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UNIT-II A  BODY FLUIDS AND CIRCULATION

    The most common circulating fluid in animals, including man, is the blood. Moreover, another fluid called lymph also helps in transport of certain materials. Blood and lymph are the fluid tissues.
I. Lymphatic system: It is an open circulatory system in which an extensive net of blind ended lymph capillaries, lymph vessels etc. are present. They send the lymph into the veins.
1. Lymph Capillaries: These are microscopic, blind ended capillaries in the inter cellular spaces of tissues. Their walls are formed by a layer of endothelial cells with porous junctions. These capillaries collect the extra cellular fluid (ECF) or interstitial fluid, microbes, proteins, absorbed fats, micro organisms (through their porous walls). The ECF in lymph capillaries is called lymph.
2. Lymph vessels: Lymph capillaries unite with one another and form lymph vessels. They lead into larger lymph ducts.

* The walls of lymph vessels and lymph ducts are thin like veins and contains valves.

* The thin layer of smooth muscles in their walls cause peristaltic waves and pump the lymph towards neck region.

a. All the lymph vessels of the lower part of the body open into a large thoracic duct or left lymphatic duct or chyliferous duct. It is the largest vessel in the lymphatic system. It deliver the lymph into the venous system at the junction of left subclavian vein and left internal jugular vein.
b. The lymph collected from left side of chest, left fore limb and left side of head is also drained into the thoracic duct.
c. Lymph from the right side of thorax, head and left fore limb is collected into right lymphatic duct. It release the lymph into the venous system at the junction of right internal jugular vein and right subclavian vein.
           Lymph is a part of blood that oozes out from the blood capillaries into the inter cellular spaces as ECF and then collected back into the venous system.
            a. Lymph supplies oxygen to tissues.
            b. Lymph supplies nutrients to tissues.

 * Lymphatic system also contains other organs such as spleen, thymus, tonsils, appendix etc. Spleen is the largest lymphatic organ and is the reservoir of blood. It also acts as a haemopoitic organ upto 5th month.

* In pregnancy, enlarged uterus exerts pressure on the abdominal veins. It results in accumulation of fluid in the legs. Hence legs appear swollen (edema).

Lymph contains water, some plasma proteins, electrolytes, certain WBC (especially lymphocytes), antibodies, enzymes, nutrients, some coagulation factors etc. RBC are absent.


An emulsion of chilomicrons (triglycerides) and lymph in lacteals
(= lymph capillaries in villi) is called chyle.

3. Functions of lymphatic system:
a. Lymph returns the absorbed nutrients into blood.
b. It returns the waste materials collected from tissues to blood.
c. Transports lymphocytes from lymphoid glands to blood.
d. Transports the digested fats (absorbed through lacteals) to blood.
e. Destroys the invading harmful microbes.
II. Blood Clotting: As long as blood is in blood vessels, it is in fluid state. When a blood vessel is injured a number of physiological processess are initiated to promote hemostasis. Injured or broken blood vessel releases collagen into blood. They initiate vasoconstriction, formation of platelet plug and production of a web with fibrin. Vitamin K is necessary for blood clotting.

* Hemostasis is a process which causes bleeding to stop.
*  Homeostasis is the property of a system that regulates its internal 
environment and tends to maintain a stable, relatively constant condition of properties.

a. When blood vessel is damaged, the muscles in its wall contract and makes its lumen narrow.
b. The collagen released from the ruptured epithelial cells of blood vessel and platelets form a platelet plug.
c. The substances released by damaged tissue, plasma proteins and platelets start clotting process. clot is a net of fibrin fibres in which blood cells entangle.

For clotting of blood twelve substances (i.e., factors)  are required. They are ....

 I. Fibrinogen              II. Prothrombin             
 III. Thromboplastin   IV. Calcium ions.                
 V. Proaccelerin          VI. Proconvertin

 VII. Antihaemophilic globulin  VIII. Christmas factor (plasma thrombo plastin component - PTC)

 IX. Stuart - Power factor     X. Plasma thromboplastin antecedent (PTA)

 XI. Hageman's factor         XII. Fbirin stabilising factor

 Factor VI is not considered as clotting factor.

Mechanism of blood clotting:
1. The first step in the process of blood clotting is the formation of prothrombin activator. It is formed in either intrinsic pathway or extrinsic pathway.
a. In intrinsic pathway, when collagen (of damaged blood vessel) contacts the blood, factor XII is activated. It in turn activates the prothrombin activator.
b. In extrinsic pathway, when collagen contacts the blood, tissue thromboplastin is released from the damaged tissue cells. It activates factor VII. It initiates the formation of prothrombin activator.
2. Now the prothrombin activator activates the prothrombin present in blood into active thrombin in presence of calcium ions.
3. Thrombin converts the soluble fibrinogen into soluble fibrin. Now the factor XIII (released by blood platelets) replaces the hydrogen bonds of soluble fibrin by covalent bonds. Thus insoluble fibrin threads are formed. They spread over the damaged part as a net. In the meshes, the blood cells entagle and thus blood clot is formed.
4. Then the blood clot begins to contract. So that the fluid is expelled out (it is called serum). This process is called clot reaction.

5. Serum is nothing but plasma without certain plasma proteins such as prothrombin, fibrinogen etc.

* Blood in the blood vessels is not clotted because of the presence of anticoagulant called heparin. It is produced by basophils and mast cells. Heparin activates antithrombin which inactivates the thrombin, it formed in blood vessel. 

* In blood banks (and also in clinical laboratories), blood clotting is prvented by adding citrates or oxalates of sodium or EDTA (Ethylene Diamine Tetra Acetic acid). They bind to Ca++ ions and prevent blood clotting.

* Coumarins of plant origin are precursors of anticoagulants such as warfarin (antagonistic to vitamin K) which prevents the synthesis of clotting factors II, VII, IX and X in liver.

III. Circulating pathways (Open type and closed type)

IV.Heart:


1. Structure of human heart:
           Heart is the Central Pumping Organ of blood. It is a muscular organ lies in the thoracic cavity in the mediastenum. Its posterior end slightly turned to left. Heart is mesodermal in origin and is covered by a double layered pericardium (outer fibrous layer and inner serus layer). 
           The serus layer of pericardium is double layered- outer parietal layer and inner visceral layer. Perietal layer is fused with fibrous layer of pericardium. Visceral layer is attached to outer surface of heart and forms epicardium. Between these two layers, pericardial cavity is present and is filled with pericardial fluid. It reduces the friction and protects the heart from shocks.
            The wall of heart has outer epicardium, middle myocardium (formed by cardiac muscles) and inner endocardium (formed by endothelial cells).

           Human heart is formed by four chambers. They are anterior atria (left and right) and posterior ventricles (left and right). Atria and Ventricles are separated by a deep groove called coronary sulcus. The two ventricles are separated by inter ventricular grooves posteiorly each atrium projects over the ventricle as auricular appendix.

a. Atria:
i. Atria (left and right) form the anterior part of the heart. Right atrium is larger than left one.
ii. The two atria are separated from each other by an inter atrial septum. In embrayonic stage, it has a pore called foramen ovale. At the time of birth, when lungs become functional, it is closed leaving a small depression called fossa ovalis.
iii. Right atrium receives deoxygenated blood from different body parts through three major veins, namely, two precaval veins and a post caval vein. Right atrium also receives blood from wall of heart through a coronary sinus. Its opening is guarded by a valve of  Thebesius.
iv. Near the opening of post caval vein, a rudimentary Eustachian valve is present. In embrayonic stage it directs the blood from right atrium to left through foramen ovale.
v. Left atrium receives Oxygenated blood from the lungs by two pairs of pulmonary veins. They open into the left atrium through a common pore.
vi. In the right upper corner of right atrium, the pace maker called sinoatrial node (S.A. node) is present.
vii. Atria and ventricles are separated from each other by an atrio ventricular septum.

viii. In the lower left corner of the right atrium close to atrio ventricular septum an atrio ventricular node is present.

b. Ventricles:
i. Ventricles (left and right) form the posterior part of heart.
ii. Left Ventricle is large and has thick walls (than Right Ventricle) and the Left and Right Ventricles are separated by an interventricular septum.
iii. The inner surface of ventricles is raised into ridges called Columnae cornae (trabiculae cornae). Some of them are large and conical. They are called Papillary muscles.
iv. Right atrium opens into right ventricle through right atrio ventricular aperture. It is guarded by a Tricuspid valve.
v. Left atrium opens into left ventricle through left atrio ventricular aperture. It is guarded by a Bicuspid valve or Mitral valve.
vi. Tricuspid Valve and Bicuspid Valve allow the blood to flow from atria to respective ventricles only.
vii. Extending between tricuspid valve, bicuspid valve and papillary muscles, there are collagenous cords known as Chordae tendinae. They prevent the cusps of these valves to bulge into atria during ventricular systole.

viii. Arising from A.V. node, is a Bundle of His (A.V. Bundle). It extends into inter ventricular septum, divides into two branches and then, they divide further as purkinje fibres. They extend into the muscular walls of ventricles.

* S.A. node is formed by Cardiac muscle cells (Cardiomyocytes). It has ability to generate action potentials without external stimuli. Hence, it is called pace maker. As it is formed from muscle cells, it is described as Myogenic Pace maker.

* A.V. node send the action potentials received from the S.A. node into ventricular muscles. Hence A.V. node is a relay point.

c. Aortic arches: (Pulmonary arch and systemic arch)
i. Arising from the right ventricle, is a pulmonary arch. Its opening into right ventricle is guarded by a pulmonary valve. It allows the blood to flow into pulmonary arch only. Pulmonary arch transports the deoxygenated blood to lungs.
ii. Arising from the left ventricle, is a systemic arch. It turns to left side and supplies oxygenated blood to different body parts through its branches - called arteries. The opening of systemic arch into left ventricle is guarded by an aortic valve. It allows the blood to flow into systemic arch only.

iii. Pulmonary valve and aortic valve are formed by three semilunar valves each.
iv. At the point of contact, the systemic and pulmonary arches are connected by a fibrous strand called ligamentum arteriosum. It represents the remnant of ductus arteriosus
[ * In embryonic stage, systemic arch and pulmonary arch are connected by ductus arteriosus].

 

2. Working of heart/ Functioning of heart/ Cardiac Cycle
   The cyclical events that occur from the beginning of one heart beat to the beginning of next constitute a cardiac cycle. It is divisible into three phases, namely atrial systole, ventricular systole and cardiac diastole. The contraction of heart is known as systole and relaxation is known as diastole. A systole and its following diastole constitute a heart beat.
A. Atrial Systole (0.1 Sec):
        When the atria are filled with blood [right atrium with deoxygenated blood and left atrium with oxygenated blood], S.A. node (pace maker) generates an action potential which spreads over atria. Thus they contract. As a result, blood from right atrium flows into right ventricle and from  left atrium to left ventricle [during atrial systole 30% of blood reaches the ventricles and the 70% of blood flows in ventricles before atrial systole.]

B. Ventricular Systole (0.3 Sec):
i. The A.V. node receives the action potentials and conduct them to walls of ventricles through Bundle of His and Parkinje fibres. Thus ventricles contract (Now atria relaxes and are filled with blood).
ii. Thus blood flows into the aortic arches.

iii. Due to high pressure in ventricles, tricuspid valve and bicuspid valve are closed causing the first sound of heart beat - LUB.
C. Cardiac diastole (0.4 Sec):
i. Now the ventricles relax and hence, pressure in ventricles falls. As a result, pulmonary valve and aortic valve are closed causing second sound of heart beat - DUP.
ii. Now blood that reached atria flows into ventricles by the pressue of blood in atria. At this stage all the four chambers are in relaxed state.
iii. Now another cardiac cycle sets in.

In Man, the duration of Cardiac cycle is 0.8 Sec, if heart beats at the rate of 72 times per minute.

3. Cardiac Output:
a. The volume of blood pumped by each ventricle for each heart beat is known as stroke volume. It is about 70 ml.
b. The volume of blood pumped out by the heart from each ventricle per minute is known as cardiac output.

Cardiac output = Stroke volume × No. of beats per minute
                           = 70 ml × 72 times = 5,040 ml/ min.

4. Double Circulation:


In the heart of man, oxygenated and deoxygenated blood remain separate. Right side of the heart contains deoxygenated blood and left side contains oxygenated blood. In one complete circuit in the body, the blood passes twice through the heart, once through the right side and the other time through the left side. This type of circulation is called double circulation.
     It includes systemic circulation and pulmonary circulation.
i. Systemic Circulation:   The Oxygenated blood
from left ventricle goes to all organs (except alveoli of lungs) through left systemic arch and its branches (arteries). Deoxygenated blood from different organs (except from alveoli of lungs) enters the right atrium through the vena cava. This circulation is called systemic circulation (greater circulation).

ii. Pulmonary circulation: Deoxygenated blood from right atrium flows into right ventricle and goes to alveoli of lungs through pulmonary arch. From the alveoli of lungs, oxygenated blood is collected and send it to left atrium by pulmonary veins. This circulation is called pulmonary circulation (lesser circulation).
V. Portal circulation and coronary circulation:
1. portal circulation:

* A blood vessel that begins with capillaries in one organ and ends with capillaries in another organ is known as a portal vessel.
* The system of portal vessels is named after the name of the organ where it ends with capillaries. e.g.: If a portal vessel ends in kidneys, it is called renal portal vessel, if it ends in liver, it is called hepatic portal vessel.
* In man, only hepatic portal system is present.
* Hepatic portal system collects blood from the gut and supply it to the liver (From liver, it enters the systemic circulation through hepatic veins).
* The digested food absorbed in gut reaches the liver, where excess food is stored in the liver (particularly by glycogenesis) and only required amounts is released into the systemic circulation.

2. Coronary Circulation: The system of blood vessels which is confined for circulation of blood to and from the heart muscles is known as coronary circulation. Two coronary arteries (left and right) supply the blood and four coronary veins collect the blood and open into a caronary sinus. It opens into the right atrium.
VI. Regulation of heart beat:
i. Normal functioning of heart is autoregulated by the nodal tissues in the wall of heart.
ii. A neural centre in medulla oblongata of brain moderate the functioning of heart though autonomous nervous system.
iii. Signals of sympathetic nervous system increase the rate of heart beat, where as the signals of parasympathetic nervous system decreases it.
iv. Hormones like thyroxine (secreted by thyroid) and epinephrine and norepinephrine (secreted by adrenal medulla) increase the heart rate and cardiac output.
VII. Blood Vessels: Arteries and veins are considered as blood vessels.
1. Blood vessels that supply blood from heart to different body parts are called arteries. Their diameter is progressively decreases and ends with capillaries.

2. Blood vessels that return the blood from different body parts to heart are known as veins. Their diamter is progressively increases (they begin with capillaries).
3. The wall of blood vessels is formed by three layers, namely outer tunica externa (fibrous connective tissue), middle tunica media (formed by smooth muscles, thin layer in veins) and inner tunica interna (a layer of endothelium with basement membrane).
4. In arteries, tunica media has elastic laminae on either side. Veins have only one elastic lamina, inner to tunica media.
5. In veins, endothelium forms the valves.
6. The blood vessels present in the walls of larger vessels (like systemic arch) are called vasa vasorum (vessels of vessels).          

                                                                               

VIII. Blood Capillaries:
1. Capillaries are the smallest blood vessels that connect the arterioles and venules.
2. The wall of blood capillaries is formed by a layer of endothelium.
3. Rapid exchange of materials and gases takes place between blood and tissues through capillaries (because of the absence of muscles and connective tissue walls).
IX. Disorders of Circulatory System:
1. Heart Failure:
A condition in which the heart is unable to provide sufficient pumping action to distribute blood to meet the needs of the tissues of the body is known as heart failure.

It may be due to myocardial infraction (Localised death of heart tissue), hypertension, other forms of ischemic heart disease etc. Congestion of lungs is one of the main symptoms of heart failure.
2. Cardiac arrest: (Cardio pulmonary arrest/ circulatory arest): Cessation of normal circulation of blood due to sudden stoppage of contraction of heart is described as cardiac arrest.
3. Angina pectoris: Chest pain due to narrowing of blood vessels to the heart is the main symptom of angina pectoris. It is the warning signal of insufficient blood supply to heart muscles. It is due to smoking, diabetes, high cholesterol, hypertension etc.
4. Hyper tension/ Blood pressure: Normal blood pressure of a healthy man is 120 mm hg (systolic)/ 80 mm Hg (diastolic). Hyper tention is a chronic medical condition, in which pressure on walls of arteries is increased. It affects the heart, brain, kidneys etc.
5. Coronary artery disease/ CAD/ Atherosclerotic heart disease:
CAD is due to accumulation of cholesterol/ fat, calcium and fibrous tissue along the wall of coronary arteries, thus their lumen become narrow resulting in ischemia (insufficient supply of blood). It leads to cardiac muscle damage. 

Posted Date : 02-08-2021

గమనిక : ప్రతిభ.ఈనాడు.నెట్‌లో కనిపించే వ్యాపార ప్రకటనలు వివిధ దేశాల్లోని వ్యాపారులు, సంస్థల నుంచి వస్తాయి. మరి కొన్ని ప్రకటనలు పాఠకుల అభిరుచి మేరకు కృత్రిమ మేధస్సు సాంకేతికత సాయంతో ప్రదర్శితమవుతుంటాయి. ఆ ప్రకటనల్లోని ఉత్పత్తులను లేదా సేవలను పాఠకులు స్వయంగా విచారించుకొని, జాగ్రత్తగా పరిశీలించి కొనుక్కోవాలి లేదా వినియోగించుకోవాలి. వాటి నాణ్యత లేదా లోపాలతో ఈనాడు యాజమాన్యానికి ఎలాంటి సంబంధం లేదు. ఈ విషయంలో ఉత్తర ప్రత్యుత్తరాలకు, ఈ-మెయిల్స్ కి, ఇంకా ఇతర రూపాల్లో సమాచార మార్పిడికి తావు లేదు. ఫిర్యాదులు స్వీకరించడం కుదరదు. పాఠకులు గమనించి, సహకరించాలని మనవి.

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