🎓 Prepared by students from Boğaziçi University

What Is the Excretory System?

The excretory system removes metabolic waste, excess water, and toxins from the body to keep its internal chemistry stable. The kidneys are its main organs, filtering blood continuously and producing urine as a byproduct. Every day, your kidneys filter about 180 liters of blood plasma to keep just the right amount of water, salts, and waste in your body.

Short answer

The excretory system, centered on the kidneys, filters blood to remove waste products like urea, excess salts, and water, forming urine that is passed out via the ureters, bladder, and urethra.

How the Kidney Makes Urine (Nephron Function)

1
Blood enters glomerulus
Blood flows into a knot of capillaries inside the nephron under high pressure.
↓
2
Filtration
Water, salts, glucose, urea, and small molecules are pushed out of the blood into the Bowman's capsule; blood cells and proteins stay in the blood.
↓
3
Reabsorption
As the filtrate flows through the tubule, useful substances like glucose, water, and salts are reabsorbed back into the blood.
↓
4
Secretion
Extra waste ions and drugs are actively added from the blood into the tubule.
↓
5
Urine formation
What remains — water, urea, and salts — becomes urine, flowing into the collecting duct.
↓
6
Excretion
Urine travels through the ureter to the bladder, then leaves the body via the urethra.

Step-by-step worked examples

An adult filters about 180 liters of plasma per day but only excretes about 1.5 liters of urine. What happened to the rest?

1) 180 L of filtrate is produced by filtration in the glomeruli each day
2) About 99% of that filtrate — roughly 178.5 L — is reabsorbed back into the blood in the tubules (water, glucose, salts)
3) Only about 1% remains as waste-concentrated fluid
4) That remaining ~1.5 L becomes urine and is excreted.

A person is dehydrated. How do the kidneys respond?

1) Blood volume and water content drop, blood becomes more concentrated
2) The pituitary gland releases more antidiuretic hormone (ADH)
3) ADH makes the kidney tubules and collecting ducts reabsorb more water back into the blood
4) Less water is lost as urine, and the urine produced becomes darker and more concentrated.

Someone eats a very salty meal. How do the kidneys restore balance?

1) Blood salt (sodium) concentration rises above normal
2) The kidneys detect the excess sodium in the filtrate
3) Less sodium is reabsorbed in the tubules, so more stays in the filtrate
4) The extra sodium is excreted in the urine along with extra water, restoring normal blood salt levels.

Flashcards

Quick quiz

Q1.Where does blood filtration first occur in the kidney?

Correct answer: B. The glomerulus is the capillary network in the nephron where blood is first filtered.

Q2.What happens to glucose during normal kidney function?

Correct answer: B. In a healthy kidney, filtered glucose is completely reabsorbed in the tubule, so none appears in normal urine.

Q3.Which waste product is a major nitrogen-containing compound removed in urine?

Correct answer: B. Urea, produced from excess amino acids in the liver, is a key nitrogenous waste filtered out by the kidneys.

Q4.What hormone increases water reabsorption when the body is dehydrated?

Correct answer: C. ADH signals the kidney tubules to reabsorb more water, concentrating the urine and conserving body water.

📄Download this topic as a printable worksheet (PDF)Summary + 10 questions + answer key — print it, share it in class.↓

Study better with Bounlu apps

Notek

The full card deck, worked steps and AI-tutor support for “What Is the Excretory System?” are in Notek — study by hand before your exam.

Get it free

PromyNever miss an exam or homework deadline with Promy: AI turns the dates in your inbox into reminders automatically.

Common mistakes

✗

Thinking kidneys just remove water. — Correct: Kidneys remove metabolic wastes (like urea), excess salts, and water together while keeping useful substances in the blood.

✗

Believing all filtered fluid becomes urine. — Correct: About 99% of the filtrate is reabsorbed back into the blood; only ~1% becomes urine.

✗

Confusing the ureter with the urethra. — Correct: The ureter carries urine from kidney to bladder; the urethra carries it from the bladder out of the body.

✗

Assuming glucose normally appears in urine. — Correct: Healthy kidneys reabsorb all filtered glucose — glucose in urine can be a sign of diabetes.

FAQ

What is the excretory system?

It's the body system, centered on the kidneys, that filters blood and removes waste products, excess water, and salts as urine.

How do the kidneys filter blood (the 'formula' for urine formation)?

The process is: filtration in the glomerulus → reabsorption of useful substances → secretion of extra waste → urine formation and excretion.

What are examples of what the excretory system removes?

Urea from protein breakdown, excess salts like sodium, extra water, and some drugs or toxins.

How is urine amount calculated or regulated?

It's regulated by hormones like ADH: more ADH means more water reabsorbed and less, more concentrated urine; less ADH means more dilute urine.