4.3 Urine Formation

Urine formation is the process by which the kidneys filter blood, reabsorb useful substances, and secrete waste products to produce urine. It involves three main processes: glomerular filtration, tubular reabsorption, and tubular secretion.

Definition

Urine Formation: Urine formation is the process by which the kidneys produce urine from blood. It involves three main processes: glomerular filtration, tubular reabsorption, and tubular secretion.

Glomerular Filtration: Glomerular filtration (pressure filtration) takes place in the renal capsule under pressure. The pressure comes from the blood pressure and is known as hydrostatic pressure.

Tubular Reabsorption: Selective reabsorption is the process by which certain substances that have been filtered out of the blood during ultrafiltration are reabsorbed.

Tubular Secretion: Tubular secretion is the process by which certain substances e.g., ammonium and hydrogen ions are secreted mainly by the tubular epithelial cells of loop of Henle into the lumen of the tubule.

Main Idea

• Glomerular filtration — filters blood under pressure
• Tubular reabsorption — reabsorbs useful substances
• Tubular secretion — secretes waste products

The formation of urine results in the production of urine with a pH range from 4.8 to 7.5.

Core Concept

• Glomerular filtration = pressure filtration in renal capsule
• Hydrostatic pressure = blood pressure
• Glomerular capillaries = have exceptionally high blood pressure
• Efferent arteriole = diameter half of afferent arteriole
• Glomerular filtrate = filtered fluid in capsule
• Selective reabsorption = reabsorption of useful substances
• Tubular secretion = secretion of waste products
• **pH of urine** = 4.8 to 7.5
• **Cortical nephrons** = 70–80%
• **Juxtamedullary nephrons** = 20–30%

Detailed Explanation

Glomerular Filtration

• Glomerular filtration (pressure filtration) takes place in the renal capsule under pressure.
• The pressure comes from the blood pressure and is known as hydrostatic pressure.
• Glomerular capillaries have exceptionally high blood pressure than any other part of capillary bed in the body.
• The diameter of efferent arteriole is half as compared to the afferent arteriole.
• As the blood enters the narrow capillaries, pressure rises.
• Due to such a high pressure, water and small solute molecules (ions, glucose, and amino acids) are filtered out of the glomerular capillaries.
• These are collected into the Bowman's capsule.
• Larger molecules like proteins, as well as red blood cells and platelets, are left behind in the blood.
• The filtered fluid in the capsule is called glomerular filtrate.
• It has a chemical composition similar to that of blood plasma.

Selective Reabsorption

• Selective reabsorption is the process by which certain substances that have been filtered out of the blood during ultrafiltration are reabsorbed.
• These substances include:
• Glucose
• Amino acids
• Vitamins
• Inorganic salts
• Some water
• As only certain substances are reabsorbed, it is known as selective reabsorption.

Tubular Secretion

• Tubular secretion is the process by which certain substances e.g., ammonium and hydrogen ions are secreted mainly by the tubular epithelial cells of loop of Henle into the lumen of the tubule.
• However, to some extent this process also occurs in convoluted tubule.
• The main purpose of this secretion is to maintain the pH of the urine.
• Normal urine has pH range from 4.8 to 7.5.

Mechanism of Urine Concentration

• Water is reabsorbed along the whole length of the nephron.
• But the formation of hypertonic urine is dependent on the reabsorption of water from the loop of Henle and collecting duct.
• This is achieved by countercurrent multiplier mechanism.
• Due to the counter current, filtrate moving in limbs of loop of Henle and the blood moving in the capillaries of vasa recta, water is greatly (approx. 99.5%) reabsorbed.
• As fluid travels up the ascending limb, sodium chloride is transported actively out of the limb into the surrounding area.
• This movement is controlled by aldosterone.
• This causes increase in the concentration of water in filtrate and decrease in concentration of water in kidney interstitium.
• As a result, water passes out of the descending limb by osmosis.
• This movement of water is also promoted by antidiuretic hormone which is secreted from posterior lobe of pituitary.

Science Titbits

• Cortical nephrons — found in the cortex; have their renal corpuscle in the superficial renal cortex; have relatively short loops of Henle; 70 to 80% nephrons in human kidney are cortical; under normal conditions of water availability, the cortical nephrons deal with the control of blood volume by forming diluted urine.
• Juxtamedullary nephrons — have their renal corpuscle close to the junction of the cortex and medulla; have long loop of Henle which extends deep into the medulla; relatively rare and only comprise 20–30% of the nephrons in the human kidney; when water is in short supply, increased water retention occurs through juxtamedullary nephrons.

Intermediate Level

Three Processes of Urine Formation

Glomerular Filtration

Selective Reabsorption

Tubular Secretion

Mechanism of Urine Concentration

Types of Nephrons

Formula / Mathematical Form

Not applicable to this topic.

Solved Example

Not applicable to this topic (descriptive topic).

Important Points

• 1. Urine formation involves three processes.
• 2. Glomerular filtration = pressure filtration.
• 3. Hydrostatic pressure = blood pressure.
• 4. Glomerular capillaries have high blood pressure.
• 5. Efferent arteriole is half the diameter of afferent arteriole.
• 6. Water, ions, glucose, and amino acids are filtered.
• 7. Proteins, RBCs, and platelets are not filtered.
• 8. Glomerular filtrate is similar to blood plasma.
• 9. Selective reabsorption reabsorbs useful substances.
• 10. Tubular secretion secretes waste products.
• 11. Urine pH is 4.8 to 7.5.
• 12. Countercurrent multiplier mechanism concentrates urine.
• 13. About 99.5% of water is reabsorbed.
• 14. Aldosterone controls sodium chloride transport.
• 15. ADH promotes water reabsorption.
• 16. ADH is secreted from posterior pituitary.
• 17. Cortical nephrons = 70–80%.
• 18. Juxtamedullary nephrons = 20–30%.
• 19. Cortical nephrons form diluted urine.
• 20. Juxtamedullary nephrons retain water.

Remember

• - Urine formation = filtration + reabsorption + secretion
• - Glomerular filtration = pressure filtration
• - Hydrostatic pressure = blood pressure
• - Efferent arteriole = half diameter of afferent
• - Glomerular filtrate = similar to blood plasma
• - Selective reabsorption = reabsorbs useful substances
• - Tubular secretion = secretes waste products
• - **Urine pH** = 4.8 to 7.5
• - Countercurrent multiplier = concentrates urine
• - **99.5%** = water reabsorbed
• - Aldosterone = controls sodium chloride transport
• - ADH = promotes water reabsorption
• - ADH = from posterior pituitary
• - **Cortical nephrons** = 70–80%
• - **Juxtamedullary nephrons** = 20–30%

Common Mistakes

• 1. Confusing filtration and reabsorption — Filtration = filtering blood; reabsorption = returning useful substances.
• 2. Thinking proteins are filtered — Proteins are not filtered.
• 3. Mixing up afferent and efferent arterioles — Afferent brings blood; efferent takes blood away.
• 4. Forgetting the role of hydrostatic pressure — It drives filtration.
• 5. Mixing up the types of nephrons — Cortical (70–80%) and juxtamedullary (20–30%).
• 6. Confusing aldosterone and ADH — Aldosterone controls sodium chloride; ADH promotes water reabsorption.
• 7. Thinking urine pH is neutral — Urine pH is 4.8 to 7.5.
• 8. Mixing up the functions of nephrons — Cortical nephrons form diluted urine; juxtamedullary nephrons retain water.
• 9. Thinking the efferent arteriole is wider — The efferent arteriole is half the diameter of the afferent arteriole.
• 10. Believing all water is reabsorbed — About 99.5% of water is reabsorbed.

Quick Revision

Urine formation involves three main processes: glomerular filtration, tubular reabsorption, and tubular secretion. Glomerular filtration takes place in the renal capsule under pressure. The pressure comes from the blood pressure and is known as hydrostatic pressure. Glomerular capillaries have exceptionally high blood pressure. The diameter of the efferent arteriole is half compared to the afferent arteriole, so as the blood enters the narrow capillaries, pressure rises. Due to such high pressure, water and small solute molecules (ions, glucose, and amino acids) are filtered out of the glomerular capillaries and collected into Bowman's capsule. Larger molecules like proteins, as well as red blood cells and platelets, are left behind in the blood. The filtered fluid is called glomerular filtrate, which has a chemical composition similar to that of blood plasma. Selective reabsorption is the process by which certain substances that have been filtered out of the blood during ultrafiltration are reabsorbed. These substances include glucose, amino acids, vitamins, inorganic salts, and some water. Tubular secretion is the process by which certain substances e.g., ammonium and hydrogen ions are secreted mainly by the tubular epithelial cells of the loop of Henle into the lumen of the tubule. The main purpose of this secretion is to maintain the pH of the urine. Normal urine has a pH range from 4.8 to 7.5. The formation of hypertonic urine is dependent on the reabsorption of water from the loop of Henle and collecting duct. This is achieved by the countercurrent multiplier mechanism. About 99.5% of water is reabsorbed. Aldosterone controls the movement of sodium chloride. Antidiuretic hormone promotes water reabsorption. There are two types of nephrons: cortical nephrons (70–80%) and juxtamedullary nephrons (20–30%).

60-Second Revision

• Urine formation = filtration + reabsorption + secretion
• Glomerular filtration = pressure filtration
• Hydrostatic pressure = blood pressure
• Efferent arteriole = half diameter of afferent
• Glomerular filtrate = similar to blood plasma
• Selective reabsorption = reabsorbs useful substances
• Tubular secretion = secretes waste products
• Urine pH = 4.8 to 7.5
• Countercurrent multiplier = concentrates urine
• 99.5% = water reabsorbed
• Aldosterone = controls sodium chloride transport
• ADH = promotes water reabsorption
• ADH = from posterior pituitary
• Warning: Efferent arteriole is half the diameter of afferent arteriole

NEXT TOPIC

• Chapter:
• 4. Urinary System of Man

• Topic:
• 4.3.1 MECHANISM OF URINE CONCENTRATION

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