Biology 20 · Excretion and movement
Filter everything, take back what you need
The kidney's design looks wasteful — it filters out glucose and salts it has no intention of losing, then reclaims them. Doing it that way means one filter can handle anything, and the fine control happens afterwards.
- 1. The words, first
- 2. What each part of the nephron does
- 3. Water and salt balance
- 4. Muscles and movement
- 5. What costs marks
The words, first
The idea: Filtration, reabsorption and secretion are three distinct processes happening in three parts of the nephron.
| Word | What it means |
|---|---|
| Nephron | The functional unit of the kidney; about a million per kidney, each doing the whole job. |
| Glomerulus | A knot of capillaries where blood is filtered by pressure. |
| Bowman's capsule | The cup that catches the filtrate. |
| Filtrate | What is forced out of the blood — water, glucose, salts, urea. Not proteins or blood cells, which are too large. |
| Reabsorption | Taking useful substances back out of the filtrate and into the blood. |
| Secretion | Actively adding substances from the blood into the filtrate — the fine-tuning step. |
| Loop of Henle | Builds a salt concentration gradient in the medulla, which is what makes concentrated urine possible. |
| Collecting duct | The final stretch, where ADH decides how much water is recovered. |
| ADH | Antidiuretic hormone. Makes the collecting duct permeable to water, so more is reabsorbed and urine is more concentrated. |
| Aldosterone | Increases sodium reabsorption; water follows by osmosis, raising blood volume and pressure. |
| Sarcomere | The repeating unit of a muscle fibre, between two Z-lines. |
| Sliding filament model | Actin is pulled past myosin. Neither filament changes length; the overlap increases. |
| Antagonistic pair | Two muscles with opposite effects. Necessary because muscles can only pull. |
What each part of the nephron does
The idea: Filter indiscriminately, then reclaim selectively. The order is what makes the fine control possible.
Filtration. Blood pressure in the glomerulus forces water, glucose, amino acids, salts and urea into the capsule. Proteins and blood cells are too large. Finding protein in urine therefore points to damage at the glomerulus itself.
Proximal tubule. Essentially all the glucose and amino acids are reclaimed by active transport, along with most of the water and salt. Glucose only reaches the urine when blood levels are so high that these transporters are saturated — which is why it is a sign of diabetes.
Loop of Henle. Sets up a salty medulla. Nothing pumps water directly; the loop makes the surroundings concentrated so that water can later be drawn out by osmosis.
Distal tubule and collecting duct. Fine-tuning by secretion, and then the ADH-controlled water recovery as the duct passes back through the salty medulla.
Water and salt balance
The idea: Two hormones, two routes to the same end. ADH moves water directly; aldosterone moves salt and lets water follow.
ADH. Released when the blood becomes too concentrated. It makes the collecting duct permeable to water, so more is reabsorbed: less urine, and more concentrated urine. Alcohol suppresses ADH, which is why it dehydrates.
Aldosterone. Released when blood pressure or sodium is low. It increases sodium reabsorption in the distal tubule, and water follows by osmosis, raising blood volume and pressure.
Negative feedback. Both are corrective loops: a deviation is detected, a hormone is released, the deviation is reduced, and the signal to release stops. The same logic runs blood sugar and temperature, and it is the central idea of Biology 30.
Why the liver matters here. Excess amino acids are deaminated in the liver, producing toxic ammonia, which the liver immediately converts to the far safer urea. The kidney removes urea; it does not make it.
Muscles and movement
The idea: Muscles only pull, which is why they come in opposing pairs — and a fibre shortens without any filament shortening.
Sliding filament model. Myosin heads bind to actin, pivot, release and rebind, walking the actin inwards. The filaments keep their own length; the sarcomere gets shorter because the overlap grows.
What it needs. Calcium ions to expose the binding sites on actin, and ATP both to power the pivot and to release the head afterwards. Without ATP the heads stay bound, which is why rigor mortis happens.
Antagonistic pairs. The biceps bends the elbow and the triceps straightens it. Neither can push, so the other has to undo the movement.
Muscle types. Skeletal is striated and voluntary; cardiac is striated, involuntary and self-exciting; smooth is unstriated and involuntary, and it is what performs peristalsis.
What costs marks
The idea: Two organ mix-ups and two mechanisms.
- Saying the kidney makes urea. The liver does.
- Saying the loop of Henle pumps water. It pumps salt; water follows by osmosis.
- Saying filtration is selective. It sorts by size only. Selection happens in reabsorption.
- Saying muscles push. They pull, which is why antagonistic pairs exist.