Frequently asked questions on the circulatory system and immunity (SHS Biology).


What are the three basic features of animal circulatory systems?

The three basic features of animal circulatory systems are:

  • a specialized fluid medium for transporting molecules, exemplified by the blood of vertebrates;
  • a muscular heart for pumping the fluid; and
  • tubular vessels for distributing the fluid pumped by the heart.

List and describe the functions of the three types of vessels in a cardiovascular system.

  • Arteries carry blood away from the heart,
  • Capillaries exchange their contents with tissue fluid, and
  • Veins return the blood to the heart.

What is the difference between an open and closed circulatory system? Why do you think humans could not function with an open circulatory system?

  • In an open circulatory system, there is no distinction between blood and interstitial fluid. Vessels from the heart release haemolymph directly into body spaces and the fluid is subsequently collected and re-enters the heart. In a closed circulatory system, blood is channeled in blood vessels leading to and from the heart and is distinct from the interstitial fluid.
  • In an open circulatory system, most of the fluid pressure generated by the heart dissipates when the blood is released into the body spaces. Consequently, blood flows relatively slowly. Humans could not function as we do with such a system, because we would not be able to distribute oxygen efficiently throughout the body, nor would we be able to eliminate the wastes we produce.

Contrast a one-circuit circulatory pathway with a two-circuit pathway.

  • The one-circuit pathway utilizes a heart with one atrium and one ventricle to send blood to the gill capillaries and then the systemic capillaries in a single loop.
  • The two-circuit pathway pumps blood to both the pulmonary and systemic capillaries simultaneously

Distinguish among atria, ventricles, arteries, and veins.

  • Atria are chambers of the heart that receive blood returning to the heart.
  • Ventricles are chambers of the heart that pump blood from the heart.
  • Arteries are vessels of circulatory systems that conduct blood away from the heart at relatively high pressure.
  • Veins are vessels of circulatory systems that carry blood back to the heart.

Distinguish among the systemic, pulmocutaneous, and pulmonary circuits.

  • The systemic circuit of the circulatory system is the circuit from the heart to most of the tissues and cells of the body and back to the heart.
  • The pulmocutaneous circuit goes from the heart to the skin and lungs or gills in amphibians and back to the heart.
  • The pulmonary circuit goes from the heart to the lungs and back to the heart.
Structure of the mammalian heart IMG 0.1

Why can’t you give type A blood to a type B recipient?

  • A type B recipient has anti-A antibodies in the plasma,
  • They will react with the donor’s red blood cells, causing agglutination.

Outline the life cycle of an erythrocyte.

  • Erythrocytes originate as pluripotent stem cells in the red bone marrow.
  • In humans and other mammals, they lose their nucleus, cytoplasmic organelles, and ribosomes as they mature, becoming essentially a membrane-bound haemoglobin reservoir that is not capable of protein synthesis.
  • At the end of their lifespan of about 4 months, erythrocytes are engulfed by macrophages, a type of leukocyte, in the spleen, liver, and bone marrow.

How does the body compensate for a lower-than-normal level of oxygen in the blood?

  • Low oxygen content triggers a negative feedback mechanism to increase erythrocytes in the blood.
  • The kidneys are stimulated to synthesize the hormone erythropoietin (EPO), which stimulates stem cells in the bone marrow to increase erythrocyte production.
  • EPO synthesis is stopped when the oxygen content of the blood rises above normal levels.

Contrast the structure and function of the right and left ventricles.

  • The wall of the left ventricle is thicker than the wall of the right ventricle, and it generates a greater pressure than the right ventricle.
  • The right ventricle pumps blood into the pulmonary circuit, which takes blood only to the lungs for gas exchange, while the left ventricle pumps blood into the systemic circuit, which takes blood to all the cells of the body

What are the roles of leukocytes and platelets?

  • Leukocytes act as the first line of defense against invading organisms, eliminate dead and dying cells from the body, and remove cellular debris. Platelets assist in blood clotting.
  • When blood vessels are damaged, platelets stick to the collagen fibres exposed to leaking blood and recruit other platelets to the site.
  • Eventually a plug forms at the site, sealing off the damaged area.

Trace the path of blood through the heart from the vena cava to the aorta.

  • From the body, the blood moves through;

vena cava, right atrium, tricuspid valve, right ventricle, pulmonary semilunar valve, pulmonary trunk, and arteries.

  • From the lungs the blood move;

pulmonary veins, left atrium, bicuspid valve, left ventricle, aortic semilunar valve, and aorta.

What is the role of each of the four chambers of the mammalian heart in blood circulation?

  • The right atrium receives blood in the systemic circuit returning to the heart in vessels coming from the entire body, except the lungs. The right atrium pumps blood into the right ventricle.
  • The right ventricle receives blood from the right atrium, and pumps it into the pulmonary arteries going to the lungs, beginning the pulmonary circle.
  • The left atrium receives blood returning from the lungs in the pulmonary veins, completing the pulmonary circle. The left atrium pumps this blood into the left ventricle.
  • The left ventricle pumps the blood received from the left atrium into the aorta, where the blood begins its path in the systemic circuit.

Explain what happens during a heartbeat and what makes the familiar lub-dub sounds.

  • First the atria contract, then the ventricles contract, and then they both rest.
  • The lub sound occurs when the atrioventricular valves close,
  • The dub sound occurs when the semilunar valves close.

Distinguish between systole and diastole.

  • Systole is the period of contraction and emptying of the heart.
  • Diastole is the period of relaxation and filling of the heart between contractions.

Why is it important to regulate arterial blood pressure?

  • Arterial blood pressure must be regulated within limits to provide sufficient blood flow for the brain and other tissues
  • To prevent damage to blood vessels, tissues, and organs that would occur at high blood pressure.

What are the three main mechanisms for regulating blood pressure?

The three main mechanisms for regulating blood pressure are:

  • controlling cardiac output (the pressure and amount of blood pumped by the ventricles);
  • controlling the degree of constriction of the blood vessels (mostly the arterioles); and
  • controlling the total blood volume.

List the main functions of blood.

  • Blood transports substances to and from the capillaries
  • Defends against pathogen invasion
  • Helps regulate body temperature,
  • Forms clots to prevent excessive blood loss.
The path way of blood IMG 0.2

Describe how a blood clot forms.

  • Platelets accumulate at the site of injury,
  • Then release a clotting factor that results in the synthesis of thrombin.
  • Thrombin synthesizes fibrin threads that provide a framework for the clot.

What is lymph, and how does it enter the lymph capillaries?

  • Lymph is interstitial fluid, an aqueous solution containing molecules, ions, infecting bacteria, damaged cells, cellular debris, and lymphocytes.
  • Because the lymph capillaries consist of a single layer of endothelial cells, the lymphatic fluid can enter them at sites where the endothelial cells overlap when the pressure of the interstitial fluid forces the flaps open.
  • The openings produced are large enough for cells to enter

Summarize the functions of the lymphatic system.

  • The lymphatic system absorbs fats, returns excess tissue fluid to the bloodstream,
  • Produces lymphocytes,
  • Also, helps defend the body against pathogens.

What features of epithelial surfaces protect against pathogens?

  • Mucous layers are physical barriers to pathogens.
  • Mucous layers may contain toxins and other chemicals that kill pathogens.
  • The aqueous environment in contact with an epithelial surface may inhibit or kill pathogens by being acidic, or by containing enzymes or bile juices.

Give a brief description of the lymphatic system.

  • The lymphatic system consists of lymphatic vessels, which have the same structure as cardiovascular veins,
  • The lymphatic organs: red bone marrow, lymph nodes, and spleen.

What are the key differences between innate immunity and adaptive immunity?

  • Innate immunity provides a nonspecific, immediate response to a pathogen.
  • It is the first response system that comes into play when a pathogen is encountered, but it retains no memory of the encounter.
  • The innate immune responses include inflammation and specialized cells that attack pathogens.
  • Adaptive immunity, by contrast, provides a specific response to a pathogen, and it retains a memory of exposure to the pathogen so that it can respond more quickly to future attacks.
  • The adaptive immune response takes several days to become protective, in contrast to minutes for an innate immune response.

What are the usual characteristics of the inflammatory response?

  • The inflammatory response is characterized by heat, pain, redness, and swelling at the infection site.

Why does combating viral pathogens require a different response by the innate immune system than combating bacterial pathogens?

  • The complement system is a nonspecific defense mechanism in which more than 30 interacting plasma proteins are activated by molecules on the surface of pathogens.
  • Activated complement system proteins produce membrane attack complexes that insert into the plasma membrane of many types of bacterial cells and coat other types of cells with fragments of the complement proteins.
  • Membrane-attack complexes create pores that lead to loss of osmotic balance, which causes the pathogens to lyse.
  • Pathogens coated with complement protein fragments are recognized by phagocytes that then engulf and destroy the pathogens.

What are the two main strategies a host uses to protect against viral infections?

  • Viral pathogens do not have surface molecules that are distinct from those of the host.
  • The main strategies used by the host in innate immunity are interferon and natural killer cells.

Describe the general structure of an antibody molecule.

  • An antibody is a member of the immunoglobulin (Ig) family of proteins.
  • It consists of four polypeptides, two of which are identical heavy chains and two of which are identical light chains.
  • Structurally, an antibody looks like a Y.
  • Each of the two arms of the Y involves pairing of a light chain with part of the heavy chain.
  • The tail of the Y consists of the remaining parts of the heavy chains paired together.
  • Each heavy chain and light chain have a variable and constant region.
  • The variable regions, which occur at the top half of the arms of the Y, form two identical binding sites for the antigen with which the antibody can react.

 What is clonal selection?

  • Clonal selection is the process by which an antigen stimulates the production of a clone of B-cell-derived plasma cells that secrete antibodies against that antigen.
  • Clonal selection accounts for the rapid response, specific action, and diversity of antibodies seen in the adaptive immune system.
  • It also accounts for immunological memory by producing memory B cells and T cells.

How does immunological memory work?

  • Immunological memory is the aspect of adaptive im- immunity that allows for a rapid, intense immune reaction to develop if the body encounters an antigen, it has seen before.

What is immunological tolerance?

  • Immunological tolerance, a feature of the adaptive immune system, protects the body’s molecules from attack by its immune system.
  • B cells and T cells are involved in the development of immunological tolerance, but the exact process is not understood.

Explain how a failure in the immune system can result in an allergy

  • Basically, an allergy results from an overreaction of the immune system to a particular antigen.
  • The substances responsible for allergic reactions are a class of antigens known as allergens.
  • Allergens stimulate B cells to secrete IGE antibodies in high amounts.
  • IGE binds to mast cells, which are then induced to oversecrete histamine.
  • Histamine contributes to inflammation, and at high amounts, the histamine-induced inflammation can be severe and even life-threatening.

What steps accomplish a specific defense?

Specific defense requires that the immune system be able to

  • recognize,
  • respond to, and
  • remember foreign antigens.

What types of complications and disorders are associated with the functioning of the immune system? Explain each of these.

  • Tissue rejection occurs when foreign tissues and organs are rejected by the body.
  • Autoimmune diseases (e.g., rheumatoid arthritis) occur when cytotoxic T cells or antibodies mistakenly attack the body’s cells.
  • Immune deficiency occurs when the immune system is deficient, as in AIDS. Allergies occur when the body responds to environmental substances such as pollen that normally do not provoke a response.
The lymphatic system IMG 0.3

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