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The Circulatory System
1. What is circulation?
Circulation is the movement of substances like nutrients and gases within blood vessels and cavities throughout the organism.
2. Do all animals have a circulatory system?
Not all animals have a circulatory system.
Poriferans, cnidarians, platyhelminthes and nematodes (in these there are the pseudocoelom fluid but no vessels) are avascular animals. Echinoderms do not have true circulatory systems either.
3. What is the alternative means for transport of substances in animals without a circulatory system? Why is blood important for larger animals?
In animals that do not present the circulatory system the transport of substances occurs by cell to cell diffusion.
The blood is a fundamental means of substance transport for larger animals since in these animals there are tissues distant from each other and from the environment thus making diffusion impossible.
4. What are the two types of circulatory systems?
The circulatory systems can be classified into open circulatory system and closed circulatory system.
5. What is an open circulatory system?
Open circulatory system is the one in which blood does not circulate only inside blood vessels but it also falls in cavities that irrigate tissues. In the open circulatory system the blood pressure is low and generally the blood (called hemolymph) has low cellularity.
Arthropods, molluscs (the cephalopods are exception) and protochordates have open circulatory system.
The Circulatory System - Image Diversity: open circulatory system
6. What is a closed circulatory system?
A closed circulatory system is one in which blood circulates only inside blood vessels. For this reason the blood pressure is higher in animals with closed circulatory system. The cellularity of the blood is also higher with many specific blood cells.
The closed circulatory system is a feature of annelids, cephalopod molluscs and vertebrates.
The Circulatory System - Image Diversity: closed circulatory system
7. What are the advantages of the closed circulatory system over the open circulatory system?
The closed circulatory system is more efficient. Since blood circulates only inside blood vessels it can do it with more pressure reaching farther distances between the organs where hematosis happens and the peripheral tissues. In addition the circulatory speed also heightens making possible more oxygen supply to great consuming tissues, like the muscle tissues that then can perform faster movements. Animals with an open circulatory system (with the exception of insects that do gas exchange independently from the circulation) are generally slower and have a low metabolic rate.
8. What is the difference between octopuses and mussels regarding their circulatory systems? How does that difference influence the mobility of these animals?
Cephalopod molluscs, like octopuses and squids, have a closed circulatory system with blood pumped under pressure flowing within vessels. Bivalve molluscs, like mussels and oysters, have an open circulatory system (also known as lacunar circulatory system) where blood flows under low pressure since it falls in cavities of the body and does not only circulate within blood vessels. Molluscs with closed circulatory systems are larger, agile and can actively move; molluscs with open circulatory systems are smaller, slow and some are practically sessile.
9. Why, even thoough they have an open circulatory system, can flying insects like flies beat their wings with great speed?
In insects the circulatory system is open but this system does not participate in the gas exchange process and in oxygen supply to the tissues. Gases go in and out through the independent tracheal system that allows direct contact of cells with the ambient air. Therefore an insect can supply the great oxygen demand of its fast-beating wing muscles even having open circulatory system.
10. What are the typical components of a closed circulatory system?
The typical components of the closed circulatory system are the blood vessels within which blood circulates (arteries, veins and capillaries), a pumping organ (heart) and the blood or bloodlike fluid.
The Circulatory System - Image Diversity: blood vessels
11. How does the heart impel the blood?
The heart is a muscular organ that contains chambers (right atrium and right ventricle and left atrium and right ventricle) through which blood passes. The blood enters the heart in the atria, goes to the ventricles and then leaves the organ.
The blood is pumped out of the heart by the contraction of the muscle fibers that form the ventricular walls. The contraction reduces the ventricle volume thus increasing the internal pressure and the blood then flows to the exit vessels (pulmonary artery for the right ventricle and aorta for the left ventricle). When ventricular muscle fibers distend the ventricles regain their original size and receive new blood flow coming from the atria.
The Circulatory System - Image Diversity: heart structure
12. What is the difference between systole and diastole
Systole and diastole are the two stages into which the cardiac cycle is divided. Systole is the stage when the contraction of ventricular muscle fibers occurs and the ventricles are emptied. Diastole is the stage of the cardiac cycle when the ventricular muscle fibers distend and the ventricles are filled with blood.
The Circulatory System - Image Diversity: cardiac cycle
13. What are arterial vessels, arteries and arterioles?
Arterial vessels are every blood vessel that carries blood from the heart to the tissues. Arteries and arterioles are arterial vessels. Arterioles are thin arteries that end in capillaries.
Not all arteries however contain arterial blood (highly oxygenated blood). The pulmonary artery and its ramifications, arteries that carry blood from the right heart ventricle to the lungs, contain venous blood.
The Circulatory System - Image Diversity: arteries arterioles
14. What are venous vessels, veins and venules?
Venous vessels are every blood vessel that carries blood from the tissues to the heart. Veins and venules are venous vessels. Venules are thin veins that are continuous to capillaries.
In general venous vessels carry venous blood. The pulmonary veins that carry blood from the lungs to the left atrium of the heart however contain arterial blood.
The Circulatory System - Image Diversity: veins venules
15. What are the capillaries of the vascular system?
Capillaries are small blood vessels that perform exchange of substances between the blood and the body tissues. Capillaries are neither arteries nor are they veins since they have distinct features. In capillaries the wall is made of a single layer of endothelial cells through which substances are exchanged. These vessels receive blood from the arterioles and drain to the venules.
The Circulatory System - Image Diversity: blood capillaries
16. What is the part of the vascular system that performs exchange of gases and other substances with the tissues?
Only capillaries perform exchange of gases and other substances with the tissues.
17. Are the arteries or the veins constituted of more muscle tissue? How different are the walls of these two types of blood vessels?
The arterial system has thicker muscle walls since within arteries the blood circulates under higher pressure. The veins are more flaccid than arteries.
From the lumen to the external layer both types of vessels are made of endothelium, muscle tissue and connective tissue. In both the endothelium is a single layer of cells. In arteries the muscle tissue portion is thicker than in veins and in these vessels the external connective tissue is thicker than in arteries.
Arteries are the pulsating blood vessels. The arterial pulse can be felt in a medical examination, for example, by the palpation of the radial artery in the internal and lateral face of the wrist near the base of the thumb.
The Circulatory System - Image Diversity: artery histology vein histology
18. What are the valves of the venous system? What is their function?
The valves of the venous system are structures inside the veins that permit blood to flow only in the normal way (from the tissues to the heart) and forbid it to return in the reverse way in favor of gravity. The valves close when the pressure of the fluid column above (after, regarding normal flux) is higher than the fluid pressure before them. Valves are thus fundamental for the returning of blood to the heart.
The Circulatory System - Image Diversity: Venous valves
19. How do the muscles of the legs and of the feet contribute to the venous return?
The muscles of the legs, mainly the muscles of the calves, contract and compress the deep veins of the legs impelling the blood to the heart.
The plantar portion of the feet retains blood and when it is compressed against the ground it impels its blood volume and aids venous return.
20. What are varices? Why are they more common in the inferior limbs?
Varix means abnormal enlargement of veins. Varices occur when excessive pressure against the normal blood flux creates enlargement of the vein and thus insufficient functioning of its valves (venous insufficiency).
Varices are more common in the veins of the inferior limbs since the fluid column above these vessels is higher. This is the reason why people that spend much time standing (e.g., surgeons) are more susceptible to varices.
In general varices are not the apparent superficial veins that appear in the leg of varix patients. These apparent vessels are the consequences of internal varices (venous insufficiency) in the deep internal veins of the legs and they appear because the blood flux is diverted to superficial veins. (Popularly however apparent superficial veins are called varices.)
The Circulatory System - Image Diversity: varices
21. What is the lymphatic system?
The lymphatic system is a network of specialized valved vessels that drain interstitial fluid (lymph). The lymphatic system is also responsible for the transport of chylomicrons (vesicles that contain lipids) made after the absorption of fats by the intestinal epithelium.
In the way of the lymphatic vessels there are ganglial-like structures called lymph nodes that contain many cells of the immune system. These cells filter impurities and destroy microorganisms and cellular wastes. The lymphatic vessels drain to two major lymphatic vessels, the thoracic duct and the right lymphatic duct, that in their turns drain into tributary veins of the superior vena cava.
The Circulatory System - Image Diversity: lymphatic system
22. Why in inflammatory and infectious conditions may clinical signs related to the lymphatic system occur?
The lymph nodes, or lymph glands, have lymphoid tissue that produces lymphocytes (a type of leukocyte). In inflammatory and infectious conditions the enlargement of lymph nodes of the lymphatic circuits that drain the affected region due to the reactive proliferation of leukocytes is common. This enlargement is known as lymphadenomegaly and sometimes it is accompanied by pain. The search for enlarged or painful lymph nodes is part of the medical examination since these findings may suggest inflammation, infection or other diseases.
The Circulatory System - Image Diversity: lymphadenopathy
23. Which are the heart chambers respectively where the entrance and the exit of blood occur?
The heart chambers through which blood enters the heart are the atria. There are the right atrium and the left atrium.
The heart chambers through which the blood exits the heart are the ventricles. There are the right ventricle and the left ventricle.
The Circulatory System - Image Diversity: heart chambers
24. Concerning the thickness of their walls how different are the heart chambers?
The ventricle walls are thicker than the atrium walls since ventricles are structures responsible for the pumping of the blood to the lungs or tissues. The muscular work of the ventricles is harder and their muscle fibers develop more.
The left ventricle is more muscular than the right ventricle because pumping blood to the lungs (the right ventricle task) is easier (needs less pressure) than pumping blood to the other tissues of the body (the left ventricle task).
25. What is vena cava? Which type of blood circulates within the vena cava?
The vena cava are either of two large veins that debouch into the right atrium. The superior vena cava drains all blood that comes from the head, the superior limbs, the neck and the superior portion of the trunk. The inferior vena cava carries blood drained from the inferior portion of the trunk and the inferior limbs.
Venous blood circulates within the vena cava.
The Circulatory System - Image Diversity: vena cava
26. Which is the first (human) heart chamber into which blood enters? Where does the blood go after passing that chamber? What is the name of the valve that separates the compartments? Why is that valve necessary?
The venous blood that comes from the tissues arrives in the right atrium of the heart. From the right atrium the blood goes to the right ventricle. The valve that separates the right ventricle from the right atrium is the tricuspid valve (a valvular system made of three leaflets). The tricuspid valve is necessary to prevent returning of blood to the right atrium during systole (contraction of ventricles).
The Circulatory System - Image Diversity: heart circulation
27. What is the function of the right ventricle? To where does the right ventricle pump the venous blood?
The function of the right ventricle is to get venous blood from the right atrium and pump the blood to be oxygenated in the lungs.
The venous blood is carried from the right ventricle to the lungs by the pulmonary artery and their ramifications.
28. What is the valve that separates the right ventricle from the pulmonary artery? Why is that valve important?
The valve that separates the right ventricle and the base of the pulmonary artery is the pulmonary valve. The pulmonary valve is important to prevent blood from the pulmonary circulation to flow back to the heart during diastole.
The Circulatory System - Image Diversity: pulmonary valve pulmonary artery
29. Do the arteries that carry blood from the heart to the lungs contain arterial or venous blood? What happens to the blood when it passes through the lungs?
Arteries of the pulmonary circulation are arteries that carry venous blood and not arterial blood.
When the blood passes within the alveolar capillaries of the lungs hematosis (oxygenation) occurs and carbon dioxide is released to the exterior.
30. What and how many are the pulmonary veins?
The pulmonary veins are part of the pulmonary circulation. They are vessels that carry oxygen-rich (arterial) blood from the lungs to the heart. There are four pulmonary veins, two that drain blood from the right lung and other two that drain the left lung. The pulmonary veins debouch into the left atrium bringing arterial blood to the heart. Although they are veins they carry arterial blood and not venous blood.
The Circulatory System - Image Diversity: pulmonary veins
31. To which heart chamber does the blood go after leaving the left atrium? What is the valve that separates these compartments?
The arterial blood that has come from the lungs to the left atrium passes then to the left ventricle.
The valve between the left ventricle and the left atrium is the mitral valve, a bicuspid (two leaflets) valve. The mitral valve is important because it prevents the regurgitation of blood to the left atrium during systole (contraction of the ventricles).
The Circulatory System - Image Diversity: mitral valve
32. What is the function of the left ventricle? Where does the blood go after leaving the left ventricle?
The function of the left ventricle is to get blood from the left atrium and to pump the blood under high pressure to the systemic circulation. After leaving the left ventricle the blood enters the aorta, the largest artery of the body.
The Circulatory System - Image Diversity: aorta
33. What is the valve that separates the aorta from the heart? What is the importance of that valve?
The valve between the left ventricle and the aorta is the aortic valve. The aortic valve prevents the retrograde flux of blood to the left ventricle during diastole. Besides, as the aortic valve closes during diastole, part of the retrograde blood flux is impelled through the coronary ostia (openings), orifices located in the aorta wall just after the valvular insertion and contiguous to the coronary circulation responsible for the blood supply of the cardiac tissues.
The Circulatory System - Image Diversity: coronary ostia coronary circulation
34. Is the ventricle lumen larger during systole or during diastole?
Systole is the stage of the cardiac cycle on which the ventricles contract. So the lumen of these chambers is reduced and the pressure upon the blood within them is heightened.
During diastole the opposite occurs. The muscle fibers of the ventricles relax and the lumen of these chambers enlarges helping the entrance of blood.
35. What is the stage of the cardiac cycle during which the ventricles are filled?
The filling of the ventricles with blood occurs during diastole.
The Circulatory System - Image Diversity: cardiac cycle
36. Of which type of tissue is the heart made? How is this tissue oxygenated and nutrified?
The heart is made of striated cardiac muscle tissue. The heart muscle is called the myocardium and it is oxygenated and nutrified by the coronary arteries. The coronary arteries come from the base of the aorta and ramify around the heart penetrating the myocardium.
Diseases of the coronary arteries are severe conditions.
The Circulatory System - Image Diversity: myocardium
37. Which are the two main metabolic gases transported by the blood?
The main metabolic gases transported by the blood are molecular oxygen (O2) and carbon dioxide (CO2).
38. How do respiratory pigments act?
Respiratory pigments are oxygen-carrying molecules present in the blood. When the oxygen concentration is high, for example, in the pulmonary alveoli, the respiratory pigments bind to the gas. In conditions of low oxygen concentration, e.g., in tissues, the respiratory pigments release the molecule.
In the human blood the respiratory pigment is hemoglobin, present within the red blood cells.
39. How different are oxyhemoglobin and hemoglobin? Where is it expected to find a higher concentration of oxyhemoglobin, in peripheral tissues or in the lungs?
Oxygen-bound hemoglobin is called oxyhemoglobin. In the lungs the oxygen concentration is higher and so there is a higher oxyhemoglobin concentration. In the peripheral tissues the situation is the reverse, the concentration of oxygen is lower and there is more free hemoglobin.
The Circulatory System - Image Diversity: oxyhemoglobin
40. What is hemoglobin F? Why does the fetus need a different hemoglobin?
Hemoglobin F is the hemoglobin found in the mammalian fetus and hemoglobin A is the normal hemoglobin. Hemoglobin F has higher affinity for oxygen.
The fetus needs hemoglobin capable of extracting oxygen from the mother’s circulation. Therefore the fetus uses hemoglobin F since it has higher affinity for oxygen than the mother’s hemoglobin.
The Circulatory System - Image Diversity: hemoglobin F
41. In high altitudes is it necessary for the blood to have more or less hemoglobin?
In high altitudes the air is rarefied and oxygen concentration is lower than in low altitudes. In this situation the efficiency of the respiratory system must be greater and thus the organism synthesizes more hemoglobin (and more red blood cells) trying to get more oxygen. This phenomenon is known as compensatory hyperglobulinemia.
The compensatory hyperglobulinemia is the reason why athletes that will compete in high altitudes need to arrive in the place some days before the event so there is time for their body to make more red blood cells and they will be less affected by the effects of the low atmospheric oxygen concentration (fatigue, reduced muscular strength).
42. What is the substance that stimulates the production of red blood cells? Which is the organ that secretes it? Under what conditions does this secretion increase?
The substance that stimulates the production of red blood cells by the bone marrow is erythropoietin. Erythropoietin is a hormone secreted by the kidneys. Its secretion is increased when there is deficient tissue oxygenation (tissue hypoxia) caused either by reduced oxygen availability (as it occurs in high altitudes) or by internal diseases, as in pulmonary diseases.
43. Why is carbon monoxide toxic for humans?
Hemoglobin “likes” carbon monoxide (CO) much more than it likes oxygen. When there is carbon monoxide in the inhaled air it binds to hemoglobin forming carboxyhemoglobin by occupying the binding site where oxygen would bind. Due to the higher hemoglobin affinity for carbon monoxide thus (e.g., in intoxication from car exhausts) there is no oxygen transport and the individual undergoes hypoxia, loses conscience, inhales more carbon monoxide and may even die.
Intoxication by carbon monoxide is an important cause of death in fires and in closed garages.
44. What is the stage of cellular respiration during which carbon dioxide is liberated?
In aerobic cellular respiration the release of carbon dioxide happens in the transformation of pyruvic acid into acetyl-CoA (two molecules) and in the Krebs cycle (four molecules). For each glucose molecule, six carbon dioxide molecules are made.
45. How is carbon dioxide released by cellular respiration transported from the tissues to be eliminated through the lungs?
In vertebrates almost 70% of the carbon dioxide is transported by the blood in the form of bicarbonate, 25% bound to hemoglobin and 5% dissolved in the plasma.
46. What is the difference between double closed circulation and simple closed circulation?
Double closed circulation, or closed circulation, is that in which the blood circulates through two associated and parallel vascular systems: one that carries blood to and takes blood from the peripheral tissues (the systemic circulation) and the other that carries blood to and takes blood from the tissues that perform gas exchange with the environment, e.g, the lungs (pulmonary circulation). Double circulation occurs in amphibians, reptiles, birds and mammals.
Simple closed circulation, or simple circulation, is the one in which the tissues that perform gas exchange are associated in series with the systemic circulation, as in fishes.
47. How many chambers does the fish heart have?
The fish heart is a tube made of two consecutive chambers: one atrium and one ventricle.
48. Does the fish heart pump venous or arterial blood?
The venous blood coming from the tissues enters the atrium and passes to the ventricle that then pumps the blood towards the gills. After oxygenation in the gills the arterial blood goes to the tissues. So the fish heart pumps venous blood.
49. Why is the fish circulation classified as a simple and complete circulation?
Complete circulation is that in which there is no mixture of venous blood and arterial blood. Simple circulation is that in which the blood circulates only in one circuit (as opposed to the double circulation that have two circuits, the systemic circulation and the pulmonary circulation). In fishes the circulatory system is simple and complete.
50. How many heart chambers does the amphibian heart have?
The amphibian heart has three heart chambers: two atria and one ventricle.
51. Why can the amphibian circulation be classified as double and incomplete?
The amphibian circulation is double because it is composed of systemic and pulmonary circulations: respectively, heart-tissues-heart and heart-lungs-heart. Since amphibians have only one ventricle in the heart, venous blood taken from the tissues and arterial blood coming from the lungs are mixed in the ventricle that then pumps the mixture back to the systemic and to the pulmonary circulations. The amphibian circulation is classified as incomplete because venous and arterial blood mix in the circuit.
The blood oxygenation in amphibians occurs also in the systemic circulation since their skin is a gas exchange organ.
52. What is the difference between the amphibian heart and the reptile heart?
The reptiles have double and incomplete circulation too, three heart chambers (two atria and one ventricle). The reptile heart however presents the beginning of a ventricular septation that partially separates a right and left region of the chamber. With the partial ventricular septation the mixture of arterial with venous blood in the reptile heart is less than in amphibians.
53. How many chambers do the bird heart and the mammalian heart have? Concerning temperature maintenance what is the advantage of the double and complete circulation of these animals?
The bird and the mammalian hearts are divided into four chambers: right atrium, right ventricle, left atrium and left ventricle.
Birds and mammals are homeothermic, i.e., they control their body temperature. The four-chambered heart and the double circulation provide the supply of more oxygenated blood to the tissues making possible a higher metabolic rate (mainly cellular respiration rate). Part of the energy produced by the cellular respiration is used to maintain the body temperature.
54. Concerning the mixture of arterial with venous blood what is the difference between the human fetal circulation and the adult circulation?
In the human fetal circulation there are two communications between arterial and venous blood characterizing an incomplete circulation. One of them is the oval foramen, an opening between the right and the left atria of the fetal heart. The other is the arterial duct, a short vessel connecting the pulmonary artery to the aorta. These communications close a few days after birth and so they are not present in the adult heart.
55. How is heart contraction triggered?
Heart contraction is independent from neuronal stimulus (although it can be modulated by the autonomous nervous system). In the heart there are pacemaker cells that trigger by themselves the action potentials that begin the muscle contraction. These cells are concentrated at two special points of the heart: the sinoatrial node (SA node) located in the superior portion of the right atrium and the atrioventricular node (AV node) located near the interatrial septum.
The action potentials generated by depolarization of the SA node cells propagate cell to cell throughout the atria producing the atrial contraction. The atrial depolarization also propagates to the AV node that then transmits the electric impulse to the ventricles through specialized conduction bundles of the interventricular septum (the bundle of His) and then to the Purkinje fibers of the ventricle walls causing ventricular contraction. (The atrial contraction precedes the ventricular contraction for blood to fill the ventricles before the ventricular contraction.)
The repolarization of the SA node makes the atria relax and then the ventricles relax too.
The Circulatory System - Image Diversity: heart conduction
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