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# 13.45: Urinary System

Difficulty Level: At Grade Created by: CK-12
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How is it determined what's waste and what's not?

Shown above is a major process of maintaining homeostasis. Getting rid of waste and excess water. Such a basic process is actually very complex. It involves an intricate exchange of material through the kidney.

### Urinary System

The kidneys are part of the urinary system, which is shown in Figure below. The main function of the urinary system is to filter waste products and excess water from the blood and excrete them from the body.

The kidneys are the chief organs of the urinary system.

#### Kidneys and Nephrons

The kidneys are a pair of bean-shaped organs just above the waist. A cross-section of a kidney is shown in Figure below. The function of the kidney is to filter blood and form urine. Urine is the liquid waste product of the body that is excreted by the urinary system. Nephrons are the structural and functional units of the kidneys. A single kidney may have more than a million nephrons!

Each kidney is supplied by a renal artery and renal vein.

#### Filtering Blood and Forming Urine

As shown in Figure below, each nephron is like a tiny filtering plant. It filters blood and forms urine in the following steps:

1. Blood enters the kidney through the renal artery, which branches into capillaries. When blood passes through capillaries of the glomerulus of a nephron, blood pressure forces some of the water and dissolved substances in the blood to cross the capillary walls into Bowman’s capsule.
2. The filtered substances pass to the renal tubule of the nephron. In the renal tubule, some of the filtered substances are reabsorbed and returned to the bloodstream. Other substances are secreted into the fluid.
3. The fluid passes to a collecting duct, which reabsorbs some of the water and returns it to the bloodstream. The fluid that remains in the collecting duct is urine.

The parts of a nephron and their functions are shown in this diagram.

#### Excretion of Urine

From the collecting ducts of the kidneys, urine enters the ureters, two muscular tubes that move the urine by peristalsis to the bladder (see Figure above). The bladder is a hollow, sac-like organ that stores urine. When the bladder is about half full, it sends a nerve impulse to a sphincter to relax and let urine flow out of the bladder and into the urethra. The urethra is a muscular tube that carries urine out of the body. Urine leaves the body through another sphincter in the process of urination. This sphincter and the process of urination are normally under conscious control.

### Summary

• The kidneys filter blood and form urine. They are part of the urinary system, which also includes the ureters, bladder, and urethra.
• Each kidney has more than a million nephrons, which are the structural and functional units of the kidney.
• Each nephron is like a tiny filtering plant.

### Practice I

Use these resources to answer the questions that follow.

• http://www.hippocampus.org/Biology \begin{align*}\rightarrow\end{align*} Biology for AP* \begin{align*}\rightarrow\end{align*} Search: The Human Kidney
1. Describe the following:
1. renal artery
2. renal vein
3. nephron
4. ureter
5. loop of the nephron
2. Describe the structure of the nephron.
3. Describe how the nephron filters blood.
4. How many nephrons are in a kidney?
1. How does the urinary system work?
2. What is urea? Where does urea come from?
3. What is the primary role of the kidneys?

### Review

1. Describe how nephrons filter blood and form urine.

2. State the functions of the ureters, bladder, and urethra.

### Notes/Highlights Having trouble? Report an issue.

Color Highlighted Text Notes

### Vocabulary Language: English Spanish

Hollow, sac-like organ that stores urine until it is excreted from the body.

Bowman’s capsule

A cup-like sac at the beginning of the nephron in the kidney; performs the first step in the filtration of blood to form urine.

glomerulus

A network of capillaries at the beginning of the nephron in the kidney; performs the first step of filtering blood.

nephron

Structural and functional unit of the kidney; filters blood and forms urine.

renal artery

The blood vessel that brings blood into the kidneys.

renal tubule

Part of a nephron where some of the filtered substances are reabsorbed and returned to the bloodstream.

ureter

Muscular, tube-like organ of the urinary system; moves urine by peristalsis from a kidney to the bladder.

urethra

Muscular, tube-like organ of the urinary system; carries urine out of the body from the bladder; in males, it also carries sperm out of the body.

urinary system

Organ system that includes the kidneys and is responsible for filtering waste products and excess water from the blood and excreting them from the body.

urination

Process in which urine leaves the body through a sphincter at the end of the urethra.

urine

Liquid waste product of the body; formed by the kidneys and excreted by the other organs of the urinary system.

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