Process of Urine Formation in Humans

 

 

Process of Urine Formation in Humans

Introduction

Urine formation is the primary function of the kidneys. It involves the removal of metabolic wastes, excess salts, and excess water from the blood while conserving useful substances. The process occurs in the nephrons, the structural and functional units of the kidney.

Objectives of Urine Formation

• Removal of nitrogenous wastes
• Maintenance of water balance
• Maintenance of electrolyte balance
• Regulation of blood volume
• Regulation of blood pressure
• Maintenance of acid-base balance
• Maintenance of homeostasis

Overview of Urine Formation

Urine formation occurs through three major processes:

1. Glomerular Filtration (Ultrafiltration)
2. Selective Tubular Reabsorption
3. Tubular Secretion

Flow Chart

Blood
↓
Glomerular Filtration
↓
Filtrate Formation
↓
Tubular Reabsorption
↓
Tubular Secretion
↓
Urine Formation
↓
Collecting Duct
↓
Renal Pelvis
↓
Ureter
↓
Urinary Bladder

Structures Involved in Urine Formation

Renal Corpuscle

Consists of:

• Glomerulus
• Bowman's capsule

Function:

• Filtration of blood

Renal Tubule

Consists of:

1. Proximal Convoluted Tubule (PCT)
2. Loop of Henle
3. Distal Convoluted Tubule (DCT)
4. Collecting Duct

Function:

• Reabsorption and secretion

STEP 1: Glomerular Filtration (Ultrafiltration)

Definition

The process by which plasma is filtered from glomerular capillaries into Bowman's capsule under high pressure is called glomerular filtration or ultrafiltration.

Site

• Glomerulus
• Bowman's capsule

Why Does Filtration Occur?

The glomerulus possesses:

1. High Hydrostatic Pressure

Approximately 55 mm Hg

This pressure is generated because:

• Afferent arteriole is wider.
• Efferent arteriole is narrower.

Thus, blood enters easily but leaves with resistance.

Filtration Membrane

The filtration membrane consists of:

A. Capillary Endothelium

• Fenestrated (contains pores)

B. Basement Membrane

• Thin, selectively permeable

C. Podocytes

• Inner layer of Bowman's capsule
• Possess filtration slits

These three layers together form the filtration barrier.

Substances Filtered

Filtered

✓ Water

✓ Glucose

✓ Amino acids

✓ Vitamins

✓ Electrolytes

✓ Urea

✓ Uric acid

✓ Creatinine

Not Filtered

✗ Blood cells

✗ Platelets

✗ Plasma proteins

Because they are too large to pass through filtration slits.

Glomerular Filtrate

The fluid entering Bowman's capsule is called glomerular filtrate.

Composition is almost similar to plasma except:

• No plasma proteins
• No blood cells

Glomerular Filtration Rate (GFR)

Definition

Volume of filtrate formed per minute by both kidneys.

Normal Value: 125 mL/minute or 180 litres/day

Importance of GFR

Used to assess kidney function.

Reduced GFR indicates:

• Kidney disease
• Renal failure
• Reduced filtration capacity

STEP 2: Selective Tubular Reabsorption

Definition

Movement of useful substances from filtrate back into blood is called tubular reabsorption.

Importance

If reabsorption did not occur:

• Entire body water would be lost.
• Valuable nutrients would be excreted.

Of the 180 litres filtered daily, only about 1–1.5 litres are excreted as urine. Thus, more than 99% of filtrate is reabsorbed.

Reabsorption in Different Parts of Nephron

A. Proximal Convoluted Tubule (PCT)

Most Important Site

Approximately 65–70% reabsorption occurs here.

Reabsorbed Completely

• Glucose
• Amino acids
• Vitamins

Reabsorbed Largely

• Water
• Sodium ions
• Chloride ions
• Potassium ions

Special Features

PCT cells possess:

• Numerous mitochondria
• Brush border microvilli

These increase absorptive capacity.

B. Loop of Henle

Descending Limb

Permeable to Water

Water moves out into medulla.

Impermeable to Electrolytes

Result: Filtrate becomes concentrated.

Ascending Limb

Impermeable to Water

Permeable to Salts

Na⁺ and Cl⁻ move out.

Result: Filtrate becomes dilute.

Significance

Creates medullary osmotic gradient.

Essential for urine concentration.

C. Distal Convoluted Tubule (DCT)

Reabsorbs:

• Sodium
• Water
• Bicarbonate ions

Under hormonal control.

D. Collecting Duct

Reabsorbs:

• Water
• Urea (partially)

Produces concentrated urine.

Counter Current Mechanism

Definition

The flow of filtrate in opposite directions in the two limbs of the loop of Henle forms the counter-current system.

Components

1. Loop of Henle

2. Vasa Recta

Both act together.

Function

Maintains: High medullary osmotic gradient

Approximately:

• Cortex = 300 mOsm/L
• Inner medulla = 1200 mOsm/L

This gradient helps reabsorb water and concentrate urine.

STEP 3: Tubular Secretion

Definition

Transfer of substances from blood into nephron tubules is called tubular secretion.

Sites

Mainly:

• PCT
• DCT
• Collecting duct

Substances Secreted

Hydrogen ions (H⁺)

Helps maintain pH.

Potassium ions (K⁺)

Maintains electrolyte balance.

Ammonia (NH₃)

Helps acid-base regulation.

Creatinine

Waste elimination.

Drugs and Toxins

Examples:

• Penicillin
• Certain medicines

Importance

·       Removes unwanted substances that escaped filtration.

·       Maintains acid-base balance.

·       Maintains ionic balance.

Formation of Final Urine

After:

1. Filtration
2. Reabsorption
3. Secretion

The remaining fluid is called urine.

Composition of Normal Urine

Daily Volume

Approximately: 1–1.5 litres/day

Colour: Pale yellow, Due to pigment: Urochrome

pH

Approximately: 6.0, Range: 4.5–8.0

Specific Gravity

1.003–1.030

Major Constituents

Constituent	Percentage
Water	95%
Solutes	5%

Solutes Present

• Urea
• Uric acid
• Creatinine
• Sodium chloride
• Potassium salts
• Phosphates
• Sulphates

Normally Absent

• Glucose
• Proteins
• Blood cells

Presence indicates pathology.

Hormonal Regulation of Urine Formation

1. Antidiuretic Hormone (ADH)

Produced by: Hypothalamus, Released from: Posterior pituitary

Function

Increases water reabsorption in:

• DCT
• Collecting duct

Result: Less urine, more concentrated urine

2. Aldosterone

Produced by: Adrenal cortex

Function

Increases:

• Sodium reabsorption
• Water retention

Result: Reduced urine volume

3. Atrial Natriuretic Factor (ANF)

Produced by: Atria of heart

Function

Promotes sodium loss.

Inhibits:

• Renin
• Aldosterone

Result: Increased urine output

Juxtaglomerular Apparatus (JGA)

Definition

A specialized structure formed by:

• Distal convoluted tubule
• Afferent arteriole

Function

Secretes: Renin, When:

• Blood pressure falls
• Blood volume decreases

Renin-Angiotensin-Aldosterone System (RAAS)

Renin
↓
Angiotensinogen
↓
Angiotensin I
↓
Angiotensin II
↓
Aldosterone secretion
↓
Water and sodium retention
↓
Blood pressure rises

Micturition (Urination)

Definition

The process of expelling urine from the urinary bladder is called micturition.

Steps

1. Urine enters bladder.
2. Bladder stretches.
3. Stretch receptors stimulated.
4. Micturition reflex initiated.
5. Internal sphincter relaxes.
6. External sphincter voluntarily relaxed.
7. Urine expelled.

Summary of Urine Formation

Process	Site	Main Function
Glomerular filtration	Glomerulus	Filtrate formation
Reabsorption	PCT, Loop, DCT	Recovery of useful substances
Secretion	PCT, DCT, Collecting duct	Addition of wastes to filtrate
Concentration	Loop of Henle & Collecting duct	Water conservation
Excretion	Urethra	Removal of urine

 

HIGH-YIELD NEET FACTS

✓ Normal GFR = 125 mL/min

✓ Daily filtrate formed = 180 L/day

✓ Urine produced = 1–1.5 L/day

✓ More than 99% filtrate is reabsorbed

✓ PCT is the major site of reabsorption.

✓ Glucose is normally completely reabsorbed.

✓ Descending limb is permeable to water.

✓ Ascending limb is impermeable to water.

✓ Counter-current mechanism concentrates urine.

✓ ADH increases water reabsorption.

✓ Aldosterone increases sodium reabsorption.

✓ JGA secretes renin.

✓ Normal urine contains approximately 95% water and 5% solutes.

MOST IMPORTANT ONE-LINERS

1. Urine formation occurs by filtration, reabsorption and secretion.
2. Glomerular filtration takes place in the renal corpuscle.
3. Normal GFR is 125 mL/minute.
4. About 180 litres of filtrate are formed daily.
5. PCT is the major site of reabsorption.
6. Loop of Henle and vasa recta establish counter-current mechanism.
7. ADH promotes water reabsorption and concentrated urine formation.
8. Aldosterone increases sodium reabsorption.
9. JGA secretes renin during low blood pressure.
10. Only 1–1.5 litres of urine are excreted from 180 litres of filtrate formed daily.

Last-Minute Revision Mnemonic

"F-R-S-C-E"

F → Filtration
R → Reabsorption
S → Secretion
C → Concentration of urine
E → Excretion (micturition)