Excretory Products and Their Elimination

What is Excretory Products and Their Elimination?

The human body constantly performs various metabolic activities such as respiration, digestion, and circulation. These activities produce not only useful substances like energy and enzymes but also harmful by-products. If these waste materials are not removed from the body in time, they can become toxic and interfere with normal biological functions.

What is Excretion?

Excretion is the biological process of eliminating metabolic waste products from the body. These wastes primarily include nitrogenous substances like ammonia, urea, and uric acid, which are formed as by-products of protein and nucleic acid metabolism.

Why is Excretion Necessary?

1. To maintain homeostasis (internal balance)
2. To prevent accumulation of harmful substances
3. To regulate ionic balance, pH, and fluid volume

Types of Excretory Products

All living organisms carry out metabolic reactions to sustain life. These biochemical processes, especially those involving proteins and nucleic acids, generate nitrogenous waste products that are harmful if allowed to accumulate. The form of nitrogenous waste produced and eliminated varies among different organisms and depends largely on their habitat, water availability, and evolutionary adaptations.

Main Types of Excretory Products

The major nitrogenous excretory products found in animals are:

1. Ammonia (NH₃)
2. Urea
3. Uric Acid

1. Ammonia (NH₃)

Highly toxic and requires a large amount of water for excretion.
Organisms: Ammonotelic
Examples: Bony fishes, amphibians, aquatic insects

2. Urea

Less toxic, soluble in water.
Requires less water compared to ammonia.
Organisms: Ureotelic
Examples: Humans, mammals, amphibians, cartilaginous fishes

3. Uric Acid

Least toxic and almost insoluble in water.
Conserves water – excreted as paste or crystals.
Organisms: Uricotelic
Examples: Birds, reptiles, insects

Human Excretory System

The human body produces a variety of waste materials during metabolic processes, especially from the breakdown of proteins and nucleic acids. To maintain internal balance (homeostasis) and prevent toxicity, these wastes must be efficiently removed from the body.

The human excretory system plays a vital role in this function by filtering blood, forming urine, and eliminating nitrogenous wastes, excess water, and salts from the body.

Main Excretory Organ in Humans:

The Kidney

The kidney is the primary excretory organ in humans. Each human has a pair of kidneys located in the abdominal cavity, on either side of the vertebral column. These organs filter blood and produce urine.

Parts of the Excretory System:

Kidneys – Filter blood to form urine.
Ureters – Transport urine from kidneys to bladder.
Urinary bladder – Temporarily stores urine.
Urethra – Releases urine from the body.

Functions of the Human Excretory System

Elimination of nitrogenous wastes like urea, uric acid, and creatinine
Osmoregulation – maintaining water and salt balance
Regulation of blood pH
Maintenance of blood pressure (via renin hormone)
Hormonal function (e.g., erythropoietin production)

Structure of the Kidney

The kidney is the primary organ of the human excretory system. It performs the essential function of filtering blood and forming urine, which helps in the elimination of metabolic wastes. Each person has two bean-shaped kidneys, located in the abdominal cavity on either side of the vertebral column, just below the diaphragm.

Excretory Products and Their Elimination

Location and Size

Each kidney is about 10–12 cm long, 5–7 cm wide, and weighs around 120–170 grams.
The right kidney is usually slightly lower than the left to accommodate the liver.

External Structure

Bean-shaped organ with:
A convex lateral surface
A concave medial surface containing the hilum (entry/exit point for blood vessels, nerves, and ureter)

Internal Structure (Longitudinal Section)

The kidney is internally divided into three main regions:

1. Cortex (Outer Region)

Light-colored, granular region
Contains Bowman’s capsule and convoluted tubules of nephrons

2. Medulla (Middle Region)

Darker, striated region
Composed of 8–18 renal pyramids (cone-shaped structures)
Pyramids open into calyces, which collect urine

3. Renal Pelvis (Inner Region)

Funnel-shaped cavity that collects urine from calyces
Connected to the ureter

Blood Supply

Renal artery brings oxygenated, unfiltered blood to the kidney.
Renal vein carries filtered, deoxygenated blood away.
Each kidney receives about 1/4 of the cardiac output to ensure efficient filtration.

Functions of the Kidney (Briefly)

Filtration of blood
Formation and concentration of urine
Regulation of electrolytes, water balance, and blood pressure
Secretion of hormones like renin and erythropoietin

Nephron – The Structural and Functional Unit

The nephron is the basic structural and functional unit of the kidney. It is responsible for filtering blood, forming urine, and helping in osmoregulation and excretion. Each human kidney contains about 1 to 1.5 million nephrons, ensuring efficient and continuous blood filtration.

Why is the Nephron Important?

The nephron performs critical tasks such as:
Removing nitrogenous wastes (like urea)
Regulating the ionic and water balance of the body
Maintaining blood pressure and pH
Basic Structure of a Nephron

A nephron is composed of two main parts:

1. Renal Corpuscle

Bowman's Capsule: A cup-like structure that surrounds a capillary network.
Glomerulus: A tuft of capillaries inside the Bowman's capsule; site of blood filtration.

2. Renal Tubule

Proximal Convoluted Tubule (PCT): Site of reabsorption of glucose, amino acids, and water.

Loop of Henle:

Descending limb: Permeable to water
Ascending limb: Permeable to salts
Distal Convoluted Tubule (DCT): Involved in selective secretion and reabsorption.
Collecting Duct: Receives urine from many nephrons and transports it to the renal pelvis.

Main Processes Performed by the Nephron

Ultrafiltration (in Bowman's capsule)
Selective Reabsorption (mainly in PCT)
Tubular Secretion (in DCT)
Urine Concentration (in loop of Henle and collecting duct)

Urine Formation

Three Main Steps: Filtration, Reabsorption, and Secretion

The process of urine formation is one of the most essential functions performed by the nephrons in the kidneys. It involves removing metabolic wastes, excess salts, and toxins from the blood while conserving water and useful substances like glucose and amino acids.

Urine formation is a step-wise process that ensures homeostasis by maintaining the body’s water-electrolyte balance and eliminating nitrogenous wastes like urea and uric acid.

The Three Main Steps of Urine Formation

1. Glomerular Filtration

Also called ultrafiltration
Occurs in the Bowman's capsule
Blood pressure forces water and small solutes (glucose, salts, urea) out of the glomerulus into the glomerular filtrate
Large molecules like proteins and blood cells are not filtered

2. Tubular Reabsorption

Takes place mainly in the Proximal Convoluted Tubule (PCT)
Selective reabsorption of useful substances like glucose, amino acids, and water
Helps conserve essential nutrients and water

3. Tubular Secretion

Occurs in the Distal Convoluted Tubule (DCT) and collecting duct
Involves active secretion of ions (K⁺, H⁺), drugs, and creatinine into the tubule
Maintains acid-base balance and helps in detoxification

Urine Composition and Elimination

The final step in the excretory process is the elimination of urine, which is the liquid waste product formed by the kidneys after filtering the blood. Urine formation involves the careful balance of filtration, reabsorption, and secretion in the nephrons.

Once urine is formed, it must be stored and eliminated from the body through a well-coordinated system. This process not only removes metabolic waste products but also helps maintain water balance, electrolyte concentration, and blood pH.

Composition of Normal Urine

Urine is a clear to yellowish fluid with the following composition:

Component Approximate % or Range