Chapter 8: Respiration and Circulation.

Chapter 8: Respiration and Circulation

Choose the correct alternative.

The muscular structure that separates the thoracic and abdominal cavity is _______.

pleura

diaphragm

trachea

epithelium

Two

What is the minimum number of the plasma membrane that oxygen has to diffuse across to pass from air in the alveolus to haemoglobin inside a RBC?

Three

Four

Five

________ is a sound–producing organ.

Larynx

Pharynx

Tonsils

Trachea

The maximum volume of gas that is inhaled during breathing in addition to T.V is _______.

residual volume

I.R.V.

G.R.V.

Vital capacity

_______ muscles contract when the external intercostal muscles contract.

Internal abdominal

Jaw

Muscles in bronchial walls

Diaphragm

Movement of cytoplasm in unicellular organisms is called __________.

diffusion

cyclosis

circulation

thrombosis

Which of the following animals do not have closed circulation?

Earthworm

Rabbit

Butterfly

Shark

Diapedesis can be seen in _________ cell.

RBC

WBC

Platelet

Neuron

Pacemaker of heart is _________.

SA node

AV node

His bundle

Purkinje fibers

Which of the following is without a nucleus?

Red blood corpuscle

Neutrophil

Basophil

Lymphocyte

Cockroach shows which kind of circulatory system?

Open

Closed

Lymphatic

Double

Diapedesis can be seen in _________ cell.

RBC

WBC

Platelet

Neuron

Opening of inferior vena cava is guarded by _______.

bicuspid valve

tricuspid valve

Eustachian valve

Thebesian valve

_______ wave in ECG represent atrial depolarization.

P

QRS complex

Q

T

The fluid seen in the intercellular spaces in human is _________

blood

lymph

interstitial fluid

water

Match the Respiratory surface to the organism in which it is found.

Respiratory surface	Organism
Plasma membrane	Insect
Lungs	Salamander
External gills	Bird
Internal gills	Amoeba
Trachea	Fish

Solution: 

Respiratory surface	Organism
Plasma membrane	Amoeba
Lungs	Bird
External gills	Salamander
Internal gills	Fish
Trachea	Insect

Very short answer question.

Why does trachea have ‘C’ shaped rings of cartilage?

Solution: 

The trachea has ‘C’ shaped rings of cartilage as they prevent the trachea from collapsing.

Why is respiration in insect called direct respiration?

Solution: 

In insects, the respiratory system is independent of its circulatory system as blood does not play a direct role in oxygen transport but the tracheal tubes directly transport oxygen to the entire body. Therefore, respiration in the insect is called direct respiration.

Why is a gas exchange very rapid at the alveolar level?

Solution: 

Alveoli are lined by layer of simple squamous epithelial epithelium. This thin, single layer of epithelium allows the rapid exchange of gases in alveolar region.

Name the organ which prevents the following the entry of food into the trachea while eating.

Solution: 

Epiglottis

Short answer question.

Why is it advantageous to breathe through the nose than through the mouth?

Solution: 

i. Nose filters and warms the inhaled air. Hair in the nose prevents the entry of microbes, dust, and other impurities which may harm the lungs.

ii. Mouth lacks any such structures for filtering and warming the air that is inhaled during inspiration. Hence, it is advantageous to breathe through the nose than through the mouth.

Identify the incorrect statement and correct it,

a. A respiratory surface area should have a large surface area.

b. A respiratory surface area should be kept dry.

c. A respiratory surface area should be thin, maybe 1mm or less.

Solution: 

b. A respiratory surface should be kept dry.

Correct statement – A respiratory surface should be moist in order to facilitate the exchange of gases.

Given below is the characteristic of some modified respiratory movement. Identify them.

Spasmodic contraction of muscles of expiration and forceful expulsion of air through nose and mouth.

Solution: 

Sneezing

Given below is the characteristic of some modified respiratory movement. Identify them.

An inspiration followed by many short convulsive expirations accompanied by facial expression.

Solution: 

Crying

Write a note on blood plasma.

Solution: 

Human blood consists of plasma and blood corpuscles or blood cells

Plasma: It is a straw-coloured, slightly alkaline, viscous fluid. It constitutes 55% of the blood.

Plasma consists of water, proteins (albumin, globulin, properdin, prothrombin, fibrinogen), inorganic salts (Na, K, Mg, Ca, Fe, Mn and Cl^–, HCO^–₃, PO₄^3-), food (glucose, amino acids, fatty acids, triglycerides), wastes (urea, uric acid and creatinine), regulators (hormones, enzymes, vitamins), anticoagulants (heparin), cholesterol and antibodies, dissolved gases (O₂, CO₂, N₂) Plasma contains 90% water, 7-8% proteins, inorganic salts – 1% and other substances 1-2%.

Blood Corpuscles: It constitutes 44% of the blood. Blood corpuscles are of three types as given below:

1. RBC (Red Blood Corpuscles) or Erythrocytes:

1. Erythrocytes are the most abundant cells in the human body.

2. They are circular, biconcave and enucleated (in camel and llama they are nucleated).

3. The red colour or RBCs is due to an oxygen-carrying pigment, the haemoglobin, in their cytoplasm.

4. In males, the RBC count is about 5.1–5.8 million/mm³ (per μL) and in females about 4.3–5.2 million/mm³.

5. The average life span of RBCs is 120 days.

6. The process of formation of RBCs is called erythropoiesis.

7. RBCs are produced from haemocytoblasts/reticulocytes.

8. The erythropoeitic organ of the foetus is the liver and spleen and in the adult, it is mainly the red bone marrow.

9. Vitamin B₁₂, folic acid and heme protein are required for the production of RBCs. The old and worn-out RBCs are destroyed in the liver and spleen (graveyard of RBCs).

10. Polycythemia is the condition in which the number of RBCs increase and erythrocytopenia is a decrease in number of RBCs.

11. The hormone erythropoietin produced by the kidney cells stimulates the bone marrow for the production of RBCs.

12. Mature erythrocyte is devoid of nucleus, mitochondria or other membrane bound cell organelles. Its cytoplasm (stroma) is rich in haemoglobin and O₂ carrying proteinaceous pigment that gives red colour to the RBCs and blood. It also contains an enzyme, carbonic anhydrase.

13. Erythrocytes are responsible for the transport of respiratory gases O₂ and CO₂, maintaining pH and viscosity of blood. They also contribute in the process of blood clotting.

14. The ratio of the volume of RBCs to the total blood volume of blood is hematocrit. It is different for men and women.

2. WBC (White Blood Corpuscles) or Leucocytes:

1. Leucocytes are colorless, nucleated and amoeboid cells larger than RBCs.

2. These are colorless, irregular nucleated cells and show polymorphism (exist in variable forms)

3. Due to their amoeboid movement, they can move out of the capillary walls by a process called diapedesis.

4. A normal adult has on average, 5000-11000 WBCs per mm³ of blood.

5. Decrease in the number of WBCs (<4000) is called leucopenia (common in HIV, AIDS, and TB patients or those exposed to radiations, shock, etc.). Temporary increase in the number of WBCs is called as leucocytosis. It is due to infection. It also occurs during pregnancy and in newborn babies. An uncontrolled increase in the number of WBCs is a type of blood cancer called leukemia. WBCs are mainly concerned with defense mechanisms i.e. protection.

3. Blood Platelets or Thrombocytes:

1. Thrombocytes are cellular fragments formed from the large cells called megakaryocytes.

2. These are produced in the bone marrow. They are very small, oval shaped cell fragments without a nucleus.

3. Normal count of thrombocytes in human blood is about 2.5 – 4.5 lakh / mm³ of blood. If the number of thrombocytes decreases than normal, the condition is called thrombocytopenia. This condition causes internal bleeding (hemorrhage).

4. Platelets secrete platelet factors which are essential in blood clotting. They also seal the ruptured blood vessels by the formation of platelet plug/ thrombus. They secrete serotonin a local vasoconstrictor.

5. Functions of Blood: Blood perform various functions like transport, homeostasis and protection.

Explain blood clotting in short.

Solution: 

1. Clotting or coagulation is the process of converting liquid blood into a solid form. This process may be initiated by contact of blood with any foreign surface (intrinsic process) or with damaged tissue (extrinsic process).

2. Intrinsic and extrinsic processes involve the interaction of various substances called clotting factors by a stepwise or cascade mechanism.

3. There are in all twelve clotting factors numbered as I to XIII (factor VI is not in active use). Interaction of these factors occurs in a cascade manner leading to the formation of the enzyme thrombin.

4. Thromboplastin helps in the formation of enzyme prothrombinase. This enzyme inactivates heparin and it also converts inactive prothrombin into its active thrombin.

5. Thrombin converts soluble blood protein fibrinogen into insoluble fibrin. Fibrin forms a mesh in which platelets and other blood cells are trapped to form the clot.

Describe pericardium.

Solution: 

The heart is enclosed in a membranous sac called the pericardium. The pericardium is formed of two main layers – outer fibrous and inner serous pericardium. Serous pericardium is soft, moist, and elastic. It is formed of squamous epithelium and is further divisible into two layers as a parietal and visceral layer. Parietal and visceral layers of serous pericardium are separated by a pericardial space. This space is filled with pericardial fluid (about 50ml) which acts as a shock absorber and protects the heart from mechanical injuries. It also keeps the heart moist and acts as a lubricant.

Describe valves of human heart.

Solution: 

Both the atria open into ventricles of their respective sides by atrioventricular apertures. The atrio-ventricular apertures are guarded by cuspid valves.

i. Cuspid valves:

These are bicuspid and tricuspid valves. The bicuspid valve also known as the mitral valve is present in the left atrio-ventricular aperture. Tricuspid valve is present in the right AV aperture.

ii. Eustachian valve:

It is present on the opening of the post-caval vein (inferior vena cava).

iii. Thebesian valve:

It guards the opening of the coronary sinus into the right atrium.

iv. Semilunar valves:

These three valves guard the opening between the right ventricle and pulmonary artery and left ventricle and aorta.

What is the role of papillary muscles and chordae tendineae in the human heart?

Solution: 

The bicuspid and tricuspid valves are connected to chordae tendineae which in turn are connected to the papillary muscles present on the ventricular wall. Chordae tendineae and papillary muscles regulate the opening and closing of valves.

Explain in brief the factors affecting blood pressure..

Solution: 

The factors affecting blood pressure are:

1. Cardiac output:

The normal cardiac output is 5 litres/min. An increase in cardiac output increases systolic pressure.

2. Peripheral resistance:

It depends upon the diameter of blood vessels. A decrease in the diameter of arterioles and capillaries under the effect of vasoconstrictors like vasopressin or ADH cause increase in peripheral resistance and thereby increase in blood pressure.

3. Blood volume:

Blood loss in accidents decreases blood volume, and thus the blood pressure.

4. Viscosity of blood:

Blood pressure is directly proportional to the viscosity of blood.

5. Age:

Blood pressure increases with age due to the increase in inelasticity of blood vessels.

6. Venous return:

The amount of blood brought to the heart via the veins per unit time is called the venous return. It is directly proportional to blood pressure.

7. Length of blood vessel:

Blood pressure is also directly proportional to the total length of the blood vessel. Blood pressure can also be affected by vasoconstriction or vasodilation.

8. Gender:

Females have slightly lower BP than males before the age of menopause. However, the risk of high B. P. increases in the females after menopause sets in.

Give scientific reason.

Closed circulation is more efficient than open circulation.

Solution: 

1. In open circulation, blood is not enclosed in blood vessels but pumped directly into the cavity called haemocoel whereas, in the closed type of circulation, blood flows within the blood vessels and does not come in direct contact with cells and body tissues.

2. Therefore, in closed blood circulation blood flows under high pressure and allows the blood to pass faster and achieve a high level of distribution within the body.

Thus, closed circulation is more efficient than open circulation.

Human heart is called as myogenic and autorhythmic.

Solution: 

1. The human heart is capable of generating a cardiac contraction independent of the nervous system. It can generate its own rhythm due to the presence of nodal tissues.

2. The nodal tissue SA node (Sinoatrial node) is capable of generating the wave on contraction and making the pace of contraction.

Thus, human heart is myogenic and autorhythmic.

Person who has undergone a heart transplant needs lifetime supply of immunosuppressants.

Solution: 

a. Immunosuppressants are the drugs that reduce the level of immune activity and the risk of rejection of foreign bodies such as transplant organs.

b. After transplantation, there is a risk of graft rejection as the body may recognize the transplanted organ/tissue as foreign and may trigger an immune response thereby damaging the transplanted organ.

Therefore, the heart recipient has to rely upon lifetime supply of immunosuppressants.

Arteries are thicker than veins.

Solution: 

i. Arteries carry oxygenated blood away from the heart to the body.

ii. The blood pumped out by the heart is under high pressure and to withstand this pressure arteries are thick-walled.

iii. Veins carry deoxygenated blood from the body back to the heart.

iv. They are thin-walled as the blood that flows through veins is under low pressure. Hence, arteries are thicker than veins.

Left ventricle is thick than all other chambers of heart.

Solution: 

i. The thickness of the myocardium of the four chambers varies according to the functions of each chamber.

ii. The thin-walled atria deliver blood into adjacent respective ventricles.

iii. As compared to the right ventricle, the left ventricle pumps blood at great distances to all other parts of the body at higher pressure, and resistance to blood flow is larger. Therefore, the left ventricle is thick as it requires strength to withstand the high pressure.

Distinguish between open and closed circulation.

Solution: 

Open circulation	Closed circulation
1. In open circulation, blood is circulated through the body cavities (haemocoels).	1. In closed circulation, blood circulates the blood vessels and does not come in direct contact with cells and body tissues.
2. The blood flows with low pressure.	2. The blood flows with high pressure.
3. Exchange of material takes place directly between blood and cells or tissues of the body.	3. Exchange of material between blood and body tissues is through intermediate fluid called lymph.
4. It usually does not contain any respiratory pigment like haemoglobin so it does not transport respiratory gases	4. It contains respiratory pigments like haemoglobin for transportation of respiratory gases.
e.g. Arthropods and molluscs	e.g. All vertebrates, higher molluscs and annelids

Distinguish between Arteries and Veins

Solution: 

Arteries	Veins
1. They carry blood away from the heart to various parts/organs of the body.	1. The carry blood towards the heart from various parts /organs of the body.
2. Blood flows under great pressure.	2. Blood flows under less pressure.
3. They are thick-walled.	3. They are thin-walled.
4. Arteries branch into arterioles and further into fine capillaries	4. Venules are small vessels that continue from capillaries and merge to form veins.
5. These are deeply situated except a few like the radial, brachial, femoral, etc. which are superficially located.	5. Mostly superficial in location.
6. They carry oxygenated blood, except pulmonary artery.	6. They carry deoxygenated blood, except the pulmonary vein.
7. Tunica media is comparatively thicker.	7. Tunica media is comparatively thinner.
8. They do not have valves.	8. They have valves to prevent the backflow of the blood.

Distinguish between Blood and lymph.

Blood	lymph
1. It is reddish in colour.	1. It is pale yellow in colour.
2. It has two main components – fluid plasma and formed elements (blood cells).	2. It has almost similar composition to the blood except for RBCs, platelets, and some proteins.
3. It flows through blood vessels.	3. It flows through lymph vessels.
4. It transports materials from one organ to another.	4. It transports material from tissues cells to blood and vice-versa.

Distinguish between

Blood capillary and lymph capillary

Solution: 

Blood capillary	lymph capillary
1. Its diameter is smaller than lymph capillary.	1. Its diameter is larger than blood capillary.
2. It contains blood.	2. It contains lymph.
3. It is less permeable than lymph.	3. It is more permeable than blood capillary.
4. Blood capillaries provide oxygen and other substances to the tissues.	4. Lymph capillaries absorb the excess of tissue fluid.

Distinguish between

Intrinsic and extrinsic process of clotting

Solution: 

Intrinsic pathway	Extrinsic pathway
1. It is stimulated by damage to blood vessels.	1. It is stimulated by damage to tissue outside the vessel.
2. It is more complex and takes more time than the extrinsic pathway.	2. It occurs rapidly as it has fewer steps as compared to the intrinsic pathway.
3. Tissue factor is not involved in the activation of the intrinsic pathway.	3. Tissue factor also known as thromboplastin activates extrinsic pathways.
4. It involves factor VIII, IX, XI, and XII.	4. It involves factors VII, X, and V.

Long answer question:

Smita was working in a garage with the doors closed and automobiles engine running. After some time she felt breathless and fainted. What would be the reason? How can she be treated?

Solution: 

1. Smita felt breathless and fainted due to the presence of an excess carbon monoxide released from automobile engines.

2. Carbon monoxide can be fatal if not treated. The affected person can be treated by administering pure oxygen. This will speed up the separation of carbon monoxide from hemoglobin.

Shreyas went to a garden on a wintry morning. When he came back, he found it difficult to breath and started wheezing. What could be the possible condition and how can he be treated?

Solution: 

1. The symptoms – difficulty in breathing, wheezing indicate that Shreyas could be suffering from asthma.

2. Inhalers in which open-air passage ways are used to treat asthma.

Why can you feel a pulse when you keep a finger on the wrist or neck but not when you keep them on a vein?

Solution: 

1. Pulse is the series of pressure waves that travel through arteries due to ventricular systole.

2. It is strongest in the arteries closer to the heart and gradually becomes weak in arteries away from the heart and will be the weakest till it reaches the vein.

A man’s pulse rate is 68 and cardiac output is 5500 cm³ . Find the stroke volume.

Solution: 

Cardiac output = Stroke volume × Heart rate

∴ 5500 = Stroke volume × 68

Stroke volume = 5500 / 68

= 80.882 ≈ 80.88mL

Which blood vessel of the heart will have the maximum content of oxygen and why?

Solution: 

1. Pulmonary vein carries the maximum content of oxygen.

2. Pulmonary circulation moves deoxygenated blood from the heart to the lungs for oxygenation and it returns to the heart as oxygenated blood. Systemic circulation pushes the oxygenated blood from the heart towards various body parts (except lungs) and returns back to the heart as deoxygenated blood.

3. Pulmonary vein is the only blood vessel that carries freshly oxygenated blood from the lungs to the heart for distribution to the body.

If the duration of the atrial systole is 0.1 sec and that of complete diastole is 0.4 sec, then how does one cardiac cycle complete in 0.8 sec?

Solution: 

1. One cardiac cycle includes atrial systole, ventricular systole, and joint/complete diastole.

2. The duration for atrial systole is 0.1 sec, duration for complete diastole is 0.4 sec, which means if one cardiac cycle completes in 0.8 sec then the duration for ventricular systole is 0.3 sec.

3. Therefore, the duration of one cardiac cycle

= Atrial systole + Ventricular systole + Complete diastole

= 0.1 sec +0.3 sec + 0.4 sec = 0.8 sec

4. Also the relaxation period shortens as the heart beats faster whereas the durations of atrial systole and diastole shortens slightly. Hence, one cardiac cycle completes in 0.8 sec.

How blood is kept moving in the large veins of the legs?

Solution: 

1. The blood in the large veins of legs is kept moving by the means of azygos system (located on either side of the vertebral column and drains the viscera within the mediastinum, as well as the back and thoracoabdominal walls).

2. It serves as a bypass for the inferior vena cava that drains blood from the lower body.

3. Several small veins link the azygos system directly with inferior vena cava. Large veins drain the lower limbs and abdomen, conducts blood into the azygos system.

4. If the inferior vena cava or hepatic portal vein becomes obstructed, the azygos system returns blood from the lower body to the superior vena cava.

Describe the histological structure of the artery, vein, and capillary.

Solution: 

The three structural layers of a generalized blood vessel from innermost to outermost are the tunica interna (intima), tunica media, and tunica externa. Modifications in this basic design account for the five types of blood vessels and the structural and functional differences among the various vessel types. In the transverse section of an artery, three layers can be seen. They are:

T. S. of Artery, Vein and Capillary-

1. Tunica externa or tunica adventitia:

It is a thick, tough layer of collagen fibers.

2. Tunica media:

It is the middle layer made up of smooth muscle fibers and a network of elastic fibers. This thick muscular and elastic layer makes the arterial wall pulsatile.

3. Tunica interna or intima:

The innermost tunica interna is a single layer of flat compact endothelial cells surrounding the lumen. The angular margin around the lumen shows tessellations. Arterial lumen is devoid of valves and blood flows through it rapidly and with high pressure.

What is blood pressure? How is it measured? Explain the factors affecting blood pressure.

Solution: 

The pressure exerted by blood on the wall of the blood vessels is called blood pressure.

It is measured by the sphygmomanometer. It is usually measured from the arteries.

The factors affecting blood pressure are:

1. Cardiac output:

The normal cardiac output is 5 litres/min. An increase in cardiac output increases systolic pressure.

2. Peripheral resistance:

It depends upon the diameter of blood vessels. A decrease in the diameter of arterioles and capillaries under the effect of vasoconstrictors like vasopressin or ADH causes an increase in peripheral resistance and thereby increase in blood pressure.

3. Blood volume:

Blood loss in accidents decreases blood volume, and thus the blood pressure.

4. Viscosity of blood:

Blood pressure is directly proportional to the viscosity of blood.

5. Age:

Blood pressure increases with age due to an increase in the inelasticity of blood vessels.

6. Venous return:

The amount of blood brought to the heart via the veins per unit time is called the venous return. It is directly proportional to blood pressure.

7. Length of blood vessel:

Blood pressure is also directly proportional to the total length of the blood vessel. Blood pressure can also be affected by vasoconstriction or vasodilation.

8. Gender:

Females have slightly lower BP than males before the age of menopause. However, the risk of high B. P. increases in the females after menopause sets in.

Describe human blood and give its functions.

Solution: 

Human blood consists of plasma and blood corpuscles or blood cells

Plasma: It is a straw-coloured, slightly alkaline, viscous fluid. It constitutes 55% of the blood. Plasma consists of water, proteins (albumin, globulin, properdin, prothrombin, fibrinogen), inorganic salts (Na, K, Mg, Ca, Fe, Mn and Cl^–, 

HCO3-

 and 

PO43-

), food (glucose, amino acids, fatty acids, triglycerides), wastes (urea, uric acid and creatinine), regulators (hormones, enzymes, vitamins), anticoagulants (heparin), cholesterol and antibodies, dissolved gases (O₂, CO₂, N₂) Plasma contains 90% water, 7-8% proteins, inorganic salts – 1% and other substances 1-2%.

Blood Corpuscles: It constitutes 44% of the blood. Blood corpuscles are of three types as given below:

1. RBC (Red Blood Corpuscles) or Erythrocytes:

a. Erythrocytes are the most abundant cells in the human body.

b. They are circular, biconcave and enucleated (in camel and llama they are nucleated).

c. The red colour or RBCs is due to an oxygen-carrying pigment, the haemoglobin, in their cytoplasm.

d. In males, the RBC count is about 5.1–5.8 million/mm³ (per μL) and in females about 4.3–5.2 million/mm³.

e. The average life span of RBCs is 120 days.

f. The process of formation of RBCs is called erythropoiesis.

g. RBCs are produced from haemocytoblasts/reticulocytes.

h. The erythropoeitic organ of the foetus is the liver and spleen and in the adult, it is mainly the red bone marrow.

i. Vitamin B₁₂, folic acid and heme protein are required for the production of RBCs. The old and worn-out RBCs are destroyed in the liver and spleen (graveyard of RBCs).

j. Polycythemia is the condition in which the number of RBCs increase and erythrocytopenia is a decrease in the number of RBCs.

k. The hormone erythropoietin produced by the kidney cells stimulates the bone marrow for production of RBCs.

l. Mature erythrocyte is devoid of nucleus, mitochondria or other membrane-bound cell organelles. Its cytoplasm (stroma) is rich in haemoglobin and O₂ carrying proteinaceous pigment that gives the red colour to the RBCs and blood. It also contains an enzyme, carbonic anhydrase.

m. Erythrocytes are responsible for the transport of respiratory gases O₂ and CO₂, maintaining pH and viscosity of blood. They also contribute in the process of blood clotting.

n. The ratio of the volume of RBCs to the total blood volume of blood is hematocrit. It is different for men and women.

2. WBC (White Blood Corpuscles) or Leucocytes:

a. Leucocytes are colorless, nucleated, and amoeboid cells larger than RBCs.

b. These are colourless, irregular nucleated cells and show polymorphism (exist in variable forms)

c. Due to their amoeboid movement they can move out of the capillary walls by a process called diapedesis.

d. A normal adult has on average, 5000-11000 WBCs per mm3 of blood.

e. Decrease in the number of WBCs (<4000) is called leucopenia (common in HIV, AIDS, and TB patients or those exposed to radiations, shock, etc.). A temporary increase in the number of WBCs is called leucocytosis. It is due to infection. It also occurs during pregnancy and in newborn babies. An uncontrolled increase in the number of WBCs is a type of blood cancer called leukemia. WBCs are mainly concerned with defense mechanisms i.e. protection.

3. Blood Platelets or Thrombocytes:

a. Thrombocytes are cellular fragments formed from the large cells called megakaryocytes.

b. These are produced in the bone marrow. They are very small, oval-shaped cell fragments without a nucleus.

c. Normal count of thrombocytes in human blood is about 2.5 – 4.5 lakh / mm³ of blood. If the number of thrombocytes decreases than normal, the condition is called as thrombocytopenia. This condition causes internal bleeding (haemorrhage).

4. Platelets secrete platelet factors which are essential in blood clotting. They also seal the ruptured blood vessels by the formation of platelet plug/ thrombus. They secrete serotonin a local vasoconstrictor.

5. Functions of Blood: Blood perform various functions like transport, homeostasis, and protection.