1
Reading #8
Physiologic support systems and exercise
Introduction
Most sport, recreational, and occupational activities require a moderately intense yet sustained energy release. The aerobic breakdown of carbohydrates, fats, and proteins generates this energy for ADP phosphorylation to ATP. Without a steady rate between oxidative phosphorylation and the energy requirements of physical activity, an anaerobic-aerobic energy imbalance develops, lactic acid accumulates, tissue acidity increases, and fatigue quickly ensues. Two factors limit an individual’s ability to sustain a high level of exercise intensity without undue fatigue:
• Capacity for oxygen delivery
• Capacity of specific muscle cells to generate ATP aerobically
Understanding the role of the ventilatory, circulatory, muscular, and endocrine systems during exercise enables us to appreciate the broad range of individual differences in exercise capacity. Knowing the energy requirements of exercise and the corresponding physiologic adjustments necessary to meet these requirements provides a sound basis to formulate an effective physical fitness program and evaluate one's physiologic and fitness status before and during such a program.
Part 1. Pulmonary System and Exercise
Pulmonary Structure and Function
If oxygen supply depended only on diffusion through the skin, it would be impossible to support the basal energy requirement, let alone the 3- to 5-liter oxygen uptake each minute to sustain a world class 5-minute per mile marathon pace. The remarkably effective ventilatory system meets the body’s needs for gas exchange.
The ventilatory system, depicted in Figure 1, regulates the gaseous state of our “external” environment for aerating fluids of the “internal” environment during rest and exercise. The major functions of the ventilatory system include:
• Supply oxygen for metabolic needs
• Eliminate carbon dioxide produced in metabolism
• Regulate hydrogen ion concentration to maintain acid-base balance
Anatomy of Ventilation
The "term pulmonary ventilation" describes how ambient air moves into and exchanges with air in the lungs. About 1 foot (0.3 m) represents the distance between the ambient air just outside the nose and mouth and the blood flowing through the lungs. Air entering the nose and mouth flows into the conductive portion of the ventilatory system. Here it adjusts to body temperature, and becomes filtered and almost completely humidified as it moves through the trachea. The trachea is a short one-inch diameter tube that extends from the and divides into two tubes of smaller diameter called bronchi. The bronchi serve as primary conduits within the right and left lungs. They further subdivide into numerous bronchioles that conduct inspired air through a tortuous, narrow route until it eventually mixes with the air in the alveoli, the terminal branches of the respiratory tract.
Lungs
The lungs provide the surface between blood and the external environment. Lung volume varies between 4 and 6 liters (amount of air in a basketball) and provides an exceptionally large moist surface. For example, the lungs of an average-sized person weigh about 1 kg, yet if spread out as in Figure 2, they would cover a surface of 60 to 80 m2. This equals about 35 times the surface of the person, and would cover almost one-half a tennis court! This represents a considerable interface for aeration of blood because during any one second of maximal exercise, no more than 1 pint of blood flows in the lung tissue’s fine network of blood vessels.
Alveoli
Lung tissue contains more than 300 million alveoli each. These elastic, thin-walled, membranous sacs provide the vital surface for gas exchange between the lungs and blood. Alveolar tissue has the largest blood supply of any organ in the body. In fact, the lung receives the entire output of blood from the heart (cardiac output). Millions of thin-walled capillaries and alveoli lie side by side, with air moving on one side and blood on the other. The capillaries form a dense mesh that covers almost the entire outside of each alveolus. This web becomes so dense that blood flows as a sheet over each alveolus. Once blood reaches the pulmonary capillaries, only a single cell barrier, the respiratory membrane, separates blood from air in the alveolus. This thin tissue-blood barrier permits rapid gas diffusion between the blood and alveolar air.
During rest, approximately 250 mL of oxygen leaves the alveoli each minute and enter the blood, and about 200 mL of carbon dioxide diffuse in the reverse direction into the alveoli. When trained endurance athletes perform heavy exercise, about 20 times the resting oxygen uptake transfers across the respiratory membrane. The primary function of pulmonary ventilation during rest and exercise is to maintain a fairly constant, favorable concentration of oxygen and carbon dioxide in the alveolar chambers. This ensures effective gaseous exchange before the blood leaves the lungs for its transit throughout the body.
Mechanics of Ventilation
The lungs do not merely suspend in the chest cavity. Rather, the difference in pressure within the lungs and the lung-chest wall interface causes the lungs to adhere to the chest wall interior and literally follow its every movement. Any change in thoracic cavity volume thus produces a corresponding change in lung volume. Because lung tissue does not contain voluntary muscle, the lungs depend on accessory means to alter their volume. The action of voluntary skeletal muscle during inspiration and expiration alters thoracic dimensions, which brings about changes in lung volume.
Inspiration
The diaphragm, a large, dome-shaped sheet of muscle makes an airtight separation between the abdominal and thoracic cavities. During inspiration, the diaphragm muscle contracts, flattens out, and moves downward up to 10 cm toward the abdominal cavity. This enlarges the chest cavity and makes it more elongated. The air in the lungs then expands reducing its pressure (referred to as intrapulmonic pressure) to about 5 mm Hg below atmospheric pressure.
Inspiration concludes when thoracic cavity expansion ceases and intrapulmonic pressure increases to equal atmospheric pressure.
Expiration
Expiration, a predominantly a passive process, occurs as air moves out of the lungs. It results from the recoil of stretched lung tissue and relaxation of the inspiratory muscles. This makes the sternum and ribs swing down, while the diaphragm moves back toward the thoracic cavity. These movements decrease the volume of the chest cavity, compressing alveolar gas and move it out through the respiratory tract into the atmosphere. During ventilation in moderate to heavy exercise, the internal intercostal muscles and abdominal muscles act powerfully on the ribs and abdominal cavity. This triggers a more rapid and greater depth of exhalation.
Respiratory muscle actions change thoracic dimensions to create a pressure differential between the inside and outside of the lung to drive airflow along the respiratory tract. Greater involvement of the pulmonary musculature (as occurs during progressively heavier exercise), causes larger pressure differences and concomitant increases in air movement.
Lung Volumes and Capacities
Figure 3 depicts the various lung volume measures that reflect one’s ability to increase the depth of breathing. The figure also shows average values for men and women while breathing from a calibrated recording spirometer that measures oxygen uptake by the closed-circuit method. Two types of measurements, static and dynamic, provide information about lung function dimensions and capacities. Static lung function measures evaluate the dimensional component for air movement within the pulmonary tract, and impose no time limitation on the subject. In contrast, dynamic lung functions evaluate the power component of pulmonary performance during different phases of the ventilatory excursion.
Static Lung Volumes
During measurement of static lung function the spirometer bell falls and rises with each inhalation and exhalation to provide a record of the ventilatory volume and breathing rate. Tidal volume (TV) describes air moved during either the inspiratory or expiratory phase of each breathing cycle. For healthy men and women, TV under resting conditions usually ranges between 0.4 and 1.0 liters of air per breath.
After recording several representative TVs, the subject breathes in normally and then inspires maximally. This additional volume of about 2.5 to 3.5 liters above the inspired tidal air represents the reserve for inhalation, termed the inspiratory reserve volume (IRV). The normal breathing pattern begins once again following the IRV. After a normal exhalation, the subject continues to exhale and forces as much air as possible from the lungs. This additional volume, the expiratory reserve volume (ERV), ranges between 1.0 and 1.5 liters for an average-sized man (and 10 to 20% lower for a woman). During exercise, TV increases considerably because of encroachment on IRV and ERV, particularly the IRV.
Forced vital capacity (FVC) represents the total air volume moved in one breath from full inspiration to maximum expiration, or vice versa, with no time limitation. Although values for FVC can vary considerably with body size and body position during the measurement, average values usually equal 4 to 5 liters in healthy young men and 3 to 4 liters in healthy young women. FVCs of 6 to 7 liters are not uncommon for tall individuals, and values of 7.6 liters have been reported for a professional football player and 8.1 liters for an Olympic gold medalist in cross-country skiing. Large lung volumes of some athletes probably reflect genetic influences because static lung volumes do not change appreciably with exercise training.
Dynamic Lung Volumes
Dynamic measures of pulmonary ventilation depend on two factors:
• Volume of air moved per breath (tidal volume)
• Speed of air movement (ventilatory rate)
Airflow speed depends on the pulmonary airways' resistance to the smooth flow of air and resistance offered by the chest and lung tissue to changes in shape during breathing.
Forced Expiratory Volume-To-Forced Vital Capacity Ratio
Normal values for vital capacity can occur in severe lung disease if no limit exists on the time to expel air. For this reason, a dynamic lung function measure such as the percentage of the FVC expelled in one second (FEV1.0) is more useful for diagnostic purposes. Forced expiratory volume-to-forced vital capacity ratio (FEV1.0/FVC) reflects expiratory power and overall resistance to air movement in the lungs. Normally, the FEV1.0/FVC averages about 85%. With severe pulmonary (obstructive) lung disease (e.g., emphysema and/or bronchial asthma), the FEV1.0/FVC becomes greatly reduced, often reaching less than 40% of vital capacity. The clinical demarcation for airway obstruction equals the point at which less than 70% of the FVC can be expelled in one second.
Maximum Voluntary Ventilation
Another dynamic test of ventilatory capacity requires rapid, deep breathing for 15 seconds. Extrapolation of this 15-second volume to the volume breathed had the subject continued for one minute represents the maximum voluntary ventilation (MVV). For healthy, college-aged men, the MVV usually ranges between 140 and 180 liters. The average for women equal 80 to 120 liters. Male members of the United States Nordic Ski Team averaged 192 liters per minute, with an individual high MVV of 239 liters per minute. Patients with obstructive lung disease achieve only about 40% of the MVV predicted normal for their age and body size. Specific pulmonary therapy benefits patients because training the breathing musculature increases the strength and endurance of the respiratory muscles (and enhances MVV).
Pulmonary Ventilation
Minute Ventilation
During quiet breathing at rest, an adults" breathing rate averages 12 breaths per minute (about 1 breath every 5 s), whereas tidal volume averages about 0.5 liter of air per breath. Under these conditions, the volume of air breathed each minute (minute ventilation) equals 6 liters.
Minute ventilation (VE) = Breathing rate x Tidal volume
6.0 L•min–1 = 12 x 0.5 L
An increase in depth or rate of breathing or both significantly increases minute ventilation. During maximal exercise, the breathing rate of healthy young adults usually increases to 35 to 45 breaths per minute, although elite athletes can achieve 60 to 70 breaths per minute. In addition, tidal volume commonly increases to 2.0 liters and larger during heavy exercise, causing exercise minute ventilation in adults to easily reach 100 liters or about 17 times the resting value. In well-trained male endurance athletes, ventilation may increase to 160 liters per minute during maximal exercise. In fact, several studies of elite endurance athletes report ventilation volumes of 200 liters per minute. Even with these large minute ventilations, the tidal volume rarely exceeds 55 to 65% of vital capacity.
|Typical values for pulmonary ventilation during rest, moderate and vigorous exercise. |
|Condition |Breathing Rate |Tidal volume |Pulmonary Ventilation (L.min) |
| |(breaths/min) |(L/breath) | |
|Rest |12 |0.5 |6 |
|Moderate exercise |30 |2.5 |75 |
|Vigorous exercise |50 |3.0 |150 |
Alveolar Ventilation
Alveolar ventilation refers to the portion of minute ventilation that mixes with the air in the alveolar chambers. A portion of each breath inspired does not enter the alveoli, and thus does not engage in gaseous exchange with the blood. This air that fills the nose, mouth, trachea, and other nondiffusible conducting portions of the respiratory tract constitutes the anatomical dead space. In healthy people, this volume averages 150 to 200 mL, or about 30% of the resting tidal volume.
Because of dead-space volume, approximately 350 mL of the 500 mL of ambient air inspired in each tidal volume at rest mixes with existing alveolar air. This does not mean that only 350 mL of air enters and leaves the alveoli with each breathe. To the contrary, if tidal volume equals 500 mL, then 500 mL of air enters the alveoli but only 350 mL represents fresh air (or about one-seventh of the total air in the alveoli). Such a relatively small, seemingly inefficient alveolar ventilation prevents drastic changes in the composition of alveolar air; this ensures a consistency in arterial blood gases throughout the entire breathing cycle.
For Your Information
Respired Gases: Concentration and Partial Pressures
Gas concentration should not be confused with gas pressure.
Gas concentration reflects the amount of gas in a given volume – determined by the gas' partial pressure x solubility [Gas concentration = partial pressure x solubility]
Gas pressure represents the force exerted by the gas molecules against the surfaces they encounter.
Partial Pressure = Percent concentration x Total pressure of gas mixture
Depth Versus Rate
Increases in the rate and depth of breathing maintain alveolar ventilation during increasing exercise intensities. In moderate exercise, well-trained endurance athletes achieve adequate alveolar ventilation by increasing tidal volume and only minimally increasing breathing rate. With deeper breathing, alveolar ventilation can increase from 70% of the minute ventilation at rest to over 85% of the total exercise ventilation.
Ventilatory adjustments during exercise occur unconsciously; each individual develops a “style” of breathing by blending breathing rate and tidal volume so alveolar ventilation matches alveolar perfusion. Conscious attempts to modify breathing during general physical activities such as running usually fail and do not benefit exercise performance. In fact, conscious manipulation of breathing detracts from the exquisitely regulated ventilatory adjustments to exercise. At rest and exercise, each individual should breathe in the manner that seems most natural.
Gas Exchange
Our oxygen supply depends on the oxygen concentration in ambient air and its pressure. Ambient (atmospheric) air composition remains relatively constant at 20.93% for oxygen, 79.04% for nitrogen (includes small quantities of inert gases that behave physiologically like nitrogen), 0.03% for carbon dioxide, and usually small quantities of water vapor. The gas molecules move at great speeds and exert a pressure against any surface they contact. At sea level, the pressure of air's gas molecules raises a column of mercury to an average height of 760 mm (29.9 in.). This barometric reading varies somewhat with changing weather conditions and decreases predictably at increased altitude.
Ambient Air
|Table 2. Percentages, partial pressures, and volumes of gases in 1 liter of dry ambient |
|air at sea level. |
|Gas |Percentage |Partial Pressure (at 760|Volume of Gas |
| | |mmHg) |(mL•L-1) |
|Oxygen |20.93 |159 |209.3 |
|Carbon dioxide |0.03 |0.02 |0.4 |
|Nitrogen |79.04 |600 |790.3 |
Table 2 presents the volume, percentage, and partial pressures of gases in dry, ambient air at sea level. The partial pressure of oxygen equals 20.93% of the total 760 mm Hg pressure exerted by air, or 159 mmHg (0.2093 x 760 mm Hg); the random movement of the minute quantity of carbon dioxide exerts a pressure of only 0.2 mm Hg (0.0003 x 760 mmHg), while nitrogen molecules exert a pressure that raises the mercury in a manometer about 600 mm (0.7904 x 760 mmHg). The letter P before the gas symbol denotes partial pressure. For sea level ambient air: PO2 = 159 mmHg; PCO2 = 0.2 mmHg; PN2 = 600 mmHg.
Tracheal Air
Air entering the nose and mouth passes down the respiratory tract; it becomes saturated with water vapor that slightly dilutes the inspired air mixture. At body temperature the pressure of water molecules in humidified air equals 47 mm Hg; this leaves 713 mmHg (760 - 47) as the total pressure exerted by the inspired dry air molecules at sea level. Consequently, the effective Po2 in tracheal air decreases by about 10 mmHg from its ambient value of 159 mm Hg to 149 mmHg (0.2093 x [760 - 47 mmHg]). Humidification has little effect on the inspired PCO2 because of carbon dioxide's almost negligible concentration in inspired air.
Alveolar Air
Alveolar air composition differs considerably from the incoming breath of moist ambient air because carbon dioxide continually enters the alveoli from the blood, whereas oxygen leaves the lungs for transport throughout the body. Table 3 shows that alveolar air contains approximately 14.5% oxygen, 5.5% carbon dioxide, and 80.0% nitrogen.
|Table 3. Percentages, partial pressures, and volumes of gases in 1 liter of dry alveolar |
|air at sea level. |
|Gas |Percentage |Partial Pressure (at 760-47|Volume of Gas |
| | |mmHg) |(mL•L-1) |
|Oxygen |14.5 |103 |145 |
|Carbon dioxide |5.5 |39 |55 |
|Nitrogen |80.0 |571 |800 |
|Water | |47 | |
After subtracting vapor pressure in moist alveolar gas, the average alveolar Po2 equals 103 mmHg (0.145 x [760 - 47 mmHg]) and 39 mmHg (0.055 x [760 - 47 mmHg]) for PCO2. These values represent the average pressures exerted by oxygen and carbon dioxide molecules against the alveolar side of the respiratory membrane. They do not exist as physiologic constants, but vary slightly with the phase of the ventilatory cycle and adequacy of ventilation in various lung segments.
Gas Exchange in the Body
The exchange of gases between the lungs and blood, and their movement at the tissue level, takes place entirely passively by diffusion.
Gas Exchange in Lungs
The first step in oxygen transport involves oxygen transfer from oxygen alveoli into the blood. The alveolar Po2 equals 100 mmHg, which is less than the Po2 of ambient air. Three main reasons for the dilution of oxygen in inspired air include:
• Water vapor saturates relatively dry inspired air
• Oxygen is continually removed from alveolar air
• Carbon dioxide is continually added to alveolar air
The pressure of oxygen molecules in alveolar air averages about 60 mm Hg higher than the Po2 in venous blood entering the pulmonary capillaries. Consequently, oxygen diffuses through the alveolar membrane into the blood. Carbon dioxide exists under slightly greater pressure in returning venous blood than in the alveoli causing diffusion of carbon dioxide from the blood into the lungs. Although only a small pressure gradient of 6 mm Hg exists for carbon dioxide diffusion compared with oxygen, adequate carbon dioxide transfer occurs rapidly because of carbon dioxide's high solubility. Nitrogen, an inert gas in metabolism, remains essentially unchanged in alveolar-capillary gas.
Gas Exchange in the Tissues
In the tissues, where energy metabolism consumes oxygen at a rate almost equal to carbon dioxide production, gas pressures can differ considerably from arterial blood. At rest, the average Po2 in the muscle's extracellular fluid rarely drops below 40 mmHg, while cellular PCO2 averages about 46 mmHg. In contrast, heavy exercise can reduce the pressure of oxygen molecules in muscle tissue to 3 mmHg, whereas the pressure of carbon dioxide approaches 90 mmHg. The pressure differential between gases in plasma and tissues establishes the gradients for diffusion – oxygen leaves capillary blood and diffuses toward metabolizing cells, while carbon dioxide flows from the cell to the blood. Blood then enters the veins and returns to the heart for delivery to the lungs. Diffusion rapidly begins once again as venous blood enters the lung's dense capillary network.
Oxygen and Carbon Dioxide Transport
Oxygen Transport in the Blood
The blood transports oxygen in two ways:
1. In physical solution – dissolved in the fluid portion of the blood.
2. Combined with hemoglobin – in loose combination with the iron-protein hemoglobin molecule in the red blood cell
Oxygen Transport in Physical Solution
Oxygen does not dissolve readily in fluids. At an alveolar Po2 of 100 mm Hg, only about 0.3 mL of gaseous oxygen dissolves in the plasma of each 100 mL of blood (3 mL of oxygen per liter of blood). Because the average adult’s total blood volume equals about 5 liters, 15 mL of oxygen dissolve for transport in the fluid portion of the blood (3 mL per L x 5 = 15 mL). This amount of oxygen could sustain life for only about four seconds. Viewed from a different perspective, the body would need to circulate 80 liters of blood each minute just to supply the resting oxygen requirements if oxygen were transported only in physical solution. This represents a blood flow two times higher than the maximum ever recorded for an exercising human!
Oxygen Combined With Hemoglobin (Hb)
The blood of many animal species contains a metallic compound to augment its oxygen-carrying capacity. In humans, the iron-containing protein pigment hemoglobin constitutes the main component of the body’s 25 trillion red blood cells. Hemoglobin increases the blood’s oxygen-carrying capacity 65 to 70 times above that normally dissolved in plasma. Thus, for each liter of blood, hemoglobin temporarily “captures” about 197 mL of oxygen. Each of the four iron atoms in a hemoglobin molecule loosely binds one molecule of oxygen to form oxyhemoglobin in the reversible oxygenation reaction:
Hb + 4 O2 ––––––––> Hb4O8
This reaction requires no enzymes. The partial pressure of oxygen in solution solely determines the oxygenation of hemoglobin to oxyhemoglobin.
Oxygen-Carrying Capacity of Hemoglobin
In men, each 100 mL of blood contains approximately 15 to 16 g of hemoglobin. The value averages 5 to 10% less for women, or about 14 g per 100 mL of blood. Sex difference in hemoglobin concentration contributes to the lower aerobic capacity of women, even after adjusting for differences in body mass and fat.
Each gram of hemoglobin can combine loosely with 1.34 mL of oxygen. Thus, oxygen-carrying capacity can be calculated by knowing blood’s hemoglobin concentration as follows:
Blood’s oxygen capacity = Hb (g•100 mL-1 blood) x Oxygen capacity of Hb (1.34 mL)
For example, if the blood’s hemoglobin concentration equals 15, then approximately 20 mL of oxygen (15 g per 100 mL x 1.34 mL = 20.1) would be carried with the hemoglobin in each 100 mL of blood if hemoglobin achieved full oxygen saturation (i.e., if all Hb existed as Hb408).
Po2 and Hemoglobin Saturation
Blood’s oxygen-carrying capacity assumes that hemoglobin achieves full saturation when exposed to alveolar gas.
Figure 4 shows the relationship between percent hemoglobin saturation (left vertical axis) at various Po2s under normal resting physiologic conditions (arterial pH 7.4, 37°C) and the effects of changes in pH (Figure 6B) and temperature (Figure 6C) on hemoglobin’s affinity for oxygen. Percent saturation of hemoglobin computes as follows:
Percent saturation = (Total O2 combined with Hb (Oxygen carrying capacity of Hb) x 100
This curve, termed the "oxyhemoglobin dissociation curve", quantifies the amount of oxygen carried in each 100 mL of blood in relation to plasma Po2 (right axis). For example, at a Po2 of 90 mm Hg the normal complement of hemoglobin in 100 mL of blood is about 19 mL of oxygen; at 40 mm Hg the oxygen falls to 15 mL, and 6.5 mL at a Po2 of 20 mm Hg.
Part 2. Circulation System and Exercise
Introduction
The Greek physician Galen (Lecture 2) theorized about blood flow in the body. He believed blood flowed like the tides of the sea, surging and abating into arteries, then away from the heart and back again. In Galen’s view, fluid carried with it “humors”, good and evil that determined one’s well-being. If a person became ill, the standard practice required bloodletting to drain off the diseased humors and restore health. This theory prevailed until the seventeenth century when physician William Harvey (Lecture 2) proposed a different scenario. Experimenting with frogs, cats, and dogs, Harvey demonstrated the existence of valves in the heart that provided for one-way movement of fluid, a finding incompatible with Galen’s “ebb-and-flow” view because it suggested a circular flow of blood through the body. In a set of ingenious experiments, Harvey measured the volume of the heart chambers and counted the number of times the heart contracted in one hour. He concluded that if the heart emptied only one-half its volume with each beat, the body’s total blood volume would be pumped in minutes. These finding led Harvey to hypothesize that blood moved (circulated) within a closed system in a circular pattern throughout the body. Harvey, of course, was correct; we now know that the heart pumps the entire blood volume, approximately five liters, in one minute. Harvey’s experiments changed medical science forever, although it would take nearly two hundred more years for his ideas to play important roles in physiology and medicine.
From Harvey’s early experiments of the sophisticated research at the dawn of the twenty first century, we now know that the highly efficient ventilatory system complements a rapid transport and delivery system comprised of blood, the heart, and more than 60,000 miles of blood vessels that integrate the body as a unit. The circulatory system serves five important functions during physical activity:
1. Delivers oxygen to active tissues
2. Aerates blood returned to the lungs
3. Transports heat, a by-product of cellular metabolism, from the body's core to the skin
4. Delivers fuel nutrients to active tissues
5. Transports hormones, the body’s chemical messengers
Components of the Cardiovascular System
The cardiovascular system consists of an interconnected, continuous vascular circuit containing a pump (heart), a high-pressure distribution system (arteries), exchange vessels (capillaries), and a low-pressure collection and return system (veins). Figure 5 presents a schematic view of this system.
Heart
The heart provides the force to propel blood throughout the vascular circuit. This four-chambered organ, a fist-sized pump, beats at rest an average of 70 times a minute, 100,800 times a day, and 36.8 million times a year. Even for a person of average fitness, maximum output of blood exceeds fluid output from a household faucet turned wide open!
Functionally, the heart consists of two separate pumps: one pump (left heart pump) receives blood from the body and pumps it to the lungs for aeration (pulmonary circulation and the other pump (right heart pump) accept oxygenated blood from the lungs and pump it throughout the body (systemic circulation).
The hollow chambers of the heart’s right side (right heart) perform two important functions:
Receive blood returning from all parts of the body
Pump blood to the lungs via the pulmonary circulation for aeration
The left side of the heart (left heart) also performs two important functions:
Receive oxygenated blood from the lungs
Pump blood into the thick-walled, muscular aorta for distribution throughout the body in the systemic circulation
A thick, solid muscular wall (septum) separates the left and right sides of the heart. The atrioventricular (AV) valves situated within the heart direct the one-way flow of blood from the right atrium to the right ventricle (tricuspid valve) and from the left atrium to the left ventricle (mitral or bicuspid valve). The semilunar valves located in the arterial wall just outside the heart prevent blood from flowing back into the heart between ventricular contractions.
The relatively thin-walled, sac-like atrial chambers serve as primer pumps to receive and store blood returning from the lungs and body during ventricular contraction. About 70% of the blood that returns to the atria flows directly into the ventricles before the atria contract. Simultaneous contraction of both atria forces remaining blood into their respective ventricles directly below. Almost immediately after atrial contraction, the ventricles contract and force blood into their specific arterial systems.
Arteries
The arteries provide the high-pressure tubing that conducts oxygen-rich blood to tissues. Arteries are composed of layers of connective tissue and smooth muscle. Because of their thickness, no gaseous exchange takes place between arterial blood and surrounding tissues. Blood pumped from the left ventricle into the highly muscular yet elastic aorta circulates throughout the body via arterioles, or smaller arterial branches. Arteriole walls contain circular layers of smooth muscle that either constrict or relax to regulate peripheral blood flow. This redistribution function becomes particularly important during exercise because blood diverts to working muscles from areas that temporarily compromise their blood supply.
Capillaries
The arterioles continue to branch and form smaller and less muscular vessels called metarterioles. These tiny vessels end in capillaries, a network of microscopic blood vessels so thin they provide only enough room for blood cells to squeeze through in single file. Capillaries generally contain about 5% of the total blood volume at any time. Gases, nutrients, and waste products rapidly transfer across the thin, porous, capillary walls. A ring of smooth muscle (precapillary sphincter) encircles the capillary at its origin to control the vessel’s internal diameter. This sphincter provides a local means for regulating capillary blood flow within a specific tissue in response to metabolic requirements that change rapidly and dramatically in exercise.
Veins
The vascular system maintains continuity of blood flow as capillaries feed deoxygenated blood at almost a trickle into small veins called venules. Blood flow then increases slightly because the venous system cross-sectional area is less than for capillaries. The lower body’s smaller veins eventually empty into the largest vein, the inferior vena cava, that travels through the abdominal and chest cavities toward the heart. Venous blood draining the head, neck, and shoulder regions empties into the superior vena cava and moves downward to join the inferior vena cava at heart level. The mixture of blood from the upper and lower body then enters the right atrium and descends into the right ventricle for delivery through the pulmonary artery to the lungs. Gas exchange takes place in the lungs’ alveolar-capillary network; here, the pulmonary veins return oxygenated blood to the left heart, where the journey through the body resumes.
Venous Return
A unique characteristic of veins solves a potential problem related to the low pressure of venous blood. Because of low venous blood pressure, muscular contractions or minor pressure changes within the chest cavity during breathing compress the veins. Alternate venous compression and relaxation, combined with the one-way action of valves, provides a “milking” effect similar to the action of the heart. Venous compression imparts considerable energy for blood flow, whereas “diastole” (relaxation) allows vessels to refill as blood moves toward the heart. Without valves, blood would stagnate or pool (as it sometimes does) in veins of the extremities, and people would faint every time they stood up because of reduced blood flow to the brain.
A Significant Blood Reservoir
The veins do not merely function as passive conduits. At rest, the venous system normally contains about 65% of the total blood volume; hence, the veins serve as capacitance vessels, or blood reservoirs. A slight increase in tension (tone) by the vein’s smooth muscle layer alters the diameter of the venous tree. A generalized increase in venous tone rapidly redistributes blood from peripheral veins toward the central blood volume returning to the heart. In this manner, the venous system plays an important role as an active blood reservoir to either retard or enhance blood flow to the systemic circulation.
Varicose Veins
Sometimes valves within a vein become defective and fail to maintain one-way blood flow. This condition of varicose veins usually occurs in superficial veins of the lower extremities from the force of gravity that retards blood flow in an upright posture. As blood accumulates, these veins become excessively distended and painful, often impairing circulation from surrounding areas. In severe cases, the venous wall becomes inflamed and degenerates – a condition called phlebitis, which often requires surgical removal of the vessel. Individuals with varicose veins should avoid excessive straining exercises like heavy resistance training.
Venous Pooling
The fact that people faint when forced to maintain an upright posture without movement (e.g., standing at attention for a prolonged period) demonstrates the importance of muscle contractions to venous return. Also, changing from a lying to a standing position affects the dynamics of venous return and triggers physiologic responses. Heart rate and blood pressure stabilize during bed rest. If a person suddenly rises and remains erect, an uninterrupted column of blood exists from heart level to the toes, creating a hydrostatic force of 80 to 100 mm Hg. Swelling (edema) occurs from pooling of blood in the lower extremities and creates “back pressure” that forces fluid from the capillary bed into surrounding tissues. Concurrently, impaired venous return decreases blood pressure; at the same time, heart rate accelerates, and venous tone increases to counter the hypotensive condition. Maintaining an upright position without movement leads to dizziness and eventual fainting from insufficient cerebral blood supply. Resuming a horizontal or head-down position restores circulation and consciousness.
The Active Cool-Down
The existence of venous pooling justifies continued slow jogging or walking after strenuous exercise. A “cooling down” with rhythmic exercise facilitates blood flow through the vascular circuit (including the heart) during recovery. An “active recovery” also aids in lactic acid removal from the blood. The pressurized suits worn by test pilots and special support stockings also aid in retarding hydrostatic shifts of blood to veins of the lower extremities in the upright position. A similar supportive effect occurs in upright exercise in a swimming pool because the water’s external support facilitates venous return.
Blood Pressure
A surge of blood enters the aorta with each contraction of the left ventricle, distending the vessel and creating pressure within it. The stretch and subsequent recoil of the aortic wall propagates as a wave through the entire arterial system. The pressure wave appears in the following areas: as a pulse in the superficial radial artery on the thumb side of the wrist, in the temporal artery (on the side of the head at the temple), and/or at the carotid artery along the side of the trachea (Figure 6).
|Blood pressure classification |
|Systolic (mmHg) |Diastolic (mmHg) |Category |
| ................
................
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