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Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiration • General function is to obtain O2 for use by the body’s cells and to eliminate the CO2 the body cells produce • Encompasses two separate but related processes – Internal respiration – External respiration Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Internal Respiration • Cellular respiration • Refers to metabolic processes carried out within the mitochondria, which use O2 and produce CO2, while deriving energy from nutrient molecules • Respiratory quotient (RQ) – Ratio of CO2 produced to O2 consumed – Varies depending on foodstuff consumed Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning External Respiration • Refers to sequence of events involved in the exchange of O2 and CO2 between the external environment and the cells of the body • Four steps – Ventilation – movement of air into and out of the lungs – O2 and CO2 are exchanged between air in alveoli and blood within the pulmonary capillaries by means of diffusion – Blood transports O2 and CO2 between lungs and tissues – O2 and CO2 are exchanged between tissues and blood by process of diffusion across systemic (tissue) capillaries Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning External and Internal Respiration Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Nonrespiratory Functions of Respiratory System • • • • • • Route for water loss and heat elimination Enhances venous return Helps maintain normal acid-base balance Enables speech, singing, and other vocalizations Defends against inhaled foreign matter Removes, modifies, activates, or inactivates various materials passing through the pulmonary circulation • Nose serves as the organ of smell Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory System • Consists of – Respiratory airways leading into the lungs – Lungs – Structures of the thorax involved in producing movement of air through the airways into and out of the lungs Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory Airways • Tubes that carry air between the atmosphere and the air sacs – Nasal passages (nose) – Pharynx (common passageway for respiratory and digestive systems) – Trachea (windpipe) – Larynx (voice box) – Right and left bronchi – Bronchioles • Alveoli (air sacs) are clustered at ends of terminal bronchioles Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory Airways • Trachea and larger bronchi – Fairly rigid, nonmuscular tubes – Rings of cartilage prevent collapse • Bronchioles – No cartilage to hold them open – Walls contain smooth muscle innervated by autonomic nervous system – Sensitive to certain hormones and local chemicals Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Alveoli • Thin-walled inflatable sacs • Function in gas exchange • Walls consist of a single layer of flattened Type I alveolar cells • Pulmonary capillaries encircle each alveolus • Type II alveolar cells secrete pulmonary surfactant • Alveolar macrophages guard lumen • Pores of Kohn permit airflow between adjacent alveoli (collateral ventilation) Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Lungs • Occupy much of thoracic cavity – Heart, associated vessels, esophagus, thymus, and some nerves also occupy space • Two lungs – Each is divided into several lobes – Tissue consists of highly branched airways, the alveoli, the pulmonary blood vessels, and large quantities of elastic connective tissue • Outer chest wall (thorax) – Formed by 12 pairs of ribs which join sternum anteriorly and thoracic vertebrae posteriorly Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Lungs • Diaphragm – Dome-shaped sheet of skeletal muscle – Separates thoracic cavity from the abdominal cavity • Pleural sac – Double-walled, closed sac that separates each lung from the thoracic wall – Pleural cavity – interior of plural sac – Intrapleural fluid • Secreted by surfaces of the pleura • Lubricates pleural surfaces Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Pleural Sac Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory Mechanics • Interrelationships among pressures inside and outside the lungs are important in ventilation • Three different pressure considerations important in ventilation – Atmospheric (barometric) pressure – Intra-alveolar pressure (intrapulmonary pressure) – Intrapleural pressure (intrathoracic pressure) Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Pressures Important in Ventilation Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory Mechanics • Changes in intra-alveolar pressure produce flow of air into and out of the lungs • If this pressure is less than atmospheric pressure, air enters the lungs. If the opposite occurs, air exits from the lungs. • Boyle’s law states that at any constant temperature, the pressure exerted by a gas varies inversely with the volume of a gas. Boyle’s Law Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory Mechanics • Major inspiratory muscles – Diaphragm • Major inspiratory muscle • Innervated by phrenic nerve – External intercostal muscles • Activated by intercostal nerves • 75 % of the enlargement of the thoracic cavity during quiet respiration is due to the contraction and flattening of the diaphragm. • This expansion decreases the intrapleural pressure (down to 754). The lungs are drawn into this area of lower pressure. They expand. This increase in volume lowers the intraalveolar pressure to a level below atmospheric pressure. By this difference, air enters the lungs. • The action of accessory inspiratory muscles can further enlarge the thoracic cavity. Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Anatomy of the Respiratory Muscles Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory Mechanics • Onset of expiration begins with relaxation of inspiratory muscles – Relaxation of diaphragm and muscles of chest wall, plus the elastic recoil of the alveoli, decrease the size of the chest cavity – Intrapleural pressure increases and lungs are compressed – Intra-alveolar pressure increases. When pressure increases to level above atmospheric pressure, air is driven out – expiration occurs – Forced expiration can occur by contraction of expiratory muscles • Abdominal wall muscles • Internal intercostal muscles Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory Muscle Activity During Inspiration Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory Muscle Activity During Expiration Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Airway Resistance • Primary determinant of resistance to airflow is the radius of the conducting airway • Autonomic nervous system controls contraction of smooth muscle in walls of bronchioles (changes the radii) • Chronic obstructive pulmonary disease abnormally increases airway resistance – Expiration is more difficult than inspiration – Diseases • Chronic bronchitis • Asthma • Emphysema Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Compliance • Lungs have elastic recoil – rebound if stretched • Compliance – Refers to how much effort is required to stretch or distend the lungs – The less compliant the lungs are, the more work is required to produce a given degree of inflation – Decreased by factors such as pulmonary fibrosis Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Elastic Recoil • Refers to how readily the lungs rebound after having been stretched • Responsible for lungs returning to their preinspiratory volume when inspiratory muscles relax at end of inspiration • Depends on two factors – Highly elastic connective tissue in the lungs – Alveolar surface tension • Thin liquid film lines each alveolus • Reduces tendency of alveoli to recoil • Helps maintain lung stability – Newborn respiratory distress syndrome Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Work of Breathing • Normally requires 3% of total energy expenditure for quiet breathing • Lungs normally operate at about “half full” • Work of breathing is increased in the following situations – When pulmonary compliance is decreased – When airway resistance is increased – When elastic recoil is decreased – When there is a need for increased ventilation Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Lung Volumes and Capacities • Can be measured by a spirometer • Spirogram is a graph that records inspiration and expiration Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Lung Volumes and Capacities Description Average Value Tidal volume (TV) Volume of air entering or leaving lungs during a single breath 500 ml Inspiratory reserve volume (IRV) Extra volume of air that can be maximally inspired over and above the typical resting tidal volume 3000 ml Inspiratory capacity (IC) Maximum volume of air that can be inspired at the end of a normal quiet expiration (IC =IRV + IV) 3500 ml Expiratory reserve volume (ERV) Extra volume of air that can be actively expired by maximal contraction beyond the normal volume of air after a resting tidal volume 1000 ml Residual volume (RV) Minimum volume of air remaining in the lungs even after a maximal expiration 1200 ml Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Lung Volumes and Capacities Description Average Value Functional residual capacity (FRC) Volume of air in lungs at end of normal passive expiration (FRC = ERV + RV) 2200 ml Vital capacity (VC) Maximum volume of air that can be moved out during a single breath following a maximal inspiration (VC = IRV + TV + ERV) 4500 ml Total lung capacity (TLC) Maximum volume of air that the lungs can hold (TLC = VC + RV) Forced expiratory volume in one second (FEV1) Volume of air that can be expired during the first second of expiration in a VC determination Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning 5700 ml Variations in Lung Volume Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory Dysfunction • Two general categories of dysfunction that yield abnormal results during spirometry – Destructive lung disease – Restrictive lung disease • Additional conditions affecting respiratory function – Diseases affecting diffusion of O2 and CO2 across pulmonary membranes – Reduced ventilation due to mechanical failure – Failure of adequate pulmonary blood flow – Ventilation/perfusion abnormalities involving a poor matching of air and blood so that efficient gas exchange does not occur Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Abnormal Spirograms Associated with Obstructive and Restrictive Lung Diseases Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Pulmonary Ventilation • Minute ventilation • Volume of air breathed in and out in one minute Pulmonary ventilation = tidal volume x respiratory rate (ml/min) (ml/breath) (breaths/min) Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Alveolar Ventilation • More important than pulmonary ventilation • Volume of air exchanged between the atmosphere and the alveoli per minute • Less than pulmonary ventilation due to anatomic dead space – Volume of air in conducting airways that is useless for exchange – Averages about 150 ml in adults Alveolar ventilation = (tidal volume – dead space) x respiratory rate Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Effect of Different Breathing Patterns on Alveolar Ventilation Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Alveolar Ventilation • Alveolar dead space – Quite small and of little importance in healthy people – Can increase even to lethal levels in several types of pulmonary disease • Local controls act on smooth muscle of airways and arterioles to match airflow to blood flow – Accumulation of carbon dioxide in alveoli decreases airway resistance leading to increased airflow – Increase in alveolar oxygen concentration brings about pulmonary vasodilation which increases blood flow to match larger airflow Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gas Exchange • At both pulmonary capillary and tissue capillary levels, gas exchange involves simple diffusion of O2 and CO2 down partial pressure gradients • Partial pressure exerted by each gas in a mixture equals the total pressure times the fractional composition of this gas in the mixture Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Oxygen and Carbon Dioxide Exchange Across Pulmonary and Systemic Capillaries Caused by Partial Pressure Gradients Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gas Exchange • Additional factors that affect the rate of gas transfer – As surface area increases, the rate increases – Increase in thickness of barrier separating air and blood decreases rate of gas transfer – Rate of gas exchange is directly proportional to the diffusion coefficient for a gas Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gas Exchange • Exchange across systemic capillaries also occurs down partial pressure gradients • By equilibration in the alveoli, the oxygen in the systemic capillaries has a high partial pressure (e.g., 100) compared to tissue cells (e.g., 40). These cells are using oxygen. • The partial pressure for carbon dioxide in the systemic capillaries is low (e.g., 40) compared to the tissue cells (e.g., 46), which are making this gas through their metabolism. • By partial pressure gradients, oxygen diffuses from the systemic capillaries into the tissue cells (100 to 40, higher to lower). Carbon dioxide diffuses in the opposite direction. • Having equilibrated with the tissue cells, the blood leaving the systemic capillaries is low in oxygen and high in carbon dioxide. • This blood returns to the right side of the heart and on to the lungs. At the pulmonary capillaries, the blood acquires oxygen and releases some of its carbon dioxide. Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gas Transport • Most oxygen in the blood is transported bound to hemoglobin. Hb + O2 ↔ HbO2 (reduced hemoglobin or (oxyhemoglobin) deoxyhemoglobin) Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gas Transport • Hemoglobin combines with oxygen to form oxyhemoglobin. This is a reversible process, favored to form oxyhemoglobin in the lungs. • Hemoglobin tends to combine with oxygen as oxygen diffuses from the alveoli into the pulmonary capillaries. • A small percentage of oxygen is dissolved in the plasma. • The dissociation of oxyhemoglobin into hemoglobin and free molecules of oxygen occurs at the tissue cells. The reaction is favored in this direction as oxygen leaves the systemic capillaries and enters tissue cells. Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gas Transport Partial pressure of oxygen is main factor determining the percent of hemoglobin saturation • The percent saturation is high where the partial pressure of oxygen is high (lungs). • The percent saturation is low where the partial pressure of oxygen is low (tissue cells). At the tissue cells oxygen tends to dissociate from hemoglobin, the opposite of saturation. • This relationship is shown in the oxygen-hemoglobin dissociation curve. • The plateau part of the curve is where the partial pressure of oxygen is high (lungs). • The steep part of the curve exists at the systemic capillaries, where hemoglobin unloads oxygen to the tissue cells. Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Oxygen Hemoglobin Dissociation Curve Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gas Transport Hemoglobin promotes the net transfer of oxygen at both the alveolar and tissue levels. • There is a net diffusion of oxygen from the alveoli to the blood. This occurs continuously until hemoglobin is as saturated as possible (97.5% at 100 mm of Hg). • At the tissue cells hemoglobin rapidly delivers oxygen into the blood plasma and on to the tissue cells. Various factors promote this unloading. • An increase in carbon dioxide from the tissue cells into the systemic capillaries increased hemoglobin dissociation from oxygen (shifts the dissociation curve to the right). • Increased acidity has the same effect. • This shift of the curve to the right (more dissociation) is called the Bohr effect. • Higher temperatures also produces this shift, as does the production of BPG. • Hemoglobin has more affinity for carbon monoxide compared to oxygen. Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Gas Transport Most carbon dioxide (about 60%) is transported as the bicarbonate ion. • Carbon dioxide combines with water to form carbonic acid. The enzyme carbonic anhydrase facilitates this in the erythrocyte. Carbonic acid dissociates into hydrogen ions and the bicarbonate ion. • This two-step, reversible process is favored at the tissue cells. The reverse of this process (bicarbonate ions forming free molecules of carbon dioxide) occurs in the lungs. • 30% of the carbon dioxide is bound to hemoglobin in the blood. This is another means of transport. • About 10% of the transported carbon dioxide is dissolved in the plasma. • By the chloride shift, the plasma membrane of the erythrocyte passively facilitates the diffusion of bicarbonate ions (out of the red cell) and chloride ions. • By the Haldane effect the removal of oxygen from hemoglobin at the tissue cells increases the ability of hemoglobin to bind with carbon dioxide. Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Abnormalities in Arterial PO2 Hypoxia • Condition of having insufficient O2 at the cell level • Categories – Hypoxic hypoxia – Anemic hypoxia – Circulatory hypoxia – Histotoxic hypoxia Hyperoxia • condition of having an above-normal arterial PO2 • Can only occur when breathing supplemental O2 • Can be dangerous Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Abnormalities in Arterial PCO2 • Hypercapnia – Condition of having excess CO2 in arterial blood – Caused by hypoventilation • Hypocapnia – Below-normal arterial PCO2 levels – Brought about by hyperventilation which can be triggered by • Anxiety states • Fever • Aspirin poisoning Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Control of Respiration • Respiratory centers in brain stem establish a rhythmic breathing pattern – Medullary respiratory center • Dorsal respiratory group (DRG) – Mostly inspiratory neurons • Ventral respiratory group (VRG) – Inspiratory neurons – Expiratory neurons – Pre-Bötzinger complex • Widely believed to generate respiratory rhythm – Pneumotaxic center • Sends impulses to DRG that help “switch off” inspiratory neurons • Dominates over apneustic center Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Control of Respiration – Apneustic center • Prevents inspiratory neurons from being switched off • Provides extra boost to inspiratory drive – Hering-Breuer reflex • Triggered to prevent overinflation of the lungs – Chemical factors that play role in determining magnitude of ventilation • PO2 • PCO2 • H+ Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Control of Respiration Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Influence of Chemical Factors on Respiration Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Peripheral Chemoreceptors • Carotid bodies are located in the carotid sinus • Aortic bodies are located in the aortic arch Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Factors That May Increase Ventilation During Exercise • • • • Reflexes originating from body movement Increase in body temperature Epinephrine release Impulses from the cerebral cortex Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Factors That Influence Ventilation That Are Unrelated to Need for Gas Exchange • Protective reflexes such as sneezing and coughing • Inhalation of noxious agents which can trigger immediate cessation of breathing • Pain originating anywhere in body reflexly stimulates respiratory center • Involuntary modification of breathing occurs during expression of various emotional states • Respiratory center is reflexly inhibited during swallowing Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Dead Space Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory Anatomy Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Respiratory Muscle Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning Volume Pressure Chapter 13 The Respiratory System Human Physiology by Lauralee Sherwood ©2007 Brooks/Cole-Thomson Learning