Excretory Products and their Elimination Chapter Notes | Biology Class 11 - NEET PDF Download

All living organisms, ranging from plants to animals, insects, birds, and unicellular organisms, have mechanisms to eliminate their bodily and excess waste. Excretion is a vital process universally conducted by all living beings, essential for collecting and expelling waste products. It serves as the primary method for eliminating nitrogenous by-products generated during bodily metabolism. Metabolic waste and undigested materials stored within the body must be expelled as they can be harmful. Ammonia, urea, and uric acid are the three types of waste products eliminated from the bodies of various creatures.

• Animals accumulate substances like ammonia, urea, uric acid, carbon dioxide, water, and ions such as Na, K, Cl-, phosphate, sulphate, etc.
• Nitrogenous wastes like ammonia, urea, and uric acid are major forms of waste excretion by animals.
• Ammonia is highly toxic and requires a lot of water for elimination. Uric acid, being less toxic, can be removed with minimal water loss.

Types of Excretion in Animals

• Ammonotelism: Bony fishes, aquatic amphibians, and aquatic insects excrete ammonia through diffusion or gills.
• Ureotelism: Mammals, some terrestrial amphibians, and marine fishes mainly excrete urea through kidneys.
• Uricotelism: Reptiles, birds, land snails, and insects excrete uric acid with minimal water loss.

Excretory Structures in Animals

• Protonephridia: Found in Platyhelminthes, rotifers, annelids, and Amphioxus, primarily for osmoregulation.
• Nephridia: Earthworms' excretory structures for waste removal and fluid balance.
• Malpighian Tubules: Insects' excretory structures aiding in waste removal and osmoregulation.
• Antennal Glands: Crustaceans' excretory organs responsible for waste elimination.

Human Excretory System

The human system comprises kidneys, ureters, bladder, and urethra. 

What are the Excretory Organs and Their Functions?

The excretory organs includes the liver, lungs, skin, and kidneys, which together make up the excretory system. The diagram of excretory system in human body shows following organs:

1. Kidneys

• The pair of bean- shaped kidneys are the primary organs of excretion and are situated on both sides of the backbone.
• The blood arteries, nerves and ureters enter the kidneys through hilum.
• The foremost role of the kidneys is to expel excess water and wastes from the bloodstream through the production of urine.

2. Ureters

• These are the tubes that connect each kidney to the bladder.
• Urine is transported from the kidneys to the bladder by this slender, muscular tube by peristaltic contractions.
• Ureters prevent the backflow of urine into the kidneys through the use of ureterovesical valves.

3. Urinary Bladder

• It is a muscular sac-like structure in which urine is stored till micturition.
• The bladder receives urine from the ureters, one from each kidney.

4. Urethra

• It emerges from the urinary bladder and its role is to discharge the urine by micturition.
• It serves as common channel for urine and sperm in males.
• Sphincter muscles protect the opening of the urethra.

Urine Formation in Nephrons

Urine formation involves glomerular filtration, reabsorption, and secretion in different nephron segments. Glomerular filtration rate (GFR) is regulated by the kidneys, with most filtrate reabsorbed back into the bloodstream. Tubular cells secrete substances like H, K, and ammonia into the filtrate, aiding in maintaining body fluid balance.

Mechanism of Concentration of Filtrate

• The loop of Henle and vasa recta facilitate urine concentration through a counter-current mechanism in the medullary interstitium.
• This mechanism enables the kidneys to produce concentrated urine by maintaining an osmolarity gradient.

Regulation of Kidney Function

• The kidneys' function is controlled by hormonal feedback involving the hypothalamus, JGA, and the heart.
• ADH, renin-angiotensin mechanism, and ANF play crucial roles in regulating blood pressure, GFR, and fluid balance.

Micturition Process

Urine, produced by the nephrons, is transported to the urinary bladder for storage until a voluntary signal is initiated by the central nervous system (CNS). This signal is prompted by the bladder's stretching as it fills with urine. Stretch receptors on the bladder walls transmit signals to the CNS, which in turn sends motor messages to contract the bladder's smooth muscles while simultaneously relaxing the urethral sphincter, facilitating urine release. This process, known as micturition

Role of Other Organs in Excretion

• Lungs, liver, and skin assist in eliminating excretory wastes through processes like respiration, bile secretion, and sweat production.
• Various organs contribute to waste removal, ensuring the body maintains proper physiological balance.

Disorders of the Excretory System

• Malfunctions like uremia due to kidney failure can be treated with hemodialysis or kidney transplantation.
• Renal calculi and glomerulonephritis are examples of disorders affecting the excretory system.