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Biology LibreTexts

19.3: Introduction to the Urinary System

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    22589
  • Sculpture Garden

    This interesting outdoor art installation can be viewed at The Hague in the Netherlands. It’s a colorful piece with an unusual subject. The wrinkled structures on each side of the sculpture represent the kidneys, and the striped structure in the distance represents the urinary bladder. The red and blue tubes are blood vessels, and the tan tubes are ureters. In short, the installation is a three-dimensional depiction of the human urinary system. Only one urinary system organ is not visible in the photo. Do you know what it is?

    Screen Shot 2019-03-31 at 9.51.41 PM.png
    Figure \(\PageIndex{1}\): Image used with permission (CC BY 2.0; FaceMePLS from The Hague, The Netherlands).

    The actual human urinary system, also known as the renal system, is shown in the drawing below. The system consists of the kidneys, ureters, bladder, and urethra, which is the only structure not visible in the sculpture above. The main function of the urinary system is to eliminate the waste products of metabolism from the body by forming and excreting urine. Between 1 and 2 liters of urine are normally produced every day in a healthy individual.

    Screen Shot 2019-03-31 at 9.54.19 PM.png
    Figure \(\PageIndex{2}\): The components of the urinary system include the two kidneys which lead to two tubes called ureters which lead to a holding structure called the bladder which leads to a tube called the urethra. The urinary system is the same in males and females, except the urethra is longer in males. Image used with permission (Public Domain; NCI ).
     

    Organs of the Urinary System

    The urinary system is all about urine. It includes organs that form urine and also those that transport, store, or excrete urine.

    Kidneys

    Urine is formed by the kidneys, which filter many substances out of blood, allow the blood to reabsorb needed materials, and use the remaining materials to form urine. The human body normally has two paired kidneys, although it is possible to get by quite well with just one kidney. As shown in the figure below, each kidney is well supplied with blood vessels by a major artery and vein. Blood to be filtered enters the kidney through the renal artery, and the filtered blood leaves the kidney through the renal vein. The kidney itself is wrapped in a fibrous capsule and consists of a thin outer layer called the cortex and a thicker inner layer called the medulla.

    Screen Shot 2019-03-31 at 9.55.09 PM.png
    Figure \(\PageIndex{3}\): The structure of the kidney is specialized to filter blood and form and collect urine. The image used with permission (Public Domain; NCI ).

    Blood is filtered and urine is formed by tiny functional units called nephrons. Each kidney contains at least a million nephrons, and each nephron spans the cortex and medulla layers of the kidney. After urine forms in the nephrons, it flows through a system of converging collecting ducts. The collecting ducts join together to form minor calyces, or chambers, which in turn join together to form major calyces (see the diagram above). Ultimately, the major calyces join the renal pelvis, which is the funnel-like end of the ureter where it enters the kidney.

    Ureters, Bladder, Urethra

    After urine forms in the kidneys, it is transported through the ureters (one per kidney) to the sac-like bladder, which stores the urine until urination. During urination, the urine is released from the bladder and transported by the urethra to be excreted outside the body through the external urethral opening.

    Functions of the Urinary System

    Waste products removed from the body with the formation and elimination of urine include many water-soluble metabolic products. The main waste products are urea, a by-product of protein catabolism, and uric acid, a by-product of nucleic acid catabolism. Excess water and mineral ions are also eliminated in urine.

    Besides the elimination of waste products such as these, the urinary system has several other vital functions. These include:

    • maintaining homeostasis of mineral ions in extracellular fluid. These ions are either excreted in urine or returned to the blood as needed to maintain the proper balance.
    • regulating acid-base balance in the body. For example, when pH is too low (blood is too acidic), the kidneys excrete less bicarbonate (which is basic) in the urine. When pH is too high (blood is too basic), the opposite occurs and more bicarbonate is excreted in the urine.
    • controlling the volume of extracellular fluids, including the blood, which helps maintain blood pressure. The kidneys control fluid volume and blood pressure by excreting more or less salt and water in urine.

    Control of the Urinary System

    The formation of urine must be closely regulated to maintain body-wide homeostasis. Several endocrine hormones help control this function of the urinary system, including antidiuretic hormone, parathyroid hormone, and aldosterone.

    • Antidiuretic hormone, also called vasopressin, is secreted by the hypothalamus. One of its main roles is conserving body water. It is released when the body is dehydrated and causes the kidneys to excrete less water in urine.
    • Parathyroid hormone is secreted by the parathyroid glands. It works to regulate the balance of mineral ions in the body through its effects on several organs, including the kidneys. Parathyroid hormone stimulates the kidneys to excrete less calcium and more phosphorus in the urine.
    • Aldosterone is secreted by the cortex of the adrenal glands, which rest atop the kidneys, as shown in the figure below. It plays a central role in regulating blood pressure through its effects on the kidneys. It causes the kidneys to excrete less sodium and water in urine.
    Screen Shot 2019-03-31 at 9.56.03 PM.png
    Figure \(\PageIndex{4}\): The adrenal glands are located on top of the kidneys. They secrete aldosterone into the bloodstream, which carries it to the kidneys. Image used wtih permission (Public Domain; Pearson Scott Foresman).

    Once urine forms, it is excreted from the body in the process of urination. This process is controlled by both the autonomic and the somatic nervous systems. As the bladder fills with urine, it causes the autonomic nervous system to signal a muscle in the bladder wall to contract and the sphincter between the bladder and urethra to relax and open. This forces urine out of the bladder and through the urethra. Another sphincter at the distal end of the urethra is under voluntary control. When it relaxes under the influence of the somatic nervous system, it allows urine to leave the body through the external urethral opening.

    Summary

    • The urinary system consists of the kidneys, ureters, bladder, and urethra. The main function of the urinary system is to eliminate the waste products of metabolism from the body by forming and excreting urine.
    • Urine is formed by the kidneys, which filter many substances out of blood, allow the blood to reabsorb needed materials, and use the remaining materials to form urine. Blood to be filtered enters the kidney through the renal artery, and filtered blood leaves the kidney through the renal vein.
    • Within each kidney, blood is filtered and urine is formed by tiny structural and functional units called nephrons, of which there are at least a million in each kidney.
    • After urine forms in the kidneys, it is transported through the ureters to the bladder. The bladder stores the urine until urination when urine is transported by the urethra to be excreted outside the body.
    • Besides the elimination of waste products such as urea, uric acid, excess water, and mineral ions, the urinary system has other vital functions. These include maintaining homeostasis of mineral ions in extracellular fluid, regulating acid-base balance in the blood, regulating the volume of extracellular fluids, and controlling blood pressure.
    • The formation of urine must be closely regulated to maintain body-wide homeostasis. Several endocrine hormones help control this function of the urinary system, including antidiuretic hormone from the hypothalamus, parathyroid hormone from the parathyroid glands, and aldosterone from the adrenal glands.
    • The process of urination is controlled by both the autonomic and the somatic nervous systems. The autonomic system causes the bladder to empty, but conscious relaxation of the sphincter at the distal end of the urethra allows urine to leave the body.

    Review

    1. What organs make up the urinary system?
    2. State the main function of the urinary system.
    3. What is the primary function of the kidneys?
    4. Describe how blood enters and leaves the kidneys.
    5. What are nephrons?
    6. What happens to urine after it forms in the kidneys?
    7. Identify the functions of the urinary system besides the elimination of waste products.
    8. How is the formation of urine regulated?
    9. Explain how the process of urination is controlled.
    10.  
      What function do the adrenal glands carry out that is related to urine?

      A. They form urine and transport it to the kidneys

      B. They excrete concentrated urine into the ureters

      C. They secrete a hormone that affects the composition of urine

      D. They store urine when water and salts need to be retained by the body

    11. Explain why it is important to have voluntary control over the sphincter at the end of the urethra.

    12. Compare aldosterone to antidiuretic hormone in terms of how they affect the kidneys.

    13. If your body needed to retain more calcium, which of the hormones described in this concept is the most likely to rise in level? Explain your reasoning.

    14. True or False. Urine is composed of water, urea, and minerals only.

    15. True or False. The renal artery contains blood that was filtered by the kidney. 

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