Day 21 of 30
Excretory/urinary system
Lessons are AI-drafted and being reviewed and improved over time.
Learning Objective
Describe how the kidneys filter blood to make urine, trace the path of urine out of the body, and explain how the kidneys help keep water balance.
As your cells work, they make waste, such as carbon dioxide and urea. Urea forms when the body breaks down proteins. Excretion is the removal of these metabolic wastes. This is different from removing undigested food from the large intestine. Several organs excrete: the lungs remove carbon dioxide, the skin removes some water and salts in sweat, and the urinary system filters the blood.
The urinary system includes two kidneys, two ureters, the bladder, and the urethra. The kidneys are bean-shaped organs about the size of a fist, located toward the back, just under the ribs. Each kidney contains about a million tiny filters called nephrons. Blood flows through a nephron in a ball of capillaries, where water, salts, sugar, and wastes are pushed out of the blood into a tiny tube. Blood cells and large proteins stay behind. Then the tube returns almost everything useful, such as all the sugar and most of the water, back to the blood, while urea and extra salts stay in the tube. What is left is urine. Your kidneys filter around 150 liters or more of fluid each day, yet you make only about 1 to 2 liters of urine, because most of the water is returned.
Urine travels down two tubes called ureters into the bladder, a stretchy, muscular bag that can hold roughly 400 to 500 milliliters. When it is full, nerves signal the brain, and urine leaves the body through the urethra.
The kidneys do more than make urine. They help keep the body's water and salt balance steady. When you are dehydrated, the pituitary gland releases a hormone that tells the kidneys to return more water to the blood, so you make less and darker urine. When you drink a lot, less of that hormone is released, and you make more and paler urine. The kidneys also help control blood pressure and make a hormone that tells red marrow to produce red blood cells. This is homeostasis in action.
Common mistakes: confusing excretion with elimination of feces, and thinking the kidneys make water. Kidneys do not create anything from nothing; they filter blood and decide what to keep. Next lesson: how the immune system defends the body.
The removal of metabolic wastes from the body.
An organ that filters the blood and makes urine.
A tiny filtering unit in the kidney; each kidney has about a million.
A waste made when the body breaks down proteins.
A tube that carries urine from the kidney to the bladder.
A stretchy muscular bag that stores urine.
Instructions
Materials Needed
- Kitchen strainer
- Coffee filter
- Two bowls
- Water
- Dried beans
- Sand
- Salt
What is excretion, and how is it different from eliminating feces?
Trace the path of urine from the kidney to the outside of the body.
Your kidneys filter around 150 liters a day, but you make only 1 to 2 liters of urine. Explain.
Explain how the kidneys respond when you are dehydrated.
Name two organs besides the kidneys that excrete wastes, and what each removes.
Teaching Tip
Students often assume the kidneys make water from scratch or that urine is simply filtered blood. Emphasize the second step, in which most of the filtered fluid is returned. Connect the hormone response to the feedback loops in Day 27.
Go Deeper
Research how a dialysis machine does the work of failing kidneys, or why drinking enough water matters for kidney health.
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