This lesson introduces the anatomy of the urinary system and how it functions. This lesson also explores the role of other body systems, particularly the circulatory and endocrine systems, in aiding with urinary excretion, absorption, and filtration.
Inside the body, the kidney, ureters, bladder, and urethra make up the urinary system, which is also called the renal system. The ureters, bladder, and urethra comprise the urinary tract. This system has many functions, some of which are outlined below:
Several of these functions are performed with help from other body systems, specifically the cardiovascular and respiratory systems.
The following table outlines key characteristics of each organ that is labeled in the image.
Organ | Shape | Characteristics |
Kidney | Resembles beans, reddish-brown in color | The body has two kidneys, which excrete wastes in the urine out of the body. |
Ureters | Tubular | Send urine from the kidney to the bladder. |
Bladder | Pear (when emptied) | Stores urine until the body expels the fluid from the body. Has three openings: two for the ureters and one for the urethra. |
Urethra | Tubular | Site where urine from the urinary bladder travels to an external opening. Removes urine from the body. |
The primary organ of the urinary system is the kidney. Blood from the heart flows through the kidneys via the renal artery. As blood drains from the kidney, it exits through a series of veins, the most prominent of which is the renal vein. When urine is produced, it does not drain through the tubes through which blood flows. Rather, urine flows through two ureters before emptying into the urinary bladder. The following steps outline how the urinary system works:
Did You Know?
As a person ages, the kidneys and bladder change. This can affect functions such as bladder control and how well the kidneys filter blood. Kidney changes range from a decrease in kidney tissue to decreased filtration capacity. Bladder changes include decreased elasticity (which affects how much urine is stored) and weakened bladder muscles.
Check Your Knowledge
The kidneys do not make urine. They help regulate water balance, regulate levels of electrolytes such as sodium and potassium, and eliminate metabolic wastes. Urine is a byproduct of these functions.
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.
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!
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:
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.
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.
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.
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.
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.
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