Renal function: water balance

Published on March 31, 2024   20 min
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Hello, and welcome back to the Fundamentals of Human Physiology. My name is Jessica Briffa, and today we'll be going through the last lecture in the Kidney Function series, which is on water balance.
Before we delve into today's lecture content, let's first start with a stop-and-think question to get you thinking about the lecture content. What do you think a negative water balance will normally result in? Similarly to a negative sodium balance, a negative water balance means that water output is greater than water input. This means that a person that has a negative water balance will excrete more water, resulting in dilute urine. That is, unless there's another underlying condition, such as vomiting or diarrhea.
As depicted here, we have two separate forms of water intake, which is by the food and beverages we consume, as well as water that's generated from normal metabolic processes. Water output occurs in two ways, via insensible and sensible manners. Insensible manners include water loss via the respiratory system and skin, which is so called because we are consciously not aware that it's occurring. Whereas sensible manners include urine and feces production, which is so called because we are aware of the water loss via these processes.
Now that we've covered what water balance is and what a negative water balance means, let's consider the following question. What do you think a negative water balance will be most obvious as? As previously mentioned, a negative water balance can be due to passing large volumes of dilate urine, as well as due to vomiting and diarrhea. First, I'll go through what happens in severe water loss, and then I'll go through what the correct answer is. When water loss exceeds water intake, blood volume decreases and plasma osmolarity increases. This decrease in blood volume causes a reduction in blood pressure, leading to increases in renin and angiotensin II secretion. The latter of which, along with aldosterone, promotes sodium and chloride reabsorption in the kidneys, and thus water reabsorption, which ultimately decreases urinary output. Increased blood osmolarity and angiotensin II also stimulates the hypothalamus and vasopressin is released, which promotes renal water retention and reduced urinary output. Increased plasma osmolarity also stimulates thirst through osmoreceptors to replace water loss. This tells us that the most obvious sign of a negative water balance would be a reduction in plasma volume. However, if this is a significant amount of water loss, it's possible that cardiac venous return may also be reduced.