5.4 Physiology of Gas Exchange Flashcards
What is partial pressure? How can we calculate it?
- Partial pressure is the proportion of the pressure of a gas created by a single component of that gas
- It can be calculated as follow:
Part. Pressure = Total Pressure * Proportion of Component
State Fick’s law in words
The molar flux due to diffusion is proportional to the concentration gradient
State the equation of fick’s law
Rate of transfer = diffusing capacity * partial pressure gradient
How do we calculate diffusing capacity?
Rate of transfer / partial pressure gradient
(i.e. what volume of gas will diffuse per unit pressure?)
What are the four categories of factors that can modify the diffusing capacity of a the blood-air barrier?
- Diffusion coefficient of gases
- Changes in effective surface area
- Changes in physical properties of membrane
- Changes in O2 uptake by RBCs
List some factors that influence a person’s blood-air barrier membrane surface area
- Height
- Lung voluime
- V/Q Mismatch
- Pathology (e.g. emphysema)
List some pathologies/factors that decrease the efficiency of diffusion across the blood-air barrier
- Pulmonary congestion
- Interstitial oedema
- Membrane thickening
List some factors that influence blood-air barrier gas uptake
- Hb concentration
- Capillary transit time
What is capillary transit time?
The time available for diffusion exchange before blood returns to the heart
Normal haemoglobin values in males/females?
Males: 13-18
Females: 12-16
(g/dL)
How much O2 can be bound to 1g of haemoglobin?
~1.34mL
What percentage of O2 in blood is bound to haemoglobin?
~97%
External vs internal respiration
External: exchange of substances with external environment, between air and alveoli
Internal: exchange of substances within organism, between blood and tissue
List three factors that increase the tendency of haemoglobin to offload oxygen
- Increased H+
- Increased CO2
- Increased temperature
(all three of these indicate increased metabolic demand)
Explain the Bohr effect
CO2 and H+ increase the tendency of Hb to release its oxygen
Low CO2: oxygen binding promoted
High CO2: oxygen offloading promoted
Explain the three mechanisms of CO2 transport
- Dissolved (10%)
- Bicarbonate ion (70%)
- Haemoglobin (20%)
Is the operating concentration range of CO2 wider or thinner than the operating concentration range of O2?
Much thinner (40-46mmHg)
Explain the Haldane effect
Greater binding of O2 with Hb increases release of CO2 into the blood
Do gases exert a partial pressure when bound to haemoglobin?
No
Average minute ventilation human
5-8L/min
Typical cardiac output
5L/min
How do ventilation and blood flow increase from the apices to the bases of the lungs?
Ventilation and perfusion are highest at the base of the lungs, and lowest at the apices.
List four factors that cause regional differences in ventilation throughout the lungs
- Gravity
- Anatomical expansion (bases of lungs have more room to expand)
- Compliance (bases are more compliant)
- Breathing pattern (alveolar recruitment can differ at slow v fast filling rates)
List three factors that influence regional differences in lung perfusion
- Gravity - creates a hydrostatic gradient
- Hypoxic pulmonary vasoconstriction (redirects flow away from poorly ventilated areas)
- Lung volume
Describe West’s four zones of the lungs. Which are most common in a healthy human?
Zone 1: PA > Pa > Pv (no flow)
Zone 2: Pa > PA > Pv (flow during systole, not diastole)
Zone 3: Pa > Pv > PA (constant flow)
Zone 4: Interstitial pressure > PA (no flow)
Most common: Zone 2 and Zone 3
