CIRCULATORY SYSTEM Complete Exam Study Guide_ Guaranteed Success.

Mammalian Circulation Let’s first examine the overall organization of the mammalian cardiovascular system, beginning with the pulmonary circuit. 1. Contraction of the right ventricle pumps blood to the lungs via the 2. pulmonary arteries. As the blood flows through 3. capillary beds in the left and right lungs, it loads O2 and unloads CO2. Oxygen-rich blood returns from the lungs via the pulmonary veins to the 4. left atrium of the heart. Next, the oxygen-rich blood flows into 5. the heart’s left ventricle, which pumps the oxygen-rich blood out to body tissues through the systemic circuit. Blood leaves the left ventricle via 6. the aorta, which conveys blood to arteries leading throughout the body. The first branches leading from the aorta are the coronary arteries, which supply blood to the heart muscle itself. Then branches lead to 7. capillary beds in the head and arms (forelimbs). The aorta then descends into the abdomen, supplying oxygen-rich blood to arteries leading to 8. capillary beds in the abdominal organs and legs (hind limbs). Within the capillaries, there is a net diffusion of O2 from the blood to the tissues and of CO2 (produced by cellular respiration) into the blood. Capillaries rejoin, forming venules, which convey blood to veins. Oxygen-poor blood from the head, neck, and forelimbs is channeled into a large vein, 9. the superior vena cava. Another large vein, inferior vena cava, drains blood from the trunk and hind limbs. The two venae cavae empty their blood into right atrium, from which the oxygen-poor blood flows into the right ventricle. 2 | P a g e There are 3 pathways for blood during its circulation 1. Pulmonary Circulation It starts from the right ventricle and ends at the left atrium. • When the right ventricle contracts, the tricuspid valve closes the opening of the right atrium. The deoxygenated blood will therefore rush through the pulmonary artery through the three-flapped semilunar valve. This valve prevents the backflow of blood to the ventricle (when it relaxes). • The pulmonary artery gives rise to two branches, each branch goes to a lung, where it branches to form several arterioles which terminate in blood capillaries. • Blood capillaries spread around the alveoli, where exchange of gases takes place. Carbon dioxide and water vapor will diffuse from the blood and Oxygen will move towards it, so that blood becomes oxygenated. • Oxygenated blood returns from the lungs through the 4 pulmonary veins (two veins from each lung) to open into the left atrium. • When the left atrium contracts, blood passes to the left ventricle through the bicuspid valve. 3 | P a g e 2-Systematic Circulation It starts from the left ventricle and ends at the right atrium. • When the left ventricle contracts after being filled with oxygenated blood, the mitral valve closes. • As a result, blood under great pressure rushes from the left ventricle to the Aorta through an opening which is controlled by a semi-lunar valve to prevent the backflow of blood. • The Aorta gives rise to several arteries, some of which move upwards while others go downwards. • Arteries then branch to form smaller and smaller arterioles which end by blood capillaries. • These capillaries spread through the tissues in between the cells transporting Oxygen, water, and dissolved food substances to them. • On the other hand, products of catabolism such as Carbon dioxide resulting from oxidation of sugar and fat diffuse through the walls of blood capillaries and reach the blood which changes in color from light red to dark red, and is now called deoxygenated blood. • Blood capillaries collect to give rise to larger and larger venules and finally veins, which pour the deoxygenated blood into the superior and the inferior vena cava which carry blood to the right atrium. • When it is filled with blood, the walls of the right atrium contracts and so blood is forced to the right ventricle which become filled with deoxygenated blood. - It worth noting that contraction of the right side of the heart occurs at the same time of contraction of the left side. Therefore, pumping of the deoxygenated blood from the right ventricle, and pumping of the oxygenated blood from the left ventricle, both take place at the same time. 3. Hepatic Portal Circulation - After being absorbed by the villi of the small intestines, both glucose and amino acids are transported to the blood capillaries inside these villi. 4 | P a g e - These blood capillaries aggregate into small venules, then large venules and finally they pour the contents into the hepatic portal vein. This also receives veins from the pancreas, the spleen, and the stomach. - When it first enters the liver, the hepatic portal vein branches into venules which end with minute blood capillaries. Excess food substances which exceed the body needs, filter through the capillary walls cells and passes to the liver where they undergo certain changes. - Finally, blood capillaries unite into the hepatic vein, which leaves the liver to pour its contents into the upper part of the inferior vena cava just before it enters the right atrium. The circulatory system It consists of the heart and the blood vessels through which the blood passes. These vessels form a complete circuit. (A closed circulatory system) The heart The heart is a hollow muscular organ which lies in the middle of the chest cavity located behind the sternum (breastbone) and it is about the size of a clenched fist and consists mostly of cardiac muscle. It is enclosed in the pericardium that protects the heart and facilitates its pumping action. The heart is divided into four chambers: • The 2 atria (auricles): The upper two chambers with thin walls. Those receive blood from veins. • The 2 ventricles: The lower two chambers with thick muscular walls. Those pump blood through arteries and contract much more forcefully than the atria—especially the left ventricle, which pumps blood to all body organs through the systemic circuit. Although the left ventricle contracts with greater force than the right ventricle, it pumps the same volume of blood as the right ventricle during each contraction. 5 | P a g e - The heart is divided longitudinally into two sides by means of muscular walls called septum. - Each atrium is connected to its own ventricle through an opening which is guarded by means of a valve. Each valve consists of thin flaps. In order for these flaps not to turn inside out, the free edges of these flaps are attached to the ventricle wall by means of tendons. Thus, blood is permitted to flow only from the atrium to the ventricle not in the reverse direction. • The right valve (The tricuspid valve) is made up of three flaps, while the left valve (the bicuspid valve or the mitral valve) is similar in structure and action, except it is formed up of 2 flaps. - There are also semilunar valves at the connection of the heart with both Aorta and pulmonary artery. The heart beats regularly throughout the whole life time. Heart Wall - The heart wall is composed of three layers of tissue: the epicardium, the myocardium, and the endocardium. • The epicardium, also called the visceral pericardium, is a thin, serous membrane forming the smooth outer surface of the heart. It consists of simple squamous epithelium overlying a layer of loose connective tissue and adipose tissue. • The thick, middle layer of the heart, the myocardium, is composed of cardiac muscle cells and is responsible for contraction of the heart chambers. • The smooth inner surface of the heart chambers is the endo