Respiratory Physiology. Respiration Includes 3 separate functions: Ventilation:

Respiratory Physiology

Respiration Includes 3 separate functions: Ventilation: Breathing. Gas exchange: Between air and capillaries in the lungs. Between systemic capillaries and tissues of the body. 02 utilization: Cellular respiration.

Ventilation Mechanical process that moves air in and out of the lungs. [O2] of air is higher in the lungs than in the blood, O2 diffuses from air to the blood. C02 moves from the blood to the air by diffusing down its concentration gradient. Gas exchange occurs entirely by diffusion.

Alveoli ~ 300 million air sacs (alveoli). Large surface area (60–80 m2). Each alveolus is 1 cell layer thick. 2 types of cells: Alveolar type I: Structural cells. Alveolar type II: Secrete surfactant. Figure 16.1

Respiratory Zone Region of gas exchange between air and blood. Includes respiratory bronchioles and alveolar sacs. Must contain alveoli. Figure 16.4

Conducting Zone All the structures air passes through before reaching the respiratory zone. Warms and humidifies inspired air. Filters and cleans: Mucus secreted to trap particles in the inspired air. Mucus moved by cilia to be expectorated. Insert fig. 16.5 Figure 16.5

Compliance: Distensibility (stretchability): Ease with which the lungs can expand. 100 x more distensible than a balloon. Compliance is reduced by factors that produce resistance to distension. Elasticity: Tendency to return to initial size after distension. High content of elastin proteins. Very elastic and resist distension. Recoil ability. Physical Properties of the Lungs

Surface Tension Force exerted by fluid in alveoli to resist distension. Lungs secrete and absorb fluid, leaving a very thin film of fluid. This film of fluid causes surface tension. H20 molecules at the surface are attracted to other H20 molecules by attractive forces. Force is directed inward, raising pressure in alveoli.

Law of Laplace Pressure in alveoli is directly proportional to surface tension; and inversely proportional to radius of alveoli. Pressure in smaller alveolus greater. Insert fig. 16.11 Figure 16.11

Surfactant Phospholipid produced by alveolar type II cells. Lowers surface tension. Reduces attractive forces of hydrogen bonding by becoming interspersed between H20 molecules. As alveoli radius decreases, surfactant’s ability to lower surface tension increases. Insert fig. 16.12 Figure 16.12

Boyle’s Law Changes in intrapulmonary pressure occur as a result of changes in lung volume. Pressure of gas is inversely proportional to its volume. Increase in lung volume decreases intrapulmonary pressure. Air goes in. Decrease in lung volume, raises intrapulmonary pressure above atmosphere. Air goes out.

Lung Pressures Intrapulmonary pressure: Intra-alveolar pressure (pressure in the alveoli). Intrapleural pressure: Pressure in the intrapleural space. Pressure is negative, due to lack of air in the intrapleural space. Transpulmonary pressure: Pressure difference across the wall of the lung. Intrapulmonary pressure – intrapleural pressure. Keeps the lungs against the chest wall.

Quiet Inspiration Active process: Contraction of diaphragm, increases thoracic volume vertically. Contraction of parasternal and internal intercostals, increases thoracic volume laterally. Increase in lung volume decreases pressure in alveoli, and air rushes in. Pressure changes: Alveolar changes from 0 to –3 mm Hg. Intrapleural changes from –4 to –6 mm Hg. Transpulmonary pressure = +3 mm Hg.

Expiration Quiet expiration is a passive process. After being stretched, lungs recoil. Decrease in lung volume raises the pressure within alveoli above atmosphere, and pushes air out. Pressure changes: Intrapulmonary pressure changes from –3 to +3 mm Hg. Intrapleural pressure changes from –6 to –3 mm Hg. Transpulmonary pressure = +6 mm Hg.

Insert fig. 16.15 Pulmonary Ventilation Figure 16.15

Pulmonary Function Tests Assessed by spirometry. Subject breathes into a closed system in which air is trapped within a bell floating in H20. The bell moves up when the subject exhales and down when the subject inhales. Insert fig. 16.16 Figure 16.16

Terms Used to Describe Lung Volumes and Capacities

Pulmonary Volumes TV = 500 ml IRV = 2500-3000 ml ERV = 1100-1200 ml RV = 1200ml IC = 3500 ml FRC = 2300 ml VC = 4600 ml TLC = 5800 ml

Pulmonary volumes Minute respiratory volume = TV x RR TV (500 ml) x RR (12 breaths/min) = 6000 ml/min

Anatomical Dead Space Not all of the inspired air reached the alveoli. As fresh air is inhaled it is mixed with air in anatomical dead space. Conducting zone and alveoli where [02] is lower than normal and [C02] is higher than normal. Alveolar ventilation = F x (TV- DS). F = frequency (breaths/min.). TV = tidal volume. DS = dead space.

Restrictive and Obstructive Disorders Restrictive disorder: Vital capacity is reduced. FVC is normal. Obstructive disorder: VC is normal. FEV1 is < 80%. Insert fig. 16.17 Figure 16.17

Pulmonary Disorders Dyspnea: Shortness of breath. COPD (chronic obstructive pulmonary disease): Asthma: Obstructive air flow through bronchioles. Caused by inflammation and mucus secretion. Inflammation contributes to increased airway responsiveness to agents that promote bronchial constriction. IgE, exercise.

Pulmonary Disorders (continued) Emphysema: Alveolar tissue is destroyed. Chronic progressive condition that reduces surface area for gas exchange. Decreases ability of bronchioles to remain open during expiration. Cigarette smoking stimulates macrophages and leukocytes to secrete protein digesting enzymes that destroy tissue. Pulmonary fibrosis: Normal structure of lungs disrupted by accumulation of fibrous connective tissue proteins. Anthracosis.

Gas Exchange in the Lungs Partial pressure: The pressure that an particular gas exerts independently. PATM = PN2 + P02 + PC02 + PH20= 760 mm Hg. 02 is humidified = 105 mm Hg. H20 contributes to partial pressure (47 mm Hg). P02 (sea level) = 150 mm Hg. PC02 = 40 mm Hg. Figure 16.20

Significance of Blood P02 and PC02 Measurements At normal P02 arterial blood = 100 mm Hg. P02 level in the systemic veins is = 40 mm Hg; PC02 = 46 mm Hg. Provides a good index of lung function. Figure 16.23

Pulmonary Circulation Rate of blood flow through the pulmonary circulation is = flow rate through the systemic circulation. Driving pressure is about 10 mm Hg. Pulmonary vascular resistance is low. Low pressure pathway produces less net filtration than produced in the systemic capillaries. Autoregulation: Pulmonary arterioles constrict when alveolar P02 decreases. Matches ventilation/perfusion ratio.

Lung Ventilation/Perfusion Ratios Functionally: Alveoli at apex are underperfused (overventilated). Alveoli at the base are underventilated (overperfused). Insert fig. 16.24 Figure 16.24

Brain Stem Respiratory Centers Rhythmicity center: Controls automatic breathing. Iinteracting neurons that fire either during inspiration (I neurons) or expiration (E neurons). Insert fig. 16.25 Figure 16.25

Rhythmicity Center I neurons located primarily in dorsal respiratory group (DRG): Regulate activity of phrenic nerve. E neurons located in ventral respiratory group (VRG): Passive process. Activity of E neurons inhibit I neurons. Rhythmicity of I and E neurons may be due to pacemaker neurons.

Medullary rhythmicity center influenced by pons. Apneustic center: Promotes inspiration by stimulating the I neurons in the medulla. Pneumotaxic center: Antagonizes the apneustic center. Inhibits inspiration. Pons Respiratory Centers

Chemoreceptors Monitor changes in blood PC02, P02, and pH. Central: Medulla. Peripheral: Carotid and aortic bodies. Control breathing indirectly. Insert fig. 16.27 Figure 16.27

Central Chemoreceptors More sensitive to changes in arterial PC02. H20 + C02 H+ cannot cross the blood brain barrier. C02 can cross the blood brain barrier and will form H2C03. Lowers pH of CSF. Directly stimulates central chemoreceptors. H+ H2C03

Peripheral Chemoreceptors Are not stimulated directly by changes in arterial PC02. H20 + C02 H2C03 H+ Stimulated by rise in [H+] of arterial blood. Increased [H+] stimulates peripheral chemoreceptors.

Chemoreceptor Control of Breathing Insert fig. 16.29 Figure 16.20

Effects of Pulmonary Receptors on Ventilation Lungs contain receptors that influence the brain stem respiratory control centers via sensory fibers in vagus. Unmyelinated C fibers can be stimulated by: Capsaicin: Produces apnea followed by rapid, shallow breathing. Histamine and bradykinin: Released in response to noxious agents. Irritant receptors are rapidly adaptive receptors. Hering-Breuer reflex: Pulmonary stretch receptors activated during inspiration. Inhibits respiratory centers to prevent undue tension on lungs.

Hemoglobin 280 million hemoglobin/RBC. Each hemoglobin has 4 polypeptide chains and 4 hemes. In the center of each heme group is 1 atom of iron that can combine with 1 molecule 02. Insert fig. 16.32 Figure 16.32

Hemoglobin (continued) Methemoglobin: Lacks electrons and cannot bind with 02. Blood normally contains a small amount. Carboxyhemoglobin: The bond with carbon monoxide is 210 times stronger than the bond with oxygen. Transport of 02 to tissues is impaired.

Hemoglobin (continued) Oxygen-carrying capacity of blood determined by its [hemoglobin]. Anemia: [Hemoglobin] below normal. Polycythemia: [Hemoglobin] above normal. Hemoglobin production controlled by erythropoietin. Production stimulated by PC02 delivery to kidneys. Loading/unloading depends: P02 of environment. Affinity between hemoglobin and 02.

Oxyhemoglobin Dissociation Curve Graphic illustration of the % oxyhemoglobin saturation at different values of P02. Loading and unloading of 02. Steep portion of the sigmoidal curve, small changes in P02 produce large differences in % saturation (unload more 02). Decreased pH, increased temperature, and increased 2,3 DPG: Affinity of hemoglobin for 02 decreases. Greater unloading of 02: Shift to the curve to the right. Figure 16.34

Effects of pH and Temperature Affinity is decreased when pH is decreased. Increased temperature and 2,3-DPG: Shift the curve to the right. Insert fig. 16.35 Figure 16.35

C02 transported in the blood: HC03- (70%). Dissolved C02 (10%). Carbaminohemoglobin (20%). C02 Transport H20 + C02 H2C03 ca High PC02

Chloride Shift at Systemic Capillaries H20 + C02 H2C03 H+ + HC03- At the tissues, C02 diffuses into the RBC; shifts the reaction to the right. Increased [HC03-] produced in RBC: HC03- diffuses into the blood. RBC becomes more +. Cl- attracted in (Cl- shift). H+ released buffered by combining with deoxyhemoglobin. HbC02 formed. Unloading of 02.

Carbon Dioxide Transport and Chloride Shift Insert fig. 16.38 Figure 16.38

At Pulmonary Capillaries H20 + C02 H2C03 H+ + HC03- At the alveoli, C02 diffuses into the alveoli; reaction shifts to the left. Decreased [HC03-] in RBC, HC03- diffuses into the RBC. RBC becomes more -. Cl- diffuses out (reverse Cl- shift). Deoxyhemoglobin converted to oxyhemoglobin. Has weak affinity for H+. Gives off HbC02.

Reverse Chloride Shift in Lungs Insert fig. 16.39 Figure 16.39

Ventilation During Exercise During exercise, breathing becomes deeper and more rapid. Produce > total minute volume. Neurogenic mechanism: Sensory nerve activity from exercising muscles stimulates the respiratory muscles. Cerebral cortex input may stimulate brain stem centers. Humoral mechanism: PC02 and pH may be different at chemoreceptors. Cyclic variations in the values that cannot be detected by blood samples. Insert fig. 16.41 Figure 16.41

Lactate Threshold and Endurance Training Maximum rate of oxygen consumption that can be obtained before blood lactic acid levels rise as a result of anaerobic respiration. 50-70% maximum 02 uptake has been reached. Endurance trained athletes have higher lactate threshold, because of higher cardiac output. Have higher rate of oxygen delivery to muscles. Have increased content of mitochondria in skeletal muscles.

Acclimatization to High Altitude Adjustments in respiratory function when moving to an area with higher altitude: Changes in ventilation: Hypoxic ventilatory response produces hyperventilation. Increases total minute volume. Increased tidal volume. Affinity of hemoglobin for 02: Action of 2,3-DPG decreases affinity of hemoglobin for 02. Increased hemoglobin production: Kidneys secrete erythropoietin.