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?
Understanding Kidney Nephron Function Reference
The renal corpuscle works like a highly selective sieve, not a simple hole in a blood vessel. Its filter has three layers. Capillary walls have tiny openings, a basement membrane blocks many large molecules, and specialized cells called podocytes form narrow gaps.
Size and electrical charge both affect what crosses. Many blood proteins carry a negative charge, and the filter helps repel them. Pressure must be carefully balanced here.
If pressure falls too low, less fluid enters the nephron. If it rises too much for long periods, delicate filter structures can be damaged. Small arteries entering and leaving the glomerulus can change their diameter to keep filtration fairly stable during normal changes in blood pressure.
The early tubule has a major workload because the initial filtrate contains many substances the body cannot afford to lose. Tubule cells use membrane proteins to move solutes into nearby capillaries. Water follows when dissolved particles create an osmotic pull.
This process has limits. For example, glucose transport proteins can become saturated when blood glucose is very high. Glucose then remains in the filtrate and appears in urine.
This is one reason urine tests can help identify uncontrolled diabetes. Reabsorption is not the same for every substance. Some materials are recovered almost completely when the body is healthy, while others are adjusted according to the body's immediate needs.
The loop of Henle creates conditions that allow the kidney to conserve water later. Its two limbs handle salt and water differently, which builds a concentration gradient from the outer kidney toward the inner medulla. Filtrate flows in the opposite direction through the two limbs.
This arrangement strengthens the gradient step by step, a process called countercurrent multiplication. Blood vessels called the vasa recta run beside the loop and help preserve this gradient.
Without them, flowing blood would carry away the concentrated salts. The collecting ducts pass through this salty medulla, so their water permeability has a powerful effect on final urine concentration.
Hormone signals change nephron activity in response to daily life. After sweating, vomiting, or drinking little water, the body releases more ADH. More water can then move from the collecting duct back into the blood, producing a smaller volume of darker urine.
When blood volume or sodium level is low, aldosterone helps retain sodium, and water tends to follow. Potassium balance matters too because abnormal potassium levels can disrupt heart rhythm. Students should track the direction of movement at every nephron region.
Identify whether a substance moves from blood to tubule or from tubule to blood. It helps to separate the route of filtrate from the route of blood, since confusing these two pathways causes many errors.