Excretion

The human excretory system consists of the kidneys-- paired structures that remove nitrogenous wastes as well as excess salts and water from the blood, forming urine.

Urine, formed in the kidneys, is moved down a pair of muscular tubes, the ureters to a holding tank, the urinary bladder. Discharge of urine from the urinary bladder is under voluntary nervous control. Urine passes from the bladder via the urethra to the outside world.

The functional unit of the kidney is the renal tubule or nephron. There are millions of nephrons in each kidney and each produces a minute amount of urine.

The nephron wall consists of a single layer of epithelial cells surrounding a central, hollow lumen. Wrapped around each nephron is a system of blood vessels which allow the exchange of materials back and forth between blood vessels and lumen of the nephron.

Blood enters the kidney through the renal artery, which branches to form smaller vessels, each of which eventually leads to a bundle of capillaries called a glomerulus.

Each glomerulus is embedded in a blind, cup-like region of the nephron referred to as Bowman's capsule.

From Bowman's capsule, the nephron begins a winding path, first as the proximal convoluted tubule, then into a long U-shaped portion called the loop of Henle and then into the distal convoluted tubule and finally into a collecting duct which drains into the ureter.

Urine formation occurs as the result of three distinct processes: glomerular filtration, tubular resorption and tubular secretion.

The capillaries of the glomerulus are particularly porous and the blood in these vessels is present at unusually high pressures. During glomerular filtration, the pressure of blood in the glomerulus pushes fluid out of the capillaries across the epithelial wall of Bowman's capsule and into the lumen of the nephron.

Proteins and cells are too large to pass through the pores in the walls of the glomerular capillaries but a lot of valuable nutrients, water and wastes are forced out the capillaries and into the nephron.

Now, the kidney is a super purification plant. About 900 liters of blood pass through the kidneys each day. About 20% of this is forced out of the glomerular capillaries and into nephrons. This is a lot more fluid that any normal adult passes as urine each day. The average urine output is between 1 and 2 liters per day.

So, what happens? About 99% of what is forced out into the nephron is reabsorbed as the fluid moves in the nephron. Most of the dissolved materials are also reabsorbed.

Where glomerular filtration is nonselective, tubular reabsorption is extremely selective. The strategy is to push everything into the nephron and then selectively reabsorb salts, sugar and water that the body cannot afford to lose.

This tubular reabsorption is accomplished by transport systems embedded in the cell membranes of the epithelial cells that transport glucose, salts and other valuable nutrients back into the blood stream.

In the distal convoluted tubule, the compositiion of urine is modified by tubular secretion-- various substances are secreted back into the nephron. These include urea and urobilin (a product derived from hemoglobin breakdown which gives urine its yellow color). Certain ions such as potassium are also excreted. In this region of the kidney, the body also maintains tissue pH to some extent by secreting more or less hydrogen ions.

Urine formation is maintained by both nervous and endocrine mechanisms. Two hormones-- aldosterone and antidiuretic hormone-- that act on the distal parts of the nephron are particularly important.

For example, if you were to eat a particularly salty bit of potato chips, the increased level of sodium in your blood stream would trigger a decrease in the amount of aldosterone by your adrenal cortex leading to a decreased reabsorption of sodium by the cells of the tubule and increased concentration of salt in the urine.

The opposite occurs also-- a drop in blood sodium stimulates an increase in aldosterone production which increases the reabsorption of sodium from the tubule.

A person suffering from untreated diseases of the adrenal cortex may produce little or no aldosterone and excrete large quantities of both salt and water.

The volume of blood is determined by its water content. Blood volume decreases when too little water is reabsorbed from the nephrons back into the blood stream. This causes the blood to become too concentrated.

Receptors in the hypothalamus detect the increase in the blood's osmotic strength and direct the posterior pituitary to release antidiuretic hormone. Antidiuretic hormone increases the permeability of the distal portion of the nephron allowing more water to be reabsorbed from the nephron back into the blood stream which dilutes the blood and concentrates the urine.

Alcohol interferes with the production of antidiuretic hormone which is why beer drinking leads to frequent trips to the bathroom.

The urine that enters the ureter has a maximum salt concentration of about 2.2% compared with that of blood which has a salt concentration of about 0.9%. So, the human kidney is capable of producing hypertonic urine but cannot concentrate salts to greater than about 2.2%.

This explains, in part, why humans cannot drink sea water, which is about 3.5% salt. In order to remove this salt from the body, water from the body tissues would have to move into the nephron to dilute the sea water salt concentration from 3.5% to about 2.2%. Consequently, drinking sea water results in a net loss of water. Human kidneys are just not designed for life in the sea.

Damage to the kidneys can result from various chemical that might be ingested (including drugs that a physician might prescribe). Bacterial infections can give rise to nephritis, an inflammation of the nephrons. High blood pressure damages the glomerular capillaries. Kidney stones are also a common problem. Passing a kidney stone is supposed to be among the most painful of things that we humans suffer.

When kidneys fail, one can be given dialysis, a procedure where blood is moved through a system that attempts to mirror what a kidney is able to do. While effective, dialysis is not as good as a functioning kidney.

Kidney transplants are among the most successful of organ transplants. The socialized health care system in Great Britain denies kidney transplants to anyone over the age of 50. We were close to a similar system in the US. What do you think of this?

What do you think about how medical treatment is paid for in the US?

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