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Chapter 24 Physiology of the

Respiratory System

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Respiratory Physiology

• complex processes that help maintain homeostasis

• Respiratory function includes the following:– External respiration

• Pulmonary ventilation (breathing)

• Pulmonary gas exchange

– Transport of gases by the blood

– Internal respiration• Systemic tissue gas exchange

• Cellular respiration

– Regulation of respiration

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Pulmonary Ventilation (breathing)

• Respiratory cycle– Inspiration— moves air into the lungs

– Expiration— moves air out of the lungs

• Mechanism of pulmonary ventilation– Pulmonary ventilation mechanism must establish

two gas pressure gradients:1. pressure within alveoli of lungs is lower than atmospheric

pressure to produce inspiration

2. pressure in alveoli of lungs is higher than atmospheric pressure to produce expiration

– Pressure gradients are established by changes in size of thoracic cavity

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Inspiration— contraction of diaphragm produces inspiration— as it contracts, it makes thoracic cavity larger

• Expansion of thorax results in decreased intrapleural pressure (Pip), leading to a decreased alveolar pressure (Pa)

• Air moves into lungs when alveolar pressure (Pa) drops below atmospheric pressure (Pb)

• Compliance— ability of pulmonary tissues to stretch, making inspiration possible

Pulmonary Ventilation (breathing)

Intrapleural is the space surrounding the lungs

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Pulmonary Ventilation (breathing)

Expiration— a passive process that begins when inspiratory muscles are relaxed, decreasing size of thorax

• Decreasing thoracic volume increases intrapleural pressure (Pip) and thus increases alveolar pressure (Pa) above atmospheric pressure (Pb)

• Air moves out of lungs when Pa > Pb

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Pulmonary Volumes• Pulmonary volumes— the amounts of air moved in and out and

remaining are important to the normal exchange of oxygen and carbon dioxide

– FYI:

• Spirometer— instrument used to measure volume of air (Figure 24-10)

• Tidal volume (TV)— amount of air exhaled after normal inspiration

• Expiratory reserve volume (ERV)— largest volume of additional air that can be forcibly exhaled (between 1.0 and 1.2 liters is normal ERV)

• Inspiratory reserve volume (IRV)— amount of air that can be forcibly inhaled after normal inspiration (normal IRV is 3.3 liters)

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• Residual volume (RV)— amount of air that CANNOT be forcibly exhaled (1.2 liters)– “wind knocked out of you” = your residual volume

(along with your expiratory reserve) is forced out of your airways alveoli collapse

• Between breaths, an exchange of O2 and CO2 occurs between this trapped air in the alveoli and the blood.

Pulmonary Volumes

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Pulmonary Capacities• Pulmonary capacities— the sum of two or more pulmonary

volumes– Vital capacity— the sum of IRV + TV + ERV

• Represents the largest volume of air one can move in and out of lungs

• A person’s vital capacity depends on many factors, including the size of the thoracic cavity and posture

• Larger person has larger vital capacity• Excess fluid in abdominal cavities • Emphysema: alveolar walls stretched too much and

unable to recoil increased RV (air trapped)– Functional residual capacity— amount of air

at the end of a normal respiration

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Alveolar ventilation— volume of inspired air that reaches the alveoli

• Only this volume of air takes part in exchange of gases

– Anatomical dead space— “dead” air in passageways that do not participate in gas exchange (nose, pharynx, larynx, trachea, bronchi)

– Physiological dead space— anatomical dead space plus the volume of any nonfunctioning alveoli (caused by disease)

– Alveoli must be properly ventilated for adequate gas exchange

• Chronic obstructive pulmonary disease (COPD): chronic bronchitis and emphysema

Pulmonary Capacities

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Pulmonary Gas Exchange• Partial pressure of gases— pressure exerted

by a gas in a mixture of gases

– Dalton’s law of partial pressures— the partial pressure of a gas in a mixture of gases is directly related to the concentration of that gas in the mixture and to the total pressure of the mixture

Arterial blood Po2 + Pco2 = Alveolar Po2 + Pco2

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Pulmonary Gas Exchange

• Exchange of gases in the lungs takes place between alveolar air and blood flowing through lung capillaries

– Four factors determine the amount of oxygen that diffuses into blood:

1. The oxygen pressure gradient between alveolar air and blood

2. The total functional surface area of the respiratory membrane

3. The respiratory volume

4. Alveolar ventilation

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Pulmonary Gas Exchange

Structural factors that facilitate oxygen diffusion from alveolar air to blood:

• Walls of the alveoli and capillaries form only a very thin barrier for gases to cross

• Alveolar and capillary surfaces are large

• Blood is distributed through the capillaries in a thin layer so each red blood cell comes close to alveolar air

Alveolar blood supply. rich blood supply

to alveoli (which have been removed). The

numerous, narrow branches ensure that each

red blood cell is exposed to the alveolar air.

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How Blood Transports Oxygen– Hemoglobin

• made up of four polypeptide chains, each with an iron-containing heme group

• Some CO2 can bind to amino acids in the chains

• O2 can bind to iron in the heme groups

– Oxygenated blood contains about 0.3 ml of dissolved O2 per 100 ml of blood

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How Blood Transports CO2

– A small amount of CO2 dissolves in plasma and is transported as a solute (10%)

– < 1/4 of blood CO2 combines with amino groups of hemoglobin (20%)

• CO2 association with hemoglobin is accelerated by an increase in blood Pco2

– > 2/3 of blood CO2 is carried in plasma as bicarbonate ions (70%)

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Systemic Gas Exchange

• Exchange of gases takes place between capillaries and cells– Oxygen diffuses out of arterial

blood because the oxygen pressure gradient favors its outward diffusion

– As oxygen diffuses out of blood, blood Po2 decreases, which accelerates oxyhemoglobin dissociation to release more oxygen to plasma for diffusion to cells

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• Oxygen-hemoglobin dissociation curve. The graph represents the relationship between PO2 and O2 saturation of hemoglobin (Hb-O2 affinity). The inset shows how the graphed curve relates to oxygen transport by the blood. Notice that at high plasma PO2 values (point A), hemoglobin (Hb) is fully loaded with oxygen. At low plasma PO2 values (point B), Hb is only partially loaded with oxygen.

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Systemic Gas Exchange

• Carbon dioxide exchange between tissues and blood takes place in the opposite direction from oxygen exchange– Bohr effect— increased Pco2

decreases the affinity between oxygen and hemoglobin

– Haldane effect— increased carbon dioxide loading caused by a decrease in Po2

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• The increased PCO2 in systemic tissues decreases the affinity between Hb and O2, shown as a right shift of the oxygen-hemoglobin dissociation curve. This phenomenon is known as the Bohr effect. A right shift can also be caused by a decrease in plasma pH.

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• At the same time, the decreased PO2 commonly observed in systemic tissues increases the CO2 content of the blood, shown as a left shift of the CO2 dissociation curve. This phenomenon is known as the Haldane effect.

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Regulation of Pulmonary Function

• Respiratory control centers— the main integrators that control the nerves that affect inspiratory and expiratory muscles are located in the brainstem– Medullary rhythmicity center— generates the basic

rhythm of respiratory cycle– Basic breathing rhythm can be altered by different

inputs to medullary rhythmicity center

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Regulation of Pulmonary Function• Factors that influence breathing— sensors from the nervous system

provide feedback to medullary rhythmicity center

– Changes in the Po2, Pco2 and pH of arterial blood influence medullary rhythmicity area

• Pco2 acts on central chemoreceptors in medulla

– if it increases, result is faster breathing

– if it decreases, result is slower breathing

– Arterial blood pressure controls breathing through respiratory pressoreflex mechanism

– Hering-Breuer reflexes help control respirations by regulating depth of respirations and volume of tidal air

– Cerebral cortex influences breathing by increasing or decreasing rate and strength of respirations

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Regulation of Pulmonary Function

• Ventilation and perfusion– Alveolar ventilation— air flow to the alveoli– Alveolar perfusion— blood flow to the alveoli– Efficiency of gas exchange can be maintained

by limited ability to match perfusion to ventilation— for example, vasoconstricting arterioles that supply poorly ventilated alveoli and allow full blood flow to well-ventilated alveoli

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The Big Picture• The internal system must continually get new oxygen and

rid itself of carbon dioxide because each cell requires oxygen and produces carbon dioxide as a result of cell respiration

• Specific mechanisms involved in respiratory function:– Blood gases need blood and the cardiovascular system to be transported

between gas exchange tissues of lungs and various systemic tissues of body

– Regulation by the nervous system adjusts ventilation to compensate for changes in oxygen or carbon dioxide levels in internal environment

– Skeletal muscles of the thorax aid airways in maintaining flow of fresh air

– Skeleton houses the lungs, and the arrangement of bones facilitates the expansion and recoil of the thorax

– Immune system prevents pathogens from colonizing the respiratory tract and causing infection