Bicarbonate is easy to understand if you think about Le Chatelier's. Here's the thing:
H2O + CO2 <---> H2CO3 <---> HCO3- + H+
Okay so body cells release CO2 (remember metabolism is the oxidation of glucose to carbon dioxide and water). This CO2 is transferred to the blood and carried to the lungs in many ways. Some of it is just the gas dissolved in blood. Some is attached to hemoglobin (I believe roughly 10%), but much of it travels inside the red blood cell, where carbonic anhydrase catalyzes the first reaction, converting it to carbonic acid. As a weak acid, some of this will dissociate to bicarbonate ion. Bicarbonate ion is very soluble in the blood. When you get to the lungs, you void CO2. This pushes that reaction to the left, meaning more of the bicarbonate diffuses back into the red blood cells to create more CO2. This decrease in H+ causes an increase in pH. Conversely, the dissociation of carbonic acid as a result of too much CO2 pushing the reaction right causes a decrease in pH. The kidney regulates the amount of bicarbonate in the blood by either releasing more into the filtrate (to pass in urine) or retaining more. This happens in the DCT.
You just need to know the important ones to the best of my knowledge. I like the way EK describes what's secreted/reabsorbed in the PCT. In the PCT, we "reabsorb everything we don't want to lose" like amino acids, glucose, water, and some ions. Some things we want to lose are released there too, like drugs and toxins. For the DCT, you absolutely have to know that sodium reabsorption takes place as a consequence of aldosterone and vasopressin. As a general rule, K+ and H+ are usually secreted and not reabsorbed (usually - if not, it'd be explained in the passage) in the kidney. And water is reabsorbed everywhere but the ascending limb.
