BIOL 2420 Lecture Notes - Lecture 1: Gas Exchange, Venous Blood, Partial Pressure
26 Apr 2018
School
Department
Course
Professor
Unit 6 – Lecture 1
Gas Exchange in the Lungs and Tissues
- Breathing is the bulk flow of air into and out of the lungs
o Once air reaches the alveoli, individual gases such as oxygen and CO2 diffuse from the
alveolar air space into the blood
o Diffusion: movement of a molecule from a region of higher concentration to one of
lower concentration
- Concentrations of solutions
o Moles/liter and milliosmoles/liter
o Respiratory physiologists commonly express plasma gas concentrations in partial
pressures to establish whether there is a concentration gradient between the alveoli
and the blood
▪ Gases move from regions of higher partial pressure to regions of lower partial
pressure
The figure to the right
- Shows partial pressures of oxygen and carbon dioxide in air, the alveoli, and inside the body
- Normal alveolar PO2 at sea level is about 100 mmHg
o PO2 of deoxygenated venous blood arriving at the lungs is about 40 mmHg
o Oxygen diffuses down its partial pressure gradient from the alveoli into the capillaries
▪ Diffusion goes to equilibrium
▪ PO2 of arterial blood leaving the lungs is the same as in the alveoli (100 mmHg)
- When arterial blood reaches tissue capillaries
o Gradient is reversed
o Cells are continuously using oxygen for oxidative phosphorylation
o At rest
▪ Intracellular PO2 averages 40 mm Hg
▪ Arterial blood arriving at the cells has a PO2 of 100 mm Hg
▪ PO2 is lower in the cells so oxygen diffused down its partial pressure gradient
from plasma into cells
• Diffusion goes to equilibrium
▪ Venous blood has the same PO2 as the cells it just passed
- PCO2 is higher in tissues than in systemic capillary blood because of CO2 production during
metabolism
o Cellular PCO2 in a person at rest is about 46 mm Hg compared to an arterial plasma PCO2
of 40 mm Hg
o Gradient causes CO2 to diffuse out of cells into the capillaries
▪ Diffusion goes to equilibrium
▪ Systemic venous blood averages a PCO2 of 46 mm Hg
- At pulmonary capillaries, the process reverses
o Venous blood brining waste of CO2 from the cells has a PCO2 of 46 mm Hg
o Alveolar PCO2 is 40 mm Hg
o Because 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
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Shows partial pressures of oxygen and carbon dioxide in air, the alveoli, and inside the body. When arterial blood reaches tissue capillaries: gradient is reversed, cells are continuously using oxygen for oxidative phosphorylation, at rest. Inspired air has low oxygen content: alveolar ventilation is inadequate. Hg: water vapor pressure at 100% humidity is the same no matter what the altitude. Its contribution to total pressure in the lungs is more important as you go higher. Altitude remains constant, unless a person is travelling. Increased airway resistance: central nervous system (cns) depression that slows ventilation rate and decreases depth, alcohol poisoning, drug overdoses. Factors of surface area, diffusion distance, and membrane permeability come into play with various diseases: pathological changes that adversely affect gas exchange include, decrease in the amount of alveolar surface area available for gas exchange. Increase in the thickness of the alveolar-capillary exchange barrier. Increase in the diffusion distance between the alveolar air space and the blood.
