See The Reasons You Urinate Frequently After Drinking Any Drink

Water gets easily absorbed by your small intestines, where it will enter your bloodcirculation making your bloodvolume bigger. Imagine this: you drink 500 ml water, now your blood will be your normal bloodvolume (5–6 liters depending on gender, size, fitness) plus the 500 ml water. This is an increase of roughly 10% and will make your heart have to pomp harder to keep your blood going round. This will also raise bloodpressure and lower osmolarity.

Osmolarity is the number of particels per volume. Just to get a feeling about it: normally this will be for example 100 particles per liter volume, now the volume is increased so it has lesser particles per volume. Like everything in nature → high concentration will go to low concentratie to establish equilibruim, so that the concentration is equal. To make the concentration equal, particles are flowing from cells to the blood. This is not what the body wants. Your body has a special system for this. Osmolarity is measured by a part in the brain: the hypothalamus.

There are two common ways in which the human body constantly shows osmoregulation in the body, this is through dehydration and waterlogging. In the process of dehydration the hypothalamus is sent a signal that the water level in the body are critically low, the hypothalamus then sends a signal to pituitary glands where the antidiuretic hormone is secreted for further use. This hormone is the sent to 
the kidney and causes it to reabsorb water from the body, where the net amount of antidiuretic hormone determines the amount of water absorbed by the kidneys. As the fluid in the body become concentrated it would be noticed in the colour of the urine passed by the individual, the kidneys however do not stop absorbing water until the pituitary glands send a signal to reduce the production of the hormone. When the reduction is sensed in the body, another form of homeostasis called the negative feedback system occurs to take the body back to the desired 
set point.

Waterlogging on the other hand is the exact opposite of dehydration, in this case, the hypothalamus is signaled the level of water in the bodily fluids is too high, a signal is then sent to the pituary gland, which stops the production and secretion of antidiuretic hormones making the kidney cease the reabsorption of water from the blood. The cells in the body become waterlogged causing them to burst increasing the concentration of the blood. The hypothalamus signals the pituiry gland to end to secretion of the hormone. ADH will also act on your kidneys, making your kidneys secrete more water into your urine, filling your urine bladder.

Drinking a large volume in one go, will increase your blood going round in your blood circulatie. Now your bloodpressure raises and your osmolarity lowers. Your hypothalamus will notice this and will make your kidneys secrete more watter → you will want to go to the toilet!!

Little fact: when you drink alcohol. You drink alot of beer/volume, but the alcohol will also blocks the activation of ADH. So your kidneys will not concentrate your urine. Now you have to go to the toilet often to get rid of all the extra water in your body. Unfortunately Making you having to go to the toilet often….