EXCRETION: Regulation of water and salt balance.

Hello, dear hivers. Kindly move along with me while I discuss the importance of kidney in excretion and maintaining water and salt balance. But, firstly I would like to explain some of the inconveniences of kidney disease.

The inconveniences of kidney disease.

If one of your kidneys were to stop working because of injury or disease, you could probably still lead a normal life. However, people whose kidneys both fail are faced with a crisis: water, urea, and potassium build up rapidly in their bodies. They may continue to pass some urine, but they can not get rid of all the waste produced by normal cell processes.

^{Human kidney. BruceBlaus, CC BY SA 3.0}

Most people suffering from kidney failure hope for a transplant, but there is a shortage of donors. Until a suitable organ becomes available, patients have to rely on dialysis to filter their blood and balance their fluid intake. Dialysis is uncomfortable and inconvenient. To reduce the time they need to spend in dialysis, kidney patients must stick to the strict Giovanetti diet. This comes as something of a shock as they must limit their fluid intake to only half a litre a day, about a quarter of the amount a human adult would normally drink. They must also control their protein intake to about 30 to 40 grams per day (for example, the amount of protein in a small egg).

But perhaps the biggest problem is the need to regulate potassium. This ion is a normal constituent of the body, but large amounts cause serious problems, including heart failure. Potassium-rich foods include citrus fruits, bananas, instant coffee, peanuts, treacle and – a big blow for many people – chocolate. In this strange diet, carbohydrates are not a priority. Few patients feel like eating anyway, and this new restricted diet makes the task of finding appetising food even more difficult.

So, how does the kidney control waste and water?

The human body consists mainly of water. A person weighing 65 kg is made up of about 40 litres of water, of which 28 litres is intracellular (inside cells). The rest, the extracellular fluid, is made up of nine to ten litres of tissue fluid and two to three litres of blood plasma. The kidneys play a major role in regulating the volume and composition of these body fluids. They excrete or conserve water and salt so that the volume and composition of the blood and body fluids remain more or less constant. This is a vital aspect of homeostasis.

The kidneys also ensure that waste products do not build up by filtering the blood. Waste is allowed to pass through and then out of the body, while important substances such as glucose are reabsorbed and conserved. The removal of metabolic waste from the blood is called excretion. Urine is the end-product of all processes that occur in the kidney. So, excretion can be explained as the removal from the body of chemical waste produced by the metabolic processes within cells and which would be toxic if allowed to accumulate. You should not confuse excretion with egestion or defecation, the removal of undigested food and other debris from the intestine.

Most excreted waste leaves the body in the urine, but some waste is also lost in sweat and in air that we breathe out. Sweat contains mainly salt and water while exhaled air contains carbon dioxide and water vapour. I will concentrate on nitrogenous excretion, the removal of waste compounds that contain nitrogen. The main nitrogenous compound excreted by humans is urea. This is made in the liver following the breakdown of excess amino acids. Enzymes in the liver remove the amine (NH₂) group from amino acids in the process of deamination. The ammonia formed is a highly toxic compound that must not be allowed to build up. It immediately enters a series of reactions called the ornithine cycle, which produce the relatively harmless compound urea:

2NH₃ + CO₂ → CO(NH₂)₂ + H₂O

Ammonia + carbon dioxide → urea + water

Urea is carried in the blood from the liver to the kidneys to be excreted. In the role of the kidneys in homeostasis; they selectively eliminate water and solutes, such as sodium, potassium and chloride ions, so that the water and solute balance of the body is kept at the correct level. The need to balance the solute concentration of body fluids is called osmotic regulation, or osmoregulation.

Osmoregulation is the maintenance of a constant solute concentration within the body. This is important: if we place animal cells in a hypertonic solution, they lose water by osmosis and shrivel. If we put them in a hypotonic solution, they gain water and may burst. Both situations are harmful to cells, and so it is important that we maintain the solute concentration of our body fluids within narrow limits. Fluids are often compared with respect to their solute concentrations. A solution that has a higher solute concentration than another is said to be hypertonic. A solution that has a lower solute concentration than another is hypotonic. Solutions that have exactly the same solute concentration are isotonic. Thus seawater is hypertonic to human blood plasma, because seawater has a much higher concentration of dissolved salts.

There are two types of kidney failure, also called renal failure. In many cases of chronic kidney failure, there is a gradual decline in kidney function. Acute kidney failure is more of a crisis in which all kidney function stops much more suddenly. There are several causes of acute kidney failure: sudden loss of large amounts of fluid (blood or tissue fluid), inadequate blood flow to the kidneys, bacterial infection in the kidneys, effect of toxins, and a blockage in the urinary tract caused, for example, by damage to the ureter. There are also three main treatments for kidney failure – haemodialysis, continuous ambulatory peritoneal dialysis (CAPD) and kidney transplants.

An overview of the human urinary system

The human urinary system is shown in the figure below. The kidneys lie at the back of the abdominal cavity, just below waist level, where they are protected to some extent by the spine and the lower part of the ribcage. If you place your hands on your hips, your thumbs show the position of your kidneys. Usually, the left kidney is slightly above the right. The kidneys receive blood from the two renal arteries that branch off the aorta. The kidneys receive the largest blood supply of any organ, per gram of tissue. About 1200 cm³ of blood flows to each of them every minute. Incoming blood must be at high pressure to ensure proper kidney function: they can’t filter blood effectively if the pressure drops. Blood leaves the kidneys in the renal veins.

^{Images showing the human trunk, with positions of the organs show, and kidneys seen at the vertebral level of T12 to L3. Mikael Häggström, Public Domain}

Urine made by the kidneys is pushed down muscular tubes, the ureters, by peristalsis (rhythmic muscular contractions). The ureters empty into the bladder, a muscular bag that stores urine until it is convenient to release it. The capacity of the human bladder varies from 400 to 700 cm³ or more. When it begins to get full, stretch receptors in the walls inform the brain of the urgency of the situation. Urination, or micturition, happens when the sphincter muscles relax, allowing urine to pass out of the body through the urethra.

Oral rehydration therapy

Most people will have experienced the unpleasant symptoms that accompany a bout of mild food poisoning due to a dodgy late-night kebab, an undercooked chicken leg at a barbecue, or those prawns that lurked in the fridge just a little too long. Vomiting is caused by bacterial toxins that irritate the gut lining. In the intestine, the frequency of peristalsis increases, and the contents move along the gut rather more rapidly than usual. This doesn’t give the large intestine enough time to absorb water from the waste and the result is diarrhoea. Fortunately, for most of us, the symptoms are short-lived.

However, both vomiting and diarrhoea can be deadly if severe and prolonged. Dysentery and cholera can rapidly lead to death by dehydration as they cause the body to lose fluid faster than it can be replaced. Every day the average person consumes about two to three litres of fluid in one form or another, and we also pour a huge volume of fluid (over eight litres) into our intestines in the form of digestive juices. Diarrhoea does not allow for the efficient reabsorption of these fluids. Also, vital ions such as sodium, potassium, and chloride, collectively known as electrolytes, are lost. If untreated, this loss can lead to muscle spasms, cramps, coma, and heart failure.

Oral rehydration therapy (ORT) can be used to treat dehydration. This does not involve expensive drugs, it is simply a mixture of glucose and salt in water. In cases where the patient cannot keep anything down, the rehydration solution can be given directly into the bloodstream via a drip. This simple treatment has saved millions of lives in places where dysentery and cholera are very common.

^{cholera patient is drinking oral rehydration solution (ORS) in order to counteract his cholera-induced dehydration. Centers for Disease Control and Prevention, Public Domain}

In this post, I’ve been able to explain some of the inconveniences that can arise from a damaged kidney, how kidney functions as waste and water control in the body, and finally on how to perform the oral rehydration therapy. So, I will be pausing here for now; but in my next post, I will discuss more on the structure and functions of the mammalian kidney and also on the role of the kidney in homeostasis.

Thank you for reading.

REFERENCES

• https://en.wikipedia.org/wiki/Excretion
• https://en.wikipedia.org/wiki/Kidney_failure
• Dialysis
• Kidney transplantation
• https://www2.estrellamountain.edu/faculty/farabee/biobk/BioBookEXCRET.html
• https://www.niddk.nih.gov/health-information/kidney-disease/kidneys-how-they-work
• Functions of the kidney
• https://www.hopkinsmedicine.org/health/wellness-and-prevention/anatomy-of-the-urinary-system
• Urinary system
• https://rehydrate.org/faq/how-to-prepare-ors.htm
• Oral rehydration therapy