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Fleur Sam. Ch 18CHAPTER 18: GAS EXCHANGE AND TRANSPORT
The mechanisms of breathing include events that create bulk flow of air into and out
of the lungs. 2 gases most significant; O2 & CO2, they move between alveolar air
spaces and cells of the body.
The process can be divided into 2 components:
- Exchange of gases between compartments, which requires diffusion across cell
membranes
- Transport of gases in the blood.
Hypoxia (state of too little oxygen) is a state where the diffusion of gases between
alveoli and blood is significantly impaired or if O2 transport in the blood is
inadequate.
It frequently (not always) goes hand in hand with hypercapnia, which is elevated
concentrations of carbon dioxide.
These 2 conditions are clinical signs, not diseases.
To avoid hypoxia & hypercapnia, the body uses sensors that monitor arterial blood
composition. These sensors respond to 3 regulated variables:
1. Oxygen: arterial oxygen delivery to the cells must be adequate to support aerobic
respiration and ATP production.
2. CO2: produced as a waste product during citric acid cycle. Excretion of CO2 by the
lungs is important for 2 reasons:
a. High levels of CO2 are a central nervous system depressant
b. Elevated CO2 causes a state of acidosis (low pH)
through the following reaction:
3. pH: maintaining pH homeostasis is critical to prevent denaturation of proteins. The
respiratory system monitors plasma pH and uses changes in ventilation to alter pH.
critical for homeostasis
,18.1: Gas exchange in the lungs and tissues
Breathing is bulk flow of air into and out of the lungs. Once air reaches the alveoli,
individual gases like oxygen and CO2 diffuse from the alveolar air space into the
blood. Diffusion is movement from region of higher conc. to lower conc.
Plasma gas concentrations often expressed in partial pressures to establish whether
there is conc. gradient between alveoli and the blood. Gases move from regions of
higher partial pressure to regions of lower partial pressure.
Normal alveolar PO2 at sea level is about 100 mm
Hg. The PO2 of deoxygenated venous blood
arriving in the lungs is about 40 mm Hg. Oxygen
therefore diffuses down its partial pressure (conc)
gradient from the alveoli into the capillaries.
Diffusion goes to equilibrium and the PO2 of arterial
blood leaving the lungs is the same as in the
alveoli: 100 mm Hg.
When arterial blood reaches tissue capillaries, the gradient is reversed.
Cells are continuously using oxygen for oxidative phosphorylation.
In the cells of a person at rest, intracellular PO2 averages 40 mm Hg. Arterial blood
arriving at the cells has a PO2 of 100 mm Hg. Because PO2 is lower in the cells,
oxygen diffuses down its partial pressure gradient from plasma into cells. Diffusion
goes to equilibrium, as a result, venous blood has the same P O2 as the cells it just
passed.
PCO2 is higher in tissues than in systemic capillary blood bc of CO2 production during
metabolism. Cellular PCO2 is higher in tissues than in systemic capillary blood bc of
CO2 production during metabolism. Cellular P CO2 in person at rest is about 46 mm
Hg, compared to arterial plasma PCO2 of 40 mm Hg. The gradient causes CO2 to
diffuse out of cells into the capillaries. Diffusion goes to equilibrium, and systemic
venous blood averages a PCO2 of 46 mm Hg.
At the pulmonary capillaries, the process reverses. Venous blood bringing waste
CO2 from cells has a PCO2 of 46 mm Hg. Alveolar PCO2 is 40 mm Hg. Bc PCO2 is higher
in the plasma, CO2 moves from the capillaries into the alveoli. By the time blood
leaves the alveoli, it has a PCO2 of 40 mm Hg, identical to the PCO2 of the alveoli.
Many variables influence the efficiency of alveolar gas exchange and determine
whether arterial blood gases are normal.
1. Adequate oxygen must reach the alveoli. A decrease in alveolar PO2 means that less
oxygen is available to enter the blood.
2. There can also be a problem with the transfer of gases between the alveoli and
pulmonary capillaries.
3. Blood flow, or perfusion, of the alveoli must be adequate.
If something impairs blood flow to the lung, the body is unable to acquire the oxygen
it needs.
2 possible causes of low alveolar PO2:
, 1. Inspired air has low oxygen content
2. Alveolar ventilation is inadequate.
The first requirement for adequate oxygen delivery to the tissue is adequate oxygen
intake from the atmosphere.
Main factor that affects atmospheric oxygen content is altitude.
The partial pressure of oxygen in air decreases along with total atmospheric pressure
as you move from sea level to higher altitudes.
Water vapor (damp) pressure at 100% humidity is the same no matter what the
altitude, making its contribution to total pressure in the lungs more important as you
go higher.
If composition of inspired air is normal but alveolar P O2 is low, problem must lie within
alveolar ventilation.
Low alveolar ventilation is also known as hypoventilation and is characterized by
lower-than-normal volumes of fresh air entering the alveoli.
Pathological changes that can result in alveolar hypoventilation include decreased
lung compliance, increased airway resistance or CNS depression that slows
ventilation rate and decreases depth.
Common causes of CNS depression in young people include alcohol poisoning &
drug overdoses.
Hypoxia can be caused by hypoventilation or problems with the gas exchange
between alveoli and the blood.
In these situations, alveolar PO2 may be normal, but PO2 of arterial(slagader) blood
leaving the lungs is low. The transfer of oxygen from alveoli to blood requires
diffusion across the barrier created by type 1 alveolar cells and capillary endothelium.
The exchange of O2 and CO2 across this diffusion barrier obeys the same rules as
simple diffusion across a membrane.
Diffusion rate is directly proportional to the available surface area, the conc. gradient
of the gas and the permeability of the barrier.
From the general rules for diffusion, the 4th factor: diffusion distance can be added.
Diffusion is inversely proportional to the square of the distance, diffusion is most
rapid over short distances.
Under most circumstances, diffusion distance, surface area, and barrier permeability
in body are constants and are maximized to facilitate diffusion.
Gas exchange in the lungs is rapid, blood flow through pulmonary capillaries is slow
and diffusion reaches equilibrium in less than 1 second.
This leaves the conc. gradient between alveoli & blood as the primary factor affecting
gas exchange in healthy people.
The factors of surface area, diffusion distance, membrane permeability do come into
play with various diseases. Pathological changes that adversely affect gas exchange
include:
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