Human Excretory System – Structure and Functions

 

 

Human Excretory System – Structure and Functions

1. Introduction

Definition

The process of removal of metabolic waste products from the body is called excretion.

Metabolism continuously produces waste substances such as:

• Urea
• Uric acid
• Creatinine
• Excess salts
• Excess water
• Carbon dioxide

If these wastes accumulate in the body, they become toxic and disturb homeostasis.

The human excretory system is responsible for:

• Removal of metabolic wastes
• Regulation of water balance
• Maintenance of ionic balance
• Acid-base regulation
• Blood pressure regulation
• Maintenance of internal environment (homeostasis)

In humans, the excretory system consists of:

1. Pair of kidneys
2. Pair of ureters
3. Urinary bladder
4. Urethra

Each kidney contains approximately 1 million nephrons, the structural and functional units of the kidney.

2. Human Excretory System

Components

Organ	Number	Function
Kidneys	2	Formation of urine
Ureters	2	Transport urine to bladder
Urinary Bladder	1	Temporary storage of urine
Urethra	1	Expulsion of urine

3. KIDNEYS

Location

• Bean-shaped organs.
• Located on either side of vertebral column.
• Situated between:
• Last thoracic vertebra (T12)
• Third lumbar vertebra (L3)
• Close to dorsal abdominal wall.

·        Right kidney lies slightly lower than left kidney because of the liver.

External Features

Dimensions

Characteristic	Measurement
Length	10–12 cm
Width	5–7 cm
Thickness	2–3 cm
Weight	120–170 g

Coverings of Kidney

From outside inward:

1. Renal fascia
2. Perirenal fat capsule
3. Fibrous renal capsule

Functions:

• Protection
• Shock absorption
• Support

4. INTERNAL STRUCTURE OF KIDNEY

Hilum

A notch on the inner concave surface of kidney.

Through hilum pass:

• Renal artery
• Renal vein
• Nerves
• Lymphatics
• Ureter

Renal Pelvis

• Funnel-shaped cavity inside hilum.
• Collects urine.
• Continues as ureter.

Calyces

Cup-shaped structures receiving urine from renal pyramids.

Types:

• Minor calyx
• Major calyx

Cortex

• Outer region.
• Dark and granular.
• Contains:
• Renal corpuscles
• Convoluted tubules

Medulla

• Inner region.
• Pale appearance.
• Contains:

Medullary Pyramids

Conical masses projecting into calyces.

Columns of Bertini

Extensions of cortex between pyramids.

5. Nephron – Structural and Functional Unit of Kidney

Definition

A nephron is the microscopic unit responsible for:

• Blood filtration
• Urine formation
• Regulation of body fluids

Each kidney contains approximately one million nephrons.

Types of Nephrons

1. Cortical Nephrons

• About 85%
• Short loop of Henle
• Located mainly in cortex

2. Juxtamedullary Nephrons

• About 15%
• Long loop of Henle
• Extend deep into medulla
• Important for concentrated urine formation

6. Structure of a Nephron

Nephron consists of:

A. Renal Corpuscle (Malpighian Body)

Composed of:

Glomerulus

• Tuft of capillaries.
• Formed by afferent arteriole.
• Site of ultrafiltration.

Bowman's Capsule

• Double-walled cup-like structure.
• Encloses glomerulus.

Together they form the renal corpuscle.

B. Renal Tubule

Consists of:

1. Proximal Convoluted Tubule (PCT)

• Highly coiled.
• Rich in microvilli.
• Major site of reabsorption.

2. Loop of Henle

Has:

Descending Limb

• Permeable to water.
• Impermeable to salts.

Ascending Limb

• Impermeable to water.
• Reabsorbs NaCl.

3. Distal Convoluted Tubule (DCT)

• Selective secretion and reabsorption.
• Hormonal regulation occurs here.

4. Collecting Duct

• Receives urine from many nephrons.
• Final concentration of urine.

·        Opens into renal pelvis.

·         ·        7. BLOOD SUPPLY OF KIDNEY

Pathway:

Renal artery
↓
Afferent arteriole
↓
Glomerulus
↓
Efferent arteriole
↓
Peritubular capillaries/Vasa Recta
↓
Renal vein

Significance of Efferent Arteriole

Unlike most capillary beds: Arteriole→ Capillaries → Arteriole

This arrangement helps maintain:

• High glomerular pressure
• Efficient filtration

8. FUNCTIONS OF HUMAN EXCRETORY SYSTEM

1. Excretion of Nitrogenous Wastes

Removes:

• Urea
• Uric acid
• Creatinine
• Ammonia

Prevents toxicity.

2. Osmoregulation

Maintains:

• Water balance
• Electrolyte balance

Regulates levels of:

• Sodium
• Potassium
• Chloride
• Calcium
• Phosphate

3. Maintenance of Blood Volume

By controlling:

• Water reabsorption
• Salt retention

Kidneys maintain:

• Extracellular fluid volume
• Plasma volume

4. Regulation of Blood Pressure

Kidneys release:

Renin

Activates:
Renin–Angiotensin–Aldosterone System (RAAS)

Functions:

• Increases blood pressure
• Conserves sodium and water

5. Maintenance of Acid-Base Balance

Kidneys regulate:

• H⁺ ions
• HCO₃⁻ ions

Thus, maintaining blood pH near 7.4.

6. Endocrine Functions

Erythropoietin

Produced by kidneys.

Function:

• Stimulates RBC production in bone marrow.

Calcitriol

Active form of Vitamin D.

Function:

• Calcium absorption
• Bone health

7. Detoxification

Removal of:

• Drugs
• Toxins
• Foreign chemicals

9. Accessory Excretory Organs

Although kidneys are the principal excretory organs, other organs also help in excretion.

Organ	Waste Removed
Lungs	CO₂ and water vapour
Skin	Sweat (water, salts, urea)
Liver	Bile pigments, cholesterol
Intestine	Certain salts and pigments

10. Pathway Of Urine Flow

Blood
↓
Kidney
↓
Nephron
↓
Collecting Duct
↓
Renal Pelvis
↓
Ureter
↓
Urinary Bladder
↓
Urethra
↓
Outside Body

11. High-Yield Facts

✓ Human excretory system = 2 kidneys + 2 ureters + urinary bladder + urethra.

✓ Kidney weight = 120–170 g.

✓ Kidney size = 10–12 × 5–7 × 2–3 cm.

✓ Functional unit = Nephron.

✓ Nephrons per kidney ≈ 1 million.

✓ Renal corpuscle = Glomerulus + Bowman's capsule.

✓ Cortex contains renal corpuscles.

✓ Medulla contains pyramids.

✓ Cortex extensions between pyramids = Columns of Bertini.

✓ Long loop of Henle occurs in juxtamedullary nephrons.

✓ Vasa recta associated with juxtamedullary nephrons.

✓ Ureters transport urine from kidney to bladder.

✓ Urinary bladder stores urine.

✓ Urethra expels urine.

QUICK REVISION TABLE

Structure	Important Functions
Kidney	Urine formation
Hilum	Entry/exit of vessels and ureter
Cortex	Contains renal corpuscles
Medulla	Contains pyramids
Glomerulus	Ultrafiltration
Bowman's capsule	Collects filtrate
PCT	Maximum reabsorption
Loop of Henle	Concentration gradient
DCT	Selective secretion
Collecting duct	Final urine concentration
Ureter	Urine transport
Urinary bladder	Storage
Urethra	Urine elimination

 

MOST IMPORTANT ONE-LINERS

1. Nephron is the structural and functional unit of kidney.
2. Each kidney contains nearly one million nephrons.
3. Renal corpuscle = Glomerulus + Bowman's capsule.
4. PCT is the major site of reabsorption.
5. Juxtamedullary nephrons help in concentrated urine formation.
6. Vasa recta maintains medullary osmotic gradient.
7. Kidneys regulate water, electrolyte and acid-base balance.
8. Renin regulates blood pressure.
9. Erythropoietin stimulates RBC formation.
10. Human beings are ureotelic animals because urea is the major nitrogenous waste product.