MAO (Mono Amino Oxidase).

When there is increase in blood O₂, the parasympathanc or vagal (10th cranial) nerve inhibits S.A. node by producing acetylcholine. This compound increases contraction time and hence, heart beat is decreased (Bradycardia). After action, acetyl choline is destroyed by enzyme acetyl choline esterase (AchE). This chemical regulation of heart beat on behalf of nerves was discovered by Otto Loewi.

Vagus escape: Stimulation of vagus nerve decreases the heart rate but its continuous stimulation shows no further decrease. This phenomenon is called vagus escape.

(2) Hormonal control: Hormones from adrenal medulla adrenaline and nor adrenaline accelerate the heart beat, the latter under normal conditions and the former at the time of emergency. Thyroxine hormone also increases the heart beat by increasing energy production.

Pounding: Very fast heart beat during some conditions like anger and love.

On the basis of origin of heart beat two types of heart

(i) Neurogenic heart: In this heart-beat is initiated by a nerve impulse coming from a nerve ganglion situated near the heart. So in this wave of contraction is generated outside the heart in the ganglion. If nerve supply is cut off then heart beat stops.

Eg. Invertebrates (some annelids, most arthorpods) (H) Myogenic heart: In this heart, beat is originated by a group of muscle fibres which is situated in the wall of the heart. So in this wave of contraction is generated inside the heart. Eg. Vertebrates, Mollusca Phylum)

Blood vessels: The study of blood vessels is called Angiology. The blood vessels are of following types:

(1) Arteries: Thick walled, carrying oxygenated blood (deoxygenated in pulmonary artery) from heart to various parts of body. These blood vessels are grouped as Aorta which branches to form arteries which further divides into thinner branches called arterioles inside the organ. Average diameter of arteriole is 12mm. the arterioles further divide into smaller vessels called meta-arterioles (70 mm) which divide into capillaries. At the beginning of capillary, the "arterioles possess circular muscles called precapillary sphincter which regulates flow of blood into the capillaries which is called vasomotion. Smooth muscles of arteries innervated by sympathetic fibers, their stimulation control vasoconstriction and vasodilation. Smooth muscles of arteries and arterioles also limit bleeding from wounds by producing vascular spasm during cut. Arteries two types.

(i) Conducting or elastic arteries

(ii) Distributing or muscular arteries.

Elastic or conducting arteries receive blood from heart and do not provide it to any organ rather they provide blood to other arteries and are pressure reservoirs of blood.

Muscleless end of meta-arteriole is called thoroughfare channel or preferential channel.

The largest artery is dorsal / abdominal aorta (systemic aorta).

(2) Capillaries: It is the- smallest blood vessels, discovered by Marcello Malpighi (also layered nucleated squamous epithelial cells called endothelium resting on a basement membrane. Diameter of capillary is about 8. These are also called as exchange vessels as they are the site of exchange of material between blood and tissue because of least barrier in them. The capillaries can be grouped into two categories:

(i) Arteriolar capillary: which supplies nutrition, respiratory gases etc. to the body cells.

(ii) Veinular capillaries: which collect the metabolic wastes from the body cells.

Capillaries possess about 7% of total body blood and are present near almost all cells of body in the intercellular spaces. The tissues which are devoid of intercellular spaces are also devoid of capillary.

They are called avascular tissues. 

Capillaries are surrounded by cells of connective tissue called pericapillary cells. Some of these cells are contractile and phagocytic in nature and are called Rouget cells or pericytes.

Continuous capillaries are without fenestral aperture, hence are less permeable. These are present in organs such as lungs, muscles, connective tissues and brain tissues.

Fenestrated capillaries possess apertures/fenestra and are found in those organs where there is maximum need of permeability such as endocrine glands, intestinal 'villi, cavities of brain, kidney, ciliary body of eye.

Sinusoids are irregularly dilated capillaries found in organs where there is decrease in flow rate such as liver, spleen, bone marrow, parathyroid, pituitary gland. In liver, sinusoids are branches of venules and open into venules while in other organs, they originate from arteriole and unite to form venules.

(3) Veins: These are thin walled, carrying deoxygenated blood (oxygenated in pulmonary vein) from tissues to the heart. Venules, smallest branches, unite to form veins which in turn unite to form vena cava. The largest vein is inferior vena cava/post caval. Varicose veins is stout, blood filled painful veins specially of the limbs due to defective watch pocket valves.
 

Cardiac Cycle

The action potential causes the atria and then the ventricles to undergo contraction (systole) followed by their relaxation (diastole). The systole forces the blood to move from the atria to the ventricles and to the pulmonary artery and the aorta. The cardiac cycle is formed by sequential events in the heart which is cyclically repeated and is called the cardiac cycle.

About 70 mL of blood is pumped out by each ventricle during a cardiac cycle and it is called the stroke or beat volume. Volume of blood pumped out by each ventricle of heart per minute is called the cardiac output and it is equal to the product of stroke volume and heart rate (approx 5 litres). 

During each cardiac cycle two prominent sounds are produced which can be easily heard through a stethoscope. The first heart sound (lub) is associated with the closure of the tricuspid and bicuspid valves whereas the second heart sound (dub) is associated with the closure of the semilunar valves.

Blood pressure: The pressure exerted by the blood on the wall of the blood vessels in which it is present is called blood pressure. It is usually measured in brachial artery by an instrument called sphygmomanometer (invented by Riva-Rocci). Arterial blood pressure is of 2 types:

(1) Systolic blood pressure: It is the pressure exerted by blood on the walls of the blood, vessels due to the systole of ventricles and is equal to 120 mm Hg. During ventricular systole, there is expansion in the artery due to the uncoiling of elastic layer. Hence, the pressure is maximum in arteries but gradually decreases in capillaries and veins.

(2) Diastolic blood pressure: It is the pressure .exerted on walls of blood vessels when the ventricles are relaxed. During ventricular diastole, the uncoiled elastic layer recoils /leading to normalization of artery. Hence, blood pressure drops down to 80 mm Hg. Thus, blood pressure in normal person is systolic/ diastolic pressure i.e. 120/ 80 mmHg.

(3) Pulse pressure: The difference between systolic and diastolic pressures is called pulse pressure and its normal value is 120 - 80 mm Hg = 40 mm Hg. It provides information about the condition of arteries.

(4) Mean arterial pressure: It is the average pressure of systolic and diastolic pressures. As the blood remains in the systolic phase for shorter period and in the diastolic phase for longer period, the mean pressure of blood lies near the diastolic pressure. The normal ratio of systolic pressure to diastolic pressure to pulse pressure is about 3: 2:1. 

Electrocardiograph (ECG)

ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle.

Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart.

The P-wave represents the electrical excitation (or depolarisation) of the atria, which leads to the contraction of both the atria. The QRS complex represents the depolarisation of the ventricles, which initiates the ventricular contraction. The contraction starts shortly after Q and marks the beginning of the systole. The T-wave represents the return of the ventricles from excited to normal state (repolarisation). The end of the, T-wave marks the end of systole.

Obviously, by counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual. Since the ECGs obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease. 
 

Double Circulation

In the fishes only deoxygenated blood flow in the heart. This type of heart is called "Venous heart". In the fishes only two chambers (one auricle and one ventricle) is present in the heart. In the fishes, 'single circulation is present. 

In the aves, mammals, double blood circulation is found. Right portion of heart collects impure blood from body and sends it into the lungs. So in the right portion impure blood is present and this circulation takes place between heart and lungs. This is called pulmonary circulation.

The left portion of heart takes pure blood from lungs and distributes it to whole body. So in left portion pure blood is present and this circulation takes place between heart and body. This called systemic circulation. So in the mammals blood flow in the heart twice in a time, so it is double blood circulation.

The blood pumped by the right ventricle enters the pulmonary artery, whereas the left ventricle pumps blood into the aorta.

The deoxygenated blood pumped into the pulmonary artery is 'passed on to the lungs from where the oxygenated blood is carried by the pulmonary veins into the left atrium. This pathway constitutes the pulmonary circulation.

The oxygenated blood entering the aorta is carried by a network 'of arteries, arterioles and capillaries to the tissues from where the deoxygenated blood is collected by a system of venules, veins and vena cava and emptied into the right atrium. This is the systemic circulation. The systemic circulation provides nutrients, O₂ and other essential substances to the tissues and takes CO₂ and other harmful substances away for elimination.

A unique vascular connection exists between the digestive tract and liver called hepatic portal system.

The hepatic portal vein carries blood from intestine to the liver before it is delivered to the systemic circulation.

A special coronary system of blood vessels is present in our body exclusively for the circulation of blood to and from the cardiac musculature.

Arterial system: It involves aorta, arteries, arterioles and meta-arterioles. It supplies oxygenated blood to all parts of the body except lungs.

The left ventricle of the heart pumps the oxygenated blood into a single, question marked shaped, long vessel called left carotid-systemic aorta. It is the largest blood vessel of the body.

After ascending from the heart, the systemic aotra turns and descends down to the level of lower border of fourth lumbar vertebra. At its distal extremity, it bifurcates into right and left common iliac arteries. The sytemic aorta has following parts-

(1) Ascending aorta: It gives off left and right coronary arteries.

(2) Descending aorta: The aorta turns towards the back of heart and finally converts into dorsal aorta. The descending dorsal aorta is called thoracic aorta in throcic region and abdominal aorta in abdominal region. 

Venous system: It originates in tissues by union of capillaries and ends in the atrium of heart. It includes two major veins - superior and inferior vena cava which drain the deoxygenated blood into the right atrium.

(1) Superior vena cava (pre caval): Single, formed by the union of right and left brachiocephalic (innominate) veins. It collects blood from head, neck, arms and chest region.

(2) Inferior vena cava: It is the largest vein, originated in inferior lumbar region by the union of right and left common iliac veins and opens into right atrium by separate opening. It collects blood from all body structures below the diaphragm.

Portal System

In this system the vein starts from capillaries and ends in capillaries.

When venous blood is collected from some pad of body it is redistributed by capillaries in some other organ instead of being returned directly to the heart. This is called as portal system.

Types of Portal System

(i) Renal Portal System:

In this system vein which collect blood from posterior parts of body and legs, enter into the kidney. This vein is called renal portal vein. Now this vein divides into capillaries and form renal portal system. This system is found in lower vertebrates like Amphibians, fishes.

This system is absent in human and rabbit.

(ii) Heptaic Portal System:

It is found in all vertebrates. In this system, veins which collect blood from digestive and absorptive parts of alimentary canal, enter into the liver. 

(iii)  Hypothalamo hypophyseal portal system: This system occurs in mammalian brain. This is present between hypothalamus and anterior lobe of the pituitary gland. Hypophyseal portal vein carries blood from the hypothalamus of the brain to the anterior lobe of the pituitary gland. This portal system enables the hormones of hypothalamus to reach the anterior lobe of the pituitary gland.

is called hepatic portal vein. Now in liver it divides into capillaries and form hepatic portal system. This vein is made up of four veins

(a) Lienogastric vein: collects blood from stomach, spleen

(b) Duodenal vein: Carries blood from pancrease, duodenum

(c) Anterior Mesentric vein: From ileum, caecum and colon

(d) Posterior Mesentric vein: Rectum and anus.
 

Disorder

High Blood Pressure (Hypertension): Hypertension is the term for blood pressure that is higher than normal (120/80). In this measurement 120mm Hg (millimetres of mercury pressure) is the systolic, or pumping, pressure and 80 mm Hg is the diastolic, or resting pressure. If repeated checks of blood pressure of an individual is 140/90 (140 over 90) or higher, it shows hypertension.

Coronary Artery Disease (CAD): Coronary Artery Disease, often referred to as atherosclerosis, affects the vessels that supply blood to the heart muscle. It is caused by deposits of calcium, fat cholesterol and fibrous tissues, which makes the lumen of arteries narrower.

Angina: It is also called 'angina pectoris'. A symptom of acute chest pain appears when no enough oxygen is reaching the heart muscle. Angina can occur in men and women of any age but it is more common among the middle-aged and elderly. It occurs due to conditions that affect the blood flow.

Heart Failure: Heart failure means the state of heart when it is not pumping blood effectively enough to meet the needs of the body. It is sometimes called congestive heart failure because congestion of the lungs is one of the main symptoms of this disease. Heart failure is not the same as cardiac arrest (when the heart stops beating) or a heart attack (when the heart muscle is suddenly damaged by an inadequate blood supply).

Note:

Normal activities of the heart are regulated intrinsically, i.e., auto regulated by specialised muscles (nodal tissue), hence the heart is called myogenic.

A special neural centre in the medulla oblangata can moderate the cardiac function through autonomic nervous system (ANS). Neural signals through the sympathetic nerves (part of ANS) can increase the rate of heart beat, the strength of ventricular contraction and thereby the cardiac output.

Parasympathetic neural signals (another component of ANS) decrease the rate of heart beat, speed of conduction of action potential and thereby the cardiac output.

Adrenal medullary hormones can increase the cardiac output.

Ischaemia: Inadequate flow of blood to a part of the heart caused by obstruction to its blood supply.

Angina Pectoris: Heart pain of short duration usually located in the front of chest.

Blue baby: A Blue baby is the name given to abnormal human baby who has a hole in the ventricular septum so that pure and impure blood mix.

Dextrocardia: Human heart get displaced right side of the chest.

World's first heart transplant:

World's first heart transplant was done by a team of doctors headed by Prof. Christian Bernard-on 3rd Dec. 1967.

India's first heart transplant: 

India's first heart transplant was done by a team of doctors led by Dr. P. Venugopal on 3rd August, 1994.

LDL (Low density lipoprotein): It is harmful for our body, its high concentration in blood causes heart disease and thus termed as bad lipoprotein.

HDL (High density lipoprotein): It's higher concentration reduces the risk of heart disease thus termed as good lipoprotein.

Blood donation day - 1st October.

The action potential generated at S.A. node travels throughout the auricles at a velocity of 0.3 m/sec.

• Differences between lymph and blood

Differences between Open and Closed Circulatory System

Differences between Arteries and Veins

Superior vena cava or precaval: Brings deoxygenated blood from head and upper parts of the body into the right auricle through an opening which is Single in human and cat and two in rabbit as there are 2 precavals - right and left in rabbit.

Inferior vena cava or post caval: Drains deoxygenated blood from middle and lower parts of the body into the right auricle through a single opening which is bordered by a membranous, falciform fold which is a remnant of the foetal valve of Eustachian.

Coronary sinus: Returns deoxygenated blood from heart wall into right auricle through a single opening.

Pulmonary vein: Four pulmonary veins, two from each lung, carry oxygenated blood from the lungs and open into the left auricle through four openings. In' rabbit, the pulmonary veins open in the left auricle through 2 openings.

Pulmonary aorta/arch: Arises from upper left corner of right ventricle through a Single opening and divides into right and left pulmonary arteries which carry deoxygenated blood to the lungs for oxygenation.

Systemic aorta: Arises from upper right corner of left ventricle through a single opening and has 3 regions - ascending· aorta, arch of aorta and descending aorta. It distributes oxygenated blood to various body parts except lungs.

PDA (Patent Ductus Arteriosus): If the ligamentum arteriosus remains open, the condition is called PDA. In this case, there is mixing of blood which leads to blue baby.

Eustachian valve: Present on the opening of inferior vena cava (post caval) in the right auricle in rabbit, whereas in human, the vestige of leustachian valve is present over the opening of, post caval vein. It allows the passage of blood. in right auricle.

Haversian valve: Present in human but absent in rabbit. It is present over the opening of precaval vein and allows the passage of blood in right auricle.

Thebesian or coronary valve: Present over the opening of coronary sinus in right auricle in mammals and allows the passage of blood in right auricle.

Right A.V. valve or Tricuspid valve: Present between right auricle and right ventricle. It consists of 3 membranous flaps or cusps.

Left A.V. valve or Bicuspid or Mitral valve: Present between left auricle and left ventricle. It consists of 2 flaps or cusps. The bicuspid valve resembles-mitre or topi of bishop, hence, also called as Mitral valve.

Semilunar valves: At the base of pulmonary arch and systemic aorta, three membranous, pocket­ shaped flaps called semilunar valves are present. They allow the passage of blood from ventricles to respective blood vessels but prevent the return of blood.

Increase in Na⁺ ions in blood or in cardiac muscles, decrease heart rate.

Increase in Ca²⁺ ions in blood increase heart beat but if they are injected in cardiac muscles, heart stops in contracted phase which is called systolic arrest.

Injection of K⁺ ions in heart muscles stop impulse generation. So, heart stops in diastolic or Relax phase. 

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