Define gas exchange
diffusion of O2 and CO2 in lungs and in peripheral tissue. O2 will diffuse from alveoli into blood, CO2 will diffuse from blood into alveoli to be released into environment on exhalation.
Dalton’s law of partial pressures
- partial pressure of a gas in a mixture of gases is the pressure that gas would exert if it occupied the total volume of the mixture.
- Thus partial pressure is the total pressure multiplied by the fractional concentration of dry gas
Ficks Law of diffusion
-(V̇ x )=(D)(A)(ΔP)/ ΔX
Henry’s law
used to convert the partial pressure of gas in the liquid phase to the concentration of gas in the liquid phase
Transport of oxygen from the atmosphere to the tissues
Oxygen is breathed in through mouth/nose, travels down trachea where it is humidified, travels into the alveoli of the lungs, into the capillaries, into the pulmonary vein, to the left atrium, to the left ventricle, to the aorta, down arteries, into tissue capillaries, diffuses across capillaries into tissue.
normal values of oxygen in dry air, humidified air, alveoli, mixed venous blood, and systemic arterial blood
- Dry inspired air: 160
- Humidified tracheal air: 150
- Alveolar air: 100
- Mixed venous blood: 40
- Systemic arterial blood: 100
ways that oxygen can be carried in a solution
• Dissolved O2 -only 2% of total O2 in blood
-only form that produces partial pressure
• Bound to Hemoglobin -98% of total O2 in blood
-Hemoglobin has four subunits 2a and 2B each containing heme, and each subunit can carry one O2
variants of hemoglobin
- Methemoglobin: When the iron component of the heme moieties is in the ferric, or Fe 3+ ,state and does not bind O 2 . Caused by oxidation of Fe 2+ to Fe 3+by nitrites and sulfonamides as well as a congenitally when there is a deficiency of methemoglobin reductase
- Fetal hemoglobin (hemoglobin F, HbF): two β chains are replaced by γ chains . Allows for higher affinity for O 2 facilitating O 2movement from the mother to the fetus. HbF is the normal variant present in the fetus and is gradually replaced by hemoglobin A within the first year of life.
- Hemoglobin S.: abnormal variant of hemoglobin that causes sickle cell disease. α subunits are normal and the β subunits are abnormal. Forms sickle-shaped rods in the red blood cells, distorting the shape of the red blood cells which can result in occlusion of small blood vessels. The O 2 affinity of hemoglobin S is less than the O 2 affinity of hemoglobin A.
how combination of oxygen with hemoglobin influences the shape of the oxyhemoglobin dissociation curve
the curve increases in steepness because of increase in affinity for oxygen as each subunit of hemoglobin binds to oxygen (positive cooperation)
O 2 -binding capacity
maximum amount of O 2 that can be bound to hemoglobin per volume of blood,
Hemoglobin saturation
all 4 subunits are carrying an oxygen molecule
O 2 content
actual amount of O 2 per volume of blood
Mechanism for Oxygen loading and unloading
When the tissue needs the oxygen the hemoglobin will be triggered to be less tightly bound to the oxygen than when the tissue does not need the oxygen
Right shift
- decreased affinity of hemoglobin for O2 causing unloading of O2 in the tissues
- causes thalacemia: decreased saturation
Causes of right shift
○ Increases in P co 2: occurs with increase of metabolic activity in tissues increasing CO2 production
○ Decreases in pH: occurs with increase of metabolic activity in tissues producing H+ and decreasing pH. This then causes a decreased affinity of hemoglobin for O 2 ,
○ Increases in temperature: heat is produced by the working muscle causing more O 2 to be released to the tissue.
○ Increases in 2,3-diphosphoglycerate (2,3-DPG) concentration. 2,3-DPG is a byproduct of glycolysis in red blood cells that increases in hypoxic conditions. 2,3-DPG binds to the β chains of deoxyhemoglobin and reduces their affinity for O 2 causing delivery of O 2 to the tissues.
Causes of Left shift
increased affinity of hemoglobin for O 2 causing unloading of O 2 in the tissues to be more difficult
Causes of left shift
○ Decreases in P co 2: decrease in tissue metabolism, there is decreased production of CO 2 showing that there is decrease in O2 demand
○ Increases in pH. When there is a decrease in tissue metabolism, there is decrease in H+ made, causing pH to increase. The increase shows that less O2 is needed and therefore it remains more tightly bound.
○ Decreases in temperature. When tissue metabolism decreases, less heat is produced and less O 2 is unloaded in the tissues.
○ Decreases in 2,3-DPG concentration. Decreases in 2,3-DPG concentration reflect decreased tissue metabolism, causing a left shift of the curve and less O 2 to be unloaded in the tissues.
○ Hemoglobin F. As previously described, HbF is the fetal variant of hemoglobin. The β chains of adult hemoglobin (hemoglobin A) are replaced by γ chains in HbF. This modification results in increased affinity of hemoglobin for O 2 , a left shift of the O 2 -hemoglobin dissociation curve, and decreased P 50 . § The mechanism based on the binding of 2,3-DPG which does not bind as avidly to the γ chains as it binds to the β chains. When less 2,3-DPG is bound, the affinity for O 2 increases. This increased affinity is beneficial to the fetus, whose Pa O 2 is low (approximately 40 mm Hg).
Role of erythropoietin
- glycoprotein growth factor that is synthesized in the kidneys and stimulates erythropoiesis by promoting the differentiation of proerythroblasts into red blood cells.
- EPO synthesis is induced in the kidney in response to hypoxia
How to calculate arterial oxygen content
- O2 capacity of Hgb= (1.34) ([hgb])
- O2 bound to hemoglobin= (O2 capacity of hgb) (%Hgb saturation)
- physcially dissolved O2= (0.003) (PaO2)
- O2 content= O2 bound to Hgb+Phycially dissolved O2
normal ranges of plasma pH, PaCO2, and bicarbonate
pH: 7.35-7.45 arterial Pco2: 45-35 mmHg, HCO3: 22-26 mmol/L,
Henderson-Hasselbach equation
pH=pka+log(base/acid)
Henderson-Hasselbach in relation to bicarb and CO2
-pH=pka+log(HCO3/ CO2*0.03) -PKA: 6.1 -justifies why normal pH is 7.4
sequential steps of ABG analysis
- Check pH 2. Check CO2, does it account for pH change 3. Check HCO3, does it account for pH change 4. If both CO2 and HCO3, determine primary disorder for compensation 5. Calculate anion gap
primary type of disorders determined by ABG
• Respiratory acidosis • Respiratory alkalosis • Metabolic acidosis -Metabolic alkalosis
