4.9.5Additional lessons
Production of Urine — IB Biology HL Revision Notes & Topical Questions
5 min read 5 parts Lesson 83 of 101
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Section 1 of 5Structure of the Kidney and Nephron
Cortex
- Cortex The outer region, which contains the glomeruli, Bowman's capsules and convoluted tubules.
Medulla
- Medulla The inner region, which contains the loops of Henle and collecting ducts.
Blood supply
- Blood supply The renal artery brings blood to the kidney,
- and an afferent arteriole enters each glomerulus while a narrower efferent arteriole leaves it.
Nephron
- Nephron Each of the million or so nephrons is a tubule that runs from Bowman's capsule through the proximal tubule, loop of Henle and distal tubule to a collecting duct.
Section 2 of 5Stages of Urine Formation
1
Ultrafiltration: high blood pressure in the glomerulus, kept high because the efferent arteriole is narrower than the afferent, forces small molecules through into Bowman's capsule.
2
The filtration barrier of fenestrated capillary endothelium, the basement membrane and podocytes holds back blood cells and large proteins.
3
Selective reabsorption: in the proximal convoluted tubule all glucose, most amino acids, and most water and salts are reabsorbed into the blood.
4
The loop of Henle creates a region of high solute concentration in the medulla, so water can leave the filtrate by osmosis.
5
In the distal tubule and collecting duct, the final adjustment of water and salt reabsorption is made under the control of ADH.
Section 3 of 5Adaptations and Mechanisms
Adaptations and Mechanisms
Proximal tubule
- Proximal tubule Cells have microvilli to give a large surface area, many mitochondria for active transport,
- and co-transport proteins that take up glucose with sodium ions.
Descending limb
- Descending limb It is permeable to water, so water leaves the filtrate by osmosis into the concentrated medulla.
Ascending limb
- Ascending limb It is impermeable to water,
- and sodium and chloride ions are actively pumped out into the medulla, building up the high solute concentration.
Collecting duct
- Collecting duct As the duct passes through the medulla, water leaves by osmosis, giving urine more concentrated than blood when ADH is high.
| Part of nephron | Main function |
|---|---|
| Glomerulus and Bowman's capsule | Ultrafiltration |
| Proximal tubule | Selective reabsorption |
| Loop of Henle | Creates a salt gradient in the medulla |
| Collecting duct | Water reabsorption controlled by ADH |
Section 4 of 5Worked Example
Question
- In one person the glomeruli filter 180 dm³ of fluid each day, but only 1.5 dm³ of urine is produced.
- Calculate the percentage of the filtrate that is reabsorbed.
- Volume reabsorbed = filtrate − urine = 180 − 1.5 = 178.5 dm³.
- Percentage = 178.5 ÷ 180 × 100.
- = 99.17%, which is 99.2% to three significant figures.
Answer About 99.2% of the filtrate is reabsorbed, which shows how efficiently the nephron conserves water.
Section 5 of 5Exam Checklist & Tips
4.9.5 – Key Points for the Exam
- Ultrafiltration Filtration under pressure through the glomerular capillary wall and basement membrane.
- Glomerular filtrate The fluid in Bowman's capsule, with the same composition as plasma but no large proteins.
- Selective reabsorption Return of useful substances from the filtrate to the blood.
- Loop of Henle The part of the nephron that builds the solute gradient in the medulla.
Common Mistakes
- Saying that protein is found in normal filtrate; large proteins stay in the blood.
- Saying that water is actively transported; water moves by osmosis and solutes may be actively transported.
- Mixing up the afferent arteriole (wider, into the glomerulus) and the efferent arteriole (narrower, out of it).
- Believing that most reabsorption happens in the collecting duct, when most occurs in the proximal tubule.
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