I re-watched one of my all-time favorite movies the other night: Unforgiven. After William Munney (Clint Eastwood) shoots his first victim, the camera zooms in on the fallen cowboy as he begins complaining about how thirsty he is, begging his companions for water. In a moment of compassion, Munney agrees to put down his gun to allow the cowboy’s friends to bring him a canteen.
You’ve probably all seen a similar scene before in another movie, if not this one (hopefully you’ve never seen it in person). Victims of gunshot wounds, or other wounds that involve a drastic loss of blood, are often portrayed as being very thirsty. I’m not sure if the reason why this occurs is common knowledge, but in case it’s not, I thought I would write a quick explanation.
First, a little about water in the body. The cells in our body not only contain water, but also are surrounded by what is called interstitial fluid. This fluid bathes the cells in a “seawater” type solution that contains water, sodium (Na), amino acids, sugars, neurotransmitters, hormones, etc. The cell is normally in an isotonic, or balanced, state in relation to its extracellular environment, meaning water doesn’t generally leave or enter the cell at large rates.
Water is also an important constituent of blood. It is essential for keeping blood volume at a level that allows for proper functioning of the heart. If volume gets too low, the atria of the heart don’t fill completely, and the heart cannot pump properly.
The need to keep the fluid balance in the body at a regular level results in the occurrence of two types of thirst that affect us when that equilibrium is disturbed: osmometric thirst and volumetric thirst. Osmometric thirst occurs when the osmotic balance between the amount of water in the cells and the water outside the cells becomes disturbed. This is what happens when we eat salty pretzels. The Na is absorbed into the blood plasma, which disrupts the osmotic balance between the blood plasma and the interstitial fluid. This draws water out of the interstitial fluid and into the plasma, now upsetting the balance between the cells and the interstitial fluid. The result is water leaving the cells to restore the balance.
The disruption in the interstitial solution is recognized by neurons called osmoreceptors, located in the region of the anterior hypothalamus. They send signals that cause us to drink more water, in order to restore the osmotic balance between the cells and the surrounding fluid. In the case of pretzel eating, if we don’t drink more water, eventually the excess Na is simply excreted by the kidneys.
Now, to the graver situation of a gunshot wound, and the other type of thirst: volumetric. Volumetric refers to the volume of the blood plasma, which is highly dependent upon water content of the body. As mentioned above, maintaining an adequate blood plasma volume is essential to proper functioning of the heart. If it gets too low, the heart can’t pump effectively.
When someone is injured and loses a lot of blood volume (known as hypovolaemia), less blood reaches the kidneys. This causes the kidneys to secrete an enzyme called renin, which enters the blood and catalyzes a hormone called angiotensinogen to convert it into a hormone called angiotensin. One form of angiotensin (angiotensin II) causes the pituitary gland and adrenal cortex to secrete hormones that prompt the kidneys to conserve water as a protective measure. Angiotensin II also affects the subfornical organ (SFO), an organ that lies just outside the blood-brain barrier. Through the SFO angiotensin II stimulates thirst.
There are also receptors in the heart that recognize decreases in blood plasma. Known as atrial baroreceptors, they detect reductions in blood plasma volume and subsequently stimulate thirst by signaling neurons in the medulla. So, when someone is shot and losing a lot of blood, it is because of the decrease in blood plasma volume that brain regions are stimulated through both of the above pathways to stimulate thirst.
Processes that stimulate thirst are really much more complicated than this brief explanation. But, I thought this was enough to give a general idea of why salty foods and gunshot wounds have similar effects on our desire to drink water.