LESSON 6 CARDIOVASCULAR AND LYMPHATIC SYSTEMS
INTRODUCTION
In order to perform the many functions of the body, the body must have a very efficient way of providing energy to each of the cells of the body. The cells of each tissue and organ receive energy from the food substances that reach them after being taken into the body. Food contains potential energy that can be converted into kinetic energy whenever it is needed. This conversion of potential energy into the kinetic energy inside the cells occurs when food, water, and oxygen combine inside the cells during the chemical processes of metabolism. The organs of the body need small amounts of other nutrients like vitamins for metabolism to occur quickly. Each cell of each organ is dependent on a constant supply of food, water, oxygen, and other nutrients in order to provide sufficient energy to work correctly.
How the body assures that adequate supplies of oxygen, water, and food will be delivered to its cells is a function of the cardiovascular system. The cardiovascular system consists of about 12 pints of a fluid called blood, many miles of vessels to deliver the blood and then return to the heart, and a double phase pump called the heart, to transport food, water, and oxygen to all organs and cells of the body. About ½ cup of blood is moved through the blood vessels with each contraction of the heart. Blood vessels in the lungs take-up the oxygen that has been breathed in from the air, and blood vessels in the intestines absorb food and water from the digestive tract. Blood vessels carry waste materials such as carbon dioxide and nitrogen from the cell and transport these substances to the lungs and kidneys, where they can be expelled from the body.
BLOOD VESSELS AND THE CIRCULATION OF BLOOD
Blood Vessels
There are three major kinds of blood vessels in the body. These are arteries, veins, and capillaries. Arteries are the large blood vessels that lead blood away from the heart. Their walls are strong and are made of connective tissue, elastic fibers, and an innermost layer of epithelial cells called endothelium. Arteries are strong to withstand the high pressure required to carry blood away from the heart to every single cell of the body. The elastic walls of the arteries allow them to expand as the heart muscle forces blood throughout the body. Smaller branches of arteries are called arterioles. Arterioles are thinner than arteries and carry the blood to the smallest of blood vessels, called the capillaries.
Capillaries have walls that are only one cell thick. These delicate, microscopic vessels carry oxygenated blood from the arteries and arterioles to the body cells. Their walls are thick enough to allow passage of oxygen and other nutrients out of the bloodstream and into the lymph fluid surrounding the cells and then into the cell. Once inside the cells, the nutrients are burned in the presence of oxygen and water to release the needed energy for functioning of the cell. As discussed above, this process is called metabolism. At the same time, waste products such as carbon dioxide, nitrogen and water, pass out of the cells and into the one cell thick capillaries. The blood then flows back from the cell to the heart by small veins called venules, and then into larger vessels called veins.
Veins have thinner walls than arteries. They carry deoxygenated blood toward the heart from the cells. Veins have much lower blood pressure than arteries, about 15 mm/hg at their beginning to about 5 mm/hg at the vena cava. In order to keep blood moving back toward the heart, veins have many valves that prevent the backward flow of blood and keep the blood moving toward the heart. Muscular action greatly helps the movement of blood in veins toward the heart.
Circulation of Blood
Arteries, arterioles, veins, venules, and capillaries, together with the heart, form a circulatory system for the flow of blood. Deoxygenated blood flows into the heart cavity through two large vessels called the vena cava, coming from the body’s capillaries to the heart. The blood becomes deoxygenated in the tissue capillaries where oxygen leaves the blood and enters the cells.
Deoxygenated blood enters the right side of the heart and travels through that side and exits into the pulmonary artery. This vessel then divides in two-- one branch leading to the left lung, the other branch to the right lung. The arteries continue dividing and subdividing in the lungs, forming smaller blood vessels called arterioles and finally reaching the tiny capillaries of the lungs. The pulmonary artery is unusual in that it is the only artery in the body that carries deoxygenated blood.
While passing through the lung’s capillaries, blood absorbs the oxygen that entered the body during respiration. The newly oxygenated blood returns to the heart through pulmonary veins. The pulmonary veins are unusual in that they are the only veins in the body that carry oxygenated blood. The circulation of blood through the vessels from the heart to the lungs and then back to the heart again is known as the pulmonary circulation.
Oxygenated blood enters the left side of the heart from the two pulmonary veins. The muscles in the left side of the heart pump the blood out of the heart through the largest single vessel in the body, the aorta. The aorta moves upward (ascending aorta), then arches over dorsally (aortic arch), and finally downward just anterior to the vertebral column (the descending aorta). The aorta divides into numerous branches called arteries that carry the oxygenated blood to all parts of the body.
The relatively large arterial vessels branch to form smaller arterioles. The arterioles, still containing oxygenated blood, branch into smaller capillaries, which are near to the body cells. Oxygen leaves the blood and passes through the thin capillary walls to the lymph fluid and then enters the body cells. There, food is broken down in the presence of oxygen and water to release energy in the form of Adenosine triphosphate (ATP). This chemical process is accomplished in the mitochondria of the cells and is most often called the Kreb cycle or citric acid cycle.
One product of this process is carbon dioxide (CO2). CO2 is produced in the cell but a large amount is harmful to the cell if it remains. It must thus be expelled from the cells and into the capillaries at the same time that oxygen is entering the cell. As the blood makes its way back toward the heart in the venules and veins, it is full of waste gas CO2. In nature, plants are dependent on CO2, so it is not wasted. The circuit is completed when deoxygenated blood re-enters the heart from the vena cava. This circulation of blood from the body organs to the heart and back again is called systemic circulation.
Anatomy of the Heart
The heart weighs about one pound, and it is roughly the size of a doubled-up fist. It lies just behind the breastbone in the mediastinum area of the thoracic cavity. The heart consists of four chambers: two upper chambers called atria and two lower chambers called ventricles. It is actually a double phase pump bound into the heart muscle. The primary phase is the movement of blood from the heart to the lungs. This pump is called the sinoatrial node. The secondary phase is the movement of blood from the heart to the cells of the body. This pump is called the atrioventricular node.
Blood must pass through each chamber in a definite pattern. The primary phase station, sinoatrial node, is located high in the wall on the right side of the heart. When it is activated, the two top chambers (atrial) of the heart contract and send deoxygenated blood to the lungs where the blood picks up oxygen and releases carbon dioxide. The newly oxygenated blood returns to the left side of the heart. The secondary pump station, atrioventricular nodes, then activates and sends a contraction wave through the lower heart chambers that forces the oxygenated blood out of the aorta to the body. At the cellular level, the blood loses its oxygen and then returns via the veins and venules to the right side of heart, where the process is repeated over and over again as the primary and secondary stations continue to activate and contract in a normal rhythmatic process. In this process, deoxygenated blood is sent out to the lungs and oxygenated blood is sent to every single cell of the body over and over again.
Deoxygenated blood enters the heart through the two largest veins in the body, the vena cava. The superior vena cava drains blood from the superior portion of the body, and the inferior vena cava carries blood from the inferior parts of the body.
The vena cava brings deoxygenated blood into the right atrium, the upper right chamber of the heart. The right um contracts to force blood through the tricuspid valve into the right ventricle, which is the lower right chamber of the heart. The tricuspid valve allows for a one-way passage of the blood, so that the blood flows in only one direction. As the right ventricle contracts, it pumps deoxygenated blood out through the pulmonary valve into the pulmonary artery. The tricuspid valve stays closed preventing blood from moving back into the right atrium. The pulmonary artery then branches to carry deoxygenated blood to each lung.
The blood that enters the lung's capillary bed from the pulmonary artery will soon lose a large quantity of carbon dioxide into the lung tissue. Carbon dioxide is then expelled from the lungs during exhalation. Oxygen enters the capillaries of the lungs and is brought back to the heart by the pulmonary vein. There are several pulmonary veins that transport oxygen-rich blood back to the left side of the heart from the lungs. The newly oxygenated blood enters the left atrium of the heart from the pulmonary veins. The walls of the left atrium contract to force blood through the bicuspid valve into the left ventricle.
The left ventricle is much more muscular than the other chambers of the heart. It must pump blood with such force that the blood travels to the smallest of arteries in all parts of the body. As the blood leaves the left ventricle it is passed through the aortic valve and into the aorta, which will finally branch to carry blood to all parts of the body. There is, at this time, a great back washing of blood from the aorta. This forced back washing of blood is responsible for sending the richest oxygenated blood into the coronary arteries. The aortic valve prevents the back washed blood from returning to the left ventricle.
The four chambers of the heart are separated by partitions called septa (singular: septum). The interatrial septum separates the two upper chambers (atria). The interventricular septum is a muscular wall that comes between the two lower chambers (ventricles).
The heart wall is composed of three layers. The endocardium is a smooth layer of endothelial cells that lines the inside of the heart and the heart valves. The pericardium is a fibrous and membranous sac that surrounds the heart. It has two layers, the visceral pericardium, which is attached to the heart, and the parietal (parietal means wall) pericardium, which lies next to the outer fibrous layer. The pericardial cavity (between the visceral and the parietal pericardium) usually contains 15 ml. of fluid. This fluid is responsible for the lubrication of the pericardium while the heart beats.
Physiology of the Heart: Heartbeat and Heart Sounds
The double pump system has a contraction and relaxation phase. The contraction phase is called systole. Systole occurs as the walls of the right and left ventricles contract to push blood out of the heart into the pulmonary artery and the aorta. Both the tricuspid and the mitral valves are closed during systole, thus preventing the back flow of blood
back into the atria. During systole, blood is forced from the heart into the aorta and pulmonary artery, then into the arteries, arterioles, capillaries, venules and veins.
The relaxation phase is called diastole. Diastole occurs when the ventricle walls relax after contraction and blood flows into the heart from the vena cava and the pulmonary veins. The tricuspid and bicuspid valves are open during diastole when blood enters the right and left atria. Some of the blood will flow naturally into the ventricles. The pulmonary and aortic valves are closed during diastole. This cardiac cycle occurs about 72 times per minute. The heart pumps about ½ cup of blood with each contraction. This means that about 6 quarts of blood are pumped by the heart in one minute, 80 gallons per hour, and more than 2000 gallons per 24 hour days.
The beat of the heart as felt through the walls of the arteries on the skin’s surface is called the pulse. The pulse is a wave of blood that travels within the arteries as the heart contracts in systole. The pulse is best felt at the wrist on the radius side or at the neck’s carotid artery just to the side of the larynx or the Adam's Apple.
The heart valves produce low audible sounds such as “lub, dub” that can be heard when listening to a normal heart through a stethoscope. The “lub” is the closure of the tricuspid and mitral valves at the beginning of systole and the “dub” is the closure of the aortic and pulmonary valves at the end of systole. The”lub” sound is called the first heart sound and the “dub” is the second heart sound because the normal cycle of the heartbeat starts with the beginning of systole. Abnormal heart sounds are known as heart murmurs. Murmurs have a “lub, dub, swish” sound or sometimes a “lub, swish, dub” sound. They are usually detected when getting a physical from a medical doctor.
Conduction System of the Heart
Although the heart does contain nerves that can affect its rate, they are not primarily responsible for its beat. It is known that the heart starts beating 12 days after fertilization in the embryo before the heart is supplied with nerves, and it will continue to beat even when the nerve supply is cut.