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Benjamin S.
Medical Student, Tutor specializing in sciences and standardized testing
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MCAT
TutorMe
Question:

In the midst of a bacterial infection, a dendritic cell is presenting antigen to a T-cell to activate it. Is the dendritic cell using MHC class I or II? Is the T-cell a CD4+ or CD8+ cell?

Benjamin S.
Answer:

The dendritic cell will be using MHC class II to present to a CD4+ T helper cell. Remember that a bacterium is an external antigen (relative to a cell), and MHC class I is used to present internal threats, while class II presents external threats. Also, recall that CD4+ T cells are involved in activating humoral immunity, while CD8+ T cells, or cytotoxic T cells, are used in cell-mediated immunity.

Biochemistry
TutorMe
Question:

A patient presents to the ER after ingesting an entire bottle of aspirin (acetylsalicylic acid). On initial examination you notice that the patient is breathing rapidly. a. What is the direct cause of the rapid breathing? b. What type of acid-base disorder would you expect the overbreathing to cause? Why? c. When you check the patient's arterial blood gas, would you expect to find acidosis or alkalosis? Why?

Benjamin S.
Answer:

a. When ingested, the acetylsalicylic acid will disassociate into salicylate and acetic acid. Salicylate directly stimulates the respiratory control center of the brain, causing rapid breathing. b. Overbreathing causes respiratory alkalosis as too much CO2 is being expelled, causing a decrease in proton concentration in the blood. c. An arterial blood gas would reveal metabolic acidosis, due to the acetic acid formed from the disassociation of acetylsalicylic acid. This will become especially apparent when the salicylate-induced overbreathing ceases, as there will then be no compensation.

Biology
TutorMe
Question:

Beginning with the right atrium of the heart, describe how blood flows through the body, making sure to mention oxygenation, delivery to the systemic circuit, and return to the heart.

Benjamin S.
Answer:

Starting in the right atrium, blood will travel through the tricuspid valve into the right ventricle. It then is pushed through the pulmonary semilunar valve into the pulmonary artery, which will take it to the lungs where it is oxygenated. It will then travel via the pulmonary vein into the left atrium, through the bicuspid (mitral) valve, and into the left ventricle. From here, blood is pushed through the aortic semilunar valve into the aorta, which brings it to tissues throughout the body. The aorta gives off many arteries, each of which split multiple times, getting progressively smaller, all the way to the terminal branches known as capillaries. Here, the blood will drop off oxygen and pick up carbon dioxide, as well as exchange any other needed nutrients or waste products. The capillary then feeds into venules (small veins) which will progressively become larger as they join with other veins. This occurs until the largest veins are formed; the superior and inferior vena cavae, which each deliver blood into the right atrium.

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