Oxygen and Carbon Dioxide Flashcards
What is partial pressure?
the pressure of an individual gas in a given volume at a constant temperature
Total pressure is divided into partial pressure of O2 and N2. What are the relative percentages of each that make up total pressure?
pO2 = 160 mm Hg = 21% total pressure
pN2 = 600 mm Hg = 79% total pressure
describe the diffusion of gases in ventilation
the respiratory system functions to maintain the partial pressure of O2 and CO2, so O2 will diffuse into capillaries from alveoli, and CO2 will diffuse into alveoli from capillaries
3 differences between alveolar air vs atmospheric air
alveolar air has less O2, more CO2, and is humidified
what is the function of humidifying air as it is drawn through the respiratory passages?
water vapor at normal body temp is 47mm Hg, so when it is added to the mix of gas in the alveoli, it serves to dilute the other gases
what is the mix of gases found in the alveoli?
14% O2
75% N2
5% CO2
6% H2O
describe the solubility of CO2 and O2 in blood
CO2 is highly soluble in blood, O2 is less so
Name 5 things that affect the rate of diffusion of a gas
1) solubility of gas in the fluid
2) surface area of the barrier
3) distance of diffusion (membrane thickness)
4) molecular weight of gas
5) temperature (irrelevant because body temp is constant)
why is aqueous diffusion the limiting factor?
respiratory gases are highly soluble in lipids (cells/tissue)
what is tidal volume?
normal volume of air displaced between inhalation and exhalation; “renewed” air
what is functional residual capacity?
the volume of air that stays in the lungs; it acts as a sort of buffer that prevents sudden changes in the alveolar gas mix
what 2 factors control the amount of oxygen in the alveoli at a given time?
ventilation from breathing and rate of absorption from the blood
As metabolism increases, what happens to blood O2 and CO2?
O2 decreases and CO2 increases
as metabolism increases, what happens to the O2 and CO2 partial pressure of the lung?
lung O2 partial pressure decreases and CO2 partial pressure increases
What is VA/Q?
- the ventilation (VA) perfusion (Q) ratio
- can be expressed as a function of partial pressures of O2 and Co2 in an alveolus
what happens when VA/Q = 0?
- no air reaching the alveolus; 0/Q = 0
- partial pressure of both gasses in the alveolus equilibrate with that of the pulmonary blood
- no respiration takes place
what happens when VA/Q = infinity (VA/0)?
- no blood perfuses an alveolus, so no gas exchange happens
- no respiration takes place
what is non-perfused blood called?
shunted blood
what happens when VA/Q approaches 0?
- the physiologic shunt occurs
- the amount of shunted blood that passes through the pulmonary circulation per minute = physiologic shunt
what happens when the VA/Q ratio is lower than normal?
- supply is not sufficient to oxygenate all perfusion
- result is shunted blood
what happens when the VA/Q ratio moves away from 0?
- physiologic dead space
- not all inspired air is available for gas exchange (that air is called physiological dead space air)
what happens when the VA/Q ratio is greater than normal?
- physiological dead space
- supplied ventilation is greater than perfusion
What equation describes physiologic dead space
the Bohr equation
In what part of the lungs is a physiologic shunt normal?
- lower portion of the lungs due to greater blood flow and slightly insufficient ventilation
- can be minimized by exercise
in what part of the lungs is a physiologic dead space normal?
- top portion of the lung due to lower blood flow
- can be minimized by exercise
- *physiologic dead space actually also exists in the lower portion of the lung
blood leaves the alveoli with what partial pressure of O2?
104 mm Hg
what is the average arterial partial pressure of O2?
95 mm Hg
what is the average venous partial pressure of O2?
40 mm Hg
partial pressure of O2 in the tissues is around 23 mm Hg. Why is this not problematic?
intracellularly, only 1-3 mm Hg are generally required to support cell processes
what component of RBCs helps make O2 more soluble?
hemoglobin
how much of all O2 in the blood is bound to hemoglobin?
97%
the rest is freely dissolved
describe hemoglobin-oxygen disassociation
high partial pressure of O2 favors hemoglobin binding of O2; low partial pressure of O2 favors disassociation of O2.
the total content of O2 in the arteries is dependent on what?
the partial pressure of free O2 (the amount of O2 hemoglobin is carrying)
what form is oxygen in when it binds to hemoglobin?
elemental (diatomic) form; as O2, so 2 oxygens bind at each of the 4 hemoglobin sites
describe the cooperative binding of O2 to hemoglobin
- for each site occupied, the affinity of the following site for oxygen is increased.
- the opposite is true of hb release of O2: for each O2 that disassociates, the easier it is for the following molecules to disassociate
for the delivery of oxygen, is increasing hg/saturation of hg more or less powerful than increasing the partial pressure of O2 in blood?
increasing hg or saturation of hb is more powerful
what is the utilization coefficient?
- the percentage of blood that gives up its O2
- at rest, this is usually 25% but can increase to 85% during exercise
describe the O2 disassociation curve
- as partial pressure of O2 exposed to hb increases, the saturation of hb increases
- eventually, hb will reach total saturation; this gives the curve a sigmoidal shape
- the steep section of the curve represents cooperative binding
- hb acts as a buffer to resist changes in tissue PO2
T or F:
fetal hemoglobin tends to bind more O2
true
describe the Bohr effect
- the affinity of hemoglobin’s binding to O2 is inversely related to acidity and the concentration of CO2
- in other words, more O2 is released in tissues with high CO2 and low pH
carbon dioxide is ___ times more soluble in blood than O2
20 times more soluble
what percent of CO2 is converted to carbonic acid, and what enzyme catalyzes that reaction?
- 93%; the rest is dissolved directly into the blood
- catalyzed by carbonic anhydrase
what happens to the 5-7% of CO2 that dissolves directly into the blood?
- it can bind to plasma proteins or enters RBCs and binds hg
- forms carbaminohemoglobin
- as RBCs pass through O2 rich pulmonary circulation, CO2 disassociates from hg and O2 takes its place
describe carbon monoxides affinity for hg
- CO has a great affinity for hg compared to CO2 or O2 and is thus, retained by the hg
- ventilation with O2 can force off the CO
describe the difference between the haldane effect and the bohr effect
- haldane effect states that deoxygenated blood has an increased ability to carry CO2 and oxygenated blood carries less CO2
- bohr effect states that the affinity of hg’s binding to O2 is inversely related to acidity and the concentration of CO2
