One immersion effect that almost all divers can relate to is, the appropriately named, P Phenomenon (Bookspan, J. 1995). We have all found ourselves 60 feet underwater and only ten minutes into our dive when the urge is upon us. While sometimes this may have a lot to do with the two liter bottle of soda we drank on the dive boat before gearing up, it is more often due to a string of physiological events that occur from being immersed in water. Let's explain.
First, you need to understand that veins (as opposed to arteries) are capacitance vessels. This means that they are able to stretch to accommodate increased volumes of blood. When we stand on land, a certain amount of blood pools in our legs due to gravity's pull on the entire column of blood. The blood in our legs "feels" the weight of all the blood above it. As we learned in "Gas Laws", this increase in volume causes a proportional increase in pressure. Thus, the pressure in our legs is higher than the pressure found in the vessels of our chest. This means that the transmural pressure (the pressure difference from inside to outside) is different for our chest and for our legs.
When we enter the water however, this effect is counteracted. Gravity still acts on our bodies, but the increased water pressure acts to cancel out the pressure differences between our thorax and our legs. When the pressure difference is counteracted, the volume of blood in our legs must decrease accordingly. This blood is then dispersed throughout the rest of the body. Our bodies have very intricate methods of regulating the volume of blood that exists. When the increased volume of blood due to this blood centralization enters the heart, it causes distension of the upper chambers called atria. These atria contain special cells that secrete a substance that will increase diuresis (urine output) to counteract the perceived increase in blood volume. This substance is a chemical called atrial natriuretic factor (ANF). In addition, the increased pressure in the atria, specifically the left atria, triggers a response by the baroreceptors that are located there. These send a response to our hypothalamus to decrease the production of antidiuretic hormone (ADH). This hormone, also known as vasopressin, acts to concentrate and decrease fluid volume output. By decreasing its production, this effect is lessened and we get increased fluid volume output. In other words, our bodies produce more urine.
One last chemical system we need to consider is the Renin-Angiotensin-Aldosterone System. This is an important system for counterbalancing the two chemical systems we just discussed, and assuring that we do not lose to much water and sodium. When our kidneys sense a reduction in blood flow due to a loss of fluid, they send out an enzyme called renin. This enzyme starts a chemical cascade that ultimately activates a chemical called angiotensin. Angiotensin causes our blood vessels to constrict, which in turn causes an increase in our blood pressure. In a very short time, this increased blood pressure stimulates the adrenal gland to secrete another chemical, aldosterone. Aldosterone signals the kidneys to save sodium and water. Typically immersion suppresses this system, but a decrease in fluid due to an increase in diuresis will stimulate the system to restrict loss.
In addition to the chemical systems that influence diuresis, there are some environmental factors and personal factors that can influence these effects. First, the temperature of the water can play a big part. When diving in cold water, the vessels in our limbs constrict in an effort to conserve heat. This has the effect of increasing the volume of blood in our thorax and consequently increases the volume of blood in the atria. Hence, cold water will increase the effects previously described and increase diuresis. Diving in warm water has the opposite effect. Because temperature is such an important factor, you can step into a cold shower and feel the effects without any immersion.
The type of water one is diving in also has a minor effect. Salt water is denser than fresh water. Thus, it will have a somewhat increased ability to decrease the transmural pressure and enhance the fluid equalization effect.
Another factor is the overall hydration of the person before entering the water. If the individual is not well hydrated, the body will work to decrease fluid loss. However, if the individual has taken in large quantities of fluid, the effects can be magnified.
Age and emotional state of the diver can also play minor roles. Older individuals (60+years) have a tendency to have increased diuresis as compared to their younger counterparts. The emotional state of the diver can also be a factor if the diver is feeling some type of emotional stress. This is common in new divers who are still a bit apprehensive about their skills. Emotional stress causes chemicals to be released (norepinephrine and epinephrine) which also signal the kidney to increase fluid output.
All of these various factors come together and collectively effect our body's system of fluid balance. Unfortunately, although the processes are understood, there is still little we can do to combat the final outcome of having to "go!"