The Urinary System

Nephrons and Urine Formation

The functional and structural unit of a kidney is a nephron. One kidney contains more than one million nephrons. An illustration of these functional units is shown below:

The nephron consists of two parts: the renal corpuscle and the renal tubule. The renal corpuscle can be divided into the glomerulus and glomerular capsule (or Bowman’s capsule). The glomerulus is a type of capillary bed that functions as a filtration system, filtering solutes as blood enters the kidneys from the renal artery. Surrounding the glomerulus is Bowman’s capsule. The renal tubule is a duct that connects to the glomerulus and terminates at the tip of the medullary pyramid. This tubule is divided into the following four regions: (1) proximal convoluted tubule, (2) loop of Henle, (3) distal convoluted tubule, and (4) collecting duct.

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Keep In Mind

It is helpful to think of each nephron as a tiny filtering structure. Each nephron filters blood and forms urine. With more than one million nephrons in a single kidney, it is no wonder the kidneys are so efficient at filtering and excreting wastes from blood!

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The components that make up the nephron filter blood and form urine. The following steps outline the pathway for urine formation. The steps are divided into three processes:

1. Glomerular filtration:

   1. Blood enters the kidney though the renal artery.
   2. This artery branches off into capillaries, allowing blood to flow into the glomerulus of the nephron.
   3. Blood pressure forces water and solutes (smaller than proteins) to diffuse from blood across the capillary walls and through pores of Bowman’s capsule into the tubule.

2. Tubular reabsorption:

   1. The filtered fluid flows toward the proximal tubule. This is the major site of reabsorption of water and solutes such as glucose, amino acids, and certain ions.
   2. The fluid travels to the loop of Henle, which is another site of reabsorption.
   3. Next, the fluid reaches the distal convoluted tubule. Reabsorption and secretion take place in this segment.

3. Tubular secretion:

   1. In the final segment, the collecting duct, fluid that remains in the duct is called urine. Reabsorption of some water and its return to the bloodstream may happen at this segment.
   2. At this site, creatinine and other nitrogenous wastes are actively secreted into the urine so they can be excreted out of the body.

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Did You Know?

About 180 liters of blood pass through the nephrons of the kidney each day. This explains why much of this fluid and its contents must be reabsorbed.

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Urine Excretion and ADH

After blood is filtered through the nephron and the byproduct of urine is produced, urine accumulates in the collecting ducts of the nephron. Eventually, urine enters the ureters, which are muscular tubes. With help from muscle contractions, the ureters contract to move urine into the bladder. Urine is stored until the bladder is about half full.

Upon reaching this level, a neural impulse is transmitted telling a sphincter in the bladder to relax and allow urine to exit the bladder. Contraction of this sphincter, which is a muscular tube, is under involuntary control. Urine flows from the bladder into the urethra, which expels urine out of the body. A second sphincter enables urine to leave the body. This process is known as urination.

Be Careful!

The urethra in males and females are different sizes due to the reproductive anatomy. The male urethra is about 20 centimeters long. It passes through the length of the penis and terminates at the end of the penis, where urine is removed from the body. The female urethra is about four centimeters long.

ADH

Recall that the urinary system works closely with the cardiovascular system to filter blood and return important substances back to the bloodstream during tubular reabsorption. To help maintain water and solute concentration either excreted from or reabsorbed by the body, the urinary system works with hormones that are part of the endocrine system to regulate this process. One of these hormones is the antidiuretic hormone, also known as ADH. This hormone is secreted from the posterior pituitary gland, which is found at the base of the brain.

One of the most important functions of ADH is to regulate urine concentration and volume by controlling how much water is reabsorbed in the tubules of the nephrons. The following image shows how ADH controls urine formation.

As shown in the image, when ADH is present, there is an increased permeability of water at the distal convoluted tubule and collecting duct. This causes more water to be reabsorbed and retained. It also decreases the volume of urine produced and concentrates the urine. The opposite occurs when ADH is not actively communicating with the kidneys and regulating urine formation.

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Did You Know?

ADH control of urine formation is intimately connected to diabetes insipdus. When people have this disease, ADH does not communicate properly with the kidneys. As a result, symptoms include excessive thirst and frequent urination.

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Urinalysis

Medical professionals can determine diseases that affect the urinary system by conducting a urinalysis. This type of test can reveal disease that does not necessarily present observable symptoms. Diseases confirmed through urinalysis include diabetes mellitus, different types of glomerulonephritis, and urinary tract infections. Both macroscopic and microscopic urinalysis can be performed.

Macroscopic Urinalysis

• The first part of this testing involves visual observation of the urine. Normal fresh urine is pale to dark yellow in color. It is also clear and not cloudy. If the color is turbid or the urine is cloudy, there may be excess protein in the urine or the presence of a bacterial infection. Red or brown urine is considered abnormal. It could indicate that blood is present in the urine.
• A urine dipstick test is another type of macroscopic urinalysis. With this test, a plastic dipstick or paper strip is inserted into the urine sample. There are chemicals on the dipstick that cause it to change color when certain substances are present in the urine (at a specific concentration). Medical professionals can compare the color of the dipstick to a standard chart to analyze a urine sample.
• When the liquid is removed, this sediment is mounted to a microscope slide and analyzed using a microscope. Typically, this test is performed to look at blood cells in the urinary tract, bacteria, parasites, or even tumor cells. This test also helps confirm the diagnosis of various urinary problems like kidney disease, cancer, microbial infections, and liver disease.

Microscopic Urinalysis

• This type of urinalysis requires the use of a light microscope. Typically, a urine sample is spun down, or centrifuged, in a test tube. This causes a sediment consisting of red blood cells, fat cells, and other large particles to aggregate and separate from the liquid portion of the urine.

Let’s Review

• The urinary system eliminates wastes from the body, regulates blood and water levels, and secretes hormones that directly influence various physiological processes in the body.
• The circulatory and endocrine systems work with the urinary system to perform various functions.
• Nephrons are functional units and structures of the kidneys that play a large role in filtration, reabsorption, and secretion.
• After blood enters the kidneys through the renal artery, it is filtered in the glomerulus. Then, it travels through the proximal tubule, the loop of Henle, and the distal convoluted tubule before accumulating as urine in the collecting duct.
• The kidneys form urine as a byproduct, which travels through the ureter before being stored in the bladder and eventually excreted from the body via the urethra.
• Urinalysis is a method used to evaluate the quality of urine and help diagnose various urinary health problems.

Urinary System Flashcards