Each kidney holds about one million tiny filters. Together they clean the blood, keep what the body needs and send the rest out as urine. The nephron does all of it in three steps.
Chapter 14 of the PECTAA Biology 12 book is the Human Urinary System. This note covers the nephron and urine formation (sections 14.1.1 to 14.1.3). It is written for FSc Part 2 (2nd year, Class 12) students in Lahore, Faisalabad, Sargodha, Rawalpindi, Multan, Gujranwala and the other Punjab boards. Gujrat district comes under the Gujranwala board. PECTAA writes the books. Your own board sets your paper.
What to know
The kidney. A kidney is a reddish brown, bean shaped organ. Internally it has the renal cortex (the outer portion, about a third of the kidney's mass) and the renal medulla (the inner portion, two-thirds of the kidney) with cone-shaped renal pyramids. The renal pelvis is a funnel-shaped space in the centre where urine is collected. Blood enters by the renal artery and leaves by the renal vein.
The nephron. It is the functional unit of the kidney. It has two main portions.
- Renal corpuscle. The glomerulus is a capillary network from an afferent renal arteriole. The Bowman's capsule is a cup-shaped, double-walled sac around the glomerulus.
- Renal tubule. It has three connected portions: the proximal convoluted tubule (highly coiled, dips into the medulla), the loop of Henle (a U-shaped hairpin with a descending and an ascending limb), and the distal convoluted tubule (highly coiled again).
The distal tubules of many nephrons drain into one collecting duct. Collecting ducts merge into papillary ducts, which discharge into the renal pelvis.
Two types of nephron. Cortical nephrons are about 85 percent of all nephrons. They lie mostly in the cortex and have short loops of Henle. Juxtamedullary nephrons start deep in the cortex near the medulla and have long loops of Henle that reach deep into the medulla.
Blood supply. The renal artery gives afferent arterioles. Each leads to the glomerulus. Unlike most capillary beds, the glomerular capillaries drain into an efferent arteriole. The efferent arteriole branches into peritubular capillaries around the proximal and distal tubules. In juxtamedullary nephrons the efferent arteriole also gives vasa recta, long straight capillaries beside the loops of Henle. The kidneys receive about 20 to 25 percent of the total cardiac output.
Step 1: glomerular filtration (ultrafiltration). The efferent arteriole is narrower than the afferent arteriole, so blood in the glomerular capillaries stays under high pressure. This pressure forces water, urea, glucose, amino acids, vitamins and salts (Na⁺, K⁺, Cl⁻) into the Bowman's capsule. About one-fifth (20 percent) of the fluid part of blood enters as glomerular filtrate. Plasma proteins such as albumin and globulins, and blood cells, are too large and stay in the blood.
Step 2: selective reabsorption. Useful materials move from the renal tubule back into the peritubular capillaries.
- Most reabsorption is in the proximal convoluted tubule. About 75 percent of the water returns by osmosis. Glucose, amino acids and salts (sodium, potassium, calcium) return by active transport.
- The descending limb of the loop of Henle reabsorbs more water.
- The ascending limb is impermeable to water but actively moves salts out.
- The distal tubule takes back some water and salts.
- The collecting duct is the final site of water reabsorption.
Step 3: tubular secretion. This is the opposite of reabsorption. Epithelial cells of the distal convoluted tubule take materials from the surrounding blood and secrete them into the filtrate. These include hydrogen ions, ammonium, potassium ions, and toxins or drug residues such as penicillin. Secreting hydrogen ions helps to keep the acid-base balance (pH) of the blood.
Leaving the body. Collecting ducts, renal pelvis, ureter, urinary bladder, then urethra. A normal adult passes 1 to 2 L of urine a day.
Key points
| Step | Where | What happens |
|---|---|---|
| Glomerular filtration | Glomerulus to Bowman's capsule | High pressure forces out water and small solutes; proteins and cells stay |
| Selective reabsorption | Tubule to peritubular capillaries | Useful materials return; collecting duct is the final water site |
| Tubular secretion | Distal tubule | H⁺, NH₄⁺, K⁺, toxins and drug residues enter the filtrate |
The kidneys also act as osmoregulatory organs. They form dilute urine when body fluids are hypotonic and concentrated urine when body fluids are hypertonic and there is dehydration.
Worked examples
Example 1. Why does the glomerulus work at high pressure?
- The efferent arteriole is narrower than the afferent arteriole.
- Blood leaves more slowly than it arrives.
- So the pressure in the glomerular capillaries stays relatively high.
Example 2. Name the parts of the nephron in order from the Bowman's capsule to the collecting duct.
- Proximal convoluted tubule.
- Loop of Henle: descending limb, then ascending limb.
- Distal convoluted tubule, which drains into the collecting duct.
Example 3. Suppose 100 mL of filtrate reaches the proximal tubule. How much water is left after the 75 percent reabsorption? (Worked from the book value.)
- Water reabsorbed = 75 percent of 100 mL = 75 mL.
- Left in the tubule = 100 - 75 = 25 mL.
- The collecting duct can take back more.
Example 4. Why is no albumin found in the glomerular filtrate?
- Plasma proteins such as albumin and globulins are too large.
- They cannot pass through the capillary walls.
- They stay in the blood.
Common mistakes in the exam
- Calling the glomerulus a tubule. It is a capillary network. The tubule is the renal tubule.
- Mixing the two arterioles. Blood goes in by the afferent and out by the efferent arteriole.
- Saying all of the blood is filtered. About one-fifth (20 percent) of the fluid part of blood is filtered.
- Mixing reabsorption and secretion. Reabsorption returns materials to the blood. Secretion adds materials to the filtrate.
- Wrong limb. The ascending limb is impermeable to water. The descending limb takes back water.
- Leaving out the cortical and juxtamedullary types. Give the 85 percent value and the loop lengths.
Exam technique
- Draw the nephron first and label the corpuscle, tubule parts and collecting duct.
- Write the three steps as numbered headings, then add one location and one value for each.
- Use the exact terms: ultrafiltration, glomerular filtrate, peritubular capillaries.
- Check your board's notice for the sections in your paper.
Practice questions
- Name the two parts of the renal corpuscle. Answer: The glomerulus and the Bowman's capsule.
- Where does most reabsorption take place? Answer: In the proximal convoluted tubule, where about 75 percent of the water returns by osmosis.
- Name two substances secreted into the filtrate. Answer: Any two of hydrogen ions, ammonium, potassium ions, toxins or drug residues such as penicillin.
- What fraction of the fluid part of blood is filtered into the Bowman's capsule? Answer: About one-fifth (20 percent).
- Which type of nephron has long loops of Henle? Answer: Juxtamedullary nephrons.
Ready to practise this? Continue in 2nd Year Biology.