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## 20.2 GAS EXCHANGE ACROSS RESPIRATORY SURFACES

### Learning Objectives

By the end of this section, you will be able to:

Name and describe lung volumes and capacities

Understand how gas pressure influences how gases move into and out of the body

The structure of the lung maximizes its surface area to increase gas diffusion. Because of the enormous number of alveoli (approximately 300 million in each human lung), the surface area of the lung is very large (75 m2). Having such a large surface area increases the amount of gas that can diffuse into and out of the lungs.

## BASIC PRINCIPLES OF GAS EXCHANGE

Gas exchange during respiration occurs primarily through diffusion. Diffusion is a process in which transport is driven by a concentration gradient. Gas molecules move from a region of high concentration to a region of low concentration. Blood that is low in oxygen concentration and high in carbon dioxide concentration undergoes gas exchange with air in the lungs. The air in the lungs has a higher concentration of oxygen than that of oxygen-depleted blood and a lower concentration of carbon dioxide. This concentration gradient allows for gas exchange during respiration.

Partial pressure is a measure of the concentration of the individual components in a mixture of gases. The total pressure exerted by the mixture is the sum of the partial pressures of the components in the mixture. The rate of diffusion of a gas is proportional to its partial pressure within the total gas mixture. This concept is discussed further in detail below.

## LUNG VOLUMES AND CAPACITIES

Different animals have different lung capacities based on their activities. Cheetahs have evolved a much higher lung capacity than humans; it helps provide oxygen to all the muscles in the body and allows them to run very fast. Elephants also have a high lung capacity. In this case, it is not because they run fast but because they have a large body and must be able to take up oxygen in accordance with their body size.

Human lung size is determined by genetics, gender, and height. At maximal capacity, an average lung can hold almost six liters of air, but lungs do not usually operate at maximal capacity. Air in the lungs is measured in terms of lung volumes and lung capacities (Figure 20.12 and Table 20.1). Volume measures the amount of air for one function (such as inhalation or exhalation). Capacity is any two or more volumes (for example, how much can be inhaled from the end of a maximal exhalation).

Figure 20.12.

Human lung volumes and capacities are shown. The total lung capacity of the adult male is six liters. Tidal volume is the volume of air inhaled in a single, normal breath. Inspiratory capacity is the amount of air taken in during a deep breath, and residual volume is the amount of air left in the lungs after forceful respiration.

Table 20.1. Lung Volumes and Capacities (Avg Adult Male)

Volume/Capacity Definition Volume (liters) Equations

Tidal volume (TV) Amount of air inhaled during a normal breath 0.5 –

Expiratory reserve volume (ERV) Amount of air that can be exhaled after a normal exhalation 1.2 –

Inspiratory reserve volume (IRV) Amount of air that can be further inhaled after a normal inhalation 3.1 –

Residual volume (RV) Air left in the lungs after a forced exhalation 1.2 –

Vital capacity (VC) Maximum amount of air that can be moved in or out of the lungs in a single respiratory cycle 4.8 ERV+TV+IRV

Inspiratory capacity (IC) Volume of air that can be inhaled in addition to a normal exhalation 3.6 TV+IRV

Functional residual capacity (FRC) Volume of air remaining after a normal exhalation 2.4 ERV+RV

Total lung capacity (TLC) Total volume of air in the lungs after a maximal inspiration 6.0 RV+ERV+TV+IRV

Forced expiratory volume (FEV1) How much air can be forced out of the lungs over a specific time period, usually one second ~4.1 to 5.5 –

The volume in the lung can be divided into four units: tidal volume, expiratory reserve volume, inspiratory reserve volume, and residual volume. Tidal volume (TV) measures the amount of air that is inspired and expired during a normal breath. On average, this volume is around one-half liter, which is a little less than the capacity of a 20-ounce drink bottle. The expiratory reserve volume (ERV) is the additional amount of air that can be exhaled after a normal exhalation. It is the reserve amount that can be exhaled beyond what is normal. Conversely, the inspiratory reserve volume (IRV) is the additional amount of air that can be inhaled after a normal inhalation. The residual volume (RV) is the amount of air that is left after expiratory reserve volume is exhaled. The lungs are never completely empty: There is always some air left in the lungs after a maximal exhalation. If this residual volume did not exist and the lungs emptied completely, the lung tissues would stick together and the energy necessary to re-inflate the lung could be too great to overcome. Therefore, there is always some air remaining in the lungs. Residual volume is also important for preventing large fluctuations in respiratory gases (O2 and CO2). The residual volume is the only lung volume that cannot be measured directly because it is impossible to completely empty the lung of air. This volume can only be calculated rather than measured.

Source : opentextbc.ca

## 22.3 THE PROCESS OF BREATHING

### Learning Objectives

By the end of this section, you will be able to:

Describe the mechanisms that drive breathing

Discuss how pressure, volume, and resistance are related

List the steps involved in pulmonary ventilation

Discuss the physical factors related to breathing

Discuss the meaning of respiratory volume and capacities

Define respiratory rate

Outline the mechanisms behind the control of breathing

Describe the respiratory centers of the medulla oblongata

Describe the respiratory centers of the pons

Discuss factors that can influence the respiratory rate

Pulmonary ventilation is the act of breathing, which can be described as the movement of air into and out of the lungs. The major mechanisms that drive pulmonary ventilation are atmospheric pressure (Patm); the air pressure within the alveoli, called alveolar pressure (Palv); and the pressure within the pleural cavity, called intrapleural pressure (Pip).

## MECHANISMS OF BREATHING

The alveolar and intrapleural pressures are dependent on certain physical features of the lung. However, the ability to breathe—to have air enter the lungs during inspiration and air leave the lungs during expiration—is dependent on the air pressure of the atmosphere and the air pressure within the lungs.

## PRESSURE RELATIONSHIPS

Inspiration (or inhalation) and expiration (or exhalation) are dependent on the differences in pressure between the atmosphere and the lungs. In a gas, pressure is a force created by the movement of gas molecules that are confined. For example, a certain number of gas molecules in a two-liter container has more room than the same number of gas molecules in a one-liter container (Figure 22.3.1). In this case, the force exerted by the movement of the gas molecules against the walls of the two-liter container is lower than the force exerted by the gas molecules in the one-liter container. Therefore, the pressure is lower in the two-liter container and higher in the one-liter container. At a constant temperature, changing the volume occupied by the gas changes the pressure, as does changing the number of gas molecules. Boyle’s law describes the relationship between volume and pressure in a gas at a constant temperature. Boyle discovered that the pressure of a gas is inversely proportional to its volume: If volume increases, pressure decreases. Likewise, if volume decreases, pressure increases. Pressure and volume are inversely related (P = k/V). Therefore, the pressure in the one-liter container (one-half the volume of the two-liter container) would be twice the pressure in the two-liter container. Boyle’s law is expressed by the following formula:

P1V1 = P2V2

In this formula, P1 represents the initial pressure and V1 represents the initial volume, whereas the final pressure and volume are represented by P2 and V2, respectively. If the two- and one-liter containers were connected by a tube and the volume of one of the containers were changed, then the gases would move from higher pressure (lower volume) to lower pressure (higher volume).

Figure 22.3.1 – Boyle’s Law: In a gas, pressure increases as volume decreases.

Pulmonary ventilation is dependent on three types of pressure: atmospheric, intra-alveolar, and interpleural. Atmospheric pressure is the amount of force that is exerted by gases in the air surrounding any given surface, such as the body. Atmospheric pressure can be expressed in terms of the unit atmosphere, abbreviated atm, or in millimeters of mercury (mm Hg). One atm is equal to 760 mm Hg, which is the atmospheric pressure at sea level. Typically, for respiration, other pressure values are discussed in relation to atmospheric pressure. Therefore, negative pressure is pressure lower than the atmospheric pressure, whereas positive pressure is pressure that it is greater than the atmospheric pressure. A pressure that is equal to the atmospheric pressure is expressed as zero.

Intra-alveolar pressure is the pressure of the air within the alveoli, which changes during the different phases of breathing (Figure 22.3.2). Because the alveoli are connected to the atmosphere via the tubing of the airways (similar to the two- and one-liter containers in the example above), the interpulmonary pressure of the alveoli always equalizes with the atmospheric pressure.

Figure 22.3.2 – Intrapulmonary and Intrapleural Pressure Relationships: Alveolar pressure changes during the different phases of the cycle. It equalizes at 760 mm Hg but does not remain at 760 mm Hg.Intrapleural pressure is the pressure of the air within the pleural cavity, between the visceral and parietal pleurae. Similar to intra-alveolar pressure, intrapleural pressure also changes during the different phases of breathing. However, due to certain characteristics of the lungs, the intrapleural pressure is always lower than, or negative to, the intra-alveolar pressure (and therefore also to atmospheric pressure). Although it fluctuates during inspiration and expiration, intrapleural pressure remains approximately –4 mm Hg throughout the breathing cycle.

Source : open.oregonstate.education

## A&P II: Chapter 22

Start studying A&P II: Chapter 22 - The Respiratory System. Learn vocabulary, terms, and more with flashcards, games, and other study tools.

## A&P II: Chapter 22 - The Respiratory System

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The main site of gas exchange is the ________.

A) alveolar duct

B) respiratory bronchiole

C) alveoli D) alveolar sacs

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C) alveoli

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The loudness of a person's voice depends on the ________.

A) length of the vocal folds

B) force with which air rushes across the vocal folds

C) thickness of vestibular folds

D) strength of the intrinsic laryngeal muscles

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B) force with which air rushes across the vocal folds

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1/61 Created by ribscake

### Terms in this set (61)

The main site of gas exchange is the ________.

A) alveolar duct

B) respiratory bronchiole

C) alveoli D) alveolar sacs C) alveoli

The loudness of a person's voice depends on the ________.

A) length of the vocal folds

B) force with which air rushes across the vocal folds

C) thickness of vestibular folds

D) strength of the intrinsic laryngeal muscles

B) force with which air rushes across the vocal folds

The walls of the alveoli are composed of two types of cells, type I and type II alveolar cells. The function of type II alveolar cells is to ________.

A) replace mucus in the alveoli

B) protect the lungs from bacterial invasion

C) trap dust and other debris

D) secrete surfactant

D) secrete surfactant

Complete the following statement using the choices below. Air moves out of the lungs when the pressure inside the lungs is ________.

A) equal to the pressure in the atmosphere

B) less than the pressure in the atmosphere

C) greater than the intra-alveolar pressure

D) greater than the pressure in the atmosphere

D) greater than the pressure in the atmosphere

Which of the following is true regarding normal quiet expiration of air?

A) It depends on the complete lack of surface tension on the alveolar wall.

B) It is a passive process that depends on the recoil of elastic fibers that were stretched during inspiration.

C) It requires contraction of abdominal wall muscles.

D) It is driven by increased blood CO2 levels.

B) It is a passive process that depends on the recoil of elastic fibers that were stretched during inspiration.

Which of the following maintains the patency (openness) of the trachea?

A) pseudostratified ciliated epithelium

B) C-shaped cartilage rings

C) surfactant production

D) surface tension of water

B) C-shaped cartilage rings

Intrapulmonary pressure is the ________.

A) difference between atmospheric pressure and respiratory pressure

B) pressure within the pleural cavity

C) pressure within the alveoli of the lungs

D) negative pressure in the intrapleural space

C) pressure within the alveoli of the lungs

The relationship between gas pressure and gas volume is described by ________.

A) Dalton's law B) Charles' law C) Boyle's law D) Henry's law C) Boyle's law

The statement, "in a mixture of gases, the total pressure is the sum of the individual partial pressures of gases in the mixture" paraphrases ________.

A) Charles' law B) Henry's law C) Boyle's law D) Dalton's law D) Dalton's law

Surfactant helps to prevent the alveoli from collapsing by ________.

A) humidifying the air before it enters

B) protecting the surface of alveoli from dehydration and other environmental variations

C) warming the air before it enters

D) interfering with the cohesiveness of water molecules, thereby reducing the surface tension of alveolar fluid

D) interfering with the cohesiveness of water molecules, thereby reducing the surface tension of alveolar fluid

For gas exchange to be efficient, the respiratory membrane must be ________.

A) between 5 and 6 micrometers thick

B) at least 3 micrometers thick

C) 0.5 to 1 micrometer thick

D) The thickness of the respiratory membrane is not important in the efficiency of gas exchange.

C) 0.5 to 1 micrometer thick

With the Bohr effect, more oxygen is released because a(n) ________.

A) decrease in pH (acidosis) weakens the hemoglobin-oxygen bond

B) increase in pH (alkalosis) strengthens the hemoglobin-oxygen bond

C) decrease in pH (acidosis) strengthens the hemoglobin-oxygen bond

D) increase in pH (alkalosis) weakens the hemoglobin-oxygen bond

A) decrease in pH (acidosis) weakens the hemoglobin-oxygen bond

The local matching of blood flow with ventilation is ________.

A) ventilation-perfusion coupling

B) the Bohr effect

C) the Haldane effect

D) chloride shifting

A) ventilation-perfusion coupling

In the plasma, the quantity of oxygen in solution is ________.

A) only about 1.5% of the oxygen carried in blood

B) not present except where it is combined with carrier molecules

C) about equal to the oxygen combined with hemoglobin

D) greater than the oxygen combined with hemoglobin

A) only about 1.5% of the oxygen carried in blood

Which of the following is the leading cause of cancer death for both men and women in North America?

A) lung

Source : quizlet.com

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James 11 month ago

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