Gas Exchange and Transport Flashcards
PO2 and PCO2 in alveoli and arterial blood just after gas exchange
Same partial pressures
Venous blood arriving at the alveoli has around what PO2 & PCO2?
Low PO2 (~ 40 mm Hg) and higher PCO2 (~ 46 mm Hg)
Factors that influence alveolar gas exchange
- The amount of oxygen that reaches the alveoli
- The ability for gas to diffuse between the alveoli and blood
- Adequate perfusion of the alveoli (are there enough functional capillaries around the alveoli)
Factors that that influence the amount oxygen that reaches the alveoli
- Composition of inspired air
- alveolar ventilation (rate and depth of breathing, airway resistance, lung stretchiness)
Factors that influence the ability for gas to diffuse between alveoli and blood
- surface area
- diffusion distance (affected by barrier thickness, amount of fluid through which gasses must diffuse)
Emphysema
Destruction of alveoli —> less SA for gas exchange
Fibrotic lung disease
Thickened alveoli membrane —> slower gas exchange
Lung becomes less stretchy —> decreased ventilation
Pulmonary edema
Increase in fluid between alveoli and epithelial cells of capillary which increases distance for gas diffusion
Leads to increased arterial PCO2 b/c CO2 is more soluble in water than O2
Asthma
Increased airway resistance due to bronchoconstriction decreases amount of oxygen that arrives at the alveoli
Hypoxic hypoxia
Low arterial partial pressure of oxygen
Causes:
High altitude, hypoventilation, abnormal alveolar perfusion
Anemic hypoxia
Decreased total amount of oxygen bound to hemoglobin
Causes:
Blood loss, anemia, carbon monoxide poisoning
Ischemic hypoxia
Reduced blood flow
Causes:
Heart failure, shock, thrombosis
Histotoxic hypoxia
Failure of cells to use oxygen because cells have been poisoned
Causes:
Cyanide and other metabolic poisons (flea dip!)
How does oxygen get loaded onto Hb?
Diffuses across alveolar membrane into plasma where it briefly dissolves. Diffuses across RBC membrane where it is loaded onto Hb.
Oxygen carrying capacity
The amount of oxygen bound to Hb
What influences oxygen carrying capacity?
- amount of oxygen dissolved in plasma (determines % saturation of Hb)
- amount of hemoglobin (which determines the number of Hb binding sites)
Total number of Hb binding sites is calculated from…?
Hb content per RBC x # of RBCs
How many heme groups are on each Hb?
An Hb has four subunits and each subunit has one heme group so thats four binding sites per Hb
How many coordination sites does iron have?
6
4 nitrogen
1 histidine side chain
1 for molecular oxygen
How would removal of histidine side chains affect oxygen heme binding?
Inhibit
How does Hb increase the oxygen carrying capacity of the blood?
PO2 in alveoli and plasma will equilibrate so when Hb binds oxygen in the RBC, equilibrium will shift to replenish the oxygen in the plasma increasing the overall oxygen carrying capacity
How does Hb change when binding oxygen?
Conformational change which increases the affinity for the next oxygen
Hb binding curve
S-shaped/sigmoidal curve characteristic of cooperative binding
Shows that each consecutive oxygen binding increases the affinity for oxygen of the next binding site when picking up oxygen and vice versa when dropping off oxygen
Partial pressure of oxygen leaving the lungs
100 mm Hg
Partial pressure of oxygen returning to the lungs
40 mm Hg
Normal percent oxygen reserve
75% of Hb binding sites are occupied by oxygen normally so that this oxygen reserve can be released to tissues with low oxygen (PO2 < 40 mm Hg) such as during exercise
What percentage of binding sites exchange oxygen in the lungs?
25% or 1 binding site per Hb
What does the Hb binding site affinity change with each subsequent O2 picked up/dropped off?
Stepwise conformational changes of Hb structure
Hb left shift
Higher of affinity of Hb for O2
Good for pick up in lungs (up to max), bad for drop-off at tissues
Hb right shift
Lower affinity of Hb for oxygen
Good for drop-off at tissues, bad for pick-up in lungs
How does fetal Hb differ from maternal/adult Hb?
Has two gamma units in place of two beta units which have a lower affinity for BPG
Fetal Hb binding curve is shifted left compared to maternal Hb binding curve
Fetal blood supply receives oxygen from maternal blood supply
Factors that shift Hb binding curve right
Decreasing pH (more acidic, such as during exercise/formation of lactic acid), increasing temperature, increasing partial pressure of CO2, increasing BPG
2, 3 BPG
Binds in place of oxygen which facilitates the drop off of oxygen to the tissues
High altitude & exercise induced adaptation
What factors influence the percent saturation of Hb?
PCO2, pH, Temperature, and BPG
How might you change the total number of Hb binding sites?
Increase the Hb content per RBC
Increase the number of RBCs via hormone EPO (increase hematocrit)
Myoglobin
Monomeric heme protein in the sarcoplasm of muscle cells
Has a higher affinity for oxygen than hemoglobin
Since myoglobin only has one binding site, oxygen bonding is not cooperative
There is a certain PO2 threshold at which myoglobin drops off oxygen (~20 mm Hg)
Animals living at high altitudes tend to have ____ shifted Hb binding curves.
Left
Short and long term human adaptations to high altitude
Short - increase in breathing rate, hyperventilation
Long - increased EPO leading to increased RBC production/hematocrit, increased angiogenesis
Ways in which CO2 is carried in the blood
- as bicarbonate (HCO3-) - 70-90%
- dissolved in blood plasma - 5-10%
- bound to amino groups in hemoglobin (carbaminohemoglobin) - 5-15%
H+ binding reduces the affinity of Hb for oxygen
How is carbon dioxide converted to bicarbonate?
In the RBC, carbon dioxide reacts with water to become carbonic acid. Carbonic acid is then converted into bicarbonate and hydrogen ions. (H2CO3 —> HCO3- + H+). Both reactions are catalyze by carbonic anhydrase.
How is plasma neutrality maintained when bicarbonate ions move into the plasma?
Negatively charged chloride moves into the RBC from the plasma making the net charge change = 0
Why does carbon dioxide unbind from Hb and diffuse out of the RBC?
Law of mass action due to carbon dioxide diffusing out of the plasma, into the alveoli, and being exhaled
How does Hb buffer H+ ions?
Histidine on Hb can pick up protons to prevent acidification thus acting as a buffer
Respiratory acidosis
Breathing rate is insufficient to clear carbon dioxide at the rate that it is being made
Hypoventilation
Metabolic acidosis
Caused by the build up of ketones, ingestion of acid, kidney failure, excessive lactic acid fermentation
Why does carbon dioxide diffuse out of the plasma, into the alveoli, and then into the air to be exhaled?
The partial pressure of carbon dioxide in the air is always lower than in the alveoli so carbon dioxide moves down its pressure gradient when moving into the air
Oxygen dissolves better in what temperature water?
Cold
How do chemosensors in the carotid body signal to medullary centers to increase ventilation?
(Very very) Low partial pressure of oxygen in the blood signals to the K+ channels in the carotid body to close causing the cell to depolarize. Depolarization of the membrane opens voltage-gated calcium channels. Calcium aids in the exocytosis of neurotransmitters which trigger an action potential on a sensory neuron that signals to the medullary centers to increase ventilation.
Aortic chemoreceptors also monitor partial pressure of oxygen. Both also monitor CO2(respiratory acidosis) and pH (metabolic acidosis).
Central chemoreceptors monitor the partial pressure of which gas in the cerebrospinal fluid?
Carbon dioxide
This is in contrast to carotid and aortic chemoreceptors (aka peripheral) which also monitor oxygen and pH.
What types of neurons signal to the respiratory control centers and what effect does that have?
Sensory neurons signal to respiratory control centers to change ventilation
