BIOL 3542 Lecture Notes - Lecture 6: Fick'S Laws Of Diffusion, Mechanoreceptor, Skeletal Muscle
26 Jun 2018
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Human Physiology II
Chapter 18: Gas Exchange and Transport
Gas Exchange in the Lungs and Tissues
Lower Alveolar PO2 Decreases Oxygen Uptake
Composition of Inspired Air
1st requirement for adequate oxygen delivery to tissues is adequate oxygen intake from
atmosphere
altitude main factor that influences atmospheric oxygen content
partial pressure of oxygen in air decreases with total atmospheric pressure moving up
from sea level
vapour pressure at 100% humidity same regardless of altitude
Alveolar Ventilation
Hypoventilation: low alveolar ventilation
characterize by lower-than-normal volumes of fresh air entering alveoli
caused by decreased lung compliance, increased airway resistance, CNS depression that
slows ventilation rate and decreases depth
Diffusion Problems Cause Hypoxia
transfer of oxygen from alveoli to blood requires diffusion across barrier created by type I
alveoli cells, capillary endothelium
diffusion rate surface area x concentration gradient x barrier permeability
diffusion rate 1/distance2 (diffusion is fastest over short distances)
diffusion distance, surface area, barrier permeability constant (maximized to facilitate
diffusion)
concentration gradient between alveoli, blood primary factor affecting gas exchange
pathologies that affect gas exchange include:
1. decrease in amount of alveolar surface area available for gas exchange
2. increase in thickness of alveolar-capillary exchange barrier
3. increase in diffusion distance between alveolar air space, blood
Surface Area
irritating effect of smoke chemicals, tar in alveoli activates alveolar macrophages that release
elastase, other proteolytic enzymes that destroy elastic fibers of lung, induce apoptosis of
cells, breaking down walls of alveoli, resulting in high-compliance/low-elastic recoil lungs
with fewer, larger alveoli, less surface area for gas exchange (emphysema)
Diffusion Barrier Permeability
excess fluid increases diffusion distance between alveolar air space, blood
fluid accumulation may occur inside alveoli or interstitial compartment between alveolar
epithelium, capillary
Pulmonary Edema: accumulation of interstitial fluid increases diffusion distance, slows gas
exchange
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if pulmonary blood pressure rises, normal filtration/reabsorption balance disrupted
when capillary hydrostatic pressure increases, more fluid filters out of capillary
if filtration increases too much, lymphatics unable to remove all fluid, excess
accumulates in pulmonary interstitial space, creating pulmonary edema
in severe cases, if edema exceeds tissue’s ability to retain it, fluid leaks from interstitial
space into alveolar air space, flooding alveoli
normally inside alveoli is moist surface lined by very thin layer of fluid with surfactant
owith alveolar flooding, fluid layer becomes much thicker, impairs gas exchange
alveolar flooding can occur with leakage when alveolar epithelium damaged (ex. from
inflammation, inhalation of toxins)
Adult Respiratory Distress Syndrome (ARDS): hypoxia due to fluid accumulation severe, cannot
be corrected by oxygen therapy
Gas Solubility Affects Diffusion
movement of gas molecules from air to liquid directly proportional to:
1. pressure gradient of gas
2. solubility of gas in liquid
3. temperature (constant)
when gas placed in contact with water under pressure gradient, gas molecules move from one
phase to the other
if gas pressure higher in water than air, gas molecules leave water
if gas pressure higher in air than water, gas dissolves in water
Partial Pressure of the Gas in Solution: concentration of oxygen dissolved in water at any given
PO2
Solubility: ease with which gas dissolves in liquid
if very soluble, large numbers of gas molecules go into solution at low gas partial
pressure
if less soluble, even high partial pressure may cause only few molecules of gas to
dissolve in liquid
oxygen not very soluble in water/any aqueous solution
low solubility driving force for evolution of oxygen-carrying molecules in blood
carbon dioxide 20x more soluble in water than oxygen
oxygen’s low solubility means very little of it can be carried dissolved in plasma, and it’s
slower to cross increased diffusion distance present in pulmonary edema
diffusion of oxygen in alveolar capillaries doesn’t have time to come to equilibrium
before blood has left capillaries resulting in decreased arterial PO2 even though alveolar
PO2 may be normal
increased diffusion distance may not significantly affect carbon dioxide exchange
Gas Transport in Blood
gases that enter capillaries first dissolve in plasma
without hemoglobin in red blood cells, blood would be unable to transport sufficient oxygen
to sustain life
Mass Flow: amount of x moving per minute
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mass flow = concentration X volume flow
calculate mass flow of oxygen travelling from lungs to cells using oxygen content of
arterial blood X cardiac output
Mass Balance: if the amount of a substance in body is to remain constant, any gain must be
offset by an equal loss
arterial oxygen transport – cell use of oxygen = venous oxygen transport
arterial oxygen transport – venous oxygen transport = oxygen consumption
Fick Equation: oxygen consumption (QO2) = cardiac output (CO) X (arterial oxygen content –
venous oxygen content)
Hemoglobin Binds to Oxygen
total blood oxygen content = dissolved oxygen content + oxygen bound to hemoglobin (Hb)
hemoglobin binds reversibly to oxygen (Hb + O2 HbO2)
hemoglobin tetramer with 4 globular protein chains (globins) each centered around iron-
containing heme group
central iron atom of each heme group binds reversibly with one oxygen molecule
one hemoglobin molecule can bind 4 oxygen molecules
Oxyhemoglobin (HbO2): hemoglobin bound to oxygen
Oxygen Binding Obeys the Law of Mass Action
Hb + O2 HbO2
as concentration of free O2 increases, more oxygen binds to hemoglobin, equation shifts
right, producing more HbO2
if concentration of O2 decreases, equation shifts left, hemoglobin releases oxygen, amount of
oxyhemoglobin decreases
once arterial blood reaches tissues, dissolved oxygen diffuses out of systemic capillaries into
cells which have lower PO2
this decreases plasma PO2, disturbs equilibrium of oxygen-hemoglobin binding
reaction by removing O2 from left side of equation
equilibrium shifts left, causing hemoglobin to release oxygen
PO2 of cells determines how much oxygen unloaded from hemoglobin
as cells increase metabolic activity, their PO2 decreases, hemoglobin releases more oxygen to
them
Hemoglobin Transports Most Oxygen to the Tissues
extra O2 reserve for when oxygen demand increases (ex. exercise)
PO2 Determines Oxygen-Hb Binding
amount of oxygen that binds to hemoglobin depends on:
1. PO2 in plasma surrounding red blood cells
2. number of potential Hb binding sites available
Percent Saturation of Hemoglobin: percentage of available hemoglobin binding sites occupied by
oxygen
primarily determined by plasma PO2
total # oxygen-binding sites depends on # of hemoglobin molecules in red blood cells
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Document Summary
1st requirement for adequate oxygen delivery to tissues is adequate oxygen intake from atmosphere altitude main factor that influences atmospheric oxygen content. Partial pressure of oxygen in air decreases with total atmospheric pressure moving up from sea level vapour pressure at 100% humidity same regardless of altitude. Characterize by lower-than-normal volumes of fresh air entering alveoli. Caused by decreased lung compliance, increased airway resistance, cns depression that slows ventilation rate and decreases depth. Concentration gradient between alveoli, blood primary factor affecting gas exchange pathologies that affect gas exchange include: decrease in amount of alveolar surface area available for gas exchange. 2. increase in thickness of alveolar-capillary exchange barrier increase in diffusion distance between alveolar air space, blood. Diffusion barrier permeability excess fluid increases diffusion distance between alveolar air space, blood fluid accumulation may occur inside alveoli or interstitial compartment between alveolar epithelium, capillary.
