Chapter 24: Physiology of the Respiratory System

RESPIRATORY PHYSIOLOGY Definition: complex, coordinated processes that help maintain homeostasis 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

PULMONARY VENTILATION Respiratory cycle (ventilation, breathing) Inspiration: movement of air into the lungs Expiration: movement of air out of the lungs Mechanism of pulmonary ventilation The pulmonary ventilation mechanism must establish two gas pressure gradients One in which the pressure within the alveoli of the lungs is lower than atmospheric pressure to produce inspiration One in which the pressure in the alveoli of the lungs is higher than atmospheric pressure to produce expiration

PULMONARY VENTILATION: MECHANISM Pressure gradients are established by changes in the size of the thoracic cavity that are produced by contraction and relaxation of muscles Boyle’s law: the volume of gas varies inversely with pressure at a constant temperature Inspiration: contraction of the diaphragm produces inspiration; as it contracts, it makes the thoracic cavity larger Compliance: ability of pulmonary tissues to stretch, thus making inspiration possible https://www.youtube.com/watch?v=N5xft2fIqQU

PULMONARY VENTILATION: MECHANISM (cont.) Expiration: a passive process that begins when the inspiratory muscles are relaxed, which decreases the size of the thorax Increasing thoracic volume increases the intrapleural pressure and thus increases alveolar pressure above the atmospheric pressure Air moves out of the lungs when alveolar pressure exceeds the atmospheric pressure

PULMONARY VENTILATION Pulmonary volumes: normal exchange of oxygen and carbon dioxide depends on the presence of normal volumes of air moving in and out and the remaining volume Tidal volume (TV): amount of air exhaled after normal inspiration Expiratory reserve volume (ERV): largest volume of additional air that can be forcibly exhaled (1.0 to 1.2 L is normal ERV) Inspiratory reserve volume (IRV): amount of air that can be forcibly inhaled after normal inspiration (normal IRV is 3.3 L) Residual volume: amount of air that cannot be forcibly exhaled (1.2 L)

PULMONARY GAS EXCHANGE Partial pressure of gases: pressure exerted by a gas in a mixture of gases or a liquid (Figure 24-14) Law of partial pressures (Dalton’s law): 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 and PCO2 equal alveolar PO2 and PCO2

PULMONARY GAS EXCHANGE (cont.) 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 The oxygen pressure gradient between alveolar air and blood The total functional surface area of the respiratory membrane The respiratory minute volume Alveolar ventilation Structural facts that facilitate oxygen diffusion from the alveolar air to the blood The walls of the alveoli and capillaries form only a very thin barrier for gases to cross The alveolar and capillary surfaces are large The blood is distributed through the capillaries in a thin layer so that each red blood cell comes close to alveolar air

HOW BLOOD TRANSPORTS GASES Oxygen and carbon dioxide are transported as solutes and as parts of molecules of certain chemical compounds Hemoglobin (Hb) Composed of four polypeptide chains (two alpha chains, two beta chains), each with an iron-containing heme group CO2 can bind to amino acids in the chains, and oxygen can bind to iron in the heme groups Transport of oxygen Oxygenated blood contains approximately 0.3 ml of dissolved O2 per 100 ml of blood Hb increases the oxygen-carrying capacity of blood

HOW BLOOD TRANSPORTS GASES (cont.) Transport of carbon dioxide A small amount of CO2 dissolves in plasma and is transported as a solute (10%) Less than one fourth of blood CO2 combines with NH2 (amine) groups of Hb and other proteins to form carbaminohemoglobin (20%) (Figure 24-22) CO2’s association with Hb is accelerated by an increase in blood PCO2 (Figure 24-23) More than two thirds of the CO2 is carried in plasma as bicarbonate ions (70%) (Figures 24-24 to 24-26)

SYSTEMIC GAS EXCHANGE Exchange of gases in tissues takes place between arterial blood flowing through tissue capillaries and cells Oxygen diffuses out of arterial blood because the oxygen pressure gradient favors its outward diffusion As dissolved oxygen diffuses out of arterial blood, blood PO2 decreases, which accelerates oxyhemoglobin dissociation to release more oxygen to plasma for diffusion to cells

SYSTEMIC GAS EXCHANGE (cont.) CO2 exchange between tissues and blood takes place in the opposite direction from oxygen exchange Bohr effect: increased PCO2 decreases the affinity between oxygen and Hb Haldane effect: increased CO2 loading caused by a decrease in PO2

REGULATION OF PULMONARY FUNCTION Respiratory control centers: the main integrators controlling the nerves that affect the inspiratory and expiratory muscles are located in the brainstem Medullary rhythmicity center: generates the basic rhythm of the respiratory cycle Consists of two interconnected control centers Inspiratory center stimulates inspiration Expiratory center stimulates expiration

REGULATION OF PULMONARY FUNCTION (cont.) The basic breathing rhythm can be altered by different inputs to the medullary rhythmicity center (Figure 24-30) Input from the apneustic center in the pons stimulates the inspiratory center to increase the length and depth of inspiration Pneumotaxic center in the pons inhibits the apneustic center and inspiratory center to prevent overinflation of the lungs

REGULATION OF PULMONARY FUNCTION (cont.) Arterial blood pressure controls breathing through the respiratory pressoreflex mechanism Hering-Breuer reflexes help control respirations by regulating depth of respirations and the volume of tidal air Cerebral cortex influences breathing by increasing or decreasing the rate and strength of respirations Ventilation and perfusion (Figure 24-33) 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 (e.g., vasoconstricting arterioles that supply poorly ventilated alveoli and allow full blood flow to well-ventilated alveoli)

THE BIG PICTURE: RESPIRATORY PHYSIOLOGY AND THE WHOLE BODY The internal system must continually acquire new oxygen and rid itself of CO2 because each cell requires oxygen and produces CO2 as a result of energy conversion Specific mechanisms involved in respiratory function Blood gases need blood and the cardiovascular system to be transported between gas exchange tissues of the lungs and various systemic tissues of the body Regulation by the nervous system adjusts ventilation to compensate for changes in oxygen or CO2 in the internal environment The skeletal muscles of the thorax help the airways maintain the flow of fresh air The skeleton houses the lungs, and the arrangement of bones facilitates the expansion and recoil of the thorax The immune system prevents pathogens from colonizing the respiratory tract and causing infection