Because oxygen rich blood needs to be separated from the oxygen poor blood. Septum separates the left and right sides of the heart and prevents blood to flow from the right to left sides or vice versa.The left side of the heart is the pump for systemic circulation, the circulation of blood throughout the body except for the air sacs of the lungs. The left ventricle ejects blood into the aorta, and blood then flows into systemic arteries, arterioles, capillaries, venules, and veins, which carry it back to the right atrium. The right side of the heart is the pump for pulmonary circulation, the circulation of blood through the lungs. The right ventricle ejects blood into the pulmonary trunk, and blood then flows into pulmonary arteries, pulmonary capillaries, and pulmonary veins, which carry it, back to the left atrium.
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Why is there a hole between the right and left atrium in a foetus?
Anatomically speaking foramen ovale is an opening between the two atria in a foetus, and exists because the interatrial septum of the foetus heart is incomplete. Foramen ovale plays very important part in a foetus circulation. Foetus receives oxygen only through the umbilical vein (http://www.indiana.edu/~anat550/cvanim/fetcirc/fetcirc.html). Most of the blood passes from the right atrium into the left atrium through the foramen ovale. Here, it mixes with a small quantity of blood returning through the pulmonary circulation. The blood then passes into the left ventricle, from which it is pumped into the aorta and through the body of the fetus. Some blood entering the right atrium passes into the right ventricle and out of the heart via the pulmonary trunk. Because the lungs of the fetus are not functional, only a small portion of blood continues through the pulmonary circulation (the resistance to blood flow is very high in the collapsed fetal lungs). Most of the blood in the pulmonary trunk passes through the ductus arteriosus into the aortic arch, where it mixes with blood coming from the left ventricle. Blood is returned to the placenta by the two umbilical arteries that arise from the internal iliac arteries.
Why are heart strings made out of the same material found in tendons?
Why is the wall of the left ventricle thicker than that of the right?
The wall of the right ventricle (Figure 1.) is relatively thin, and in sectional view it resembles a pouch attached to the massive wall of the left ventricle. When the right ventricle contracts, it moves toward the wall of the left ventricle. This compresses the blood within the right ventricle, and the rising pressure forces the blood through the pulmonary valve and into the pulmonary trunk. This mechanism moves blood very efficiently at relatively low pressures, which are all that one needs to move blood around the pulmonary circuit. Higher pressures would actually be dangerous, because the pulmonary capillaries are very delicate.
A comparable pumping arrangement would not be suitable for the left ventricle, because six to seven times as much force must be exerted to propel blood through the systemic circuit. The left ventricle, which has an extremely thick muscular wall, is round in cross section. When the left ventricle contracts, two things happen: The distance between the base and apex decreases, and the diameter of the ventricular chamber decreases. The forces generated are quite powerful, more than enough to force open the aortic valve and eject blood into the ascending aorta. As the powerful left ventricle contracts, it also bulges into the right ventricular cavity. This intrusion improves the efficiency of the right ventricle’s efforts. Individuals whose right ventricular musculature has been severely damaged may continue to survive because of the extra push provided by the contraction of the left ventricle.
Figure 1. Anterior view of the frontal section showing internal anatomy of the human heart
Why does the heart have coronary arteries?
Heart works continuously, and cardiac muscle cells require reliable supplies of oxygen and nutrients which is delivered through coronary arteries.
3) Describe and explain the function of the coronary circulation (L3 – 5.2)
The coronary circulation (http://www.vhlab.umn.edu/atlas/physiology-tutorial/coronary-circulation.shtml) supplies blood to the muscle tissue of the heart. During maximum exertion, the oxygen demand rises considerably, and the blood flow to the heart may increase to nine times that of resting levels. The coronary circulation includes an extensive network of coronary blood vessels. The left and right coronary arteries (Figure 2.) originate at the base of the ascending aorta, within the aortic sinus, as the first branches of this vessel. Blood pressure here is the highest found anywhere in the systemic circuit and this pressure ensures a continuous flow of blood to meet the demands of active cardiac muscle tissue.The coronary circulation provides blood flow to the myocardium. The main arteries of the coronary circulation are the left and right coronary arteries; the main veins are the cardiac veins and the coronary sinus.The wall of the heart has its own supply of systemic blood vessels to meet its vital needs. The myocardium is supplied with blood by the right and left coronary arteries. These two vessels arise from the ascending part of the aorta, at the location of the aortic valve. The coronary arteries encircle the heart within the atrioventricular sulcus, the depression between the atria and ventricles. Two branches arise from both the right and left coronary arteries to serve the atrial and ventricular walls.
Figure 2. Coronary arteries
4) With reference to question AC5.2 describe and explain the sequence of events that lead to a heart attack (L3 – 5.2).
Atherosclerosis is a disease of the blood vessels (Hansson GK 2005). When plaques, fatty deposits of cholesterol, accumulate inside the vessel walls, they narrow or block the lumen, reducing blood flow. When plaque ruptures causes a clot to form within the vessel (Figure 3). A clot that is attached to the vessel wall is called a thrombus; if it loosens and floats in the bloodstream. When floating clot lodge in a coronary artery, completely blocking blood flow. Therefore less oxygen-rich blood is delivered to the heart causing cardiac tissue to die. Dead cardiac tissue ceases to conduct electricity, so the contraction impulse cannot pass. A ventricle that cannot contract completely cannot move blood efficiently, and the result is reduced cardiac output.
Figure 3. Atherosclerosis
5) Distinguish between Pulmonary and systemic circulatory systems. For each name the major blood vessels to and from the heart. For the systemic system name the arteries and corresponding veins that service four named organs. Describe the function of the hepatic portal vein. (L3 – 6.1).
Blood can take one of two pathways from the heart: the pulmonary circuit toward the lungs or the systemic circuit toward the tissues. The purpose of the pulmonary circuit is to exchange carbon dioxide in the blood for oxygen from the environment. The systemic circuit brings this oxygen (and nutrients) to the tissues and then removes carbon dioxide from them.The pulmonary circulation includes blood vessels that transport blood to the lungs for gas exchange and then back to the heart and it does not directly serve the metabolic needs of body tissues (systemic circulation). It consists of the right ventricle that ejects the blood, the pulmonary trunk with its pulmonary valve, the pulmonary, arteries that transport deoxygenated blood to the lungs, the pulmonary capillaries within each lung, the pulmonary veins that transport oxygenated blood back to the heart, and the left atrium that receives the blood from the pulmonary veins. When deoxygenated blood enters the pulmonary circulation as it is pumped from the right ventricle into the large pulmonary trunk (Figure 4a), which divides to form the right and left pulmonary arteries. In the lungs, the pulmonary arteries subdivide into the lobar arteries. The lobar arteries accompany the main bronchi into the lungs and then branch profusely, forming first arterioles and then the dense networks of pulmonary capillaries that surround and cling to the delicate air sacs. It is here that oxygen moves from the alveolar air to the blood and carbon dioxide moves from the blood to the LA alveolar air. As gases are exchanged and the oxygen content of the blood rises, the blood becomes oxygenated, then four pulmonary veins complete the circuit by returning the reach of oxygen blood into the left atrium of the heart. The systemic circulation (Figure 4b) begins when oxygen-rich blood enters the left. This oxygen-rich blood then enters the left ventricle and, during ventricular systole, is pumped through the aortic arch to the body. After passing through the capillaries, venous blood returns to the superior and inferior vena cava. These large veins drain into the right atrium, where blood re-enters the pulmonary circuit.
Organs
Artery
Vein
Brain
Carotid artery
Vertebral artery
Jugular vein
Vertebral vein
Liver
Hepatic artery common
Hepatic vein
Spleen
Splenic artery
Splenic vein
Kidneys
Renal artery
Renal vein
Figure 4
Pulmonary circulation b) Systemic circulation c) Hepatic portal circulation
The hepatic portal circulation carries venous blood from the gastrointestinal organs and spleen to the liver. A vein that carries blood from one capillary network to another is called a portal vein. The hepatic portal vein receives blood from capillaries of gastrointestinal organs and the spleen and delivers it to the sinusoids of the liver (Figure 4c). After a meal, hepatic portal blood is rich in nutrients absorbed from the gastrointestinal tract. The liver stores some of them and modifies others before they pass into the general circulation. For example, the liver converts glucose into glycogen for storage, reducing blood glucose level shortly after a meal. The liver also detoxifies harmful substances, such as alcohol, that have been absorbed from the gastrointestinal tract and destroys bacteria by phagocytosis.
6) Clearly label the following diagram, next page add arrows indicating the direction of blood flow and colours to indicate the type of blood present in each vessel (L3 – 6.1)
7)
a) Explain why arteries have thick walls and a narrow lumen, while veins have thins walls and a large lumen and capillaries have walls that are a single cell thick.
The arteries have thicker smooth muscle and connective tissue layers than veins, to handle the higher pressure the arteries are under.Arteries are blood vessels on the output side of the heart. Arteries closest to the heart have large diameters and thick walls because the heart’s pumping cause them to stretch and recoil with each beat. Farther from the heart, diameter and wall thickness both decrease, because this distance reduces the fluid pressure from the heart. As the vessels get smaller, the ratio of the inner surfaces of their lumen to the volume that lumen holds goes from a small surface-to-volume ratio to a large surface-to-volume ratio. This directly affects blood flow, because there is more surface area to create friction and drag in smaller, more numerous vessels called arterioles. Arterioles lead to capillaries, the smallest blood vessels. The wall of a capillary is one cell layer thick, and the lumen is barely big enough for one blood cell. They are important functionally because they are the only blood vessels whose walls permit vital exchange of gases, nutrients, and waste across the blood vessel wall. Because the walls are relatively thin, the diffusion distances are small, and exchange can occur quickly. In addition, blood flows slowly through capillaries, allowing sufficient time for diffusion or active transport of materials across the capillary walls.
Blood leaving capillaries collects in larger vessels called venules venules, small veins that drain blood from capillaries to larger veins and veins heading back toward the heart. At this point, circulation resembles the flow of water from rivulets into creeks, then into rivers,
and eventually to the sea. As the veins get bigger, the walls thicken slightly. Because the
veins are beyond the capillaries, the heart’s pumping cannot put much pressure on venous blood. Therefore, the veins are not as thick as arterial walls. The blood in the veins is moving with barely any pressure, so the veins do not need to be terribly strong.
b) explain why veins have a series of valves and arteries do not (L3 6. 2)
Figure 5
The veins of the systemic circuit, which return blood to the heart, are under much lower pressure than arteries. The low venous pressure is insufficient to return blood to the heart, particularly from the lower limbs. Veins pass between skeletal muscle groups that provide a massaging action as they contract (Figure 5). As the veins are squeezed by contracting skeletal muscles, a one-way flow of blood to the heart is ensured by the presence of venous valves. Venous valves also prevent the backflow of blood. The effect of the massaging action of skeletal muscles on venous blood flow is described as the skeletal muscle pump.
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