Background Information
System Structure and Function
Need for Circulatory and Ventilation Systems
Each metabolically active cell in the body needs a steady supply of oxygen. Because only a tiny fraction of these cells lie in close enough proximity to the body surface to obtain oxygen directly, a ventilation system is necessary to bring oxygen into the body, and a circulatory system is required to distribute it to all cells.
Even at rest, our tissues have incredible demands for nutrients and energy sources. Blood nourishes the tissues, providing various materials needed for energy and building molecules. It also transports products made in different glands and organs, and circulates cells involved in immune response. Additionally, blood collects wastes such as carbon dioxide and excess nitrogen (urea) and takes them to organs responsible for their removal 4. Our often-underappreciated heart has the endless task of distributing this vital fluid; each minute it pumps approximately five liters of blood throughout the body. The heart is situated at the center of the circulatory system, a series of vessels so finely spread throughout the body that each metabolically active cell is within 100 micrometers of one of its branches 5. Blood vessels exist in a number of sizes with a variety of specific functions, all with the general purpose of distributing and collecting blood to each of the cells in the body.
The Heart
The heart is a four-chambered muscle set up as a pair of pumps. The left side collects blood from the lungs and sends it out through the body; the right collects this blood back from the body and sends it to the lungs. Each side of the heart has an atrium, a thin-walled collection chamber and a ventricle, a thicker-walled pump 6. Both sides of the heart contract simultaneously and follow the same pattern. The beginning point is arbitrary since the process is cyclical.
Blood returning to the heart from the pulmonary vein (left side) and venae cavae (right side) collects in the atria and flows into the ventricles through the open atrioventrical valves. The atria contract, then the ventricles, increasing the flow into the ventricles until the pressure in the ventricles exceeds that in the atria and the atrioventricular valve between them shuts. This, combined with the continued contraction, causes the pressure within the ventricle to exceed that in the aorta and pulmonary artery. The semilunar valves then open, blood is ejected from the ventricles to the arteries, and the valves close again after the ventricles relax and pressure drops 7. Blood again flows into the atria and the cycle continues. To simplify, both atria contract, then both ventricles, and blood flows out of the heart. When the body is at rest, each contraction lasts about one second, resulting in a measurable pulse of 60-70 "waves" (beats) per minute 8.
The Circulatory System
The heart lies at the center of a remarkable set of plumbing, extending to every organ, tissue, and extremity in the body. Blood from the left side of the heart moves through the systemic circulatory system and diffusion occurs across capillaries in body tissues such as organs, muscles, and the brain. Blood also flows through the coronary arteries located directly on the heart itself, providing the cardiovascular tissue with necessary oxygen and nutrients. The blood exiting the right side of the heart moves through the pulmonary circulatory system and gas and nutrient exchange occurs in capillaries in the lungs.
Arteries carry blood away from the heart; the pulmonary artery carries blood to the lungs and the aorta carries blood to the systemic circulatory system 9. These large arteries branch into successively smaller channels, then to smaller branches called arterioles, then eventually to capillary beds, where molecular exchange occurs. Arteries are characterized by their relatively large diameter, thick layer of smooth muscle, high-pressure flow, and lack of valves 1 0. As blood flows through the arterial system to the capillary beds, the total cross-area for flow increases while blood velocity and pressure decrease proportionally 1 1. Because they have the highest total cross-sectional area, the blood flows through the capillary beds slowly and diffusion occurs on a single-cell level. Small veins called venules receive the low-pressure blood from capillary beds and it flows into progressively larger veins until reaching the heart. Unlike arteries, veins have larger diameters with thin walls, contain low-pressure flow, and have a series of valves to keep blood circulating back towards the heart. Deoxygenated blood from the lungs flows back in to the heart through the pulmonary vein, blood from the body through the venae cavae 1 2. A blood cell traversing the entire system would go through both the pulmonary and systemic systems; its journey would take approximately one minute 1 3.
Heart Rate Regulation
The majority of the heart is composed of cardiac muscle; the cells in this tissue are unique because they possess the ability to spontaneously contract and relay electrical signals without nervous system input. This myogenic muscle contraction allows the heart to beat every minute of every day of our lives, beginning just twenty-two days after fertilization and continuing until death 1 4. There is a particularly active cluster of cells located in the tissue in the right atrium called the sinoatrial (SA) node, which serves as the pacemaker of the heart. These cells generate action potentials, which are transmitted simultaneously through the atrial tissue and cause its contraction. This electrical signal continues through the myocardium to the atrioventricular (AV) node, and, after a brief delay, it is sent through the ventricles, triggering a contraction that ejects a volume of blood through the pulmonary and systemic systems 1 5.
Components of Blood
Since it is responsible for moving cells and molecules throughout the body, blood is a fluid with many components. The liquid portion of blood, plasma, composes approximately 55% of the fluid, and cellular components (erythrocytes, leucocytes, and platelets) make up the rest 1 6. Erythrocytes, or red blood cells, make up the majority of the cellular portion. They have the demanding task of carrying oxygen from the lungs to the body's tissues, as oxygen itself is not very soluble in plasma. Leucocytes, white blood cells involved in the immune response, are also present in blood, as well as platelets, cell fragments that form clots to stop bleeding at the site of injury 1 7.
Blood is responsible for moving a number of molecules in addition to oxygen. When oxygen is used in the process of internal respiration (discussed in a later section) to produce chemical energy, carbon dioxide is created as a waste product. This waste is collected and removed by the circulatory system, as well as other forms of waste, such as urea (excess nitrogen). Blood also transports nutrients necessary for energy and building tissues, such as amino acids, glucose, and lipids. Additionally, blood moves molecules made in specific tissue cells throughout the body. Hormones, for example, are produced in particular glands but are needed in target cells that may be far removed from the gland, and antibodies must be distributed throughout the body or to specific areas when there is an immune response. Blood also has a role in temperature regulation; the arterioles can expand and contract to gain and release heat 1 8.
The Lung
The ventilation system consists of two lungs, each of which contains a system of branching airways and hundreds of millions of alveoli, which are air-filled sacs specialized for gas exchange. Regular inhalation is necessary, as it brings in approximately half a liter of air, a mix of a number of gases, including the oxygen necessaryto create cellular energy. The air enters the pharynx then trachea through the mouth or nose, flows to either the left or right bronchi, then to progressively smaller branches called bronchioles, which end in clusters of alveoli. There are approximately 23 forks along this pathway 1 9. Each alveolus is surrounded by a sheet of capillaries that is fed with oxygen-poor blood pumped by the right heart. Gas exchange occurs at the alveolus-capillary interface 2 0.
Lung tissue is passive, not muscular, so its ability to take in and expel air for gas exchange relies on the surrounding muscles and pressure properties of the lung and chest wall. The lungs are in constant contact with the chest wall, although the contact is indirect because they are separated by a thin layer of pleural fluid, creating a closed system. The lung and chest wall are both elastic in nature; the lung constantly pulls inward, and the chest wall constantly pulls outward. The muscles involved—the diaphragm, abdomen, and intercostal muscles—increase and decrease the volume of the thoracic cavity cyclically 2 1. Since volume and pressure are inversely proportional and the lung cavity is a closed system, these changes in chest wall cause a change in the volume of the lungs. Upon inspiration (breathing in), the diaphragm contracts and pulls down, increasing the volume of the thoracic cavity. The pressure in the cavity decreases (as the inverse function of the cavity volume increasing), causing the lungs themselves to increase in volume. This increase in volume creates a partial vacuum, which air flowing from the trachea must fill to counter. This refreshes some of the air around the alveoli, where gas exchange occurs. The expiration process then proceeds in reverse 2 2. During normal inspiration, the diaphragm contracts approximately 1 centimeter, but can move up to 10 centimeters during forced exhalation 2 3. Just as the heart increases stroke rate and volume with physical activity, the lung also increases its inspiration/exhalation volume and rate to maintain sufficient oxygen levels
Alveoli and Gas Exchange
The process of gas exchange in the alveoli relies upon diffusion. Diffusion occurs throughout the body to move various molecules, but is particularly important in the lungs, where it freshens the blood's supply of oxygen while removing carbon dioxide waste. Diffusion is simply the movement of molecules from a region of higher to lower concentration so as to work towards equilibrium. Often, diffusion occurs across a membrane, which separates regions of different concentration. For diffusion to occur, the membrane must be permeable to the diffusing substance. Not all materials can support the diffusion of a particular substance; however, small, uncharged molecules such as oxygen and carbon dioxide can diffuse across most biological membranes, even without the presence of special transporters like channels or pores.
The air entering the lungs has a higher concentration of oxygen than the blood in the capillaries embedded in the alveoli; consequently, oxygen molecules move from the oxygen-rich air of the alveoli to the oxygen-deficient blood of the capillaries. The opposite is true for the concentrations of carbon dioxide, so diffusion removes the carbon dioxide waste from the blood in the capillaries into the alveoli. The newly oxygenated blood returns to the heart for distribution to tissues; an exhalation from the lungs expels the carbon dioxide from the body. The process proceeds in reverse in the capillary beds throughout the body, where internal respiration continually uses oxygen and produces carbon dioxide in the pursuit of the chemical energy molecule ATP (see below) 2 4.
Alveoli are specially adapted for this process, as they have a spherical shape to create a large surface area for gas exchange. In addition, the boundary of the alveoli are only a single cell layer thick, minimizing the distance through which diffusion must occur. Each alveolus is associated with a capillary bed to ensure close proximity to the circulatory system 2 5. The lungs collectively have approximately 300 million alveoli, which create a massive surface area roughly the size of a tennis court (approximately 80 square meters) 2 6. The importance of surface area is a key concept for understanding both the circulatory and respiratory systems. Effective diffusion and molecular delivery would be impossible without the intricate branching of both systems, because of the large surface area that is obtained from highly branched architectures.
Cellular Energy and the Need for Circulation and External Respiration
Every action performed by the human body requires chemical energy. All of our cells depend upon ATP (adenosine triphosphate) to fuel their functions. The process of aerobic respiration involves the breakdown of sugar in the presence of oxygen within the mitochondria of the cell to release ATP. This chemical energy must be created at a steady rate, which depends on the body's needs; for example, demand is higher during exercise. The lungs are necessary to supply cells with steady access to oxygen and a means to remove carbon dioxide waste, and the heart and associated vessels are needed to circulate these materials throughout the body.
As the body uses more oxygen to create greater amounts of ATP, it also creates more carbon dioxide waste. The heart can respond to this increased demand by producing higher rates of bloodflow. A region of the brain called the medulla monitors carbon dioxide levels in the brain and sends a signal through the cardiac nerve when it detects an increase. This signal travels to the sinoatrial (SA) node and causes the heart to increase both the number of beats per minute and volume of blood per contraction. When the medulla detects a decrease in carbon dioxide, it sends a signal through the vagus nerve to slow contractions in the SA node 2 7.
Health Consequences and the Heart and Lung
The final sections will be covered by the students through in-class research. Useful resources (both web and print) are located in the appendices. The following paragraphs contain a brief introduction to preventable diseases of the heart and lung, as the focus is on health and awareness.
Cardiovascular-related diseases (damage to the heart or blood vessels) are the leading cause of death in the United States; collectively heart disease and stroke are responsible for more than 40% of all adult deaths in the nation 2 8. Coronary heart disease results from arteriosclerosis, the accumulation of cholesterol and lipid deposits (collectively called plaque) in the coronary arteries. This buildup causes the arteries to narrow and harden, eventually causing myocardial ischemia, low oxygen concentration in the tissue due to reduced blood supply to the heart. If blood flow becomes interrupted through blockage or rupture, the heart tissue will actually die (myocardial infarction), and if the region of infarction is sufficiently large (such as the region served by the major coronary arteries) the infarction can destroy enough muscle tissue to cause cardiac arrest. These events are commonly referred to as having a heart attack 2 9. Plaque can build up in other major arteries as well; a stroke is caused by a blockage of blood flow to the brain.
While heart disease is not entirely preventable—there are numerous genetic factors involved—there are several risk factors that can be reduced by a healthy lifestyle: elevated blood lipids, high blood pressure (hypertension), smoking cigarettes, and having diabetes.
Poor decisions—choosing to smoke, in particular—can lead to several chronic and sometimes fatal lung conditions. Emphysema is a condition where the alveolar structure has been ruptured throughout the lung, creating empty air spaces instead. People with emphysema often have chronic bronchitis (long-term inflammation of the bronchioles) as well. The reduction of surface area for gas exchange combined with swelling and irritation makes the lungs much less efficient 3 0. Smoking can cause fatal damage to the lungs. Lung cancer is the leading cause of cancer in the United States, and smoking is responsible for 80-90% of cases. Toxins from tobacco smoke damage lung cells over time and lead to mutations that cause damaged cells to grow out of control 3 1.
Organ Transplantation
The heart and lungs are clearly necessary for human survival. However, as noted in the previous section, a number of conditions can cause failure. Transplantation is possible for either or both organs when all other health options have been exhausted. In 1984, the United States Congress passed the National Organ Transplant Act to maintain a national registry for organ matching 3 2. As of July 15 th, 2011, 111,943 people are waiting for organ transplants. Of these, 3,185 are waiting for a heart, 1,769 for a lung, and 66 for both 3 3.
These transplants are nearly always taken from deceased donors who have volunteered the posthumous use of their organs. Donors must be registered on their state registry (links available through http://www.organdonor.gov) and in the event of brain death, the hospital contacts the Organ Procurement and Transplantation Network to search for a matching candidate 3 4. The American Academy of Neurology has a very specific set of standards that determine what constitutes brain death: "the irreversible loss of function of the brain, including the brainstem." 3 5 Organs are matched with candidates through a detailed selection process, involving factors such as blood type, immune system compatibility, size of organs, time waiting on the candidate list, and distance between donor and recipient (since quick movement of the organ from donor to recipient is critical for most organs) 3 6.
Doctors performed 2,333 heart transplants in 2010. Heart transplantation is generally successful, with 88% of patients surviving the first year and 75% surviving five years. Lung transplants are not as common due to the low number of organs available; 1,769 patients received lung transplants in 2010 but the five-year survival rate is considerably lower than for the heart—just 54% 3 7.
Organ transplantation remains a difficult issue due to medical and ethical factors. The organ recipient must have a compatible blood type and take a number of immune system-suppressing drugs to ensure that their body does not reject or attack its donated organ. An organ donor must be careful to maintain (or in many cases, improve) their health after receiving a donation. Organ donation remains an issue of controversy in regards to personal belief as well. The United States Department of Health and Human Services states that "most major religions in the United States support organ donation and consider donation as the final act of love and generosity toward others, 3 8" but since only 40% of Americans are registered donors 3 9, there seems to be a persistent issue of belief or education.
Since the limiting factor in organ replacement is the number of organs available, there is a great deal of research going into the production of artificial organs and tissue engineering for replacement and repair. Synthetic heart machines can be used as a short-term bridge between heart failure and heart transplantation, though no permanent options currently exist. Certain components of the heart have been successfully reproduced, however, such as replacement valves and pacemakers, and stents can be used to open up blocked or narrowed blood vessels. Scientists have also been able to grow living tissue in vitro with the hopes of someday being able to graft patches onto dead or damaged organs in the body 4 0.
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