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Chapter 18: Gas Exchange and Transport (Gas Exchange in the Lungs and…
Chapter 18: Gas Exchange and Transport
Gas Exchange in the Lungs and Tissues
Hypoxia: state of too little oxygen
Hypercapnia: elevated concentrations of carbon dioxide
sensors respond to oxygen, CO2 and pH to monitor arterial blood composition
breathing: bulk flow of air into and out of the lungs
concentration gradient is the most important factor in moving gases
possible causes of low alveolar P O2: the inspired air has low oxygen content or alveolar ventilation is inadequate
hypoventilation: low alveolar ventilation; characterized by lower-than-normal volumes of fresh air entering the alveoli
diffusion rate is directly proportional to the available surface area, the concentration gradient of the gas and the permeability of the barrier; diffusion is rapid over short distances
decrease in the amount of alveolar surface area available for gas exchange, and increase in the thickness of the alveolar-capillary exchange barrier and an increase in the diffusion distance between the alveolar air space and the blood adversely affect gas exchange
pulmonary edema: accumulation of interstitial fluid increases the diffusion distance and slows gas exchange
the inside of the alveoli is a moist surface lined by a very thin layer of fluid with surfactant
movement of gas molecules from air into a liquid is directly proportional to: the pressure gradient of the gas, the solubility of the gas in the liquid and temperature
Gas Transport in the Blood
mass flow: amount of something moving per minute
O2 transport requires that the O2 that is dissolved in plasma and that O2 is bound to hemoglobin
hemoglobin is a tetramer with four globular protein chains, each centered around an iron-containing heme group
oxyhemoglobin: hemoglobin bound to oxygen
as the concentration of free O2 increases, more O2 binds to the hemoglobin and more oxyhemoglobin is produced
the amount of O2 that binds to hemoglobin depends on the P O2 in the plasma surrounding the red blood cells and the number of potential hemoglobin binding sites available in the red blood cells
percent saturation of hemoglobin: what percentage of the available hemoglobin binding sites are occupied by O2
oxygen hemoglobin saturation curves: obtained when researches expose samples of hemoglobin to various P O2 levels and quantitatively determine the amount of O2 that binds
changes in binding affinity are reflected by changes in the shape of the oxyhemoglobin saturation curve
Bohr effect: a shift in the hemoglobin saturation curve that results from a change in pH
chronic hypoxia: extended periods of low oxygen
CO2 is converted into bicarbonate ions which provide additional means of CO2 transport from cells to lungs and the bicarbonate ions act as a buffer for metabolic acids which helps to stabilize the body's pH
carbonic anhydrase: an enzyme which converts CO2 into bicarbonate ions
hemoglobin acts as a buffer and binds hydrogen ions which prevents large changes in the body's pH
Regulation of Ventilation
contemporary model for the control of ventilation
respiratory neurons in the medulla control inspiratory and expiratory muscles
neurons in the pons integrate sensory information and interact with medullary neurons to influence ventilation
the rhythmic pattern of breathing arises from a neural network with spontaneously discharging neurons
ventilation is subject to continuous modulation by various chemoreceptor and mechanoreceptor linked reflexes and by higher brain centers
nucleus tractus solitarius: controls muscles of inspiration and receives sensory input from peripheral chemo and mechanoreceptors
pontine respiratory groups: provide tonic input to the medullary networks to help coordinate a smooth respiratory rhythm
ventral respiratory group (VRG): acts as the basic pacemaker for the respiratory rhythm, controls muscles used for active expiration or for inspiration during vigorous exercise; nerve fibers work to keep upper airways open during breathing
peripheral chemoreceptors located in the carotid and aortic arteries sense changes in the P O2, pH and P CO2 of the plasma
central chemoreceptors: located in the brain and respond to changes in the concentration of CO2 in the cerebrospinal fluid