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Kidney Anatomy
& Nephron Structure

Explore the internal architecture of the kidney, the functional unit called the nephron, and how they work together to filter blood and form urine.

Anatomy & Physiology

Why Kidneys Matter

Your two kidneys are bean-shaped organs located in the retroperitoneal space, each roughly the size of your fist. Despite comprising only about 0.5% of total body weight, they receive approximately 20-25% of cardiac output at rest.

Key Functions of the Kidney

Filtration
Removes metabolic wastes such as urea, creatinine, and uric acid from the blood.
Regulation
Maintains water balance, electrolyte concentrations, and blood pH within narrow limits.
Endocrine
Produces erythropoietin (EPO), activates vitamin D, and secretes renin for blood pressure control.

Gross Anatomy: Three Regions

Renal artery Renal vein Ureter Cortex Medulla Renal Pelvis

Cortex vs. Medulla

Cortex Medulla
Location Outer region Inner region
Appearance Granular (glomeruli) Striated (tubules)
Key Structures Glomeruli, PCT, DCT Loop of Henle, collecting ducts
Blood Supply Rich capillary network Vasa recta
Osmolarity ~300 mOsm/L (isotonic) Up to ~1200 mOsm/L

The Nephron: Functional Unit

Each kidney contains roughly 1 million nephrons. A nephron is the microscopic structural and functional unit responsible for filtering blood and producing urine. It consists of two main parts:

Renal Corpuscle
The filtering component: contains the glomerulus (capillary tuft) enclosed within Bowman's capsule.
Renal Tubule
The processing component: a long tube (PCT, loop of Henle, DCT) that modifies the filtrate into final urine.

Nephron Architecture

Glomerulus Bowman's Capsule Afferent arteriole Efferent arteriole PCT Descending limb Loop of Henle Ascending limb DCT Collecting duct To renal pelvis CORTEX MEDULLA

The Renal Corpuscle

The renal corpuscle is where filtration begins. Blood enters the glomerulus via the afferent arteriole and leaves via the efferent arteriole. The filtration membrane has three layers:

1
Fenestrated endothelium: Pores in glomerular capillaries allow plasma to pass but block blood cells.
2
Basement membrane: A charged gel layer that repels large proteins like albumin.
3
Podocytes (visceral layer): Specialized cells with filtration slits that provide the final size barrier.

Glomerular Filtration Pressure

Filtration is driven by net filtration pressure (NFP), determined by the balance of hydrostatic and osmotic pressures across the glomerular membrane:

Tubular Reabsorption & Secretion

Reabsorption
Movement of substances from the tubular fluid back into peritubular capillaries. About 99% of water, glucose, and amino acids are reclaimed.
Secretion
Movement of substances from peritubular capillaries into the tubular fluid. Removes excess \( \text{H}^+ \), \( \text{K}^+ \), drugs, and toxins.

Segment-by-Segment Processing

1
PCT (Proximal Convoluted Tubule): Reabsorbs ~65% of filtrate: glucose, amino acids, \( \text{Na}^+ \), \( \text{HCO}_3^- \), and water (obligatory).
2
Loop of Henle (descending): Permeable to water but not solutes. Water leaves by osmosis, concentrating the filtrate.
3
Loop of Henle (ascending): Impermeable to water. Actively pumps out \( \text{Na}^+ \), \( \text{K}^+ \), \( \text{Cl}^- \), diluting the filtrate.
4
DCT (Distal Convoluted Tubule): Fine-tunes \( \text{Na}^+ \)/\( \text{K}^+ \) balance under aldosterone. Secretes \( \text{H}^+ \) for pH control.
5
Collecting Duct: ADH controls water permeability. With ADH: concentrated urine. Without ADH: dilute urine.

Cortical vs. Juxtamedullary

Cortical (~85%) Juxtamedullary (~15%)
Glomerulus location Outer cortex Near cortex-medulla border
Loop of Henle Short: barely enters medulla Long: extends deep into medulla
Vasa recta Sparse or absent Well-developed
Primary role General filtration Concentrating urine

Renal Blood Supply Pathway

Renal Artery Segmental A. Interlobar A. Arcuate A. Interlobular A. Afferent Arteriole GLOMERULUS Efferent Arteriole Peritubular Caps Renal Vein

How to Trace the Nephron

Follow this step-by-step approach to trace filtrate from blood to urine:

1
Start at the glomerulus: Blood is filtered through the filtration membrane into Bowman's capsule.
2
Follow through the PCT: Most reabsorption occurs here: glucose, amino acids, \( \text{Na}^+ \), water.
3
Descend the loop: Water leaves (descending). Solutes are pumped out (ascending). This creates the medullary gradient.
4
Pass through the DCT: Hormones (aldosterone, PTH) fine-tune ion balance.
5
Enter the collecting duct: ADH determines how much water is reabsorbed. Final urine drains to the renal pelvis.

Example: Calculating GFR

Given the following pressures in a glomerulus, calculate the net filtration pressure and estimate if filtration occurs:

Glomerular HP
\( P_{GH} = 55 \) mmHg
Capsular HP
\( P_{CH} = 15 \) mmHg
Osmotic P
\( \pi_{GC} = 30 \) mmHg

Example: Urine Volume Calculation

If GFR is 125 mL/min and 99% of filtrate is reabsorbed, how much urine is produced per day?

Common Mistakes

✕ Confusing arterioles
The afferent arteriole brings blood TO the glomerulus; the efferent carries it AWAY. Remember: A = Arrival, E = Exit.
✕ Mixing up limbs
The descending limb is permeable to water (not solutes). The ascending limb pumps out solutes (impermeable to water). They do opposite jobs.
✕ Forgetting secretion
Students often think urine = filtrate minus reabsorption. But secretion adds substances (like drugs, \( \text{H}^+ \)) to the tubule as well.

Nephron Segment Summary

Segment Reabsorbs Secretes Hormone
PCT Glucose, amino acids, \( \text{Na}^+ \), \( \text{H}_2\text{O} \) \( \text{H}^+ \), toxins -
Descending loop \( \text{H}_2\text{O} \) (osmosis) - -
Ascending loop \( \text{Na}^+ \), \( \text{K}^+ \), \( \text{Cl}^- \) - -
DCT \( \text{Na}^+ \), \( \text{Ca}^{2+} \) \( \text{K}^+ \), \( \text{H}^+ \) Aldosterone, PTH
Collecting duct \( \text{H}_2\text{O} \), urea \( \text{K}^+ \), \( \text{H}^+ \) ADH, Aldosterone

Question 1 of 5

Which structure is the functional unit of the kidney?

Question 2 of 5

The descending limb of the loop of Henle is permeable to:

Question 3 of 5

If glomerular hydrostatic pressure is 60 mmHg, capsular hydrostatic pressure is 18 mmHg, and blood colloid osmotic pressure is 32 mmHg, what is the net filtration pressure?

Question 4 of 5

Which hormone increases water permeability of the collecting duct, allowing the kidney to produce concentrated urine?

Question 5 of 5

Which type of nephron is primarily responsible for creating the medullary osmotic gradient needed to concentrate urine?