Urine formation is a very sophisticated process that takes place in the kidneys. This important process provides a mechanism for the body to get rid of metabolic wastes and toxins, which can be deadly if allowed to accumulate in the body.The kidneys have the very important responsibility for extracting toxins and other waste products from the blood.

Blood performs many crucial functions within the body and is largely responsible for sustaining health and life.

In addition to defending the body against diseases, blood is also responsible for transporting oxygen, hormones and other essential nutrients around the body.

It is, therefore, important that blood consistently has the correct chemical composition and is free of wastes and toxins, which can be dangerous to the body. Maintaining the correct volume and concentration of blood and removing wastes and toxins from it are essential functions of the kidneys. The kidneys are able to do this by converting unwanted blood plasma into urine and expelling it from the body.
Basically, the process of urine formation takes place in three (3) stages, as blood plasma flows through the nephrons.

Nephrons are microscopic tube-like structures in the kidneys that filter the blood and cause wastes to be removed. They are the most basic structures of the kidney's anatomy. Each kidney contains over one million nephrons.

As the blood plasma flows through the nephrons, its composition changes. Fluids comprising excess water, salt and metabolic waste products are extracted from the blood as it enters the Bowman's capsule in the nephrons.

This fluid in the in the Bowman's capsule is calledglomerular filtrate. It is similar to blood plasma except that it has almost no protein.As the fluid moves through the renal tubule it is called tubular fluid. It is different form glomerular filtrate because of substances which are removed and added by the tubule cells.Finally, when the fluid moves into the collecting tubule it is called urine.

The three (3) distinct stages of urine formation are referred to as: (1) glomerular filtration, (2)tubular reabsorption and secretion, and (3) water conservation.

Glomerular filtration is a process in which water and some other substances in the blood plasma pass from the capillaries of the glomerulus into the Bowman's capsule. Very small molecules can pass through the filtration membrane into the Bowman's capsule. This includes water, electrolytes, glucose, fatty acids, amino acids, nitrogenous wastes, and vitamins.

The second stage of urine formation is tubular reabsorption and secretion. This involves removal and addition of chemicals, after glomerular filtrate leaves the Bowman's capsule and enters the renal tubule. The Renal tubule is very long, which increases its absorptive surface area. It reabsorbs about 65% of the glomerular filtrate, while it removes some substances from the blood.

Tubular reabsorption is the process of reclaiming water and other substances from the tubular fluid (glomerular filtrate which passes from the Bowman's capsule to the renal tubule) and returning them to the blood.

Sodium reabsorption is the key to everything else. It creates the environment for water and other substances to be reabsorbed.
Glucose is transported along with sodium irons by carriers called sodium-glucose transport proteins. Normally all glucose in the tubular fluid is reabsorbed and there is none in the urine.
Water reabsorption is a significant function of the kidney. The amount of water reabsorption is continually regulated by hormones according to the body's state of hydration. The more hydrated the body, the less water is reabsorbed, and vice versa.
Nitrogenous wastes such as urea diffuses through the renal tubule with water. The kidneys remove about 50% of the urea in the blood thus keeping its concentration down to a safe level, but not completely clearing the blood of it. Almost all uric acid is first reabsorbed by the renal tubule but later parts of the nephron secretes it back into the tubular fluid.Creatinine is not reabsorbed at all. It is too large to diffuse through water channels in the plasma membrane, and there are no transport proteins for it. All creatinine filtered by the glomerulus is, therefore, excreted in the urine.
The third and final stage of urine formation is water conservation. The kidneys are not only responsible for eliminating metabolic wastes from the body but they also prevent excessive water loss, in doing so. This is very important in maintaining the body's fluid balance. Urine is made up mostly of water. It plays a significant role in the entire process of waste elimination. If, however, too much water is removed from the body, it results in dehydration, which could lead to other serious medical conditions.
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