Excretory/Urinary System

Background Information:

As animals perform their various metabolic processes, protein and nucleic acid, both of which contain nitrogen, are broken down. While some of the nitrogen is used to manufacture new nitrogen-containing molecules, much of it cannot be used for this purpose and must be disposed of as waste. Typically, the first nitrogen-containing molecule that forms is ammonia (NH3, which is very water-soluble, forming NH4OH, a strong base. In some way, this ammonia must be gotten rid of before it raises the pH of the body fluids. Because ammonia is so water-soluble, aquatic animals often can get rid of it just by diffusion into the surrounding water. That’s one reason why the water in your aquarium gets “bad” and needs to be changed, and why not changing the water could kill the fish. However, ammonia doesn’t readily go from body fluids into air, so terrestrial animals need other ways of getting rid of nitrogenous wastes.

Purine Structure
Structure of Purine
The two most common substances used by terrestrial animals to get rid of excess nitrogen are urea and uric acid. Many animal species that aren’t terribly concerned about water-loss, including humans, convert the ammonia to urea, which is water-soluble and excreted in a water-based solution. Other organisms such as birds, insects, or lizards, especially if they live in an arid area, must conserve water whenever possible, thus convert the NH3 to uric acid. Uric acid is not water-soluble, thus can be excreted with little, if any, water with it. This is the white goo in bird droppings. While the major portion of human nitrogenous waste is in the form of urea, humans typically excrete some uric acid, too. Uric acid is another kind of purine like the adenine and guanine in our DNA (structure to the right).

Gout is a disorder in which humans start to accumulate more than the usual amount of uric acid (caused by either the body manufacturing excess uric acid or the kidneys not excreting enough of it) and since it’s not water-soluble, it gets stored in the body, frequently in toe joints, causing pain and deformation of the joints involved as well as the formation of kidney stones. Traditionally, people who had gout were put on diets low in purines to try to help alleviate the condition, but according to the Merck Manual, now these people are doped up with drugs rather than given nutritional counseling: [“Drugs are so effective in lowering the serum urate concentration that rigid restrictions of the purine content of the diet usually is unnecessary.”]. Typically, gout is treated with colchicine, a deadly poison (see further notes below)! Caffeine and its relatives, theobromine (in cocoa), and theophylline (in tea) are classified as xanthines (a subgroup within the purines), thus it would make sense that people with gout should be counseled to avoid coffee, tea, and chocolate.

Some insects, notably blowfly larvae (larvae of those shiny green or blue flies) excrete their nitrogenous wastes as allantoin, another purine. Allantoin is known to be a “cell-proliferant,” thus is used to help wounds to heal. For hundreds of years, people have recognized that the presence of blowfly larvae in a gangrenous wound actually helped it to heal better. From about the turn of the century until the invention of a lot of synthetic drugs, blowfly larvae were raised aseptically, and used to treat severe wounds. With the increase in availability of chemicals after World War II, the use of blowfly larvae declined, but I’ve heard of several cases lately where, for some reason, this treatment was necessary and/or preferred over synthetic drugs. It has been found that the fly larvae only eat dead, gangrenous tissue, leaving the live, healthy tissue, and since their nitrogenous waste is allantoin, that stimulates the wound to heal, usually with less scaring. In this procedure, small, sterile larvae are introduced into the wound and, if needed, traded for other small ones when they get big.

We excrete nitrogenous wastes via our kidneys. Our kidneys are located on either side of the spine, just up under the bottom ribs. They are well supplied with blood via the renal artery and renal vein. Urine made in the kidney collects in the renal pelvis within the kidney, then flows down the ureter to the bladder where it is stored until voided. From the bladder, the urine flows to the outside via the urethra, (which in the male also serves as part of the reproductory tract).

Excretory System
Human Excretory System
(clipart edited from Corel Presentations 8)
The kidney is composed of an outer layer, the cortex, and an inner core, the medulla. The kidney consists of repeating units (tubules) called nephrons. The “tops” of the nephrons make up or are in the cortex, while their long tubule portions make up the medulla. To the right is a diagram of an individual nephron. Each nephron has a closely associated blood supply. Blood comes in at the glomerulus and transfers water and solutes to the nephron at Bowman’s capsule. In the proximal tubule, water and some “good” molecules are absorbed back into the body, while a few other, unwanted molecules/ions are added to the urine. Then, the filtrate goes down the loop of Henle (in the medulla) where more water is removed (back into the bloodstream) on the way “down”, but the “up” side is impervious to water. Some NaCl (salt) is removed from the filtrate at this point to adjust the amount in the fluid which surrounds the tubule. Capillaries wind around and exchange materials with the tubule. In the distal tubule, more water and some “good” solutes are removed from the urine, while some more unwanted molecules are put in. From there, the urine flows down a collecting duct which gathers urine from several nephrons. As the collecting duct goes back through the medulla, more water is removed from the urine. The collecting ducts eventually end up at the renal pelvis which collects the urine from all of them. The area where the collecting ducts enter the renal pelvis is a common area for formation of kidney stones, often giving them a “staghorn” shape.

Antidiuretic hormone (ADH) from the pituitary is one factor influencing urine production. ADH promotes water retention by the kidneys, and its secretion is regulated by a negative feedback loop involving blood water and salt balances. ADH helps the kidney tubules reabsorb water to concentrate the urine. When the blood water level is too high (when you’ve been drinking a lot of liquids), this acts as a negative feedback to inhibit the secretion of ADH so more water is released. Ethanol also inhibits secretion of ADH, so a person who consumes a lot of alcoholic beverages could excrete too much water (and maybe even become dehydrated). Many diuretics work by interfering with ADH production, thus increasing the volume of urine produced. These diuretic effects are one reason why a person drinking beer (alcohol) or coffee (caffeine) needs to urinate more frequently.

When a person’s kidneys cease functioning, due to illness or other causes, renal dialysis can be used on a short-term basis to filter the person’s blood. This is not a perfect process; it can’t do everything a person’s kidneys can. Typically a person is put on renal dialysis as a temporary measure to extend the person’s life until a kidney transplant can be found. While life-saving, this procedure is often very inconvenient and stressful for the person. It requires spending long periods of time, several days a week, hooked up to the dialysis machine: the person’s blood must actually pass into the dialysis machine so the wastes can be filtered out, and then the blood is returned to the person’s body. This, combined with symptoms caused by the renal failure (the inability of the person’s kidneys to function) often preclude working at a job to earn the money to pay for the treatment. People can get by with one kidney, and the closest tissue match for a kidney transplant is often a sibling. However, as one former student who was a kidney-transplant recipient pointed out, even kidney transplants don’t last “forever”. Besides the constant workings of the person’s immune system to reject this foreign tissue, whatever disease caused the problem in the first place will probably eventually also affect the transplanted kidney. Since the same donor can’t provide another new kidney, this may mean going back on dialysis and hoping a matching donor (accident victim) can be found before it’s too late.


Some Excretory System Diseases and Disorders:


More information on colchicine


References:

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Copyright © 1996 by J. Stein Carter. All rights reserved.
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