Circulatory System

This system is concerned with the circulation of body fluids to distribute various substances to various body parts.               

Functions of Circulatory System

(1) Transport of various substances such as nutrients, waste products, respiratory gases, metabolic intermediates (Such as lactic acid from muscle to liver), vitamins, hormones etc.

(2) Regulation of body pH by means of buffer, body temperature homeostasis, water balance etc.

(3) Prevention of disease by means of antibodies and antitoxins.

(4) Support or turgidity to certain organs like penis and nipples.

Types of Circulation

Circulatory system in various groups of animals can be classified as follows :

(1) Intracellular circulation : Occurs inside the individual cells where the distribution of substances is through cyclosis of cell cytoplasm. Example – Protozoans.

(2) Extracellular circulation : When the distribution of the substances occurs inside the body through extracellular or intracellular fluids. This is of following types –

(i) Extra organismic circulation : Canal system in porifera, water vascular system in Echinoderms and gastrovascular system in coelenterates.

(ii) Intra-organismic circulation : It involves circulation of body fluids. It is of following types –

(a) Parenchymal circulation : In platyhelminthes, the fluid filled spaces present in the mesodermal parenchyma tissue between body wall and internal organs are used in the distribution of substances.

(b) Coelomic circulation : Coelomic fluid is concerned with the transport of substances. Example – pseudocoelomic fluid in the roundworms and haemolymph in Arthropods.

               (c) Blood vascular system : It contains blood and a pumping structure (heart) for circulation of materials inside the body. It is open circulatory system and closed circulatory system.

Blood circulation in vertebrates

Blood circulation was discovered by William harvey. In case of vertebrates, blood circulation is of closed type, which can be grouped into two categories :

(1) Single circulation

(2) Double circulation

Double circulation in mammals can be divided into three parts :

(i) Cardiac circulation : The amount of blood present in the heart. Its value is 8%.

(ii) Pulmonary or lesser circulation : The amount of blood present in the surrounding of lungs and pulmonary blood vessels. Its value is 12%.

(iii) Systemic or greater circulation : The amount of blood which circulates in the rest part of the body. Its value is 80%. It can be divided into three parts –

Arterial circulation – 15%

Capillary circulation – 5%

Venous circulation – 60%

Heart

The form, structure and function of heart exhibits much variation. The characteristics of heart of fishes, amphibians, reptiles, birds and mammals is presented in the following table.

Shape and position : Reddish, roughly conical, highly muscular, mesodermal hollow organ of the size of one’s fist. Its average weight in males is about 300 gm. and in females about 250 gm. It lies behind the sternum in the mediastinum space of thoracic cavity in between the two lungs. The broader base faces upward and backward. The narrower apex is directed downward, forward and slightly towards left, lying between 5^th and 6^th ribs and rests on the diaphragm. The heart is about 12 cm (5 inch) long, 9 cm (3.5 inch) wide and 6 cm (2.2 inch) thick.

Protective covering : Heart is enclosed in a tough, 2 layered fibroserous sac, the pericardium. The outer layer is non-distensible fibrous pericardium and inner layer is thin serous pericardium which further consists of outer parietal layer (attached to fibrous pericardium) and inner visceral layer (adhered to the heart).

Between the parietal and visceral layers, occurs a narrow potential space, the pericardial cavity which is derived from coelom and is filled with serous pericardial fluid for frictionless movement and protection from shock and mechanical injury.

Histology : The heart wall consists of connective tissue, blood vessels and cardiac muscle fibres in 3 different layers ­– Epicardium, Myocardium and Endocardium.

(1) Epicardium : The outermost epicardium, also called visceral layer of the serous pericardium, is the thin, transparent outer layer of the wall. It is composed of mesothelium and connective tissue. Visceral pericardium, joined to myocardium by connective tissue.

 (2) Myocardium : Middle, highly vascular layer, composed of cardiac muscle fibres joined together by intercalated disc. The connective tissue in myocardium acts as cardiac skeleton. Myocardium is thickest where the endocardium is thinnest

(3) Endocardium : Innermost layer lining the cavity of heart and consisting of endothelium of squamous cells resting on thin basement membrane of loose connective tissue.

External structure : Human heart is 4-chambered and is divided by septa into two halves – right and left. Each half has one darker, thin walled auricle in the broader upper region and one lighter, thick-walled ventricle in the narrower lower region.

Sinus venosus and conus/truncus/bulbus arteriosus are accessory chambers in the heart of lower vertebrates (fishes and amphibians). In rabbit, sinus venosus is formed in the embryo but later it becomes a part of wall of right auricle.

               In frog, sinus venosus spreads upon most of the dorsal side of heart and conus arteriosus lies obliquely upon the ventral surface of right atrium.

Internal structure 

       (1) Auricles : Atria are thin walled. They act as reservoirs for blood entering the heart. Right auricle is bigger than left auricle and both are separated by a myomembranous partition called Interatrial or interauricular septum. During embryonic stage, at the place of this septum, there are present septum primum and septum secondum having a gap (aperture) called foramen ovale between them. From the opening of inferior vena cava upto foramen ovale, there is a flap called Eustachian flap which prevents the blood in the foetal heart go to lungs because in foetal life, lungs are not functional purification of blood is done by placenta.

      ਊt the time of birth, there is closure of foramen ovale but there remains depression on posterior part of the right surface of interauricular septum in rabbit. In man this depression is present on both the side. because of least regenerative power in human being. The depression towards right atrium is called fossa ovalis and depression towards left atrium is called fossa lunata.

       The inner surface of auricles is smooth. A network of muscular ridges called musculi pectinati or trabeculi pectinati occurs internally in the region of the auricular appendages and give comb like appearance.

$\bullet$ PFO (Patent Foramen Ovale) or septal defect : In case there is no closure of foramen ovale, then disease is called PFO. In this condition, there is mixing of blood after birth which gives bluish appearance to the body called as Cyanosis. Such child is called Blue Baby.

(2) Ventricles : The right and left ventricles are demarcated by an interventricular septum which is obliquely curved towards right, so that the left ventricle is larger than right one. However, the cavity of left ventricle is relatively smaller and nearly circular because the myocardium of left ventricle is 3 times thicker than right ventricle whose cavity is larger and somewhat crescentic.

       The walls of the ventricles are internally raised into a number of thick, muscular, column shaped projections called columnae carnae or trabecular carnae and a few large muscular elevations called papillary muscles or musculli papillares which are 3 in right ventricle and 2 in left ventricle. These muscles act as anchors for chordae tendinae.

       Numerous, strong, inelastic thread like tendons present in the mammalian heart but absent in frog.

$\bullet$ Regurgitation : If there is weakening of papillary muscles or breaking of chordae tendinae, then AV valves revert into auricles. So, blood goes in opposite direction, it is called regurgitation. Sometimes, there is narrowing of valves. So, there remains gap between the valves which causes regurgitation.

$\bullet$ Moderator band : Right ventricle contains a prominent muscular trabeculum called moderator band which extends from the interventricular septum to anterior papillary muscle.

Major blood vessels associated with heart : The blood vessels that enter or leave the heart are called Great Blood Vessels.

(1) 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.

(2) 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.

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

(4) 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.

(5) 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.

(6) 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.

$\bullet$ Ligamentum arteriosus : During foetal life, because the lungs are non-functional hence blood of pulmonary aorta comes into systemic aorta through a small duct called ductus botalli or ductus arteriosus soon after birth, deposition of elastin fibre blocks this duct, forming a new structure called ligamentum botalli or ligamentum arteriosus.

$\bullet$ 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.

Valves : The valves present in the mammalian heart are tendinous cords.

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

(2) 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.

(3) 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.

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

(5) 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.

       (6) 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.

Nodal tissue : The nodal tissue consists of the following –

(1) Sinu-auricular or S.A. node : Also called as pacemaker, node of keith and flack, heart of heart, brain of heart, pulsation centre. It is located in the right wall of right atrium below the opening of superior vena cava. This is the place where sinus venosus is incorporated in the wall of right atrium in the embryo. S.A. node is the main tissue of heart and has highest degree of autorhythmicity (generates beating impulse at the rate of 70-80 times/minute) but least conductivity. The rhythmic impulses produced are called as Sinus rhythmia. In frog S.A. node is present in sinus venosus.

(2) Atrio-ventricular node or A.V. node : Also called reserve pacemaker, node of Twara and Aschoff. Discovered by Lewis Kent. It lies in the right atrium near the junction of interauricular and interventricular septum close to the opening of coronary sinus. It is concerned with the conduction of cardiac impulses generated by S.A. node, but it can also generate the impulse at the rate of 40-60/minute. These impulses produced are rhythmic and called nodal rhythmia. In frog, A.V. node is absent.

(3) Bundle of His or A.V. bundle : Discovered by His. It arises from A.V. node, descends in the interventricular septum and bifurcates into two branches innervating the wall of right and left ventricle respectively. The myocardium of atria and ventricles are discontinuous and this bundle is the only muscular connection between the two. It is concerned with the conduction of impulse from atria to the tip of ventricle but can also generate impulse at the rate of 35-40/minute. The impulses produced are non-rhythmic.

(4) Purkinje fibres : Numerous, modified muscle fibres which act as sympathetic nerve fibres. They arise from branches of bundle of His and provide impulse to myocardium of ventricles. They can also generate non-rhythmic impulse at a rate of 30-35/minute.

Working of nodal tissue : S.A. node spontaneously initiates a wave of contraction which is conducted along the tracts of special muscle fibres called internal pathways over both the auricles at a rate of 1m/sec. The impulse generated travels first in the right atrium than in left atrium. So, right atrium contracts first but the contraction ends simultaneously in both atria. As the musculatures of atria and ventricles are discontinuous and are separated by a septum of fibrous connective tissue, called annular pad in mammals, the wave of contraction is received by A.V. node from myocardium of atria and is provided to bundle of His. The impulses reach the A.V. node about 0.03 seconds after their origin from S.A. node. The A.V. node generates a fresh wave of contraction which passes over both the ventricles along the bundle of His and its ramifications at the rate of 1.5 to 4 m/sec. The Purkinje fibres bring about the contraction of ventricles from the apex of heart which passes quickly towards the origin of pulmonary and systemic arches forcing blood into them.

S.A. node not only acts as pacemaker but also establishes the basic rhythm at which the heart beats. In case of degeneration of S.A. node, A.V. node can generate impulse but it will lead to abnormal beating (arrhythmia). The failure of atrial impulse to pass into ventricles for a few seconds to few hours is called ventricular escape or stokes-adams syndrome leading to delayed pick up of heart beat. In such conditions, artificial pacemaker (Lithium Battery) is placed underneath the patient’s chest.

$\bullet$ Ectopic pacemaker : If any cardiac muscle other than the conducting tissue (nodes) generates impulse, then extra beats are heard. Such muscles are called Ectopic pacemaker.

In mammals, conducting system of the heart has S.A. node, A.V node and complicated system of conducting fibres. But in frog, it has only S.A. node and system of conducting fibres is simple.

Heart beat : The spontaneous and rhythmic contraction and relaxation of the heart to pump out and receive blood to and from the body is called Heart beat. Depending upon the nature of control of the heart beat, hearts are of 2 types – Neurogenic and Myogenic or autorhythmic.

Origin and conduction of heart beat : Initiation of heart beat is under special bundles of cardiac muscles called nodal tissue or autorhythmic cells. They act as pace maker so setting the rhythm for the entire heart and they form conducting system.

Heart beat rate : Heart beat/minute or number of cardiac cycles/minute. Females have higher heart rate than males.

Normal heart beat rate $\to$ Rhythmia

Abnormal heart rate $\to$ Arrhythmia

Decrease in heart rate $\to$ Bradycardia

Increase in heart rate $\to$ Tachycardia

Heart beat rate in some animals

Elephant – 29/min.

Human – 70-80/min.

Foetus (human) – 140-150 / min.

New born baby – 115-130/min.

Horse – 35-40/min.

Rat – 300-500/min.

Frog – 64/min.

Rabbit – 200/min.

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 CO_૒, 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 parasympathetic or vagal (10^th 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.

$\bullet$ 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.

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

Factors affecting heart rate

(1) Heart rate increases with increase in basal metabolic rate (BMR).

(2) Heart beat rate increases as the size of the animals body decreases.

(3) Decrease in pH also increases heart rate.

(4) Heart rate increases with increase in temperature.

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

(6) 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.

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

(8) H⁺ ions reduce force of contraction of heart.

(9) Increased inspiration, muscular exercise, low oxygen tension, injection of adrenaline, thyroxine, sympathin – all increase heart rate.

(10) Increased expiration, during sleep, injection of acetylcholine decrease heart rate.

(11) Stenosis – Narrowing of valve is called stenosis.

(12) Alkalosis – Decreases heart rate.

(13) Anoxia – (Absence of O₂ in tissue) Increases heart rates.

(14) CO₂ more amount, decreases heart rate.

(15) CO₂ moderate amount, increases heart rate.

(16) Epinephrine and nor epinephrine increase heart rates.

(17) Thyroid hormone increases heart rate.

Cardiac cycle

During the completion of one heart beat is called as cardiac cycle. Following events are repeated in a cyclic manner during each heart beat.

(1) Auricular systole : The atria contract due to wave of contraction stimulated by S.A. node contraction of auricles drives most of their blood into respective ventricles as the A.V. valves are open. There is no backflow of blood into the large veins as the contraction begins at the upper end and passes towards ventricles and moreover, the valves present at the opening of these veins close. Also, blood is already present in large veins which offers resistance to the blood that may return from the atria. At the end of a atrial systole, there starts the relaxation of auricles (auricular diastole) and contraction of ventricles (ventricular systole) simultaneously. Atrial systole takes 0.1 second while atrial diastole is of about 0.7 seconds.

(2) Ventricular systole : The ventricles begin to contract due to a wave of contraction stimulated by A.V. node. Due to ventricular systole, the pressure of blood in ventricles immediately rises above that in the auricles. With this pressure, the bicuspid and tricuspid valves close rapidly to prevent the backflow of blood. This closure of A.V. valves at the start of ventricular systole produces first heart sound called “Lubb” or Systolic sound. The semilunar valves are also close at this time. When the pressure of blood in the ventricles exceeds that in the great arteries, the semilunar valves open and blood enters into the great arteries. This marks the end of ventricular systole which takes about 0.3 seconds. Now the ventricles start relaxing (ventricular diastole which lasts for about 0.5 sec.)

(3) Joint diastole : The ventricles and auricles are in the diastolic phase simultaneously. As the ventricular diastole progresses, the pressure in the ventricles falls below that in the great arteries. So, to prevent backflow of blood from great arteries into ventricles, the semilunar valves close rapidly. This rapid closure of semilunar valves at the beginning of ventricular diastole produces second heart sound 𠇍up” or diastolic sound.

The quality of heart sounds indicates the state of the heart valves. Defective or damaged heart valves lead to the backflow of blood either from ventricles to auricles or from aortae to ventricles. Such defects are detectable as abnormal hissing sound called “Murmur”.. Defective valves may be replaced or repaired surgically. Syphilis and Rheumatic fever cause Murmur. The instrument used to magnify and record the heart sound is called Phonocardiogram.

During joint diastole, blood from great veins and coronary sinus flows into the atria and some blood also passes from auricles into the respective relaxing ventricles due to less pressure in ventricles. This phase takes only 0.4 seconds and is also called as blood receiving period of heart. Thus a cardiac cycle is completed in 0.8 seconds.

Cardiac output : Volume of blood pumped from heart (left ventricle) into the systemic aorta in one minute is called cardiac output. It is also called minute volume. It is calculated as the product of stroke volume (amount of blood pumped by left ventricle each time it contracts) and rate of heart beat.

i.e. Cardiac output = Stroke volume × Rate of heart beat

= 70 ml × 75 times/minute = 5250 ml/minute or 5.25 liters/min.

Total amount of blood in human body is about 5 litres (7% of body weight). During mild exercise, the cardiac output rises to about 11 litres. During intense exercise, cardiac out put rises to 19.5 liters/min., and heart beat may rise to130 beat/minute.ꃊrdiac output is directly proportional to the size of the organism, metabolic rate etc. but is inversely proportional to age.

(1) Fractions of cardiac output : Amount of pure blood going to an organ per minute is called as fraction of the organ.

(i) Cardiac fraction – 200 ml/min.

(ii) Hepatic fraction (maximum) – 1500 ml/min. (28% of blood as liver is the busiest organ of body and has maximum power of regeneration).

(iii) Renal fraction – 1300 ml/min (25% of blood)’

(iv) Myofraction – 600-900 ml/min.  

(v) Cephalic organs (brain) – 700-800 ml/min.

(vi) Remaining organs – Remaining blood.

(2) Cardiac index : Cardiac output per square metre of body surface area per minute. As area of normal young adult is 1.7 metre square, so, cardiac index is 3 litres/min/square metre.

(3) Cardiac reserve : Maximum amount of blood that can be pumped by left ventricle under the conditions of maximum needs. In this condition, heart beat can go upto 250 and stroke volume can go upto 100 ml per systole. Cardiac reserve is 25-30 litres which is about 5-6 times of cardiac output.

(4) End diastolic volume (EDV) : Amount of blood present in left ventricle at the end of diastole. It is the maximum volume of the cavity of left ventricle and is equal to 120-130 ml.

(5) End systolic volume (ESV) : Amount of blood present in left ventricle at the end of systole. It is the least volume of the cavity of left ventricle and is equal to 50-60 ml.

(6) Stroke volume : (70 ml) is equal to the difference between the end diastolic volume and end systolic volume.

SV = EDV – ESV

(7) Venous return : Amount of impure blood returning to righ atrium per minute is called venous return and is equal to 5.25 litres.

Electrocardiogram (ECG)

A graphic record of electrical events occuring during a cardiac cycle is called Electrocardiogram. The instrument used for recording the heart’s electrical variations is called Electrocardiograph in which the potential differences of heart muscles are recorded by a galvanometer. In ECG, there are 2 types of waves :

(1) Depolarisation waves : They represent the generation of the potential difference. These waves appear only when both electrodes of galvanometer are in different fields. When both the electrodes are in same field, there is no deflection and wave drops down to base line.

(2) Repolarisation waves : They appear when depolarisation is over and the muscle fibre is returning to its original polarity. When both electrodes are in same polarity (means 100% repolarisation and 100% depolarisation), there is no deflection.

A normal ECG has 5 deflection waves – P, Q, R, S and T. Out of them – P, R and T waves are above the base line and are called positive waves. The Q and S waves are below base line and are called negative waves. The part of the base line between any 2 deflections is called Interval.

P wave : Indicates impulse of contraction generated by S.A. node and its spread in atria causing atrial depolarisation. The interval PQ represents atrial contraction and takes 0.1 second.

QRS complex : Indicates spread of impulse of contraction from A.V node to the wall of ventricles through bundle of His and purkinje fibres causing ventricular depolarisation. This complex also represents repolarization of S.A. node.

The RS of QRS wave and ST interval show ventricular contraction (0.3 seconds). QRS is related to ventricular systole.

T wave : Indicates repolarisation during ventricular relaxation.

Any abnormality in the working of heart alters the wave pattern of ECG. Thus, ECG is of great diagnostic value in cardiac diseases. ECG also indicates the rate of heart beat

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 120 mm. the arterioles further divide into smaller vessels called meta-arterioles (70 mm) which divide into capillaries. At the beginning of capillary, the arterioles posses 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 atreries 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).

$\bullet$ Anastomosis : If more than one arteries are supplying to one organ then branches of these arteries unite to form a network called Anastomosis. It provides many collateral or alternate pathways of blood supply. So, if there is blocking of any artery, it will not lead to necrosis.

(2) Capillaries : 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 8m. 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 fenestra/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.

Histology of arteries and veins

(1) Tunica externa or tunica adventitia : Outermost, fibrous, made up of collagen rich connective tissue and less elastin fibres. The collagen fibres give strength to the blood vessels and prevent their overdilation.

(2) Tunica media : Middle, thickest, made up of smooth involuntary muscle fibres and elastin fibres. This layer is very much variable because number of elastin fibres and muscle fibres depend upon the position of blood vessels from the heart.

(3) Tunica interna or tunica intima : Innermost, thinnest, made up of inner, single layer of simple squamous epithelial cells called endothelium resting on a basement membrane and outer layer of elastic (yellow fibrous) connective tissue. The hollow space in the blood vessel is called lumen.