Introduction to Urine, Nephron & its structure and Formation of urine

Urine – Introduction, Nephron & Formation of Urine

Urine

• Urine is a biological fluid formed as a byproduct of metabolism, primarily composed of water, urea, salts, and metabolites.

• Study of urine is called Urology.

• The urinary system consists of the kidney, the ureter, the urinary bladder, and the urethra.

• The formation of urine takes place in the kidneys and, in particular, in the nephrons, which is the basic structural and functional unit of the kidney.

• Each nephron is composed of a glomerulus, a Bowman’s capsule, and a renal tubule.

Structure of Nephron

The nephron is the microscopic structural and functional unit of the kidney responsible for filtering blood, removing waste products, balancing body fluids, and forming urine. Each human kidney contains approximately 1 to 1.5 million nephrons, which work continuously to maintain the body’s internal environment.

The nephron is mainly divided into two major parts:

1. Renal Corpuscle

2. Renal Tubule

1. Renal Corpuscle

The renal corpuscle is the initial filtering component of the nephron located in the cortex of the kidney.

It consists of:

A. Glomerulus

The glomerulus is a network of tiny capillaries supplied by the afferent arteriole and drained by the efferent arteriole.

Functions of Glomerulus

• Filters blood under high pressure

• Allows water, salts, glucose, amino acids, and waste products to pass

• Prevents blood cells and large proteins from entering the filtrate

B. Bowman’s Capsule

The Bowman’s capsule is a cup-shaped double-walled structure surrounding the glomerulus.

Functions of Bowman’s Capsule

• Collects the filtrate produced by glomerular filtration

• Passes the filtrate into the renal tubule

The fluid collected here is known as glomerular filtrate.

2. Renal Tubule

The renal tubule is a long tubular structure where reabsorption and secretion occur.

It consists of the following parts:

A. Proximal Convoluted Tubule (PCT)

The PCT is the first coiled segment after Bowman’s capsule.

Structure

• Lined by cuboidal epithelial cells

• Contains microvilli for increased absorption

Functions

• Reabsorbs:

  • Glucose

  • Amino acids

  • Sodium

  • Water

  • Vitamins

• Secretes hydrogen ions and toxins

Approximately 65–70% of filtrate reabsorption occurs in the PCT.

B. Loop of Henle

The Loop of Henle is a U-shaped segment extending into the medulla of the kidney.

It has two limbs:

1. Descending Limb

• Permeable to water

• Water is reabsorbed here

2. Ascending Limb

• Impermeable to water

• Reabsorbs sodium and chloride ions

Function of Loop of Henle

• Concentrates urine

• Maintains osmotic gradient in the kidney

C. Distal Convoluted Tubule (DCT)

The DCT is the coiled segment after the Loop of Henle.

Functions

• Reabsorbs sodium and water under hormonal control

• Maintains pH balance

• Secretes potassium and hydrogen ions

Hormones such as aldosterone act mainly on the DCT.

D. Collecting Duct

The collecting duct receives filtrate from multiple nephrons.

Functions

• Final concentration of urine

• Reabsorption of water under the influence of ADH (Antidiuretic Hormone)

• Transports urine toward the renal pelvis

Formation of Urine

1. Glomerular Filtration

• Occurs in the glomerulus of the nephron.

• Blood under pressure is filtered through the glomerular membrane.

• Water, salts, glucose, urea, creatinine, and small molecules pass into Bowman’s capsule.

• Blood cells and plasma proteins are retained in the bloodstream.

• Produces glomerular filtrate.

2. Tubular Reabsorption

• Takes place mainly in the proximal convoluted tubule, loop of Henle, and distal tubule.

• Useful substances such as glucose, amino acids, electrolytes, and most of the water are reabsorbed into the blood.

• Prevents loss of essential materials.

3. Tubular Secretion

• Occurs in the distal convoluted tubule and collecting duct.

• Waste substances like hydrogen ions, potassium ions, ammonia, drugs, and toxins are actively secreted from blood into the tubule.

• Helps regulate acid–base balance and remove toxic substances.

About 1.5 – 1.8 liters of urine is excreted out of body.

Read more about

Composition of urine, properties & disease related to it with its forensic significance - Click here

Examination of urine (Preliminary and confirmatory) - Click here

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