Answer of Question of Circulation Immunity & Gas Exchange

Answer of Question of Circulation Immunity & Gas Exchange

.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turtles, because it allows blood to be diverted away from the pulmonary circulation during diving and when the body is withdrawn into its shell.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.

Q.14. Describe The Structure of Human Heart.

Ans. The human heart is of myogenic type i.e. the heart beat is initiated in specialized

muscle cells (pace makers). Althouhg the nervous system does alter pace maker ativity, a myogenic heart will beat spontaneously if removed from the body, as against a neurogenlc heart (crustacean heart) which stops beating when cardiac ganglion which acts as pace maker in these hearts, is removed. Fig. 4.6.

The heart of man, as in other mammals, and birds, is a four chambered heart composed of three layer — an outer fibrous connective tissue layer, the epicardium; a much thick middle muscular layer of cardiac muscles myocardium; and an inner layer the endocardium composed of connective tissue and single layer of epithehal cells (endothelium). Human heart is located in the thorax and covered by a tough, fibrous sac, the pericardium. The right and left halves are two separate pumps, each containing two chambers. In each half,

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blood first flows into upper thin walled atrium, then into lower thick walled ventricle through atrio-ventricular aperture guarded by atrioventricular valves. The valve between right atrium and right ventricle is named tricuspid valve, and between left atrium and left ventricle is the bicuspid or mitral valve. The right ventricle pumps the blood into pulmonary aorta. At the entrance of pulmonary aorta ace pulmonary semilunar valves and at the entrance of aorta are aortic semilunar valves. The aorta arises from the left ventricle. All the valves open and close due to blood pressure changes when the heart contracts during each heart beat. All the valves in the heart, like the valves in veins, keep blood moving in one direction, preventing back flow.

As an average, the heart of an adult beats 70 times per minutes (100,000 times/ day or more than 2.6 billion times in a 70 year life time). It pumps about 5 liters of blood in one minute (8000 liters through 96,000 km of blood vessels each day).

Q.15. Describe events in a cardiac cycle.

Ans. The sequence of muscle contractions and relaxations in ah heart beat is called the cardiac cycle. During each cycle the atria and ventricles go throug a phase of contraction called systole and a phase of relaxation called diastole. When the atria contract (atrial systole), the ventricles relax (ventricle diastole), and ventricular systole is accompanied by atrial systole. Fig. 4.6a.

The heart beat is initiated by a small mass of tissue called “pacemaker” or sinoatrial node (SA node) located at the entrance to the right atruim (nervous innervation is not necessary). The SA node produces an action potential that spreads over both atria, causing them to contract simultaneously. The action

potential then passes to the atrioventricular node (AV node), near the  interatrial septum. From here the action potential continues through the atrioventricular bundle
 (bundle of His), at the tips of the interventricular septum divides into right and left             branches, which are  continuous with the Purkinje fibres in the Human Heart in systole & diastole. ventricular walls. Stimulation of these fibers causes the ventricles to contract simultaneously and eject blood into the pulmonary and systemic circulations. The contraction begins at the apex or tip of the ventricles and spread upward to squeeze out the blood. Fig. 4.7.


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Q.16. What do you mean by blood pressure?

Ans. Blood pressure is the force the blood exerts against the inner walls of the blood

vessels and is responsible to circulate the blood throughout the body. The term blood pressure is commonly refered to systemic arterial blood pressure. Arterial blood pressure rises and falls in a pattern corresponding to the phase of the cardiac cycle. It is higher during ventricular systol being 120 mm Hg in a healthy adult. It is called systolic pressure, while diastolic pressure is about 80 mm Hg written as 120/80.

Q.17. What is a lymphatic system? What are its main functions?

Ans. The lymphatic system of vertebrates is an extensive system of thin — walled vessels that arise as blind — ended   lymph capillaries in most tissues of the body. Lymph capillaries unite to form lymph vessels which finally drain into veins in the lower neck in case of man. Located at intervals along the lymph vessels are lymph nodes, and valves that ensure one way flow of    lymph. Fig.4.8.

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The lymphatic vessels perform four major functions:

i)  These collect and drain most of the fluid that seeps from the blood out of capillaries fluid.

H) These return small amounts of proteins that have left the cells.

Hi) These vessels transport lipids that have been absorbed from the intestine.

iv) These transport foreign particles and cellular debris to disposal centres called lymph nodes.

In addition to above mentioned parts, the lymphatic system of birds and mammals consists of lymphoid organs — the spleen, bursa of fabricius in birds or the thymus gland, tonsils and adenoids in mammals following is the summary of major components of lymphatic system.



A plethora of viral, prokaryotic, and eukaryofic parasites exists in every animals environment, and a defense (immune) system is crucial to survival. Immunity can be defined concisely as possession of tissues capable of recognizing and protecting the animal against nonself invaders. Most animals have some amount of innate (nonspecific) immunity, and vertebrates develop acquired (specific) immunity. The surface of most animals provides as physical barrier to invasion, and vertebrates have a variety of antimicrobial substances in their body secretions.

Phagocytes engulf particles and usually digest or kill them with enzymes and cytotoxic secretions. Many invertebrates have specialized cells that can perform defensive phagocytosis. Several kinds of vertebrate cells, especially macrophages and neutrophils, are important phagocytes, and cells of the mononuclear phagocyte system (RE system) reside in various sites in the body. Eosinophils are important in allergies and many parasitic infections. Basophils, mast cells! T and B lymphocytes, and natural killer cells are not phagocyte but play vital roles in defense.

An immune response is elicited by an antigen. Vertebrates demonstrate increased resistance to specific foreign substances (antigens) on repeated exposure, and the resistance is based on a vast number of specific recognition molecules; antibodies and T-cell receptors. Nonself recognition depends on markers in cell surfaces known as major histocompatibility (MHC) proteins. Antibodies are borne on the surfaces of B lymphocytes (B cells) and in solution in the blood after secretion by the progeny of B cells, the plasma cells. T-cell receptors occur only on the surfaces of T lymphocytes (T cells).

The cells of immunity communicate with each other and with other cells in the body by means of protein hormones called cytokines such as interleukins, tumor necrosis factor, and interferon -y. The two arms of the vertebrate immune response are the humoral response (TH2), involving antibodies, and the

cell-mediated response (TH1), involving cell surfaces only. When one arm is activated or stimulated, its cells produce cytokines that tend to suppress activity in the other arm. Activation of either arm requires that the antigen be consumed by an APC (antigen-presenting cell, usually a macrophage), which partially digests the antigen and presents its determinant (epitope) on the surface of the APC along with an MHC class II protein. Extensive communication by cytokines and activation (and suppression) of various cells in the response leads to production of specific antibody or proliferation of T cells with the specific receptors that recognize the antigenic epitope. After the initial response, memory cells remain in the body and are responsible for enhanced response on next exposure to the antigen.

Damage to the immune response done by HIV (human irnmumedeficiency virus) in production of AIDS (acquired immune deficiency syndrome) is due primarily to destruction of a crucial set of T cells: those bearing the CD4 coreceptor on their surface.

Inflammation is an important part of the body’s defense; it is greatly influenced by prior immunizing experience with an antigen.

People have genetically determined antigens in the surfaces of their red blood cells (ABO blood groups and others); blood types must be compatible in transfusion, or the transfused blood will be agglutinated by antibodies in the recipient.

Many invertebrates show nonself recognition by rejection of xenografts or allografts or both. In some cases they may show enhanced response on repeated exposure.

Q.18. What is immunity? What are nonspecific and specific defenses?

Ans. Immunity refers to the general ability of an animal to resists harmful attack. An animal demonstrates immunity if it possesses tissues capable of recognizing and protecting the animal against foreign (nonself) invaders. Invertebrates do not have immune systems with special cells that recognize and destroy specific foreign agents. However many have innate, internal defense mechanisms e.g; molluscs and insects have granulocytes that are highly phagocytic to foreign agents; melanization in arthropods, which involve depositing melanin around the foreign body.



Vertebrates, which are continuously exposed to microorganisms, foreign macromolecules, or cancer cells, have a complex, specific, defense system. These defenses can be divided into innate or non-specific and specific defenses. Nonspecific or innate defenses are general mechanisms that are inherited by the animal and act as a part of defense against intruders, before they can cause disease. These include physical barriers, such as; unbroken integument, mucus, inflammation, and phagocytosis; chemical barriers, enzyme action, interferon, low pH in stomach and vagina, immunoglobin A, and lysozyme; general barriers, such as, fever.

Specific defenses consist of a number of immunological mechanisms in which certain white blood cells, the lymphocytes, recognize the presence of particular foreign invader (microorganism) or substances (antigens) and act to eliminate them. Lymphocytes may directly destroy the antigens or may form specialized proteins called antibodies which either destroy the invader or target it for destruction by other cells. See table below:


Q,19. How does an antibody differ from an antigen?

Ans. Ms. An antibody also known as immunoglobin, is a specific substance that cells produce in response to the presence of an antigen. It reacts with the antigen. These are recognition glycoproteins present in the blood and tissue fluids of birds and mammals.

An antigen is a foreign (nonself) substance such as, a protein, nucleoprotein, polysaccharide and some glycolipid, to which lymphocytes respond by producing specific immunological reactions leading to its removal. Most antigens (immunogens) are proteins with molecular weight generally greater than 10,000.

Q.20. Describe structure of an antibody.

Ans. The basic antibody molecule consists of four polypeptide strands: two identical light chains and two identical heavy chains held together in a Y- shape by disulfide bonds and hydrogen bonds. The amino acid sequence towards the ends of the Y varies in both the heavy and light chains, according to the specific antibody molecule, and this variation determines with which antigen the antibody can bind. Each of the ends of Y forms a cleft that acts as the antigen binding site which is variable (according to antigen) and is called as Fab (antigen — binding fragment). Rest of the part is known as constant region

(crystallizable fragment). There are five types of heavy chains, each .determine the class of the antibody, known as 1g M, 1g G (gamma globulin), 1gA, 1gD, 19E. Fig. 4.9.


Q.21. What is meant by humoral immunity and cell — mediated immunity?

Ans.Humoral immunity is a type of immunity that results from the presence of antibodies that are soluble in blood and lymph. When a foreign invader stimulates B cells, they divide into plasma cells. Plasma cells respond to invaders by secreting specific antibodies into the blood and lymph. This humoral response defends mostly against bacteria, bacterial toxins, and viruses that enter the body fluid systms of an animal.

Cell mediate immunity is a type of immunity resulting from T cells coming into close physical contact with foreign cells or infected cells to destroy them. It can be transferred to a nonimmune individual by the transfer of cells. In this case T cells are directly involved in destroying invading cells or host cells that have been

invaded by microorganisms such as fungi, protozoa, and worms, or tissue cells transplanted from one animal to other, or cancer cells.

Q.22. How do NK (Natural Killer) cells function? or How do cell — mediated immune response functions?

Ans. Natural Killer (NK) cells, also called cytotoxic T cells, recognize cell — surface

changes on cancer cells, virus infected cells, fungi, bacteria, protozoa, or helminth parasites. Upon contact with a cancer cell, a natural killer cell releases toxic chemicals, such as perforin-1, that punch hole in the cancer cell’s plasma membrane, causing the cancer cell to die. It is due to the action of NK cells that tissue and organ transplants are rejected in birds and mammals (except in case of between identical twins who have identical sets of DNA). In the rejection


mechanism, NK cells enter the transplanted tissue through the blood vessels, recognize it as foreign, attach to the tissue, and destroy it.

T cells also regulate the activity of other parts of the immune system. For example when bacteria invade the skin through a cut, inflammation occurs, and macrophages phagocytize the bacteria. Macrophages destroy most of the antigens (bacteria), but some of the antigens are moved to the surface of the plasma membrane of both macrophage, and B cells, where they are displayed alongside the self – recognition marker. Self- recognition marker and antigen is recognized by helper T cells.


When the macrophage reacts with a helper T cell, it releases interleukin – 1 (IL1). IL1 stimulates other helper T cells to secrete interleukin – 2 (IL2), which stimulates growth and cell division of helper T cells. Some of the 1L2 acts on sensitized B cells. Sensitized B cells have recognized and processed the antigen onto their plasma membrane alongside their self – recognition marker. These stimulated B cells mature and divide into differentiated plasma cells.

Q.23. Define acquired immunity. Describe its various forms.

Ans. Acquired immunity is a type of specific immunity that develops after exposure to

a suitable antigen or is produced after antibodies are transferred from one individual to other. It can be natural or artificial, active or passive.


Naturally acquired active immunity occurs when an animal’s immune system comes into contact with an appropriate antigenic stimules. The immune system responds by producing antibodies and lymphocytes that inactivate the antigen. The immunity thus produced may be either lifelong as in measles or chicken pox or lasts for only a few years as in tetanus.

Naturally acquired passive immunity involves the transfer of antibodies from one host to another e.g. in a pregnant mammal, some of the female’s antibodies pass across the placenta to the fetus, or the baby gets some of the mother’s antibodies in colostrum. This type of immunity last for few weeks or few months only.


Artificially acquired active immunity result from immunization of an animal with a vaccine which contain killed or weakened microorganisms (e.g. polio virus) ‘ or contain toxoids (inactivated bacterial toxins)

Artificially acquired passive immunity results from introducing antibodies into an animal which have been produced in some other animal or produced in vitro, e.g. botulinum antitoxins produced in a horse are given to human suffering from botulism food poisoning.

Q.24. What is AIDS?

Ans. Acquired immune deficiency syndrome (AIDS) is a devastating disease of the immune system which according to the best available evidence, first appeared in Africa in the early 1960s. Some researchers believe it is caused by a mutant form of some previously nonfatal human virus, while other believe it evolved in the green monkey and was transferred to humans through bites. What ever its origin, the AIDS virus, HTLV-III (human T cell lymphotropic virus III), has spread rapidlly, first in Africa (where, given the many prevalent diseases, it was not recognized as novel until recently), and then to other parts of the world, notably Haiti, the United States, and Europe.

Careful epidemiological studies show that transmissions is almost always from the blood or semen of one individual to the circulatory system of another. (The virus is also present in the saliva and tears of its victims). Hence it readily spreads through transfusions of diseased blood, the reuse of hypodermic needles, and anal intercourse. While the latter mode of transmission currently accounts for the majority of deaths in the West, where the disease is largely confined to male homosexuals, it probably cannot explain its transmission in Africa, were the disease is common to both sexes. The possibility of an unknown alternative route for infection, presumably a heterosexual one, is a matter of great concern.

AIDS virus, HTLV-III, is a retrovirus — an RNA virus that carries with it the enzyme reverse transcriptase, which makes a DNA copy of the viral chromosome and inserts it into the host genome. Apparently, it preferentially infects T4 lymphocytes (one subclass of helper T cells), where it remains hidden until its virulent phase is triggered. There is c onsiderable controversy over what may trigger the virulent phase, but preliminary evidence suggests that infections by hepatitis B or by Epstein – Barr virus may be involved. In any case, once triggered, the viral DNA begins producing new viruses, which bud off from the host cells. Unlike other retroviruses, HTLV – Ill can kill the cell it infects or, in

some cases, it may cause infected cells to fuse, thereby diminishing their function. But the death of T4 cells or the disruption of their normal function is not the direct cause of AIDS fatalities; instead, victims succumb to subsequent, normally minor infections which rage unchecked because the helper T cells responsible for activating the appropriate immune defenses are either missing or no longer effective. So far as is known, AIDS is inevitably fatal, and the number of deaths among those infected by the virus is doubling every 9— 12 months.

Q.25. How does immunological memory occur?

Ans. During the first encounter with an antigen the animal responds by producing

antibodies. During this response the antigen disappears from the body due to phagocytosis etc. Most of the B cells that produce antibodies also die. However, if the same antigen enters the body a second time, the body mounts a secondary immune response that is faster and more extensive than the first or primary response. This rapid response is possible because the immune system has stored a memory in the form of memory cells (B cells) that remain in the body for life. These memory cells repidly produce large numbers of antibody secreting B cells.


Very small animals can depend on diffusion between the external environment and their tissues or cytoplasm for transport of respiratory gases, but larger animals require specialized organs, such as gills, tracheae, or lungs, for this function. Gills and lungs provide an increased surface area for exchange of respiratory gases between blood and environment. Many animals have special respiratory pigments and other mechanisms to help transport oxygen an carbon dioxide in blood. The most widespread respiratory pigment in the animal kingdom, hemoglobin, has a high affinity for oxygen at high oxygen concentrations but releases it at lower concentrations. Vertebrate hemoglobin, which is packaged in red blood cells, combines readily with oxygen in gills or lungs then releases it in respiring body tissues where the oxygen partial pressure is low. Blood carries carbon dioxide from the tissues to the lungs as bicarbonate ion, in combination with hemoglobin, and as dissolved gas

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