O2 and CO2 exchange Flashcards
How much air do we breathe in and out?
VE= VT x f
Respiratory minute volume (L/min) = tidal volume (L/breath) x respiratory rate (breathes/min)
Tidal volume = breathing strength
Respiratory rate = breathing speed
What is dead space?
Not all of the 6L of breathe will get to your alveoli, some of the inhaled air, 150ml, enters the respiratory system but doesn’t contribute to gas exchange - Dead space, fills upper airway
How much air gets to alveoli?
VA= (VT-VD) x f
VA = volume of gas per unit time that reaches alveoli, alveolar ventilation (L/min)
VD= dead space, air that can’t be used (L/breath)
Tiny rapid breaths are not helpful as most of the breath will only reach airway not alveoli hence gas exchange becomes inefficient. Slow and really deep breaths are also inefficient as more of the deep breath contributes to airway/dead space
Dalton’s law and gas diffusion
In a gas mixture (air), each gas exerts its own individual pressure, called partial pressure (P)
Dalton’s law is the pressure of a mixture of gases = the sum total of the pressures of each individual gas
Moving gas across membrane
The goal is to move gas back and forth between the alveoli and the capillaries. Transport inhaled oxygen into capillaries. Transport carbon dioxide into the alveoli to be exhaled
Gases have tendency to dissolve into solution, oxygen will dissolve into solution until partial pressure of air (140mmHg) and solution (100mmHg) are at equilibrium
Ficks law of diffusion
Gases move across the membranes between the alveoli and the capillaries by diffusion. Diffusion rate determined by surface area of the membranes, thickness of the membranes, solubility of the gas and the pressure difference between the two sides
F= (A/T) x D x (P1-P2)
F= flux (amount flowing), A= surface area, T= thickness, D= diffusion constant, P1-P2= pressure difference
Diffusion constant
Diffusion constant, D, or coeffienct for substance depends on gas solubility (S) and its molecular weight. On a per molecule basis CO2 diffuses about 20x faster than O2 due to CO2 higher solubility (nitrogen cannot dissolve in air hence doesn’t dissolve into blood)
Surface area
Bulbous structure of alveoli and the high density of capillaries creates lots of surface area, A, for gas exchange
Reduction in surface area means less contact between the air and capillaries, so oxygen exchange greatly reduced
What is emphysema?
Emphysema is a disease characterised by dilation of the alveolar spaces and destruction of the alveolar walls, example of chronic obstructive lung disease
Thickness of membranes
The blood air barrier is mostly comprised of the alveolar and capillary walls. The distance between the alveolar air and the blood is very small
Thickening, T, and scarring of alveolar membranes can affect moving of gas across membranes (rate of diffusion decreases). Can arise from chronic inflammation or exposure to industrial chemicals
Alveolar partial pressure of O2
Partial pressure depends on partial pressure of oxygen in inspired air, alveolar ventilation, oxygen consumption. The atmospheric PO2 is usually constant, so it is balance between oxygen consumption and ventilation that is most important
Exercise and blood oxygen
Low activity = reduced oxygen consumption, higher levels of oxygen in venous blood
High activity = increased oxygen consumption, lower levels of oxygen in the venous blood
Alveolar partial pressure of CO2
Alveolar partial pressure of CO2 depends on partial pressure of CO2 in inspired air, alveolar ventilation and CO2 production. Alveolar PCO2 is usually determined only by the balance between CO2 production and alveolar ventilation, because atmospheric CO2 is negligible
Gas diffusion down their pressure gradient
Partial pressure of oxygen at alveoli 100mmHg, blood coming into lung is depleted of oxygen, partial pressure ~40mmHg and is constantly diffusing until partial pressure in alveoli is similar to capillary, partial pressure among membrane reaches equilibrium
CO2 down pressure gradient out of pulmonary system into alveoli, CO2 moves into alveoli as long as pressure gradient present, here 46mmHg -> 40mmHg, continues to move out until both capillary and alveoli at 40mmHg, equilibrium
