🦴 Anatomy introductory Lesson 7 of 8 4 min read

The Urinary (Renal) System

The kidneys filter the blood through millions of nephrons to remove waste, balance water and electrolytes, and produce urine that is stored and expelled by the bladder.

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What you'll learn

  • Identify the organs of the urinary system and the path urine takes out of the body.
  • Describe the structure of a nephron and its role in urine formation.
  • Explain the three steps of urine formation: filtration, reabsorption, and secretion.
  • Summarize how the kidneys regulate water, electrolyte, and acid-base balance.
  • Relate common kidney disorders to the structures involved.

Overview

The urinary system (also called the renal system) removes metabolic wastes from the blood and precisely controls the body’s water, salt, and acid-base balance. It consists of two kidneys, two ureters, the bladder, and the urethra. By adjusting what it keeps and what it discards, the urinary system partners with the cardiovascular system to keep blood volume, pressure, and composition stable.

Organs and Urine Pathway

OrganFunction
KidneysFilter blood; form urine; regulate fluid, electrolytes, and pH
UretersCarry urine from each kidney to the bladder
BladderMuscular sac that stores urine
UrethraCarries urine from the bladder out of the body

Blood enters each kidney through the renal artery, is cleaned, and leaves through the renal vein. The kidneys receive about 20% of the heart’s output, so a large volume of blood is filtered every minute. Inside, the kidney has an outer cortex and an inner medulla arranged into cone-shaped pyramids; the processed urine drains from the pyramids into the renal pelvis, a funnel that empties into the ureter. Emptying the bladder (micturition) is partly voluntary: as the bladder fills, stretch signals create the urge to urinate, and relaxing the urethral sphincter allows release.

The Nephron

Each kidney holds roughly a million nephrons, the units that actually make urine. A nephron has two main parts:

  • Renal corpuscle — the glomerulus (a ball of capillaries) inside Bowman’s capsule, where filtration begins.
  • Renal tubule — a long, winding tube (proximal tubule, loop of Henle, distal tubule) that processes the filtrate, emptying into a collecting duct.

Each nephron is wrapped in a network of tiny blood vessels, so substances can move back and forth between the tubule and the blood at every step. The loop of Henle dips deep into the kidney’s medulla and sets up a salt gradient that lets the kidney concentrate urine, one of the body’s key water-saving mechanisms.

Urine Formation: Three Steps

Urine is produced through three sequential processes:

  1. Filtration — Blood pressure forces water and small solutes (glucose, ions, urea) out of the glomerulus into Bowman’s capsule. Large items like blood cells and proteins stay in the blood. This raw fluid is the filtrate.
  2. Reabsorption — As filtrate moves through the tubule, useful materials (most water, all glucose, needed ions) are transported back into surrounding capillaries. This is how the body avoids losing valuable substances.
  3. Secretion — Extra wastes, drugs, and surplus ions (such as H⁺ and K⁺) are actively added from the blood into the tubule, fine-tuning the urine.

What remains is urine: mostly water plus urea, excess salts, and other wastes. Hormones adjust these steps: antidiuretic hormone (ADH) from the pituitary increases water reabsorption when the body is dehydrated, linking the urinary and endocrine systems.

Fluid, Electrolyte, and Waste Balance

Beyond making urine, the kidneys:

  • Maintain blood pressure and volume by controlling how much water and sodium are retained (via the renin-angiotensin system).
  • Regulate acid-base balance by excreting hydrogen ions and conserving bicarbonate.
  • Keep electrolytes (sodium, potassium, calcium) within narrow limits.
  • Release erythropoietin, a hormone that stimulates red blood cell production, and help activate vitamin D.

A healthy adult produces roughly 1 to 2 liters of urine per day, though this varies widely with fluid intake, sweating, and temperature. Because urine reflects what the blood contains, a urinalysis is a valuable diagnostic tool: glucose in the urine can hint at diabetes, protein may signal kidney damage, and white blood cells suggest infection. The kidneys are remarkably adaptable, concentrating urine to conserve water when a person is dehydrated and producing dilute urine after drinking a large volume, keeping the body’s internal fluid environment stable.

Clinical Relevance

Kidney stones are hard mineral deposits that can lodge in a ureter, causing intense flank pain and blocking urine flow. Urinary tract infections (UTIs) commonly affect the bladder and, if untreated, can ascend to the kidneys. Chronic kidney disease and renal failure, often driven by diabetes and hypertension, gradually destroy nephrons; when filtration fails, wastes and fluid build up, and patients may require dialysis or a transplant. Because the kidneys guard the body’s fluid and electrolyte balance, their function is closely watched in emergency and critical care.

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The countercurrent multiplier concentrates urine

The loop of Henle establishes a medullary osmotic gradient through countercurrent multiplication: the thick ascending limb actively pumps out Na+, K+, and Cl- (via the NKCC2 transporter, the target of loop diuretics) but is impermeable to water, while the descending limb is permeable to water but not solute. This makes the interstitium progressively hypertonic toward the medulla, and the collecting duct then exploits that gradient. Under antidiuretic hormone (ADH), aquaporin-2 channels insert into the collecting duct, allowing water to be reabsorbed and concentrated urine to form.

RAAS, GFR, and clearance

The renin-angiotensin-aldosterone system defends blood pressure and volume: juxtaglomerular cells release renin in response to low renal perfusion, generating angiotensin II, which constricts vessels (preferentially the efferent arteriole to preserve glomerular filtration) and stimulates aldosterone-driven Na+ reabsorption. Glomerular filtration rate (GFR) is estimated clinically by the clearance of creatinine, a substance that is freely filtered and minimally secreted, so its clearance approximates GFR. Inulin clearance is the gold-standard measure because it is filtered but neither reabsorbed nor secreted.

Key terms

Kidney
A bean-shaped organ that filters blood, removes waste, and regulates fluid and electrolyte balance.
Nephron
The microscopic functional unit of the kidney that filters blood and forms urine.
Glomerulus
A tuft of capillaries in the nephron where blood plasma is filtered under pressure.
Filtration
The first step of urine formation, in which water and small solutes pass from blood into the nephron.
Reabsorption
The return of useful substances such as water, glucose, and ions from the nephron back into the blood.
Secretion
The active transfer of additional wastes and excess ions from the blood into the nephron tubule.
Ureter
The tube that carries urine from a kidney to the bladder.
Urea
The main nitrogen-containing waste product of protein metabolism, excreted in urine.

Check your understanding

5 questions · answers reveal instantly.

  1. 1.What is the functional unit of the kidney?
  2. 2.In which structure does the initial filtration of blood plasma occur?
  3. 3.Which process returns useful substances like glucose and water from the tubule back to the blood?
  4. 4.Which is the correct path of urine leaving the body?
  5. 5.Besides removing waste, the kidneys are essential for:

Citations & References

Links open publicly available educational and peer-reviewed sources.

  1. OpenStax. Anatomy and Physiology 2e. ↗
  2. National Institute of Diabetes and Digestive and Kidney Diseases (NIDDK). ↗
  3. MedlinePlus, U.S. National Library of Medicine. ↗