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Filtration, Reabsorption
and Secretion

Explore the three core processes that the nephron uses to produce urine, maintain homeostasis, and regulate blood composition.

Anatomy & Physiology

The Nephron: Functional Unit of the Kidney

Each kidney contains roughly 1 million nephrons. A nephron is a tiny tubular structure responsible for filtering blood and producing urine. It performs three essential processes: filtration, reabsorption, and secretion.

Three Core Processes at a Glance

Filtration
Blood pressure forces water, ions, glucose, and waste from the glomerulus into Bowman's capsule.
Reabsorption
Useful substances (glucose, amino acids, water, ions) are reclaimed from the filtrate back into peritubular capillaries.
Secretion
Additional wastes (\(\text{H}^+\), \(\text{K}^+\), drugs) are moved from the blood into the tubule for excretion.

Why These Processes Matter

Every day your kidneys filter approximately 180 liters of plasma. Without reabsorption, you would lose all that fluid in hours. Without secretion, toxic substances and excess ions would accumulate in the blood.

1
Fluid balance: Regulates blood volume and blood pressure.
2
Electrolyte balance: Maintains proper \(\text{Na}^+\), \(\text{K}^+\), \(\text{Ca}^{2+}\) concentrations.
3
pH regulation: Secretion of \(\text{H}^+\) and reabsorption of \(\text{HCO}_3^-\) keep blood pH near \(\approx 7.4\).
4
Waste removal: Eliminates urea, creatinine, drugs, and metabolic byproducts.

Glomerular Filtration: Step One

Bowman's Capsule Glomerulus (capillary tuft) Afferent arteriole Efferent arteriole Filtrate Contains: Water Glucose, amino acids
\( \text{Na}^+, \text{K}^+, \text{Cl}^-, \text{HCO}_3^- \)
 Urea, creatinine NOT filtered: proteins, RBCs
\( \text{Net Filtration Pressure} \approx 10 \text{ mmHg} \)

Glomerular Filtration Rate: GFR

The GFR measures how much filtrate the kidneys produce per minute. A normal GFR is approximately 125 mL/min, which equals about 180 L/day.

Tubular Reabsorption: Reclaiming the Good

Tubular Lumen Filtrate with: glucose, amino acids, Na+, water, urea Glucose
\( \text{Na}^+ \)
\( \text{H}_2\text{O} \)
 Peritubular Substances returned to the blood:
\( 100\% \text{ glucose} \)
\( 99\% \text{ water} \)
\( \approx 99\% \text{ Na}^+ \)
 Most reabsorption occurs in the Proximal Convoluted Tubule (PCT)
\( \approx 65\% \text{ of filtered Na}^+ \text{ and water reabsorbed here} \)

Active vs Passive Reabsorption

Active Transport
Requires ATP. Used for \(\text{Na}^+\), glucose, and amino acids. The \(\text{Na}^+/\text{K}^+\) ATPase pump on the basolateral membrane drives most active reabsorption.
Passive Transport
No ATP needed. Water follows \(\text{Na}^+\) by osmosis. \(\text{Cl}^-\) follows \(\text{Na}^+\) by electrochemical gradient. Urea is partially reabsorbed by diffusion.

Tubular Secretion: The Final Cleanup

Peritubular Blood Contains excess:
\( \text{H}^+ \text{ ions} \)
\( \text{K}^+ \text{ ions} \)
\( \text{NH}_3 \text{ (ammonia)} \)
 Drugs, toxins SECRETED Tubular Lumen Added to filtrate for excretion in urine Secretion is the REVERSE of reabsorption: blood → tubule

Reabsorption vs Secretion

Reabsorption Secretion
Direction Tubule → Blood Blood → Tubule
Purpose Reclaim useful substances Remove additional waste
Key substances Glucose, \(\text{Na}^+\), \(\text{H}_2\text{O}\) \(\text{H}^+\), \(\text{K}^+\), drugs
Primary location PCT (mostly) PCT and DCT
Mechanism Active and passive Mostly active

Nephron Pathway Overview

Glomerulus FILTRATION PCT Major reabsorption Loop of Henle Concentrate filtrate DCT Secretion + fine-tuning Collecting Duct
Final \( \text{H}_2\text{O} \) reabsorption
 URINE
Descending: \( \text{H}_2\text{O} \) out
Ascending: \( \text{Na}^+ \) out
ADH acts on collecting duct | Aldosterone acts on DCT
These hormones regulate how much water and \( \text{Na}^+ \) are reabsorbed

How to Trace a Substance Through the Nephron

1
Identify the substance: Is it a small solute (glucose, urea, ions) or a large molecule (protein, blood cell)?
2
Check filtration: Small molecules pass through the glomerular membrane. Proteins and cells do not.
3
Follow through the tubule: Is it reabsorbed? Glucose and amino acids are 100% reabsorbed in the PCT. Water reabsorption varies by segment and hormones.
4
Check for secretion: Substances like \(\text{H}^+\), \(\text{K}^+\), and drugs are added to the filtrate in the DCT and collecting duct.
5
Determine final fate: Urine Excretion = Filtered − Reabsorbed + Secreted. Apply the formula to predict output.

The Urine Formation Equation

Glucose Example
Filtered: yes. Reabsorbed: 100%. Secreted: no. Excreted: 0 (normally none in urine).
Creatinine Example
Filtered: yes. Reabsorbed: 0%. Secreted: a small amount. Excreted: all filtered + secreted.

Example: Tracing Glucose

A healthy individual has a blood glucose of \(90 \text{ mg/dL}\) and a GFR of \(125 \text{ mL/min}\). How much glucose is filtered per minute, and how much appears in urine?

1
Filtered glucose: \(125 \text{ mL/min} \times 0.9 \text{ mg/mL} = 112.5 \text{ mg/min}\)
2
Reabsorbed: 100% in the PCT via \(\text{Na}^+\)-glucose cotransporters (SGLT). All 112.5 mg/min are returned to blood.
3
Secreted: 0 mg/min (glucose is not secreted).
4
Excreted: \(112.5 - 112.5 + 0 = \) \(0 \text{ mg/min}\). No glucose in urine.

Example: Glucose in Diabetes

A patient with uncontrolled diabetes has blood glucose of \(350 \text{ mg/dL}\). The transport maximum (\(T_m\)) for glucose is approximately \(375 \text{ mg/min}\).

1
Filtered glucose: \(125 \text{ mL/min} \times 3.5 \text{ mg/mL} = 437.5 \text{ mg/min}\)
2
Reabsorbed: Only \(375 \text{ mg/min}\) (carriers are saturated at \(T_m\)).
3
Excreted: \(437.5 - 375 + 0 = \) \(62.5 \text{ mg/min}\) glucose in urine (glycosuria).

Example: ADH and Water Reabsorption

When blood osmolarity rises (dehydration), the posterior pituitary releases antidiuretic hormone (ADH). How does this affect the nephron?

1
ADH target: Collecting duct cells. ADH triggers insertion of aquaporin-2 water channels into the apical membrane.
2
Effect: More water is reabsorbed from the collecting duct back into the blood.
3
Result: Urine becomes more concentrated and lower in volume. Blood osmolarity decreases (homeostasis restored).

Common Mistakes

✕ Confusing Direction
Reabsorption goes tubule to blood. Secretion goes blood to tubule. Mixing these up reverses the entire concept.
✕ Filtration = Excretion
Filtration is not the same as excretion. Most filtered substances are reabsorbed. Only \(\approx 1\%\) of filtrate becomes urine.
✕ Ignoring \(T_m\)
Forgetting that carrier proteins saturate. Above the transport maximum, excess solute spills into urine regardless of reabsorption.

Summary of Key Substances

Substance Filtered? Reabsorbed? Secreted?
Glucose Yes 100% No
Urea Yes ~50% Some
\(\text{Na}^+\) Yes ~99% No
\(\text{K}^+\) Yes Most Yes (DCT)
\(\text{H}^+\) Yes No Yes (PCT, DCT)
Proteins No N/A No
Creatinine Yes No Some

Question 1 of 5

Glomerular filtration is primarily driven by which force?

Question 2 of 5

In a healthy person, which substance is completely reabsorbed from the filtrate and does NOT appear in urine?

Question 3 of 5

Tubular secretion moves substances in which direction?

Question 4 of 5

A patient has a GFR of \(125 \text{ mL/min}\) and blood glucose of \(300 \text{ mg/dL}\). If the \(T_m\) for glucose is \(375 \text{ mg/min}\), how much glucose is excreted per minute?

Question 5 of 5

ADH (antidiuretic hormone) increases water reabsorption by acting primarily on which structure?