When it comes to pH, your body likes to keep a tight control of the balance between acidity and alkalinity.
The normal range for pH in your body is between 7.35-7.45 so, very slightly alkaline. At times, this balance can be disrupted and I will be talking about what occurs.
Now, we must think about the ways you can control acidity. One is to remove/add acid, the other is to remove/add base.
So how can we do this? There are two places where we can do this — the lungs and the kidneys.
The lungs may seem like an strange place for controlling pH however, if we consider how CO2 is transported from the tissues (read more here), we see that CO2 dissociates into carbonic acid. Hence, the higher the CO2 levels in the tissues, the lower the pH gets (more acidic).
So, if we are experiencing an Acidosis (low pH), if we decrease our CO2, we can increase the pH. We do this by hyperventilating and blowing off our CO2 however, this is limited by the amount of CO2 we have in our bodies; once we have blown off all our CO2, there is no more that the lungs can do to help us compensate.
Conversely, if we experience an Alkylosis (high pH) our lungs can try to compensate by slowing down our breathing to increase our CO2 however, this can be dangerous because it can cause hypoxia (lack of oxygen). The benefit of respiratory compensation is that it happens very quickly (a few minutes) however, it has a very limited range of effectiveness.
The kidneys deal in acids and bases, they can excrete/retain H+ if needed and they also control the excretion/retention of bicarbonate (HCO3-). If you are acidotic, your kidneys will try to excrete H+ and retain HCO3-, if you are alkylotic, your kidneys will try to retain H+ and excrete HCO3-. The drawback of this is that it takes a few days to be effective but, it is a lot more powerful than respiratory compensation.
So how do we classify these imbalances? The easiest way is with an Arterial Blood Gas (Analysis of pH and gasses in an arterial blood sample).
Firstly we must deduce whether we have an Acidosis or an Alkylosis by measuring the pH. Once we have done this, we need to work out whether we have a respiratory (lungs) or a metabolic (kidney) cause and an easy way to remember this is ROME.
Respiratory = CO2 levels go Opposite to the pH, Metabolic = HCO3- Levels are Equal to the pH (go in the same direction).
Thirdly, we need to know whether the condition is compensated or uncompensated. This is whether the body is trying to normalise the imbalance or not.
Some examples:
Eg. 1: Low pH, High CO2 and Normal Bicarbonate – This is an acidosis, it is respiratory and it is not compensated (therefore, it must be acute/fast onset).
Eg. 2: Low-ish pH, High CO2 and High bicarbonate – This is a respiratory acidosis that is compensated. The pH is higher Eg. 1 because the kidneys are retaining bicarbonate and excreting H+ to try and bring the pH back to normal.
Eg. 3: High pH, Normal CO2 and High bicarbonate – This is an alkalosis, it is metabolic and it is uncompensated.
Eg. 4: High-ish pH, High CO2 and High bicarbonate - Compensated metabolic alkalosis. This person may not be breathing very fast.
Eg. 5: Low pH, Low CO2, High bicarbonate - Compensated Metabolic Acidosis. This person will be hyperventilating to try and blow off all their CO2.
Now, consider someone who has been losing acid in large amounts for several days say, from vomiting a lot (they lose acid from their stomach). This person’s pH would be high (they would be Alkalotic), they wouldn’t be breathing much because they are trying to increase the amount of acid that they have (Respiratory Compensation) and because it has been several days, their kidneys would be trying to excrete bicarbonate (it would be low) and retain H+ (Metabolic Compensation). Now, is this a metabolic or a respiratory cause? The lungs are not causing it so it is a Metabolic alkalosis.
A lot of these cases seem quite complex and may be confusing however, it is always good to break each part down in order to deduce the answer. 
In summary, pH can go up or down and this is caused by many factors however, when this does occur, the body has mechanisms in which it can attempt to restore balance.