This cheat sheet covers how the kidney nephron filters blood, reclaims useful substances, removes wastes, and helps maintain homeostasis. Students need this reference because nephron function connects anatomy, transport processes, blood pressure, and hormone control. It is especially useful for understanding urine formation, water balance, and how the kidneys regulate internal conditions.
The core pathway is filtration in the renal corpuscle, reabsorption and secretion along the tubule, and final concentration in the collecting duct. Important concepts include glomerular filtration rate, selective reabsorption, diffusion, osmosis, active transport, and countercurrent multiplication. Hormones such as ADH and aldosterone adjust water and ion balance by changing permeability and sodium reabsorption.
Key Facts
- Filtration occurs in the glomerulus when blood pressure forces water and small solutes from the blood into Bowman's capsule.
- Large proteins and blood cells normally remain in the bloodstream because they are too large to pass through the filtration membrane.
- Glomerular filtration rate, or GFR, is the volume of filtrate formed per minute, and a typical adult value is about 125 mL/min.
- The proximal convoluted tubule reabsorbs most filtered glucose, amino acids, sodium ions, chloride ions, bicarbonate ions, and water.
- Reabsorption moves useful substances from the filtrate back into the blood, while secretion moves wastes or excess ions from the blood into the filtrate.
- The descending limb of the loop of Henle is permeable to water, so water leaves the filtrate by osmosis as the surrounding medulla becomes saltier.
- The ascending limb of the loop of Henle pumps out sodium and chloride ions but is mostly impermeable to water, helping create the medullary concentration gradient.
- ADH increases water reabsorption in the collecting duct, while aldosterone increases sodium reabsorption and potassium secretion in the distal tubule and collecting duct.
Vocabulary
- Nephron
- A nephron is the microscopic functional unit of the kidney that filters blood and forms urine.
- Glomerulus
- The glomerulus is a cluster of capillaries where blood pressure drives filtration into Bowman's capsule.
- Filtrate
- Filtrate is the fluid made from filtered blood plasma that enters the nephron tubule before becoming urine.
- Reabsorption
- Reabsorption is the movement of water and useful solutes from the nephron tubule back into the blood.
- Secretion
- Secretion is the movement of extra wastes, drugs, hydrogen ions, or potassium ions from the blood into the nephron tubule.
- Osmoregulation
- Osmoregulation is the control of water and solute concentrations in body fluids to maintain homeostasis.
Common Mistakes to Avoid
- Confusing filtration with reabsorption is wrong because filtration moves material from blood into Bowman's capsule, while reabsorption returns useful material from the tubule to blood.
- Saying all filtered substances become urine is wrong because most water, glucose, amino acids, and needed ions are normally reabsorbed before urine leaves the kidney.
- Thinking the descending and ascending limbs do the same job is wrong because the descending limb mainly loses water, while the ascending limb mainly removes sodium and chloride ions.
- Assuming ADH adds water to the body is wrong because ADH reduces water loss by increasing water reabsorption from the collecting duct back into the blood.
- Ignoring membrane permeability is wrong because nephron movement depends on which parts of the tubule allow water, ions, or solutes to cross.
Practice Questions
- 1 If a person's GFR is 125 mL/min, how many milliliters of filtrate are formed in 10 minutes?
- 2 A nephron filters 180 L of fluid in one day and produces 1.5 L of urine. How many liters were reabsorbed or otherwise returned to the blood?
- 3 If ADH levels rise during dehydration, predict whether urine volume increases or decreases and explain why.
- 4 Why is it useful for the nephron to reabsorb glucose but secrete excess hydrogen ions or potassium ions?