Kidney Control

Kidney Control 💧

Every day your body has to dump toxic waste and keep its water level just right — too little water and cells shrivel, too much and they burst. Both jobs fall to the **kidneys**. (This whole topic is **Biology-only**, and the ADH and kidney-failure detail is **Higher tier**.)

Where water goes 🚰

You lose water constantly through three routes: • the **lungs** — water vapour in the air you breathe out • the **skin** — sweat (which also loses ions, and is **not** controlled) • the **kidneys** — as **urine**, the **only** route the body can adjust Urine carries away excess **water**, **ions** and the waste product **urea**. Balancing water means controlling what the kidneys let go.

Making urea 🧪

Your body can't store excess protein or amino acids. So when there are too many amino acids: 1. They are broken down (**deaminated**) in the **liver**, producing **ammonia**. 2. Ammonia is **toxic**, so the liver immediately converts it into **urea**. 3. Urea travels in the blood to the **kidneys**, which remove it in the urine.

Where is urea made?

A common exam trap: in which organ is urea actually produced?

• The liver, by deaminating excess amino acids
• The kidneys
• The pancreas
• The bladder

How the kidney makes urine 🩸

The kidney cleans blood in two stages: 1. **Filtration** — blood is filtered under pressure. Small molecules (water, glucose, ions, urea) pass out; big things (proteins, blood cells) stay in the blood. 2. **Selective reabsorption** — the kidney takes back everything the body needs: **all** the **glucose**, plus the **ions** and **water** it requires. What is left — **urea**, and excess water and ions — leaves as urine.

Label the blood flow

An interactive activity.

Filter, then reabsorb

The kidney first _____ the blood, then reabsorbs all of the _____. The waste product _____ is left behind to leave the body as urine.

The water dial: ADH 🔄 [Higher tier]

The amount of water reabsorbed is tuned by a hormone: **ADH** (anti-diuretic hormone), released by the **pituitary gland**, controlled by **negative feedback**. • Blood **too concentrated** (not enough water) → **more ADH** → kidney tubules reabsorb **more water** → a **small** volume of **concentrated** urine. • Blood **too dilute** (too much water) → **less ADH** → **less water** reabsorbed → a **large** volume of **dilute** urine.

Run the water loop [Higher tier]

An interactive activity.

The opposite case [Higher tier]

Now you drink a litre of water quickly, so your blood becomes too dilute. What happens?

• Less ADH is released, so you make a large volume of dilute urine
• More ADH is released, so you make concentrated urine
• ADH stays exactly the same
• The kidneys stop making urine

When kidneys fail 🎓 [Higher tier]

If the kidneys fail, waste builds up in the blood. Two treatments: • **Dialysis** — a machine filters the blood against a special fluid so urea diffuses out while glucose and useful ions stay. Effective, but must be repeated regularly for life. • **Transplant** — a donor kidney frees the patient from dialysis, but donors are scarce and the body may **reject** it (needing immunosuppressant drugs). Grade-9 recap: **urea** is made in the **liver** (deamination); the kidney **filters** then **selectively reabsorbs** (all glucose + needed water/ions); **ADH** tunes water by **negative feedback**.