Water, Sodium, Potassium and Chlorine

Now we're onto our final topic for BIOC3004, which deals with micronutrients (as opposed to macronutrients like carbohydrates, fats and proteins).

Water

Understand that water is the key molecule of life

Hopefully you should know that water is important- if you don't, then try going without it for a long period of time and see how well you fare then. (Alternatively, don't. If you do, I take no responsibility for your idiocy.) Water makes up most of our body- around 60% for males and 55% for females (females tend to have a larger proportion of fat). Around two-thirds of this is intracellular fluid (i.e. fluid in the cells), and the remaining third is located in the extracellular fluid (i.e. fluid outside of the cells).

Know the basic interactions of water

Water molecules, as mentioned here, are capable of forming hydrogen bonds between them. This is because the oxygen attracts electrons more strongly than the two hydrogens, making one side of water relatively negative compared to the other side. (You can also say that water has a "dipole moment," which is a fancy way of saying that one end of the molecule has the opposite charge to the other. Okay, that's probably an oversimplification, but good enough for now.) The side that is more negative is attracted to the slightly positive side of another water molecule, forming a hydrogen bond.

Know that water is a tetrahedral molecule

Chances are, you'll have learned somewhere that water is "bent." That's because when you draw the water molecule on paper, it just looks like a bent V-shape with free electrons adorning the top of the oxygen atom, like so:

In reality, however, water is a bit more complex than that. Obviously, water does not just exist in 2D: it exists in 3D. In 3D, the electrons can "spread out" to the front and back, ultimately resulting in a tetrahedral shape (in which the O is at the centre and the two Hs and two electron pairs form the corners of the tetrahedron). This tetrahedral arrangement exists not just within single molecules, but also within many water molecules bonded together. A single water molecule can form four bonds: the two Hs can bond with Os from other water molecules, and the lone pairs can bond with Hs from other water molecules. Hence, water molecules in ice also form a tetrahedral shape.

Know the requirements for water and disorders

As mentioned earlier, water is very important. As well as making up a large amount of our bodies, water is able to interact with ions and many important molecules, such as glucose, alcohols, organic acids such as lactate and pyruvate, and so on. Many reactions also produce or consume water. Chances are, there are far more uses for water than what I've explained here.

Water requirements can vary depending on diet, climate and level of physical activity. In general, we need about 1L of water for every 1kcal of energy expended. Pregnant women need even more water to maintain the amniotic fluid, and after birth they still need lots of water to keep lactation going. Infants also need more water (around 1.5L water/1kcal expended) for two main reasons: firstly, they have a lot of growing to do, and secondly, they have a large surface area to volume ratio, resulting in a relatively large loss of water through their skin (to my understanding, anyway).

We lose plenty of water every day. Every day, around 1400mL is lost through urine, and another 1250mL is lost through sweat, breathing and so on. If water loss is increased, such as in certain disease states, we could run into a lot of trouble. Losing water causes us to lose blood plasma, which in turn decreases the oxygen-carrying capacity of our blood. Aside from this, loss of water can cause us to overheat.

An example of a disease that causes water loss is cholera. Cholera still kills over 100 000 people worldwide every year, especially in places where access to clean water is an issue.

Potassium, Sodium and Chlorine

Be able to characterise types of ion pumps with examples

Potassium, sodium and chlorine move around the body a lot, as you may have gathered if you have read my posts on electrophysiology or renal physiology. Ion pumps provide one method of transport for these ions.

There are three main types of ion pumps: uniporters, synporters and antiporters. Uniporters transport one ion in one direction, just like the voltage-gated ion channels described here. Synporters transport multiple ions in the same direction, just like the Na⁺/glucose and Na⁺/amino acid cotransporters mentioned here. Just in case someone asks at a trivia night, SGLT1 is the name of one of the Na⁺/glucose synporters in the intestines. Finally, antiporters transport multiple ions but at least one is transported in the opposite direction to the others. The example in the lecture is the Na⁺/H⁺ antiporter, which regulates sodium levels and pH, but perhaps a more familiar example is the Na⁺/K⁺ ATPase.

Know the daily requirements for the ions and disorders related to the ions, particularly hypertension

The daily requirement for sodium is around 0.5-2.4g/day, less if it's a cool day and/or you're inactive, and more if you're lactating. Insufficient sodium is generally not an issue though: usually people take in too much sodium! Sodium can be found in many foods, from cereals, to meats, to dairy and a range of processed foods. High sodium levels, particularly combined with low potassium levels, can lead to hypertension (high blood pressure). However, sodium levels that are too low (hyponatremia) can result in headaches, seizures and comas. Diuretics, extreme fluid loss or being one of those rare cases who eats a really low sodium diet can all cause hyponatremia.

The daily requirement for potassium is around 2g per day for children, 2.8g for women (3.8g for lactating women) and 3.8g for men. Potassium can be found in green leafy vegetables, tea, onions, sundried tomatoes and some spices. Low levels of potassium (hypokalemia) can result in weakness, muscle cramps, arrhythmias or even paralysis. The causes of hypokalemia are similar to the causes for hyponatremia: diuretics and extreme fluid loss.

The slides didn't list a daily requirement for chloride. Chloride is plentiful in the stomach, as it makes up hydrochloric acid which is obviously pretty important for digestion. In cystic fibrosis, the chloride transporter (CTFR) is defective, reducing flow of chloride and, by extension, flow of fluids (since water goes wherever ions and other dissolved particles go), which in turn leads to the drying out of mucus in places. This can lead to lung problems, infertility and a range of other problems.

Finally, just a quick note on how sodium, potassium and chloride ions are usually regulated in the body. The kidney does the main work in regulating Na⁺/K⁺ balance, and Cl^- just follows wherever Na⁺ goes. Na⁺ can be regulated through hormones such as aldosterone, which increases the reabsorption of Na⁺ (see here for a more thorough description on the RAAS pathway, which aldosterone is part of). K⁺ is not as well regulated as Na⁺, possibly because it is generally less secreted.