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The electrical signal tetion of fils to the bottom of the atria and passesinto the ter specialized res the etestecalled the atrioventricular notes (RV nodo. amie V node transmaits hat alectical signal from the afria into thevensV nod The so transmits the signal along into another region of specializ...

The electrical signal tetion of fils to the bottom of the atria and passesinto the ter specialized res the etestecalled the atrioventricular notes (RV nodo. amie V node transmaits hat alectical signal from the afria into thevensV nod The so transmits the signal along into another region of specialized tasteles. has covn the ventricularsentum the tissue that separates the to ventibat. mahi specialized tissue within the ventricular septum is called the buentrol fils also sometimes called the atrioventricular bundle) and it splits tollent the light and left bundle branches. The bundle branches pass the signal on torne perkinje fbres, which in turn pass the electrical signal to the myocardlumthat forms the ventricles. This arrangement of specialized tissue allows the contraction of the heart o be initiated in the atria from the top downward, pushing blood into the entricles. The ventricles then contract from the bottom up, forcing blood into the aorta and pulmonary arteries. The SA node, without any input from the nervous system, will cause the heart to contract at approximately 70-80 beats per minute. In situations where the SA node is damaged, the AV node takes control and becomes the pacemaker of the heart. As noted earlier, the SA node normally acts as the pacemaker of the heart, setting its basic rate of contraction, and it is influenced by the autonomic nervous system. Stimulation from the nervous system can result in either an increase or decrease in heart rate and in the force of contraction of the heart. The electrical activity of the heart can be measured using an instrument known as an electrocardiogram (ECG). The ECG provides a graphical representation of the electrical sequence of events that occurs with each contraction of the heart (see Figure 8.3). Each of the electrical waves generated during contraction has a specific name, The vascular system is formed by a network of vessels that transport blood throughout the body (see Figure 8.4). As you follow the path of the blood away from the heart, in either the systemic or pulmonary cireuit, the structure of the vessels begins to branch out and get smaller. sets deferent vessels are divided into four main categories: arteries, arterioles. capillaries: and venules and veins. All of these different types of vessels share One common feature- a thin layer of cells that lines the inside of the vessels, a layer known as the endothelium. Arteries The arteries are vessels with very thick muscular walls, which carry blood away from the heart to the different tissues and organs of the body. Even though the walls of arteries are very thick, they are still very elastic and can stretch and then recoil back to their original diameter. This ability to stretch and recoil is important in assisting the movement of blood during diastole (the relaxation phase of the pumping of the heart). Arteries carry oxygenated blood away from the heart, with the exception of the pulmonary arteries that carry blood to the lungs for oxygenation. Blood pressure is also measured in the arteries. Systolic blood pressure is the pressure caused by the contraction of the heart, while the recoil of the arteries causes diastolic blood pressure during diastole. The arterioles are smaller than the arteries and are important in the regulation of blood distribution to the various tissues of the body. Arterioles are surrounded by rings of smooth muscle, and these rings can contract, constricting the arteriole and reducing the amount of blood flow, or relax, opening the arteriole and increasing the amount of blood flow. These rings of smooth muscle are controlled by two factors: the nervous system, and local chemical factors released by the tissues that are supplied by the arterioles. Nerves interact directly with the arterioles, and can be signaled by the central nervous system to contract or relax the layers of smooth muscle, depending on the needs of the body. For example, during exercise, the arterioles that supply the muscles that are involved in the exercise would be opened by local chemical changes and by the nervous system, increasing blood flow to the muscles. At the same time, the arterioles that supply the intestine would be constricted, reducing blood flow to this organ. Therefore, the nervous system can control the distribution of blood flow to different organs using the arterioles. Blood flow is also influenced by a process called autoregulation. The process of autoregulation refers to the effects of locally produced chemical compounds on blood flow. For example, many local chemical factors are produced in skeletal muscle during exercise. These factors are released from the muscle and will diffuse to the arterioles. These factors cause the arterioles to relax, increasing the delivery of blood to the working muscle. This is a very interesting system, in that it attempts to deliver blood directly to where it is needed. Many different chemical factors that are produced in skeletal muscle during contraction have been thought to influence blood flow. Probably the most potent stimulator of blood flow produced in skeletal muscle is nitric oxide. This compound has been the focus of much research and has been implicated in the local control of many factors, one of which is blood flow. It should also be noted that most of the time the arteries carry oxygen-rich blood, with the only exception being the pulmonary arteries that carry deoxygenated blood to the lungs. Arterioles Arterioles are vessels in the blood circulation system that branch out from arteries and lead to capillaries, where gas exchange eventually occurs. Surrounded by smooth muscle, arterioles are the primary site of vascular resistance Capillaries The smallest of the blood vessels, capillaries help to enable the exchange of water, oxygen, carbon dioxide, and other nutrients and waste substances between blood and the tissues. Capillaries The capillaries are the smallest vessels within the body and, in a sense, they have the most important function of all the vessels. Capillaries are so small that the red blood cells can barely fit through, and the walls of the capillaries are very thin (one cell thick). Despite being so small, there are millions of capillaries within the body. All body tissues have an extensive supply of capillaries. If you were to line up all of the capillaries from one person, they would form a line more than 40 000 kilometres long. It is in the capillaries where the main function of the cardiovascular system occurs, as well as the exchange of gases and nutrients with the tissues. The transfer of gases and nutrients from the blood to the tissues depends on diffu-sion. For example, the concentration of 02 is high in the blood when it arrives in the capillaries, but in the tissues there is less 02. So, the Oz leaves the blood sectioned arteriole, by diffusion and goes into the tissues where it is needed, moving from a higher concentration to a lower concentration. The diffusion pathway of 02 will be Verublood travels through the capilarisofthecapilaries. rerules. These vessel hierreturn blood to the heart for anot from the ramshout the vased.. Blood vessels that carry blood towards the heart. In the systemic circulation, veins carry deoxygenated blood towards the right side of the heart from body tissues. In the pulmonary circulation, veins carry oxygenated blood towards the left side of the heart from the lungs. rein. veins become larger as they moves wast the the capillaries. Many systems Come together to form larger veins, until they all come togetherto form either the superior or inferior vena cava. "The superior and inferior vena cavae drain the venous blood into the right The of the heart. The walls of the veins also contain smooth muscle, which allows the veins to dilate and contract similar to the arterioles. Veins have the ability to dilate and contract to make sure that enough blood is returned to the heart, so that the heart can meet the needs of the body Veins usually carry deoxygenated blood, with the only exception being the pulmonary veins that carry oxygen-rich blood. Veins are different from all other blood vessels in that they have one-way valves. Blood exerts a force on the walls of blood vessels; this blood pressure in the veins is very low, and if there were no valves, the blood could travel in the valves thus ensure that the flow of blood can only go back to the heart. wrons direction, back towards the tissues and organs of the body. These one way The Return of Blood in the Veins Back to the Heart The low pressure within the veins creates a problem for the cardiovascular system -how to get all of the blood in the veins back to the heart. There are three main tools, or systems, that the body uses to assist in the return of blood in the veins to the heart. The first tool is called the skeletal muscle pump, a general term used to describe how, with each contraction of a skeletal muscle, blood is pushed or massaged back to the heart. This occurs because of the one-way valves found within the veins. Each contraction of s muscle compresses the veins within or around that muscle, increasing the pressure within that vein. The increase in pressure moves the blood along, and because of the one-way valves, the only direction the blood can travel is back towards the heart (see Figure 8.5). The second system that the body uses to assist in the return of blood in the contracts veins to the heart is called the thoracic pump. The thoracic pump is related to breathing. With each breath taken by the respiratory system, pressure in the chest cavity is very low for a few short seconds, while the pressure in the abdominal cavity increases. The pressure within the veins that are found in the chest also drops, while the pressure in the veins within the abdominal cavity increases. This creates a difference in pressure between the veins in these two body cavities, and this pushes blood from the veins in the abdominal cavity into the veins. the veins in the thoracic cavity, again because of the one-way valves found in The final system that the body can use to assist in the return of blood from the veins to the heart is the nervous system. At times when cardiac output needs to be increased, such as during exercise, the nervous system sends a signal to the veins, causing them to slightly constrict (this response is known as venoconstriction). This slight constriction helps to return more blood back to the heart. slood is the specialized fluid that is found in the heart and all of the vessels. is main role is to act as a transport medium for 02, CO2, and nutrients. Blood is made up of two main components: plasma and blood cells. Plasma is the Auid component of blood. It is composed mostly of water and makes up about 55 percent of blood. Within the plasma you will find many different dissolved substances, such as nutrients, proteins, ions, and gases. The blood cells make up the other 45 percent of blood, with the most abundant blood cells being the red blood cells or erythrocytes. The erythrocvtes are the specialized cells that transport Oz and CO, in the blood. Erythrocytes contain a specialized protein called hemoglobin, which can bind Oz and CO2. It is this protein in erythrocytes that gives blood the ability to transport and deliver O2 to the tissues, and remove COz to the lungs. Another type of cell that is found in blood is white blood cells, or leukocytes. Leukocytes make up less than one percent of the blood and are an important part of the body's immune system. They play an important role in protecting the body from disease. Platelets are also found in blood. They are not complete cells, but fragments of cells. These platelets are important in the regulation of blood clotting. To separate blood into its component parts, the blood is collected and put into a centrifuge unit and spun at very high speeds for a few minutes. This rapid spinning results in the heaviest components of the blood (the erythrocytes) sitting at the bottom of the tube and the lightest components (the plasma) sitting at the top, as illustrated in Figure 8.6. Plaque build-up restricts blood flow to the heart muscle The heart is a working muscle that needs not only a constant supply of 02, but also fuel and nutrients. The system of vessels that supply these essential materials via blood to the heart is called the coronary circulation (see Figure 8.7). Serious health repercussions and even death can occur if a narrowing or blockage of blood vessels restricts the flow of blood to the heart muscle. The Coronary Arteries Blood is supplied to the heart through two main arteries, the right and left coronary arteries. These two arteries branch off from the aorta just above where the aorta leaves the heart. The arteries then divide multiple times, supplying blood to all regions of the myocardium with O2-rich blood. Each new branch decreases in size, and eventually these smaller vessels are called arterioles. The coronary arterioles continue to branch and divide, and become even smaller in diameter. Eventually the vessels are so small that a red blood cell can barely get through, and the walls are only one cell thick. It is in these microscopic vessels -the capillaries-that 02, CO2, and nutrients are exchanged between the blood and the myocardium. Within the myocardium there are millions of capillaries. If we were to look at one square millimetre of myocardium under a microscope, we would be able to see 3,000 to 4,000 capillaries. The Coronary Veins Once blood flows through the capillaries, the vessels become larger, as many smaller vessels come together to form larger vessels. These larger vessels that are collecting blood from the capillaries are called coronary venules. These venules are blood vessels that drain blood directly from the capillary beds. As the blood moves through the venules, the coronary venules come together and form coronary veins. Eventually, all of the coronary veins come together to form the coronary sinus. The coronary sinus then drains into the right atrium of the heart, and this completes the path of blood through the coronary circulation. Cardiovascular Disease The term "cardiovascular disease" encompasses any disease associated with the cardiovascular system. More often, people use this term to refer to coronary artery disease or coronary heart disease. Coronary artery disease (which is also known as atherosclerosis) is associated with a gradual narrowing of the coronary arteries resulting from the accumulation of hard deposits of cholesterol, called plaque, on the lining of the blood vessels. This fatty plaque builds up over the years, often as a result of a diet high in saturated fats. The process can start early in life. In fact, evidence of fatty plaque build-up has been observed in 18-year-olds! Besides a poor diet, other risk factors associated with coronary artery disease include smoking, elevated blood lipids, hypertension, family history, and physical inactivity. Each factor individually increases the risk of development of coronary artery disease, but when the factors are combined, the risk is magnified. One of the most serious possible consequences of coronary artery disease is a heart attack. What Causes a Heart Attack? If the fatty plaque coating the walls of the coronary arteries supplying oxygenated blood to the heart should rupture, this will cause a blood clot on the surface of the artery. If the plaque buildup is significant enough, the clot can partially or completely block the flow of oxygen-rich blood to that part of the heart muscle, as shown in Figure 8.8 below. A heart attack (a myocardial infarction) can result when blood flow to a section of the heart muscle becomes blocked due to plaque buildup or some other reason. If the blood flow is not restored promptly, then that section of the heart becomes damaged from lack of oxygen. If the plaque blockage is not treated and removed quickly (say, within a few hours), then the damage to the heart muscle will be irreversible. The damaged part of the heart muscle will die and will be replaced with scar tissue. The severity of the disease is determined by where this muscle death in the coronary artery occurs and how many coronary arteries are involved. Cardiovascular dynamics deals with the functioning of the cardiovascular Saran and how it adapts to meet the demands that are placed on it Essentially, the heart and the vessels must constantly adapt to accommodate the ever. changing requirements of the body. For example, during exercise, dramatic changes in cardiovascular dynamics occur. Some of the factors that are considered when discussing cardiovascular dvnamics are cardiac output, blood pressure, distribution of blood flow, and oxygen consumption. Cardiac Output (Q) The volume of blood pumped out of the left ventricle in one minute is called cardiac out put (Q), and is measured in litres per minute (L/min). At rest, a typical person's cardiac output will be approximately 5-6 L/min, but during heavy exercise, cardiac output can increase up to greater than 30 L/min. The two other factors that contribute to cardiac output are stroke volume and heart rate. Stroke volume (S) is the amount of blood that is ejected from the left ventricle in a single beat, and is measured in millilitres (mL). SV is calculated by subtracting the left ventricular end-systolic volume (LVES) from the left ventricular end-diastolic volume (LVEDV). LVES is the amount of blood remaining in the left ventricle after the contraction of the ventricle, while LVEDV is the amount of blood remaining in the left ventricle after the contraction of the left atrium. Stroke volume is regulated by three main factors, both at rest and during exercise: (1) LVEDV, (2) aortic blood pressure, and (3) the strength of the ventricular contraction. Alterations to any of these three factors will result in changes in stroke volume. LVEDV is the amount of blood that is returned to the ventricle before it contracts. The ventricle has the capacity to stretch to accommodate increases in LVEDV, and this stretching of the ventricle results in a more forceful contraction of the cardiac muscle and an increase in the amount of blood that is ejected. Therefore, the most important factor that regulates SV would be the amount of blood that is returned to the heart (venous return). During exercise, venous return increases as the result of four main factors: (1) constriction of the veins (venoconstriction); (2) the skeletal muscle pump; (3) the thoracic pump; and (4) nervous stimulation of the heart The walls ofthe veins contain some smooth muscle, and during exercise, the smooth muscle is stimulated to contract, slightly reducing the diameter of the veins and reducing the volume of blood in the veins, directing it towards the heart. The skeletal muscle pump and the thoracic pump also contribute to increases in venous return. Finally, nervous stimulation of the heart results in an increase in the force of contraction of the heart, further contributing to the increases in SV that are observed during exercise. The efficiency of SV is measured through the calculation of ejection fraction. Ejection fraction (EF) is the proportion of blood that is eiected from the left ventricle during a single heartbeat. On average, EF at rest is -50-60 percent, and it increases during exercise as the intensitv of exercise increases. During maximal exercise, EF can increase to -85 percent. Heart rate (HR) is the number of times the heart contracts in a minute beats per minute). Cardiac output (9) can be calculated as the product ofstroke volume and heart rate. For example, an average HR at rest would be -72 beats/ min, and an average SV at rest would be -71 mL. Therefore, using the equation for O. resting O would be calculated as 5,040 mL/min, or 5.04 L/min. During exercise, Q can increase to 15-25 L/min, depending on the intensity of the exercise. The increase in occurs very early in the exercise and then becomes constant at the new higher level. These increases are mediated by both an increase in SV and HR. The increase in SV also occurs very early in exercise and then maintains a plateau. The increase in HR is very similar to increases observed in SV and Q. If the exercise is very prolonged, there might be a slight decline in SV late in the exercise. Such declines result from excessive fluid loss from the body due to sweating. Despite the slight decline in SV, Q is maintained with a gradual increase in HR. Generally, the increase in O with exercise is related to the intensity of the exercise, with increases in observed with increases in exercise intensity. During prolonged exercise, Q is maintained, but significant changes are observed with HR and SV. This phenomenon is called cardiovascular drift, and is characterized by a slow and steady rise in HR and a corresponding decline in S. Cardiovascular drift results from the physiological changes associated with the increase in body temperature that occurs during the exercise. Some of these changes include decreases in plasma volume, redistribution of blood flow to the skin, and dehydration. All of these changes result in a decrease in venous return of blood to the heart, resulting in a decrease in SV. Despite the decline in SV, the body compensates through increases in HR, and Q is maintained. Bloscardiac cycle is defined as the seriest orrelaxatoscalastarouston. am. Blood Pressure beatricle is filing with blood, and a phase of contraction (systole) in which, measured in millimetres of mercury (abbreviated as mmHs). Systolic blood pressure refers to the maximum pressure observedin the alteries during the contraction phase of the ventricle (e.g., 120 mm. Diastolic blood pressure is the minimum pressure observed in the arteries during the relaxation phase of the ventricle (e.g., 80 mmlig) Blood pressure is the force exerted by the blood against the walls of the arteries and other vascular vessels. During the cardiac cycle, the dramatic changesin pressure propel the blood through the circulation. The changes create pressure waves that are measured in the arteries. A noted above, the changes are refered to as systolic and diastolic blood pressure. When blood pressure is reported of measured, it is often stated as being the systolic pressure over the diastolicpres. sure (e.g., 120/80 mm Hg). The normal range of blood pressure in humans varies from 90/60 mmHg to 120/80 mmHg. During exercise, profound changes in blood pressure can oceur depending on the type, duration, and intensity of the exercise. For example, acute aerobic or endurance exercise generally leads to a sustained increase in systolic blood pressure, but no change in diastolic blood pressure during the activity. The increase in systolic blood pressure is often proportional to the exercise intensity-meaning the greater the exercise intensity, the greater the rise in systolic blood pressure. Resistance exercise, such as weightlifting, can result in very short, but very large increases in both systolic and diastolic blood pressure. As with endurance exercise, the increase in blood pressure with resistance-trpe exercise is also proportional to the intensity of the exercise, with greater increases in blood pressure observed with higher exercise intensities. Following exercise, both endurance and resistance, there is a prolonged period where blood pressure drops below normal resting values. This phenomenon is called post-exercise hypotension, and occurs even with low- intensitv exercise. Persistently elevated (high) blood pressure, also called hypertension (blood pressure greater than 140/90 mmHg), is a major risk factor for cardiovascular disease. Hypertension can be caused by obesity, smoking, aging, kidney disease, and genetic factors. It is considered a modifiable risk factor, because through changes to lifestyle, blood pressure can be returned to near normal levels. Aerobic exercise leads to improvements in resting blood pressure in people with moderate to high blood pressure within three weeks to three months. Further improvements are also observed when modifications are made to other health behaviours, such as diet. For example, it is recommended that individuals with elevated blood pressure consume a diet low in saturated fats and cholesterol and high in fibre and complex carbohvdrates. When modifications in diet are combined with increases in aerobic exercise, improvements in blood pressure are common. However, in some extreme cases, additional medical intervention is required, and no person with elevated blood pressure should start exercising without clearance from his or her doctor. Blood Flow Distribution During exercise, the working skeletal muscle has an increased need for 02, and the cardiovascular system attempts to match the delivery of O, to meet this need by altering the blood how distribution. The increase in the delivery of Oz I achieved in two ways: an increase in Q, and a redistribution of blood how. the system increases the amount of blood flow that is directed to the working muscle while blood flow to less active organs, such as the stomach, intestine, and kidneys, is decreased (see Table 8.1 below). The redistribution of blood flow is dependent on the intensity of the exercise, with a greater amount of blood shunted towards the working muscle with increasing exercise intensities. One of the only organ systems in the human body where blood flow distribution remains unaltered during exercise is the brain. The absolute amount of blood delivered to the brain is maintained both at rest and during exercise. Table 8.1 Distribution of Cardiac Output to Various Vascular Regions Aerobic exercise leads to improvements in the cardiovascular system Regular aerobic exercise has been shown to result in improvements in efficiency of the cardiovascular system and its various components both at rest and during sub-maximal exercise, and also during maximal exercise (see Table 8.2 below). The most influential changes that occur with aerobic training and exercise are alterations in the structure of the heart. With prolonged training, increases in the mass and dimensions of the heart can be observed. Specifically, increases in ventricular volume and the thickness of the ventricle walls take place. These changes to the structure of the heart likely occur due to the persistent increases in venous return that occur during exercise. The increase in ventricular volume would lead to an increase in SV, while the increase in ventricular wall thickness would contribute to an increased force of contraction of the ventricle. Both of these factors would in turn contribute to an increase in SV and therefore Q during exercise. Other structural changes that are observed in the heart include an increase in the number of capillaries that deliver blood to the myocardium. This adaptation likely occurs in response to the increase in 02 demand because of the increase in work being performed by the heart. There has been some evidence to suggest that training may also lead to an increase in the diameter of the coronary arteries. Such an increase would also increase the delivery of blood to the myocardium. Another important and rapid adaptation with training is an increase in blood volume. Within the first few days of initiating training, measurable increases in plasma volume can be observed-increases upward of 15 percent within two days of starting training. Such increases contribute to an increase in venous return, and therefore SV and Q. Eventually, as the training continues, there is also an increase in erythrocytes. If training is stopped, blood volume returns back to pre-training levels within a week or two. Bradycardia and Tachycardia Despite the alterations in O during exercise, it remains unchanged at rest. However, the factors that contribute to Q, 5V, and HR are altered at rest, with an increase in SV and a decrease in HR. These alterations are persistent during exercise, with an increase in SV being observed during both sub-maximal and maximal exercise. HR is decreased at rest and during sub-maximal exercise. This decrease in HR is often referred to as bradycardia. Bradycardia is one of the most easily observed adaptations that occurs with training. Bradycardia is characterized by a heart rate of 60 beats per minute or less at rest, while tachycardia is a heart rate of more than 100 beats per minute at rest. Generally, a lower heart rate is regarded as an indication of an athletic or strong heart. Despite the alterations in HR at rest and during sub-maximal exercise, HR is unchanged at maximal exercise. These alterations and adaptations together indicate the cardiovascular effects of training and contribute to the increases in maximal exercise capacity that is observed following training. The respiratory system is composed of structures that allow the passage of air then outside the body to the lungs, as well as the structures therein that allow functions of the respiratory system are to: gas exchange to oceur, as shownin Fisure 89 on the facing page. The threemain supply 02 to the blood; remove CO2 from the blood; and regulate blood pH (acid-base balance). External respiration refers to the processes that occur within the lungs involving the exchange of 02 and CO2. Internal respiration refers to the exchange of gases at the tissue level, where Oz is delivered and CO2 removed. Finally, cellular respiration is the process in which the cells use 02 to generate energy through the different metabolic pathways found in the mitochondria o the cells. The actual structure of the respiratory system can be divided into two main zones. These zones include the conductive zone and the respiratory zone. The conductive zone transports filtered air to the lungs, while the respiratory zones where gas exchange occurs (see Figure 8.9). The Conductive Zone The conductive zone is composed of all of the structures that convey air from the outside of the body through to the lungs. This zone includes the mouth and nose; pharynx; larynx; trachea; primary and secondary bronchi; and tertiary bronchioles and terminal bronchioles. By the time air reaches the respiratory zone, it is at body temperature (37°C) and is almost completely saturated with moisture. This helps to maintain body temperature and protect the sensitive tissues that make up the respiratory zone. Another role of the conductive zone is to filter air that is taken in with each breath. The nasal cavity is lined with hairs to trap larger foreign bodies and prevent them from being inspired (breathed in). The Respiratory Zone The respiratory zone is composed of the respiratory bronchioles, alveolar ducts, and the alveolar sacs. All of these structures are involved with the exchange of gases between inspired air and the blood. The alveolar sacs (alveoli) are grape-like structures found within the lungs. The alveolar sacs provide a large surface area for the diffusion of gases into and out of the blood. Additionally, all respiratory organs have moist surfaces in which the gases can dissolve and diffuse. The average person has about 300 million alveolar sacs. If all of these alveolar sacs were stretched out, they would cover an area as large as a tennis court. Each of these structures is surrounded by a web of capillaries as well as a network of delicate elastic fibres. Both the wall of the alveolar sacs and the wall of each capillary surrounding these sacs are one cell thick, which provides a very short distance for gases to diffuse. Therefore, the structure of the lungs provides a large surface area and a minimal distance for the diffusion of gases to occur, maximizing the rate of gas exchange. Human lungs are found within the chest cavity, which is separated from the abdominal cavity by a large, flat, specialized muscle called the diaphragm. In response to stimulation from the brain, at the start of a breath, the diaphragm contracts, pulling downward and enlarging the thoracic (chest) cavity. At the same time, the intercostal muscles contract, moving the chest upward and outward, which also helps to enlarge the thoracic cavity. As the chest expands, the lungs expand with it. The pressure in the lungs becomes negative relative to the outside air. The lower air pressure causes air to rush into the lungs, which results in an inhalation (or inspiration) that equalizes this pressure differential. Once the lungs are inflated with air, the chest muscles and diaphragm relax and recoil to their original positions. This compresses the lungs and forces air out of the airways; this is known as exhalation or expiration. Inspiration is an active process, requiring the contraction of various respiratory muscles and therefore the expenditure of significant amounts of energy. Expiration, on the other hand, may be passive, as in quiet breathing, (which may not require much energy), or active, as in forced breathing. In quiet breathing, expiration is similar to the release of air from an inflated balloon. in forced breathing, which occurs during vigorous exercise, the passive recoil of the lungs is not fast enough to keep up with the required rate of respiration. Thus, muscles in the thoracic cavity and abdominal wall contract, actively decreasing the volume of the thoracic cavity and increasing the air pressure within the lungs. This process forces air out of the lungs rapidly; an example occurs when someone attempts to blow out birthday candles. Ventilation The combination of inspiration and expiration together is known as ventilation (VE). More specifically, Ve is the volume of air that is moved by the lungs in one minute. V+ is influenced by two factors: the volume of air in each breath, and the number of breaths taken per minute. The volume of air in each breath is known as the tidal volume (V-). At rest, a typical V- is about 0.5 L/breath, while during exercise, V- can increase up to 3 to 4 L/breath. The number of breaths taken per minute is known as the respiratory frequency (f). Under resting conditions, a typical frequency of respiration is about 12 breaths/min, while during exercise, frequency can increase up to 30 to 40 breaths/min. The Control of Ventilation Breathing results from the contraction and relaxation of the inspiratory muscles and the expiratory muscles. The contraction of muscles is dependent on stimulation from the central nervous system. In the case of the muscles involved in breathing or ventilation, there are highly specialized regions within the brain that initiate the stimulation of the muscles involved in ventilation. All aspects of breathing are closely associated with the overall need of Oz, metabolic processes, muscle activity, and the production of CO2. Control of breathing is very complex and involves many different forms of feedback from specialized sensory systems to the neural control centres within the brain. The respiratory control centres are found within the brain stem. The brain stem is the region of the brain found just above the spinal cord and it is involved in many body processes that are not under conscious control. (In other words, the brain stem is part of the autonomic nervous system). The areas of the brain stem that are important in the regulation of ventilation are the medulla oblongata and the pons. Within the medulla oblongata is the inspiratory centre and the expiratory centre. The specialized nerves that are found in the inspiratory centre spontaneously generate a rhythmic signal (in an on-off pattern) that is sent to the respiratory muscles, the diaphragm, and the external intercostals. There are also two specialized respiratory centres that are found in the pons, and they are called the pneumotaxic and apneustic centres. These centres act to ensure that the transition from inhalation to exhalation is smooth. In addition to the respiratory control centres in the brain stem, other areas of the brain can also influence ventilation. For example, the stimulation of skeletal muscles leads to stimulation of the breathing control centres of the brain, in an attempt to "turn on" respiration with the initiation of movement. There are also specialized sensory systems in place to provide feedback to respiratory control centres. These sensory systems ensure that an adequate rate of respiration has been achieved. All of these sensory systems work together to ensure that ventilation requirements are met. The primary factor that mediates ave and at thietastelung cater bag. The pries arygenated and CO голоу з алтивол. війтивонтетсоз сани ми beconetabolism and COzis removed am afesion Fifusion is defined a is. torovement of a gas, liquid, or solid from a res on of high concentrationes, concentration is called a concentration gradient. megion of low concentration through randatienovement. Such a diference. nicer concentrations of specific gases involved in respiration are measuel.The cystem called "partial pressures. Recedithat air is made pot ananas ustiFierent gases, including nitrogen, Oz, and COs. The relative percentage, ofch of these gases stays the same in air but the partial pressure ofeach gates change depending on the air pressure (barometric pressure) and baromete pressure changes depending on the weather. For example, higherlevels of Barometric pressure are observed during clear weather, while lower measure are observed during bad weather. To calculate the partial pressure ofa gas, multiply the barometric pressure by the fraction of the gas. For example the Average barometric pressure is 760 mmHg, and the fraction percentage of Oin the air is 20.93 percent. Therefore, the partial pressure of O2, or PO,, is 760 mm HIg X 0.2093 = 159.1 mmHg. (Sce other sample calculations in Tablesg) Factors Affecting the Rates of Diffusion When it comes to the respiratory system, we are interested in the diffusion pathway for gases moving from the lungs into the blood and from the bloodinto the tissue, and back. The rates of diffusion of a gas between two different areas depend on a number of factors. The first factor that must be considered is the size of the concentration gradient. As the concentration gradient increases, greater rates of diffusion are observed. The diffusion of a gas into a liquid is governed by Henry's law which states that the amount of gas that will dissolve and/or diffuse into a liquid is proportional to the partial pressure and the solubility of the gas. Furthermore. Henry's law also states that the gas will continue to dissolve and/or diffuse into the liquid until an equilibrium has been achieved, meaning that the partial pressures within the liquid and the air are the same. Using O2 as an example, the PO2 within the alveoli is 105 mmHg, while the PO2 within the deoxygenated blood as it enters the lungs is only 40 mmHg. This creates a diffusion gradient, which mediates the diffusion of O2 into the blood (see Figure 8.11 on the facing page). The second factor that must be considered is the thickness of the barrier between the two areas where diffusion occurs. Within the lung, the distance between the alveoli and the capillaries that surround each alveoli is extremely small, essentially only two cells thick, optimizing the diffusion distance. The third factor is the surface area between the two areas where diffusion occurs. The anatomy of the lung provides a huge surface area for diffusion. As mentioned, if you were to stretch out all of the alveoli from the lungs, it would provide a surface area for diffusion about the size of a tennis court. The respiratory system takes advantage of all of these factors to maximize the rate of diffusion of O2 from the air inhaled in the lungs into the blood, and the movement of COz out of the blood. Oxygen (Oz) Transport first annal mount oronservanly desert allian thieve in two g. oxygen mali inmount ofO, is actually dissolved within the plasma, or the us Firstansent ofthe blood. This only represeatsported is by bind iftie O2 loundin protein a has the capacity to bind 1.34 ml. of zand the average concentration o hemoglobin is -16 mg/100 mL of blood. mosrofore, the average O2 carrying capacity for blood is 21 4 mI 02/100 m of biber (On carrying capacity = Igb| 1.34). The maiority of the O2 transporter in the blood is bound to hemoglobin. However, keep in mind that a very smart amount of Oz is also transported in the blood plasma (-0.5 mL 02/100 mI of blood). linder diferent conditions the amount of O, bound to hemoslobin can vary. The relative amount of the O2 carrying capacity that is used is termed the percent saturation of hemoglobin (Sb02%. However, the main factor that affects SbO-% is the PO? within the blood. The relationship that describes the infuence Of PO, on SBO.% is called the oxyhemoglobin dissociation curve. This curve has an "S" shape, and essentially, the lower the PO2, the less O2 will bind to hemoglobin. This relationship ensures that Oz will be delivered to where it is needed. For example, in the arterial blood PO2 is -100 mm Hg and $BO2% is -95-100%. In the capillaries of skeletal muscle, POz would decline to -40 mmHg, and SBO2% declines to ~75%. Carbon Dioxide (COz) Transport Carbon dioxide must be moved from body tissues, where it is produced, back to the lung where it can be moved into the alveoli and then exhaled and removed from the body. There are three ways in which CO2 is carried within the blood (COz transport). First, a small amount of CO2 is found dissolved in the plasma, much like O2. Only about 5-10 percent of CO2 is transported dissolved in the plasma. The remaining 90-95 percent of CO2 diffuses into the erythrocytes. Carbon dioxide (-20%) can also bind to the hemoglobin, forming what is called carbaminohemoglobin, when there are low concentrations of 02. Having arrived in the lung, the elevated concentrations of 02 stimulate the hemoglobin to release the CO2, which then diffuses out into the alveoli and is exhaled. The third way in which CO2 is transported is through what is called the bicarbonate system. The remaining CO2 (70-75%) diffuses into the erythrocytes, but undergoes a chemical reaction with water, forming a weak acid called carbonic acid. This chemical reaction occurs because of a specialized enzyme found in the erythrocytes called carbonic anhydrase. The newly formed carbonic acid then dissociates, forming a hydrogen ion and a bicarbonate ion. The resulting H+ ion binds to the hemoglobin, while the bicarbonate ion diffuses into the plasma. At the lungs, the partial pressure of CO2 is low, and this process is reversed. Furthermore, the higher partial pressure of O2 mediates the release of the H+ ion from the CO2. Once the CO2 is returned back to its original form, it is free to diffuse out of the blood and into the alveoli, and is then exhaled. The respiratory system is so efficient that these processes occur very rapidly and do not limit exercise performance. Ventilation and the Regulation of Blood pH Ventilation plays an important role in the regulation of the pH of the blood. Blood pH is a measure of how acidic or how basic the blood is. Generally, blood pH is maintained very close to a pH of 6.4. However, situations in which large amounts of acid are released into the blood, such as during exercise (lactic acid), result in a decline in blood pH. A decrease in pH (an increase in acidity) means that there is an increase in the accumulation of H+ ions in the blood. Ventilation plays an important role in the regulation of the amount of I+ ions in the blood because of the transport of CO2 through the bicarbonate system. Recall the carbonic anhydrase reaction that was discussed previously. Essentially, if ventilation is increased, expelling extra amounts of CO2, this situation causes more H+ to combine with bicarbonate to form carbonic acid and eventually CO2 and H2O. At the same time, this lowers the concentration of H+ ions, thereby increasing pH levels. Therefore, increases in ventilation can assist in returning blood pH back to near-normal levels. During exercise, the body responds to the increased need for oxygen at the working muscles through a series of responses that attempt to match oxygen delivery with oxygen demand. With respect to the respiratory system, changes occur in pulmonary ventilation (VE), external respiration, and internal respi-ration. Exercise results in increases in pulmonary ventilation, external and internal respiration, and cellular respiration. Pulmonary Ventilation (VE) Pulmonary ventilation (Vp) is closely matched to the rate and/or intensity of the work being done. The increases in Ve that occur with sub-maximal exercise can be divided into three phases. The first phase is termed the rapid on phase. During this phase, VE is increased at a very rapid rate, almost immediately upon the onset of the activity. The second phase is characterized by a slower exponential increase from the rapid increase observed in phase one. Phase three of the response is characterized by a levelling off of Ve at a new steady-state level. The new steady-state level is predominately determined by the intensity of the exercise and the level of fitness of the individual. The increases in Ve are due initially to a rise in VT, and then, as the exercise continues, a rise in f. With more intense exercise, f will increase to a greater extent to accommodate the increased demands for VE. External Respiration Total gas exchange at the lungs (external respiration) is increased as a result of two main factors: the increase in Ve and the increase in blood flow to the lungs. The increase in gas exchange is closely matched to the increase in requirements of the working skeletal muscle. The increase in Ve serves to maintain the necessary gradients in the partial pressures of both 02 and CO2, to maintain gas exchange. The increase in Q, discussed previously, also results in an increase in blood flow to the lungs. Generally, Ve is closely matched to the delivery of blood to the lungs, thereby maintaining the partial pressures in the alveoli and in the blood, and the normal diffusion gradients. The maintenance of the diffusion gradients ensures proper oxygenation of the blood as well as the removal of CO2 (see Figure 8.12). Internal Respiration the respiratory system automatically responds to the increased need for oxygen. Internal respiration involves the exchange of gases at the level of the tissues. Essentially, the extraction of O2 at the tissues is increased. This occurs as a result of four main factors: O an increase in the PO2 gradient, (2) an increase in PCO2, (3) a decrease in pH, and (4) an increase in temperature. During exercise, the skeletal muscle increases cellular respiration, and Oz is used to generate ATP. This increase in the use of O2 results in a decline in the POz within the skeletal muscle, and increases the gradient between the PO2 within the blood and the muscle. The increase in the gradient further enhances the diffusion of 02 out of the blood into the working muscle. The increase in muscle activity results in an increase in CO2 production, a decline in pH (resulting from increases in COz and lactic acid), and an increase in temperature. These three factors influence the binding of Oz with hemoglobin and result in an increase in the unloading of 02. Essentially, the alteration in CO2, pH, and temperature contribute to what is called the Bohr shift. This shift is in reference to the oxyhemoglobin dissociation curve, in that the curve is shifted to the left. This means that, for a given PO2, more 02 will be unloaded This results in an enhanced unloading of O2 at the working muscle, contributing to the increase in internal respiration that occurs during exercise. One wav to determine how much oxygen has been delivered to skeletal muscle is to measure the amount of oxygen in the arterial blood before it arrives at the muscle, and then measure the venous blood that drains from the same muscle. The difference between the amount of 02 in the artery and vein reflects the amount of 02 that was delivered to the muscle. This is referred to as the a-v02 difference (a-vO2 diff), as illustrated in Figure 8.13 above. Adaptations to Training Regular aerobic training leads to very few adaptations in the respiratory system. Essentially, the only observable changes to respiratory function with training are in VE. Generally, training results in an increase in Vi and a decrease in f, with no observed changes in Ve at rest. During sub-maximal exercise, similar responses are observed, but in some cases slight declines in Ve are also observed. However, training does result in increases in Ve during maximal exercise. Exercise physiologists have suggested that these changes may be the result of increases in both the strength and endurance of the respiratory muscles. The lungs and the respiratory system generally are not considered to limit exercise performance in healthy individuals. However, there are a couple of disease states where respiratory system function is impaired, which results in poor physical functioning. Asthma is a disease that is characterized by spasm of the smooth muscles that line the respiratory system, an oversecretion of mucous, and swelling of the cells lining the respiratory tract. Together, these changes result in shortness ofbreath, also called dyspnea, and wheezing sounds during breathing. These events can be acute or chronic in nature. Many factors can lead to the stimulation of an attack, including exercise, allergic reaction, contaminates, and stress. Fortunately, most cases of asthma can be controlled through the use of different medications. Olympic level athletes have been diagnosed with asthma and yet are able to compete internationally. Chronic obstructive pulmonary disease (COPD) is a general term that describes a family of diseases that lead to a dramatic reduction in airflow through the respiratory system. These diseases differ from asthma in that the conditions persist and cannot be relieved as quickly or as effectively through the use of medications. Individuals with COPD cannot perform normal everyday activities without experiencing dyspnea (shortness of breath). Treatment of COPD conditions includes not only medications but also supplemental oxygen therapy for severe cases, as well as respiratory muscle training. The amount of Oz taken up and consumed by the body in the metabolic processes is Called oxygen consumption (VO2). It is equalal to the amount, of o2 inspired minus the amount of Oz expired. VOz is proportional to workload, meaning that the greater the workload, the greater the VOz or the greater the amount of Oz used by the body. In the laboratory, VOz can be determined using a computerized metabolic cart system (also called indirect calorimetry, tens syrienomeasures the amount of air expired over a period of time and the concentration of Oz in the expired air. A computer then interprets this information, and VO, is calculated using values of men and women in various sports. mathematical formulae. Table 8.4 compares the average maximal oxygen uptake Maximal Rate of Oxygen Consumption (VO2max) Theoretically, VOz is a function of both Oz delivery to and O2 uptake by the working muscle and other tissues. Another way to describe or represent Oz delivery is Q (cardiac output). Q can be thought of as the total blood flow distrib. uted throughout the body. Another way to describe Oz uptake is a-vO2 dift, which represents the average amount of O2 found in the arteries minus the average amount of O2 found in the vena cava. Therefore, the maximal rate of oxygen consumption (VO,max) would theoretically occur at maximum SV, HR, and a-vO2 diff. VO›max is more properly defined as the maximal amount of O2 that can be taken in and used for the metabolic production of ATP during exercise. To determine VO›max in the laboratory, VOz is measured using a metabolic cart and computer system, as previously described, while the participant performs incremental exercise to exhaustion. Incremental exercise means that the exercise workload progressively becomes more difficult every minute or two, much like climbing a steeper and steeper hill. Such exercise can be performed using either a cycle ergometer or a treadmill. VO2max is used as a measure of aerobic fitness, and is indicative of aerobic exercise performance. Another factor that can be measured at the same time as VO2 is the production of CO2 (VCO2). VCO2 is calculated by measuring the difference between the amount of COz expired and the amount of COz inspired. Both VO2 and VCO2 provide a considerable amount of information individually in relation to exercise, but together they provide even more information. The ratio between VCO2 and VO2 is used to calculate the respiratory exchange ratio (RER). The respiratory exchange ratio is indicative of what metabolic systems are being used within the working muscle. The amount of CO2 produced and Oz consumed varies, depending on what fuels are being used by the working muscle. When fat is being oxidized and used to produce AlP, more Oz is consumed as compared to the amount of CO2 produced. When carbohydrate is the major fuel, not as much O2 is consumed relative to the amount of CO2 produced. When only carbohydrate is being used, the ratio of VCO2 to VO2 is equal to 1. RER is close to 0.7 when the main fuel being used is fat. Therefore, RER allows a way to estimate the relative contribution of the different fuels used in skeletal muscle during exercise. Limitation of VO›max has been a much debated topic in the field of exercise physiology. Theoretically, any of the components of the different systems involved could potentially be limiting. However, research has focussed on the three main systems involved: the respiratory system, the cardiovascular system, and the metabolic system within the working muscle that uses the Oz. The respiratory system could potentially limit VOzmax through a couple of different ways, including inadequate ventilation and oxygen diffusion limita-tions. In contrast, the cardiovascular svstem could limit VOsmax because of inadequate blood flow and or cardiac out put, or inadequate oxygen-carrying capacity (hemoglobin concentration). Finally, within the working muscle, a lack of mitochondria and the metabolic systems involved with the use of Oz could O also potentially limit VO2max. There is evidence and theory to support each of these possible limitations, but most exercise physiologists support the notion that it is the cardiovascular system that limits VO›max in healthy people. More specifically, it appears that the cardiovascular system is unable to meet the demands of the working muscle and deliver adequate amounts of O2. The limitation to VOmax within the cardiovascular system appears to be related to cardiac output (Q). It should be noted that not all exercise physiologists support this view, and it is likely that this topic will continue to be debated over the next few years. As noted earlier, the delivery of O2 to the working skeletal muscle is achieved through a combination of physiological mechanisms. Ultimately, the delivery of O2 is matched to the demand of 02. However, the physiological mechanisms are not instantaneous - there is a "lag" between the initiation of exercise and the achievement of a new steady state. By "steady state" we mean sub-maximal exercise levels, where oxygen uptake and heart rate level off, where energy demands and energy production are evenly balanced, and where the body maintains a steady level of exertion for a fairly extended period of time Oxygen Deficit Eventually all systems will have been turned on and O2 delivery will be matched again with O2 demand. However, during this "lag" a phenomenon called oxygen deficit (O2 deficit) occurs. During this period, the working muscle must partially rely on metabolic systems that do not require O2 (in other words, anaerobic metabolic systems). These anaerobic systems make up the difference and compensate for the "lag" in VO2, allowing the exercise to continue at the new workload, as shown in Figure 8.14. Oxygen deficit represents the difference between the oxygen required to perform a task and the oxygen actually consumed prior to reaching a new steady state (see Figure 8.14). The trained individual will reach this steady-state plateau faster than an untrained individual and therefore will have a smaller oxygen deficit for an exercise of a given duration. Individuals can increase their aerobic capacity through proper aerobic training. During incremental exercise, such as that done during a VOzmax test, Pulmonary ventilation initially increases at a rate proportional to the increase in workload. However, eventually a point is reached where ventilation increases much more rapidly than workload. This point is called the ventilators threshold, and it normally occurs at an exercise intensity that corresponds to 65-85 percent of VOmax, depending on one's level of aerobic fitness. This increase in ventilation is thought to occur because of an increase in the accumulation of lactic acid within the blood. Lactic acid is a by-product of the anaerobic metabolic processes in the working skeletal muscle. The enersy demands of the exercise can no longer be met by only the aerobic metabolic systems. Hence, the anaerobic systems are also used to meet the increasing energy requirements of the exercise. The body increases ventilation to deal with the accumulation of the lactic acid in the blood and the corresponding drop in pH. Therefore, the ventilatory threshold is often used as a marker of an increased reliance on anaerobic metabolic systems during exercise. Blood lactate remains low (-1.0 mmol/L) initially. Eventually, a point 1s reached where concentrations rise exponentially. This point is referred to as the lactate threshold (see Figure 8.15). Interestingly, the lactate threshold is usually closely associated with the ventilatory threshold. When lactate levels begin to accumulate rapidly (shortly after the lactic acid threshold is reached), this is referred to as the onset of blood lactate accumulation (OBLA). With proper aerobic training, the OBLA curve can be shifted to the right such that OBLA occurs later and during higher levels of intense exercise. The exercise intensity where it occurs can range from 65% of VO2max to -85% of VO2max, depending on the type of exercise and the individual's fitness level.