Answer of Question Circulation Immunity & Gas Exchange

Answer of Question Circulation Immunity & Gas Exchange

Q.1.      Why don’t protozoa need a circulatory system?

Ans. Because protozoa are small, with high surface – area – to – volume ratio, all they need for gas, nutrients, and waste exchange is simple diffusion. However, as the diffusion is a slow process, diffusion is supplemented by other transfer mechanisms. For example the food vacuole moves along a fairly precise path within the cell, it distributes the products of digestion to all parts of the cell. Further, cytoplasm itself is seldom motionless.

Q.2.      How does circulation occur in sponges and cnidarians?

Ans. Sponges circulate water from the external environment through their bodies. Cnidarians, such as Hydra, have a fluid – filled internal gastrovascular cavity. The cavity supplies nutrients for all body cells lining the cavity, provides oxygen from the water in the cavity, and is a reservoir for carbon dioxide and other wastes. Simple body movements move the fluid. Fig. 4.1a, b.

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Q.3. How does circulation occur in pseudocoelomates?

Ans. Pseudocoelomates, such as rotifers, nematodes, gastrotrichs, ectoprocts, use the coelomic fluid of their body cavity for transport. Most of these animals are small, and movements of the body against the coelomic fluids. which are in direct contact

internal tissues and organs (digestive tube), produce adequate transport. Fig.4.1d.

Q. 4. How does circulation occur in a planarian?

Ans. The gastrovascular cavity of a planarian is branched and runs close to all body cells, so the diffusion distances for nutrients, gases, and wastes are short. Body movement helps distribute materials to various parts of the body. Fig. 4.1c.

Q.5. What is an open and a closed circulatory system?

Ans. The animal kingdom has two basic types of circulatory systems open and closed.

Open circulatory system

In the open circulatory system, the heart pumps the hemolymph, (circulatory fluid in place of blood) out into the body cavity (hemocoel) or at least through parts of the cavity (sinuses) where the hemolymph bathes the cells, tissues and organs rather than being carried only in the vessels. There are no small vessels or capillaries connecting the arteries with veins. Such a circulatory system is found in arthropods, most molluscs, and in many smaller invertebrate groups.

Closed circulatory system

In closed circulatory system the blood circulates in the blood vessels only. A heart pumps blood into arteries that branch and narrow into arterioles and then into a vast system of capillaries. Blood leaving capillaries enters venules and then veins that return blood to the heart. Such a circulatory system is present in annelids and all chordates except ascidians.

Q.6. Which type of circulatory system is more efficient open or closed? Why?

Ans. A closed circulatory system is more suitable for large and active animals; firstly because blood can be moved rapidly to tissues needing it. In addition, flow to various organs can be readjusted to meet changing needs by varying the diameters of blood vessels. Secondly blood pressures are much higher in closed than in open systems, fluid is constantly filtered across capillary walls into the surrounding tissue spaces. Most of this fluid is drawn back into capillaries by osmosis, and lymphatic system.

The third factor is that since there is no separation of the extracellular fluid into blood plasma and lymph in case of hemolymph, the blood volume is large and may constitute 20% to 40% of body volume. By contrast, blood volume in animals with closed circulations is only about 5% to 10% of the body volume.

Q.7.      What are the different blood vessels in a vertebrate?

The blood vessels in the vertebrate system are: arteries, arterioles, venules, veins and capillaries. Fig. 4.2.

Arteries are the blood vessels which carry blood away from the heart, whether oxygenated or deoxygenated. To withstand high pounding pressures, arteries are invested with thick layers of both elastic and tough inelastic connective tissue. The central canal of an artery and all other blood vessels is a lumen. Surrounding the lumen are three layers or tunicae: tunica interna or inner layer with a single layer of endothelial cells; tunica media or middle layer composed of elastic and smooth muscle tissue; and tunica externs or external layer which consists of connective tissue. Arteries branch and narrow into arterioles the walls of which
are mostly with smooth muscles Contractions of these muscles narrow the arterioles and reduce the flow of blood to body organs Arterioles branch to form capillaries. Capillaries are
generally composed of a single layer of endothelial cells Capillaries are the most numerous blood vessels providing an enormous surface area for exchange of gases, fluids, nutrients and wastes between the blood and body cells. Blood leaving capillaries enters venules and then to veins. Veins are also composed of three layers, but the middle layer is thin having few elastic tissues. Veins contain one or more valves which permit blood flow in only one direction i.e. towards heart.

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Q.8.      What functions do coelomic fluid and hemolymph perform in invertebrates?

Ans. Some animals, such as echinoderms, annelids, sipunculans, use coelomic fluid as a supplementary or sole circulatory system. Coelomic fluid may be identical in composition to interstitial fluid or may differ. Coelomic fluid transports gases, nutrients, and waste products. In certain invertebrates (annelids) it also functions as a hydrostatic skeleton.

Hemolymph, like the coelomic fluid transports gases, nutrients and waste products. In arthropods especially insects, hemolymph pressure assists in molting of the old cuticle and in inflation of wings. In certain jumping spiders, hydrostatic pressure of hemolymph provides a hydraulic mechanism for limb extension. Some cells that do not contain respiratory pigments help in blood clotting in some cases; some cells act as phagocytes etc.

Q.9. What type of cells are found in hemolymph? What functions do these perform?

Ans. The coelomic fluid, hemolymph, or blood of most animals (with open circulatory system) contain circulating cells called hemocytes. Some cells contain respiratory pigment such as hemoglobin and are called

erythrocytes or ABC, and are present in high numbers to facilitate oxygen transport. Some cells help in blood clotting. The number and types of blood cells vary in different invertebrates:

  • Annelid blood contain hemocytes that function as phagocytes.
  • Coelomic fluid contain coelomocytes(amoebocytes, eleocytes, lampocytes, linocytes) that function in phagocytosis, glycogen storage, encapsulation, defense responses, and excretion.
    • In molluscs, hemolymph has two general types of hemocytes; amoebocytes and granulocytes, that perform functions similar to annelid’s hemocytes as well as

nacrezation (pearl formation) in some bivalves. Fig. 4.3. a Insect hemolymph contains large Adipohemocytes number of various hemocyte CLIII

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Q.10. Give an account of characteristics of vertebrate blood and blood cells.

Ans. Vertebrate blood is a specialized type of connective tissue fluid that circulates in the blood vessels of an animal body. It performs the following important functions.

i)      Blood transports oxygen, carbon dioxide, wastes and nutrients to and from the body cells.

ii)     Blood transports hormones from endocrine glands to target cells.

Hi) Blood cells defend against harmful micro organisms, cells, and viruses i.e., bring about immunity.

iv)    Platelets in blood prevent blood loss through clotting.

v)     Blood helps regulate body temperature and pH.

Composition: When centrifuged, blood is seen to form two layers; a watery fluid part called plasma which is about 55% of blood by volume, and formed elements, mostly red blood corpuscles, white blood cells, cell fragments (platelets in mammals) which form 45% of blood.


Plasma (formed or molded); is the straw colored liquid part of the blood. The composition of the mammalian blood is:

a)      Water 90%

b)      Dissolved solids consisting of plasma proteins, such as albumen, globulins, fibrinogen) = 7%, electrolytes, amino acids, glucose and other nutrients, various enzymes, hormones, metabolic wastes = 3% (serum is plasma from which blood clotting proteins, such as fibrinogen, have been removed).


The formed elements fraction or cellular components of vertebrate blood consists of:

i)       Erythrocytes or red blood cells/ corpuscles (RBC)

ii)      Leucocytes or white blood cells (WBC)

iii)     Platelets (thrombocytes)

Erythrocytes, leucocytes and platelets all develop from a common source, the pleuripotent or multipotential stem cells in• the red marrow of bones particularly the ribs, vertebrae, breast bone, and pelvis. (Pleuripotent means that these cells have the potential to differentiate into any type of formed element).

Red blood cells/ Erythrocytes



Red blood cells are present in enormous numbers in the blood (5.4 billion per ml of blood of man, and 4.8 billion per ml in adult women). The cells vary in size, shape, and number in different vertebrates. The red blood cells of most vertebrates are nucleated, while in mammals the nucleus shrinks during development and eventually disappears altogether. Ribosomes, mitochondria (generate their ATP anaerobically) and most enzyme systems are also lost. The cell is a biconcave disc bounded by a membrane enclosing about 280 million

molecules of hemoglobin (33% of the erythrocyte by weight). The biconcave shape provides more surface area for gas diffusion than a flat or spherical one. In other vertebrates RBC’s are nucleated and shape. Some fishes and amphibians also have eunucleated RBC.

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Among all vertebrates except mammals, the salamander Amphiuma has the largest RBC, the smallest in musk deer. Avian RBC’s are oval, nucleated and larger than mammalian RBC’s. Among birds Ostrich and among mammals the elephant has the largest RBC.


The major function of erythrocytes is to pickup oxygen from the environment, bind it to hemoglobin to form oxyhemoglobin, and transport it to body tissues. Blood rich in oxyhemoglobin is bright red, and bluish when viewed through vein after giving out oxygen. Hemoglobin also carries waste carbon dioxide as carbaminohemoglobin from tissues to gills or lungs for removal from the body. The average life span of an erythrocyte is 90— 120 days.

The number of erythrocytes in the blood is controlled by the amount of oxygen the tissues receive. If the tissues are not receiving enough oxygen the kidneys convert a plasma protein to a hormone called erythropoietin, which stimulates production of erythrocytes in bone marrow. That is why humans living at high altitudes have more erythrocytes.

White blood cells (Leucocytes)

White blood cells or leucocytes form a wandering system of protection. In adults they are around 7 to 10 thousand per ml of blood (a ratio of one white cell to 700 red cells) Classified on the basis of presence of certain granules in their cytoplasm, they are of two types: granulocytes and agranulocytes.


Granulocytes have abundant granules in their cytoplasm and are of three types:

1)        Neurtrophils are the most numerous of leucocytes (40 — 70%), have fine neutrophilic granules in the cytoplasm and an irregular lobed nucleus. They are chemically attracted to sites of inflammation and are phagocytic.

2)        Basophils have coarse basophilic granules and a bent nucleus, partially constricted into two lobes. These are the least numerous (0— 1%). When in contact with a foreign substance, these release histamine and heparin. Histamine causes blood vessels to dilate and heparin prevents blood clotting.

3)        Eosinophils also called acidophils or acidocytes are leucocytes readily stained by eosin, have nucleus with two lobes connected by a slender thread of chromatin. Normally they are 1 — 4%. They are phagocyte. They also release chemicals that counteract the effects of certain inflammatory chemicals released during allergic reactions.


Agrantdocytes are of two types: monocytes and lymphocytes.

1)        Monocytes arise from stem cells in the bone marrow and give rise to mononuclear phagocyte system, which are phagocytic cells stationed around the body as macrophages in lymph nodes, spleen and lungs, and as kupfer cells in sinusoids of the liver.

2)        Lymphocytes exist in two basic forms: T lymphocytes or T cells, and B lymphocytes or B cells, both of these are crucial in acquired immune responses. T lymphocytes are associated with the thymus gland before they colonize lymphoid tissue and play their role in immune response. When B cells are activated, they divide and differentiate to produce plasma cells.

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Q.11. Describe the process of blood coagulation

Ans. In vertebrates blood coagulation is the dominant haemostatic defense. Blood clots form as a tangled network of fibers from one of the plasma proteins, fibrogen. The transformation of fibrinogen into a fibrin mesh work that entangles blood cells to form a gel – like clot is catalyzed by the enzyme thrombin, normally present in blood in an inactive form called
prothrombin, which must be activated for coagulation to occur. In this process blood platelets play a vital role.

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When the normally smooth inner surface of a blood vessel is disrupted, either by a break or by deposits of cholesterol — lipid material, platelets rapidly adhere to the surface and release thromboplastin and other clotting factors. These factors, alongwith factors released from damaged tissue and with calcium ions, initiate conversion of prothrombin to active thrombin. The catalytic conversions are complex and may involve at least 13 different coagulation factors. Several kinds of clotting abnormalities in humans are known, one of which is hemophilia, a sex — linked trait Fig. 4.4a.

0.12. Describe the heart (two chambered/ venous) and circulatory system of a bony fish (single circulation circuit)

Ans. A fish heart (except lung fishes) contains two main chambers in series, an atrium

and a ventricle. The atruim is preceded by an enlarged chamber, the sinus venosus, which collects deoxygenated blood from the venous system. The blood from venous system enters the atrium from where it enters the muscular ventricle. Blood leaves the heart via the ventral aorta from where through branch arteries it enters the gills where blood becomes oxygenated, loses carbon dioxide, and enters the dorsal aorta. The dorsal aorta distributes blood to all of the body organs, and then blood returns to the heart via the venous system.

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Because blood (venous) only passes once through the heart in a complete circuit of the body, this system is called a single circulation circuit. In this circuit the heart must provide sufficient pressure to push the blood through two sequential capillary systems, first that of the gills, and then of the body. The principal disadvantage of this system is that the gill capillaries offer so much resistance to the blood flow that blood pressure to body tissues are greatly reduced which could not support the high metabolic rates present in birds and mammals.


.Q .13. Describe a double circulatory circuit.

Ans. The heart and blood vessels changed greatly as vertebrates moved from water to land and as endothermy evolved. The principal differences in the blood vascular system involve the gradual separation of the heart into two separate pumps as vertebrates evolved from aquatic life with gill breathing to fully terrestrial life with lung breathing. The blood passes through the heart twice during its circuit through the body, so it is called a double circulation circuit.

In modern amphibians the atrium is completely separated by inter atrial septum into two atria; the right atruim receives venous blood from the body while the left atruim receives oxygenated blood from the lungs. The ventricle is undivided, but venous and arterial blood remain mostly separate by arrangement of vessels and spiral valve in the heart. However, because most amphibians absorb more oxygen through their skin than through their lungs or gills, blood returning from the skin also contributes oxygenated blood to the ventricle. The blood pumped to the rest of the body is thus highly oxygenated. Fig. 4.5b.

In most of the reptiles, the ventricle is partially divided into a right and left side, Oxygenated blood from the lungs return to the left side of the heart via the pulmonary vein and does not mix much with deoxygenated blood in the right side of the heart. When the ventricle contract, the blood is pumped out into two aortae for distribution throughout the body as well as to lungs via pulmonary artery. The incomplete separation of the ventricle is an important adaptation for reptiles, such as turtles, because it allows blood to be diverted away from the pulmonary circulation during diving and when the body is withdrawn into its shell. This conserves energy and diverts blood to vital organs during the time when the lungs cannot be ventilated. Fig. 4.5c.

In crocodilians, birds and mammals there is complete anatomical separation of the ventricle, thus systemic (to body organs) and pulmonary (to lungs) circuits are now separate circulations, each served by one half of the dual heart. This type of circulation can maintain high blood pressure so important for rapid delivery of oxygenated nutrient rich blood to tissues with high metabolic rates. Fig. 4.5d.


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