Reading #1 Introduction to Science

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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.


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.


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, 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.

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.

able 1 shows that minute ventilation does not always reflect actual alveolar ventilation. In the first example of shallow breathing, tidal volume decreases to 150 mL, yet a 6-L minute ventilation results when breathing rate increases to 40 breaths per minute. The same 6-L minute volume results by decreasing breathing rate to 12 breaths per minute and increasing tidal volume to 500 mL. Doubling tidal volume and halving the ventilatory rate, as in the example of deep breathing, again produces a 6-L minute ventilation. Each ventilatory adjustment drastically affects alveolar ventilation. In the example of shallow breathing, dead-space air represents the entire air volume moved (no alveolar ventilation has taken place.)

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.

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

Ambient Air

Table 2. Percentages, partial pressures, and volumes of gases in 1 liter of dry ambient air at sea level.



Partial Pressure (at 760 mmHg)

Volume of Gas (mL•L-1)





Carbon dioxide








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 completely saturated with water vapor, that slightly dilutes the inspired air mixture. At body temperature, for example, 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.


Partial Pressure (at 760-47 mmHg)

Volume of Gas (mL•L-1)





Carbon dioxide










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

Thus far, the discussion of blood’s oxygen-carrying capacity assumes that hemoglobin achieves full saturation with oxygen 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 normal 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 quantity falls to 15 mL, and 6.5 mL at a Po2 of 20 mm Hg.

The Bohr Effect

Figures 5B and 5C show that increases in acidity (H+ concentration and CO2) or temperature cause the oxyhemoglobin dissociation curve to shift downward to the right (to enhance unloading of oxygen), particularly in the Po2 range of 20 to 50 mm Hg. This phenomenon, known as the Bohr effect (named after its discoverer physiologist Christian Bohr), results from alterations in hemoglobin’s molecular structure.

Bohr Effect becomes important in vigorous exercise, as increased metabolic heat and acidity in tissues augments oxygen release. For example, at a Po2 of 20 mm Hg and normal body temperature (37°C), %O2 saturation of hemoglobin equals 35%. At the same Po2, with body temperature increased to 43°C (like at the end of a marathon run), hemoglobin’s saturation decreases to about 23%. Thus, more oxygen unloads from hemoglobin for use in cellular metabolism. Similar effects take place with increased acidity.

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