The topic of Mike Hawke and Ruth advocating the consumption of each other's urine came up in another forum and I decided to really explore from a human physiology perspective the advantages and/or consequences of self-urine consumption from a water balance perspective. I did a few simple reference calculations to test my assumptions and came up with a somewhat different perspective than I had before. I thought this information would be worth while sharing with the community especially since we have a number of medical and technical types. BTW, my background is that I have a PhD in wildlife toxicology and contaminant movement in the environment. However, I did teach university level human physiology classes for a couple of years and a lot of my personal research pertains to physiological controls affecting chemical exposure (fish, amphibians and birds).
Its wordy, but I hope some of you enjoy this little essay.
....In earlier posts I had publically stated my disagreement with the practice of urine drinking in survival shows such as Man Vs. Wild and more recently advocated in Man, Woman, Wild. I thought it might be fun to re-evaluate this one from a human physiology perspective and do another set of model calculations to test some of my assumptions about the implications of drinking concentrated urine to body water balance. The results sort of surprised me and is made me reconsider parts of my earlier stance on the subject.
The kidneys are the organs responsible for maintaining osmoregulation (or in other words the appropriate salt concentration in blood). Blood has a normal salt concentration of 280 mOsM. The unit here refers to milli-osmomoles per liter, such that 1 mole of salt (NaCl) contributes 2 osmomoles based on number of ions dissolved in the solvent. Blood salt concentrations and blood pH have to be kept in a very narrow range in order for you to survive. Changes in the blood salt concentration outside of the normal physiological levels cause problems such as interfering with neuron signalling (indirectly related to potassium balance) which can lead to heart failure. You also get movement of water between intra-cellular fluids and blood that plays havoc with cell membrane function, i.e. bad news.
When you over hydrate, your kidneys secrete dilute urine (as low as 50 mOsM or 5 times more diluted then your blood). When you dehydrate, your kidneys secrete concentrated urine up to a maximum concentration of 1200 mOsM or about 4 fold more concentrated than your blood. This is the body`s attempt to maintain the same salt content of the blood while working with an overall lower volume of blood in the body.
During dehydration, you are loosing much of your water to evaporation and sweat. The normal evaporation rate from lungs and across skin is about 1 L per day but this can crank up as high as 1 L per hour in high, dry heat present in the desert. Stop sweating and you overheat. Enzymes which do the chemical work of your body don`t function if the body temperature is raised beyond 108 degree F (lethal body temperature). Sweat out too much water and your blood volume becomes so low that you loose blood pressure and the brain gets oxygen starved from a lack of ability of your heart to pump adequate amounts of blood through your arteries. Life is tough, thank god your body is a wonderful machine. But like any machine it has its limits.
M. Hawke`s rational in the show was that they did not pee yet that day. If they were well hydrated in the early part of the day when most of the urine was being processed by the kidney, then their urine should be either diluted (if they were hyper-hydrated) relative to blood or about the same concentration of blood. After the concentration of waste fluid is adjusted in the kidney, the urine is passed into the bladder. Once the urine is in the bladder it is effectively isolated from the rest of the body. The body has no capacity to resorb salts or water from the bladder unless you drink the urine after urinating. So Mike`s thinking is that if the urine held in his bladder was dilute or around isomolar with blood he could drink it and effectively add that water back to his body. If the urine is isomolar with blood, the volume of urine consumed becomes equal to the effective amount of blood added to the body. Alternatively, if the urine consumed is more concentrated then blood, then the kidneys have to excrete the extra salt. In order to excrete that extra salt, the kidneys also have to shunt some body water to the bladder, causing a reduction in the net intake of fluid into your blood relative to what was consumed.
(You should have heard MC Hammer's voice reading the bold subheading
. Lets say Mike produced 300 mL of urine at a concentration of 1000 mOsM (rather concentrated). This constitutes 0.3 L * 1000 mOsM = 300 mOsmoles of salt being added to the body and 300 mL of water. On the one hand its good that he added 300 mL of water to his body. However, he also added excess salt. Considering the osmolarity of blood, the equivalent volume of blood would have 0.3 L * 280 mOsM = 84 mOsmoles of salt. Therefore, he added 216 mOsmoles (300 mosmoles -84 mOsmoles) extra salt to his blood that his kidneys need to excrete. If his kidneys were to crank up the osmolarity of the new urine created to the max possible after drinking the pee, it would require a minimum of 180 mL of body water be lost to his bladder. (That is 216 mOsMoles divided by 1200 mOsM x 1000). Thus, his net intake of blood fluids was 120 mL instead of the 300 mL of liquid actually consumed. If the urine consumed was at 1200 mOsM x 300 mL, the body would need to excrete 230 mL of water back to urine and the net influx of water to his body in this case is only 70 mL. So, I have to essentially reverse my earlier stance.
Drinking your urine will give you a net gain of body fluids although it might be small compared to your total body needs. I did take a quick look at urea toxicity, the chief waste product lost to blood, and it seems doubtful this will be a problem (in the short term) although over several weeks to months can result in chronic kidney failure, blood acidosis and other problems. However, I think this is not a survival concern given the show scenario and 1 week time frame. It would be a concern if you got into a habit of drinking concentrated urine (German pornstars - beware!).
A contrasting and illustrative example would be to run the above calculations with sea water instead of urine. Sea water has an osmolarity of 2000 -2400 mOsM which surpasses the maxim concentrating ability of the kidney to produce urine. So, if you drank 300 mL of seawater at the max range of its osmolarity (2400 mOsM), then your kidney would have to excrete the excess salt using 530 mL of water. So in the case you are actually losing body water by drinking sea water. This is a sure way to screw yourself up very quickly.
Mind you, you do have to place the urine drinking strategy in the context of the body`s requirement to replace water, particularly the water being lost to your breath and sweat. Considering a daily loss of 900 mL per day under limited perspiration to as much as 1400 mL per hour under intense dry desert heat. Total daily urine production will be between 0.3 - 1.5 L day depending on how hydrated you are, with the lower mark being what Mike and Ruth were faced. So, in other words they are losing well over 1-2L (or much more!) per day due to sweat and perspiration. They can only recover between 70-180 mL of effective body fluid by consuming all the pee they produce. Short of licking the sweat off of Ruth`s body (which I`m sure many would volunteer to do), you are still in a major deficit when it comes to the rate of blood volume and blood pressure loss relative to your ability to resupply body water by recycling your urine. The loss to perspiration is simply way too high!
In the end, you absolutely have to find a new water source. Drinking your own pee might give you an extra hour of life (saying a loss of 2L per day divided by 24 h = 166 mL lost per hour) or much less if your perspiration rate is higher than 2 L per day. Put into context then, yep, you can drink your pee, but its not going to save your butt for that much longer than not having drunk your pee. Is it worth the effort to give you 10 minutes more life?
I'd be interested in hearing other's view on the subject or even some critiques of my logic.