Lec 4 Ventilation perfusion rate Flashcards
Definition of Ventilation perfusion:
Ventilation: amount of gas going into alveoli ready for gas exchange
Perfusion: amount of blood flow to the alveoli
VA/Q
normally 4.2/5.5= 0.8
VA/Q ratios and their effect
if VA is zero- ratio is 0
if Q is zero- ratio is infinite
in both of these circumstances theres no gas exchange
Lung zones:
Upper part of lung: V low/Q LOW LOW
(higher ratio around 3)
lower part of lung: V high/ Q HIGH HIGH
(lower ratio around 0.6)
upper have larger alveoli than lower
inadequate perfusion diseases but good ventilation
Respiratory diseases and how they affect VA/Q:
high ratio
-Inadequate perfusion(higher ratio): pulmonary embolism and pulmonary thrombosis .
so, when VA/Q is high, PO2 and CO2 of alveolar air is the sameee as the the inspired PO2 and CO2
because the gas goes to the alveoli with no blood to exchange partial pressures, is it remains the same.
transport of oxygen from lungs to body tissues:
-PO2 in alveoli is higher than in capillaries, so by normal diffusion rules, it diffuses into capillary as it has less PO2 then into body tissues.
-the opposite happens in PCO2….
-After O2 is metabolized, CO2 is produced, so PCO2 rises and diffuses into capillaries, where PCO2 is higher than in alveoli, therefore CO2 diffuses back into alveoli to be exhaled
Inadequate ventilation but good perfusion
Respiratory diseases and how they effect VA/Q:
low ratio
-In emphysema and smokers… has to do with the wellness of the ALVEOLI (e.g surfactant)
- if you have low to 0 VA/Q , that means there is little gas coming in, therefore less gas available for exchange… which means there’ll be shunted blood i.e blood that doesnt undergo exchange.
- leaving the PO2 and CO2 in alveoli the same of that in venous blood
Diffusion of oxygen from the alveoli to the pulmonary capillary blood:
PO2 of alveolus is 104mm Hg
PO2 in venous blood is 40mm Hg
104-40= 64 mm Hg difference
Uptake of oxygen during exercise:
-During exercise, the oxygen needs rise up to 20 times
-diffusing capacity for oxygen reaches threefold resulting mainly from:
-increased surface area of capillaries
-ideal ventilation perfusion ratio in the upper part of lungs
diffusion of oxygen from periphral capillaries into tissue fluid:
-Arterial blood that reaches periphral tissues has PO2 of 95mmHg
Interstial tissue fluid has PO2 of 40mmHg
-arterial Oxygen loads onto interstitial fluid leaving it at 40 mmHg PO2 in the venous blood
effect of rate of blood flow on interstitial fluid PO2
-Normally flow is at 100% and PO2 is 40 in interstial fluid
-a 400% increase in blood flow will raise it to 66mmHg PO2
-The max PO2 that interstial fluid can reach is 95mmHg, cant exceed the arterial PO2
Tissue PO2 is determined by a balance between:
-the rate of oxygen transport to the tissues in the blood
-the rate of oxygen usage by the tissue
Physiological shunt vs physiological dead space
physiological shunt- wasted blood
Physiological deadspace- wasted air
wasted air= low Q
wasted blood= low VA
Diffusion of CO2 from periphral tissue into the capillaries and from the capillaries to alveoli :
-CO2 is carried in opposite direction to O2
-CO2 can diffuse 20 times faster than O2, so less pressure is needed to diffuse CO2.
CO2 pressures are:
Intracellular, interstitial, Arterial, capillary,
Intracellular PCO2- 46mmHg
Interstitial PCO2- 45mmHg
Arterial PCO2- 40mmHg and venous PCO2 is 45mmHg
this 5 pressure difference causes the capillary-alveolar diffusion of CO2
PCO2 of capillary blood then equals the PCO2 of alveolar, 40mmHg
Effect of rate of tissue metabolism and blood flow on PCO2
High metabolism does the following
PO2 decreases
PCO2 Increases
Increasing blood flow Decreases CO2 from 45 to 40 mmHg
