Week 8- Gas Exchange and Transport Flashcards
Gas Exchange in the Lungs
- Takes place between alveolar sir and blood flowing through the lung capillaries
- Physiologically, air in the lungs is not part of the body’s internal environment
- Before O2 can enter and CO2 can leave the internal environment they must cross a barrier
Respiratory Membrane Thickness
- Increased thickness= decreased diffusion
- Result of pulmonary edema
- Gas exchange is decreased
O2 & CO2 Diffusion
- Gases move in both direction through the respiratory membrane
- oxygen enters the blood from the alveolar air because the PO2 of the incoming blood (remember things move from high to low conc)
- Simultaneously, CO2 molecules exit the blood by diffusing down the pressure gradient into the alveolar air
- PCO2 of venous blood is much higher than the PCO2 of alveolar air
- This 2 way exchange of gases converts deoxygenated blood to oxygenated blood
The amount of O2 diffused into the blood each minute depends on several factors:
- The alveolar pressure gradient
- The total functional of the respiratory membrane
- The respiratory minute volume (RR/ min x the volume of air inspired per respiration)- how much we breathe to bring in that oxygen
- Alveolar ventilation
General rule: anything that decreases the alveolar PO2 tends to decrease the alveolar- blood oxygen pressure gradient, reducing the amount of O2 entering the blood
Application 1- O2 Pressure Gradient
- Alveolar PO2 decreases as altitude increases, thus less O2 enters the blood at high altitudes
- Eventually, the PO2 in the alveolar air equals the PO2 of blood
Application 2- Functional Surface Area
- Anything that decreases the functional surface area of the respiratory membrane tends to decrease oxygen diffusion into the blood
- Eg. Emphysema pt- the total functional area decreases and is one of the factors responsible for poor oxygenation
Application 3- Resp. Minute volume
- Anything that decreases RR tends to decrease blood oxygenation
- Eg. Morphine slows respirations and therefore decreases the respiratory minute volume and tends to lessen the amount of O2 entering the blood
How blood transports gases
- Blood transports O2 and CO2 either solutes or combined with other chemicals
- Immediately upon entering the blood, both O2 and CO2 dissolve in the plasma
- B/c fluids can only hold small amounts of gas most of the O2 and CO2 rapidly form a chemical union with other molecules such as hemoglobin, plasma, proteins or water
- Once they are bound to a molecule, their plasma concentration decreases and more gas can diffuse into the plasma- allowing large amounts of gases to be transported
Hemoglobin
- Reddish protein pigment found in the RBCs
- Contains iron, alpha, and beta chains
- Contains iron- O2 affinity for iron atoms, allowing the iron to act as oxygen sponge that chemically absorbs O2 molecules from the surrounding solution
- CO2 has an affinity for the alpha and beta amino acid chains, allowing HB to “sponge” the CO2 and carry it as well
Transport of O2
- HB combines with O2- forms oxyhemoglobin
- Each gram of HB can untie with 1.34ml of O2
- As a result, the exact amount of O2 in the blood depends largely on the amount of HB present
- Think in percent- normal arterial blood contains 20% O2. This means 20 mls of O2 in 100 mls of blood.
- The higher the HB percentage, naturally the higher the O2 carrying capacity of the blood is and vice versa
- At rest, fully saturated HB molecule unloads only 25% of O2, during stress/ exercise- up to 70%
Oxygen Hemoglobin Dissociation Curve
- To combine with HB, O2 must diffuse from the plasma into the RBC (millions of HB molecules are in the RBC)
- The higher the PO2 in the blood- acceleration of O2 being bound to HB
- The lower the PO2 in the blood- lowers the rate O2 is being bound to HB
O2 Dissociation Curve
- Describes the relationship between the PO2 (x axis) and the O2 saturation (y axis)
HB O2 affinity increases as more O2 binds
- This continues to move up until a max amounts is reached
- As this limit is reached, little to no more binding occurs, you will see the curve level out as all HB are saturated with O2
- This typically happens at pressures of PO2 >60 mmHg (this means that no matter how much you increase the PO2, the SPO2 will not arise any more past this level)
Factors that affect the curve
- The strength at which O2 binds to HB is affected by several factors
- This will alter or shift the shape of the curve
Rightward Shift
- Indicated the HB has a decreased affinity for O2 (doesn’t want it anymore)
- Means a higher PO2 would be required to reach the same O2 saturation of a healthy person
- Also means it is easier for the HB to release the O2 molecules
- When it needs oxygen
- Higher CO2, Lower pH, Higher temp
Why the right shift?
- Typically shifts this way during times O2is needed most- exercise, stress, shock
- C- CO2
- A- Acid
- D- DPG (factor that controls how easily/ difficult O2 is bound)
- E- Exercise
- T- Temp
Right- Bohr Effect
This curve shifts down (off) to the right when:
- high CO2
- low pH
- high temp
- high DPG (enzyme that helps to release O2) gets rid of oxygen (decreasing the affinity) releasing this enzyme
- Therefore more O2 is released to the tissues
Leftward shift
- HB has an increased affinity for O2
- Binds more easily, unloads more reluctantly
Left- Bohr Effect
The curve shifts up (over) to the left when:
- low CO2
- high pH
- low temp
- low DPG
- Therefore more O2 will be bound to HB
Why the left shift?
Patient presentations of a left shift could be:
- Carbon monoxide poisoning
- Hypothermia
- Cancers of the head and neck- due to smoking and alcohol
Temperture
High temp: right shift
Low temp: left shift
2,3-BPG
High 2,3-BPG: right shift
Low 2,3-BPG: left shift
PCO2
High PCO2: right shift
Low PCO2: left shift
Acidity [H+]
High acidity: right shift
Low acidity: left shift
Transport of CO2- Dissolved CO2
- small amount of CO2 dissolves in plasma and is transported as a solute (10% of the total volume is carried this way)
- this dissolved CO2 produces the PCO2 of blood plasma
Transport of CO2- Carbamino compounds
- 1/5 to 1/4 of CO2 in blood unites with NH2 amino acid chains
- When CO2 combines with these chains, it forms carbamino compounds
- CO2 combines with HB and creates carbaminohemoglobin
- The higher the PCO2 levels- accelerates this binding process
- The lower the PCO2 levels- slows this process
Biocarbonate
- More the 2/3 of the blood CO2 is carried in the form of bicarbonate ions (HCO3-)
How does the bicarbonate process work?
- CO2 dissolves in the plasma (water present), some molecules bind with the H2O to form carbonic acid (H2CO3)
- Some then dissociate to form H+ and Bicarbonate (HCO3-) ions
- The more CO2 that is present= higher levels of carbonic acid
- The higher levels of carbonic acid= pulls the system towards the bicarbonate ions, increasing the rate of bicarbonate formation
- This allows for more CO2 to dissolve in the plasma, increasing the CO2 carrying capacity of the blood
Bohr Effect/ Haldane Effect
- Reciprocal interrelationship between O2 and CO2 transport
Bohr Effect
- Increased PCO2 decreases the affinity between HB and O2- called a “right shift’ on the O2 HB dissociation curve
