topdogg82 said:
counter negative 
Kaplan FL 1 question 138 (1st one in BS):
Question:
The llama is a warm blooded mammal that lives in regions of unusually high altitudes, and has evolved a special type of hemoglobin that adapts it to such an existence. If curve B represents the O2 dissociation curve for a horse, then which curve represents the llama?
Answer:
A (curve that is left-shifted)
Explanation:
The key to understanding this question is that at high latitudes, the atmospheric pressure is low, meaning that there is less oxygen than at sea level. Since the partial pressure of oxygen is lower at high altitudes where the llama lives, the llama hemoglobin must be able to bind hemoglobin MORE readily at low partial pressures of oxygen. In terms of Figure 1, this means that the llama's oxygen dissociation curve will be to the left of the horses.
To be honest the altitude thing has me confused, b/c I could see either way working. The bottom line is that you need more oxygen, so initially you want hemoglobin to bind more strongly to the oxygen present since the partial pressure is lower, but then later, you want its affinity to be less so that its easier to distribute that same oxygen to the tissues.
Let me start by saying that my explanation will be more background than you need for the MCAT, but it helps you understand the information you DO need to know.
Okay, here's what's going on. In the above question, they are saying that the llama has a different type of hemoglobin, one with a naturally higher affinity for oxygen. That means the llama curve would have a leftward shift compared to a horse curve. Having a different type of hemoglobin is different than a human physiological adaptation to altitude -- we must work with the hemoglobin we already have.
In red blood cells, 2,3-BPG is a side product of glycolysis, produced by an enzyme called bisphosphoglycerate mutase. Take a look here:
http://www.med.unibs.it/~marchesi/23bpg.html
Because the blood is slightly hypoxic in high altitude, glycolysis backs up and more 2,3-BPG is produced (2,3-DPG is the older name for the same molecule -- BPG stands for bisphosphoglycerate).
Hemoglobin has 2 conformations -- oxygenated, or R (for relaxed) and deoxygenated, or T (for tight). Now the thing about 2,3-BPG is that it fits right in the middle of the hemoglobin tetramer, making it more likely to be in the T conformation. This is good in hypoxic situations because the oxygen is more readily dumped from the blood into the potentially oxygen-starved tissues. Actually, if you strip all the 2,3-BPG from blood, it has a hyperbolic curve almost identical to the myoglobin curve.
The trick to remembering what happens to the hemoglobin curve is thinking about it in terms of continuing to supply tissues with oxygen. If there's less oxygen in the blood, it helps to make it easier to dump into the tissue. By lowering the affinity of hemoglobin for oxygen, you're releasing it to the tissues more easily.
The other side of the story is fetal hemoglobin. The fetus can't breathe, obviously, but it must get oxygen. In the placental vessels, oxygen is exchanged between maternal and fetal blood, but it doesn't just go there because mommy is nice. Fetal hemoglobin is different from adult hemoglobin, and it has a lower affinity for 2,3-BPG. Now remember -- at a lower affinity for 2,3-BPG, the fetal hemoglobin has a higher affinity for oxygen, so it can strip oxygen from the maternal blood. Sure, the baby is slightly more susceptible to hypoxia, but lungless beggars can't be choosers.
