Cardiovascular System PDF: Anatomy and Blood Circulation

Chapter 02 : Cardiovasculair system The cardiovascular system consists of the heart (cardio) and the blood vessels (vascular). This is a closed system, meaning that the blood flowing through the heart and vasculature always remains within those structures as it circulates around the entire body. St...

Chapter 02 : Cardiovasculair system The cardiovascular system consists of the heart (cardio) and the blood vessels (vascular). This is a closed system, meaning that the blood flowing through the heart and vasculature always remains within those structures as it circulates around the entire body. Stable blood pressure is maintained by having a consistent total volume of blood (usually about 5.0 L) within this system at all times. As we will see, exchange between the cardiovascular system of the systemic (body) and the pulmonary (lung) circuits occurs at the capillary level of the vasculature. As we will see, blood is the fluid connective tissue that is contained within the cardiovascular system. 1-The Heart The heart is the central orchestrator of blood flow in the body. As an organ (seen image below), the heart is composed of four chambers: It has two upper chambers called the right and left atria; and two lower larger chambers called the right and left ventricles. The word atrium means a receiving room. It is useful to know that it is the atria that always receive the blood that is returning to the heart, whether from the body or the lungs. The right atrium receives deoxygenated blood returning from the body, and the left atrium receives oxygenated blood returning from the lungs. The right atrium is connected to the right ventricle below it, and the left atrium is connected to the left ventricle below that. In between them are the atrioventricular (AV) valves that ensure unidirectional (one way) flow of blood in the heart. Anatomy of the heart 1 2-Circulation of Blood in the Heart There are four chambers in the heart, 2 superior atria, and 2 large inferior ventricles. When looking at most artistic renderings of the heart, it is color coded to account for the differences in the oxygen levels of the blood in specific regions of the heart and body. Chambers and vessels carrying deoxygenated blood are usually displayed in blue, while those carrying oxygenated blood are displayed as red. As will be discussed in the blood section, the molecule hemoglobin (Hb) which is found inside red blood cells is a pigmented protein molecule that changes color. It is bright red when oxygen (O2) is bound and a deeper sort of purple (blue/red) when it is lacking O2. 3-Heart Valves There are 4 heart valves and their role is to prevent retrograde (back) flow of blood in the heart, ensuring that the flow is always in one direction - called unidirectional flow. There are 2 atrioventricular (AV) valves that are situated in between the atria and the ventricles. The valve between the right atrium and right ventricle is the right tricuspid AV valve, because it has 3 cusps. The valve between the left atrium and left ventricle is the left bicuspid AV valve, because it has 2 cusps. This bicuspid AV valve is also commonly referred to as the mitral valve. The other two heart valves are: The pulmonary valve sits in between the right ventricle and the pulmonary trunk. The aortic valve sits in between the left ventricle and the aorta. These valves open when the blood is being ejected from the ventricles, and close to prevent backflow from the arteries into the ventricles. VALVES OF THE HEART 2 4-The Circuits of the Body There are actually three circulations or circuits in the body, the two major circulations - the Pulmonary and Systemic circuits - plus the Coronary circulation, which is the circulation of the heart itself. All three of these circuits go away and come back to the heart. The Pulmonary Circuit The pulmonary circulation goes from the heart, to the lungs and returns to the heart. Succinctly, blood in this circuit goes from the right ventricle to the left atrium (RV to LA). This circuit starts in the right ventricle, where the deoxygenated blood leaves the right ventricle of the heart and travels through the pulmonary trunk (a large artery), and then branches into the L and R pulmonary arteries (seen as blue vessels in images) to deliver the deoxygenated blood to the L and R lungs respectively. In the lungs, the blood gathers oxygen (O2) wherein the blood now becomes oxygenated. It also drops off carbon dioxide (CO2) at the lungs. This blood then returns from both of the lungs to the left atrium of the heart by way of the L and R pulmonary veins (seen as red vessels in images). That is the end of the pulmonary circuit and where the systemic circuit picks up. The Systemic Circuit The other main circulation in the body is called the systemic circuit, where blood travels from the left ventricle of the heart and goes to the body and returns to the right atrium. Starting at the left ventricle, the fully oxygenated blood leaves the left ventricle through the aortic semilunar valve into the aorta (largest artery of body). Many arteries branch off the aorta as it travels around the entire body, delivering O2 and nutrients. As the systemic vessels return from the body (via veins), the blood they carry is depleted of O2 and has collected CO2 from the tissues. This deoxygenated blood is returned to the right atrium of the heart by way of 3 different vessels to the right atrium: 1) the superior vena cava (from above the heart); 2) the inferior vena cava (from below the heart); and 3) from the coronary sinus (from the heart’s own circulation). The right atrium is the end of the systemic circuit. This blood then goes directly into the right ventricle below it, which is the start of the pulmonary circuit, and the cycle repeats. Important note about arteries and veins: They are named for where they are taking blood, not the level of O2 in the blood. Arteries are vascular tubes that take blood away from the heart. Veins are vessels that return blood to the heart. It is true that most arteries carry oxygenated blood and most veins carry deoxygenated blood, but there are some important exceptions to that rule. A better understanding of blood vessels is that in the systemic circuit, arteries are carrying O2 rich blood and veins are returning O2 poor blood to the heart. While in the pulmonary circuit, it is the opposite, arteries still carry blood away from the heart, but have low O2 levels, and the pulmonary veins that return blood to the heart have very high O2 levels. 3 5-The General Sequence of Blood Vessels In both the systemic and pulmonary circuits, the blood vessels leaving and returning to the heart make a type of ‘loop’, and it usually follow this sequence: Heart > Artery > Arteriole > Capillary > Venule > Vein > Heart. For the systemic circuit, the first blood vessel that leaves the heart is the largest artery in the human body, the aorta, which is the foundational vessel of the entire systemic arterial system. The aorta branches into many arteries as is travels away from the heart to deliver O2 rich blood throughout the body. As they branch, the vessels become smaller and smaller, until they become arterioles, which is the next main category of blood vessels. As these continue to get smaller they become capillaries. The capillaries are the site of exchange with the cells of the body. the capillaries are the interchange between the O2 rich arteries (red) and the O2 poor veins (blue) of the systemic circuit. After the capillaries, the blood vessels chuck a U-ie, that is, they make a U-turn and begin to head back to the heart – so now they are veins. As the capillaries end, they merge and enlarge to become venules, which are like small veins. The venules merge and become larger and become veins. Veins are the largest type of blood vessel (diametrically). Although they have low pressure, because they are so large, the blood flow is high. This blood returning from the body is delivered to the receiving room of the systemic circuit, which is the right atrium. Blood Vessels Blood circulates inside the blood vessels, which form a closed transport system, the so-called vascular system. Arteries. As the heart beats, blood is propelled into large arteries leaving the heart. Arterioles. It then moves into successively smaller and smaller arteries and then into arterioles, which feed the capillary beds in the tissues. Veins. Capillary beds are drained by venules, which in turn empty into veins that finally empty into the great veins entering the heart. Associated Great Vessels The great blood vessels provide a pathway for the entire cardiac circulation to proceed. Superior and inferior vena cava. The heart receives relatively oxygen-poor blood from the veins of the body through the large superior and inferior vena cava and pumps it through the pulmonary trunk. Pulmonary arteries. The pulmonary trunk splits into the right and left pulmonary arteries, which carry blood to the lungs, where oxygen is picked up and carbon dioxide is unloaded. Pulmonary veins. Oxygen-rich blood drains from the lungs and is returned to the left side of the heart through the four pulmonary veins. 4  Aorta. Blood returned to the left side of the heart is pumped out of the heart into the aorta from which the systemic arteries branch to supply essentially all body tissues. 5