Week 7 Flashcards
Def: Convection
Bulk flow of gases goes through movement of medium
Oxygen transport pathway
- Air flow via convection into alveoli (Pulmonary Ventilation)
- Diffusion of O2 across respiratory epithelium
- Circulation of Oxygen through circulatory system via convection
- Diffusion of O2 into cells
Why do the partial pressure of O2 and CO2 decrease along the path of flow at each diffusion step?
There has to be a pressure difference at point of diffusion for process to occur
Purpose of lungs
Provides the interface for gas exchange between the air and the blood
Pulmonary Ventilation
Brings fresh air and O2 into contact with the gas-exchange surface and expels CO2
Purpose of Alveoli
Provide an interface for diffusion between air and blood
Characteristics of Air-blood interface
- large surface area
- very thin
Ideal Gas Law
PV=nRT
- where n is the number of moles of the gas, V is its volume, Tis its temperature (in K), and R is the gas constant
What is total barometric pressure
760mmHg or 760 Torr
How do you calculate total barometric pressure
= sum of partial pressures of each gas in a volume
Fraction of N2 in dry air
79%
fraction of O2 in dry air
21%
fraction of CO2 in dry air
0.03%
Partial pressure of gases in liquid
The partial pressure of gas that is dissolved in a liquid is equal to the partial pressure of that gas in the air phase with which the fluid is in equilibrium
How to calculate concentration of a dissolved gas
= Partial pressure x solubility
O2 and CO2 partial pressure through the circulatory system
- Atmospheric air and alveolar
- Alveolar partial pressures are very different from outside air
- 160 mmHg O2 atmospheric
- 0.3 mmHg CO2 Atm
- 104 mmHg O2 alveolar
- 40 mmHg CO2 Alv - Pulmonary Veins
- Partial pressures are very similar in alveolar air and the blood leaving the lungs (slightly less in blood)
- 104 mmHg O2
- 40 mmHg CO2 - Systemic Arteries
- Partial pressures are the same in blood leaving the lungs and entering the systemic capillary beds
- 104 mmHg O2
- 40 mmHg CO2 - Cells
- Partial Pressure in cells are very different from blood
- less than 40 mmHg O2
- more than 46 mmHg CO2 - Systemic Veins and Pulmonary Arteries
- Partial pressures are the same in venous blood leaving the systemic capillaries and blood entering the pulmonary capillary beds
- 40 mmHg O2
- 46 mmHg CO2
Flow Down Gradients
Diffusion of gases from one point to another is driven by partial pressure gradients between two points
- move from low pressure to high pressure
Def: diffusion capacity
an index of the conductance of a gas across the diffusion surface
Ficks law of the rate of diffusion
Diffusion flux rate = D x change in pressure
What factors determine diffusion capactiy
- Solubility and diffusivity of the gas (+)
- Area of diffusion surface (+)
- Thickness of diffusion surface (-)
- Properties of ventilation and/or perfusion of diffusion surface
- Temperature (+)
How and why does the O2 diffusion rate change along pulmonary capillaries
diffusion rate decreases along capillaries as blood O2 partial presure increases because the difference in pressures decreases
What factors determine venous PO2?
- Effects of diffusion
- Blood flow (convection) (+)
- Tissue metabolism (O2 consumption) (-)
Mass Balance
The contents of material in any body compartment is determined by the inputs and the outputs from that compartment
Accumulation = input + generation - output - consumption
- equation refers to absolute mass in a compartment or to rates of change
- in steady state conditions equation equals 0
Mass balance for O2 in tissues
- Input = Q x CaO2
- Generation = 0
- Output = Q x CvO2
- Consumption = VO2
VO2 = Q x (CaO2-CvO2)
Mass balance across O2 transport pathway
VO2
= O2 inhaled - O2 exhaled (in lungs)
= Lung O2 diffusion
= Q x (CaO2 - CvO2)
= Tissue O2 diffusion
= Mitochondrial O2 Consumption
What limits maximal capacity for aerobic metabolism?
maximal rate of O2 transport
Determinant of VO2
Occurs at the intersection between mass balance curve and fick’s law curve because the rate of O2 removal from the circulation must equal the rate of diffusion into the tissue
- determined by Q and O2 diffusing capacity
What determines the O2 diffusing capacity of a tissue
- Capillary density (surface area)
- Properties of blood flow
Mass Balance graph
x- axis: Venous PO2
y- axis: VO2
as the venous PO2 increases VO2 decreases
Fick’s Law graph
X- Axis: Venous PO2
y- Axis: VO2
As the venous PO2 increases VO2 increases
P cap increases VO2 increases, translates to increase venous PO2
