Friday, August 15, 2008

Part Two of A Case Study in Interconnectedness: Acidosis, global climate change, and a strong urge to pee

II. The Answers to Hyperventilation, hypertension and a strong urge to pee

First, why am I experiencing these physiological effects? I'm doing it to myself. Why am I doing it to myself? That would be a reasonable question. I like to look at pretty things under water. To see the petty things while snorkeling, I have to hold my breath and dive (freedive). Sometimes the pretty things are pretty deep. Sometimes I want to look at them for a while. Holding one's breath while in cool water at depth creates a series of interesting responses due to a) the mammalian dive reflex, b) hydrostatic pressure on the body, c) breath holding and d) our response to cold. Let's look at each of these.

a) The Mammalian Dive Reflex
Mammals share a common reflex when they hold their breath and dive underwater, especially cold water. In fact, simply placing someone's face in cool water while they hold their breath will initiate the reflex. During spring quarter one of my biology majors, Kacie, did her project on this reflex with a few volunteers. She found, as have others, that cold water is especially effective at initiating this reflex. The physiological variable that she was most able to measure was heart rate. What she found was that heart rate dropped (bradycardia) when her subjects held their breath (apneic episode) while placing their face in water. She also found that cold water created the most dramatic response. Many researchers have studied this and recorded similar results. However, other effects in this reflex include widespread peripheral vasoconstriction (narrowing of blood vessels in the skin and appendages) and a concomitant increase in blood pressure. It is thought that this reflex saves oxygen for the brain and vital organs.

b) Hydrostatic Pressure
When diving the pressure placed on the body increases one atmosphere (the total pressure that we feel at sea level from the atmosphere of the earth pressing down on us) for every 10 meters of depth. As you dive the water pressure around you increases. This increase in pressure will compress surface blood vessels in the skin and will cause pressure on tissues around the body to be fairly equal. When we are standing on the ground the pressures are not equal, gravity pushes down on the fluids in your body and you have to compensate for these effects. Without movement fluid will accumulate in your feet and ankles.

c) Breath Holding
During breath holding, our ability to take in oxygen and remove carbon dioxide from our blood is limited. We can continue to send blood to our lungs while we are holding our breath, but gas exchange rates decline. This brings up a very important chemical reaction. The formation of carbonic acid. As we hold our breath, CO2 levels in the blood increase and CO2 combines with water and forms carbonic acid. This carbonic acid disassociates leaving hydrogen ions, H+, and bicarbonate ions, HCO3-. The hydrogen ions determine pH. Thus, as we hold our breath H+ increases, causing pH to decrease (more acidic). It is this pH decrease that is most responsible for our desire to end the breath-hold and take a new breath.

d) Response to Cold
When exposed to cold air or water (especially water), a typical response is to constrict blood vessels to our periphery (skin, appendages, nose, and ears). This peripheral vasoconstriction will shunt blood to our core, decrease the temperature gradient between our surfaces and the air or water, and slow heat loss. Note: the opposite occurs when we are hot.

Now, let's use the above information to answer the questions from the case study.

1. How could peripheral vasoconstriction lead to a) the hypertension reported and b) the bradycardia reported?
a) The answer to this question has multiple parts. First, constricting blood vessels decreases their diameter which increases the friction (resistance) to blood flow. In fact, a small decrease in diameter leads to a HUGE increase in resistance to flow (the fourth power is involved here). Blood pressure is proportional to resistance, so as resistance goes up, so does blood pressure. Thus, the hypertension (in part).

b) First, bradycardia (slow heart rate) will occur during the freedive due to the mammalian dive reflex. This would occur even if you simply placed your face in cool water while holding your breath. However, there are additional reasons for the bradycardia. Constricting blood vessels in the skin and appendages will shove more blood toward the core and force more blood through the heart. The heart will stretch and contract harder in response to the stretch which will shove more blood into the arteries (it will also release a hormone -- see below) . The combination of more blood in the arteries, more resistance to blood flow and hard contractions will increase blood pressure. The increase will be sensed by stretch receptors known as baroreceptors. These receptors in the aorta and carotid arteries will create a reflex through both divisions of the autonomic nervous system, the sympathetic and parasympathetic divisions. Parasympathetic signals will increase through the Vagus nerve to the heart, which will slow heart rate. At the same time, sympathetic signals to the heart will decrease, slowing heart rate (sympathetic stimulation would increase heart rate).
2. Explain how my alkalosis develops.

Hyperventilation has a profound effect on blood CO2 levels. A brief period of hyperventilation prior to freediving is a common strategy for increasing breath-holding duration. This technique does come with risks, namely shallow-water blackout, blacking-out while returning to the surface due to low oxygen tension in the blood and a rapid decrease in blood pressure as blood returns to the p. Since CO2 and H+ levels due to the CO2 in the blood are most responsible for the urge to breathe, decreasing CO2 in the blood will delay the urge to breathe. Hyperventilation causes a decrease in blood CO2, which causes a decrease in blood H+. The decrease in H+ leads to an increase in blood pH, or alkalosis.

3. Explain what may be the cause of my slight acidosis.
The answer to this question is basically the opposite of the previous answer. Apneic episodes, breath-holding, is an example of HYPOventilation which causes blood CO2 levels to rise and blood H+ levels to rise (you can't get rid of the CO2 because you're not refreshing the air in your lungs). The increase in H+ concentration decreases pH causing acidosis. This really makes you want to breathe.
4. Using your answers from #1 and other information provided, explain why I'm making so much urine. Why might the urine be so dilute? (note: there are several mechanisms that you should come up with in answering this question)

This is a fun one. There are several reasons that I may be making more urine. I've actually pondered this just about as long as I can remember. As a child, I didn't understand why I had to pee so bad when swimming and why my urine was so dilute. I assumed that the water was being forced up my urethra and water from the pool or lake was moving into my bladder through this path. I also recall wondering if it was worse if I peed while swimming because it seemed like I quickly had to pee again. I assumed that water from the pool was displacing the water that I had peed out. I recall regularly peeing clear urine at the urinals of Fairmont Hot Springs after swimming and being amazed that I had to pee so much and so often (one possibility for the often thing, similar to why you have to pee so bad after your first urination "break the seal" when drinking alcohol, is that the internal urinary sphincter is slow to close and when you're making a lot of urine it is overwhelmed).

Back to the question. Recall that blood pressure increases during a dive (#1) due to peripheral vasoconstriction from hydrostatic pressure, the mammalian dive reflex, and cold. The increase in blood pressure and volume of blood in the chest (blood forced to the core) will stretch the heart and cause the heart to release Natriuretic Peptides. These hormones increase Na+ and H2O loss at the kidney, which will increase urine volume. In addition, an increase in BP will cause increase stretch in kindey arteries which will lead to a decrease in Renin production. That leads to a decrease in angiotensin II production (ultimately), which causes less Anti-diuretic Hormone and Aldosterone to be secreted. When these hormones decrease, the kidney reabsorbs less water and less sodium, leading to increased urine production and production of dilute urine. It is also possible that the increase in pressure at the kidney is causing an increase in glomerular filtration rate which could lead to higher urine output. Voila! Polyuria! More urine! That's why when I'm diving (or swimming to an extent) I have a lot of pee to pee.
The above answer does not explain the intense, immediate urinary urgency that I feel when I do a freedive. In fact, I haven't found a good explanation for this, but I believe that it is most associated with the apneic episodes (breath holding). When I was in high school I used to train myself in breath-holding. I don't know why, but I did. I would run in place while holding my breath. I would nearly pee myself in the process -- OK. Maybe a little pee was peed, I'm not sure. For some reason breath holding leads to an urge to urinate. In fact, it can be used to treat "shy bladder." If you have a shy bladder, you can hold your breath as you head toward the bathroom (empty your lungs about 75% first) and you should be ready to go. You can also wait until you are at the potty. After 30-40 seconds you're probably going to need to pee. Watch out for one thing, however. Your anal sphincters may relax too!

What does any of this have to do with global climate change? Study the answers to questions 2 and 3 and read part 3 when I get a chance to post it. If you made it this far, then I'm really impressed.

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