Biology UPSC, Human Excretory System UPSC CSE, Biology Notes UPSC
Table of Contents
Human Excretory System UPSC & Other Exams
Excretory System
During normal cellular activities, cells release waste products into the bloodstream. These waste products are toxic and must be removed to maintain the body’s health. The process of eliminating these cellular wastes is called excretion, and the organs involved constitute the excretory system.
Excretory Products and their Elimination
Animals produce and accumulate various waste products such as ammonia, urea, uric acid, carbon dioxide, water, and ions like Na⁺, K⁺, Cl⁻, phosphate, and sulphate. These substances arise from metabolic activities or through other means like excess ingestion. To maintain homeostasis, these wastes must be removed either completely or partially.
Forms of Nitrogenous Wastes
The method of nitrogenous waste excretion depends on the availability of water.
Ammonia:
- Highly toxic and requires large amounts of water for elimination.
- Excreted by aquatic animals such as bony fishes, amphibians, and aquatic insects through diffusion across body surfaces or gills as ammonium ions.
- Such animals are termed ammonotelic.
Urea:
- Less toxic than ammonia and requires moderate amounts of water for excretion.
- Produced by mammals, terrestrial amphibians, and marine fishes.
- Ammonia generated by metabolism is converted into urea in the liver through the urea cycle and filtered out by the kidneys.
- These animals are known as ureotelic.
- Some mammals retain a small amount of urea in the kidney matrix to maintain osmolarity.
Uric Acid:
- The least toxic nitrogenous waste, excreted with minimal water loss.
- Found in reptiles, birds, land snails, and insects, where it is excreted as a semi-solid paste or pellets.
- These animals are called uricotelic.
Human Excretory System
The human excretory system is responsible for the removal of waste products from the body to maintain homeostasis and prevent damage caused by the accumulation of toxins. In humans, the excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra. The primary organ involved in this system is the kidney.
Components of the Human Excretory System
Components of the Excretory System
Kidneys: Filter blood and produce urine.
Ureters: Transport urine from the kidneys to the bladder.
Urinary Bladder: Stores urine temporarily.
Urethra: Carries urine from the bladder to the outside of the body.
Urine Flow:
Urine formed in the kidneys travels to the urinary bladder through tube-like ureters.
It is temporarily stored in the urinary bladder.
When the bladder is full, urine is expelled from the body through the urethra, a muscular tube that ends at the urinary opening.
Urine Composition:
Volume: An adult human typically excretes 1–1.8 liters of urine daily.
Contents:
95% water
2.5% urea
2.5% other waste products, including salts and toxins.
The excretory system plays a crucial role in maintaining the body’s internal environment by removing toxic wastes and ensuring the balance of water and electrolytes.
Kidneys
Blood containing both useful and harmful substances flows to the two kidneys. In the kidneys, blood capillaries filter the waste products. Useful substances like glucose, salts, and water are reabsorbed into the bloodstream. The remaining waste materials, including urea, excess salts, and water, are excreted as urine.
Humans have two reddish-brown, bean-shaped kidneys located in the abdominal cavity (between the last thoracic and third lumbar vertebrae), on either side of the spine. This organ consists of multiple components such as Capsule, Henle’s Loop, and Nephrons.
Each kidney in an adult human measure approximately 10–12 cm in length, 5–7 cm in width, and 2–3 cm in thickness, with an average weight of 120–170 g. On the inner concave surface of the kidney lies a notch called the hilum, which serves as the entry point for the ureter, blood vessels, and nerves. Beneath the hilum is a broad, funnel-shaped space called the renal pelvis, which has projections known as calyces.
Structure of the Kidney:
- Outer surface: Convex in shape.
- Inner surface: Concave in shape.
- The kidneys are usually slightly asymmetric in position and size due to the position of the liver.
- Position: The right kidney is typically lower and more medial than the left kidney. The left kidney is usually located at the vertebral level T12 to L3, while the right kidney is slightly lower.
- Size: The left kidney is usually larger than the right kidney. The left kidney has a renal volume of about 146 cm3, while the right kidney has a volume of about 134 cm3.
- Each kidney contains functional units called nephrons, which filter blood and form urine.
- The kidney consists of two distinct zones: Outer Cortex and Inner Medulla
- The medulla is divided into conical structures known as medullary pyramids, which project into the calyces.
- Each kidney contains nearly one million nephrons, which are the functional units responsible for filtration and excretion.
Structure of the Nephron
- A nephron comprises two main parts: Glomerulus and Renal Tubule
- Glomerulus: The glomerulus is a tuft of capillaries formed by the afferent arteriole (a branch of the renal artery). Blood is carried away from the glomerulus by the efferent arteriole. The glomerulus is enclosed by a double-walled cup-like structure called the Bowman’s capsule. Together, the glomerulus and Bowman’s capsule form the Malpighian body or renal corpuscle. The efferent arteriole emerging from the glomerulus forms a network of capillaries around the renal tubule called the peritubular capillaries.
- Renal Tubule: The renal tubule begins with the Bowman’s capsule and extends further to form the following regions:
- Proximal Convoluted Tubule (PCT): A highly coiled segment close to the Bowman’s capsule.
- Henle’s Loop: A U-shaped structure with descending and ascending limbs.
- Distal Convoluted Tubule (DCT): Another coiled segment that continues from the ascending limb of Henle’s loop. The DCTs of multiple nephrons open into a collecting duct, which eventually drains into the renal pelvis through the medullary pyramids.
- One end of the tubule is attached to Bowman’s capsule and the other to the urine collecting duct of the kidney.
Function of the Kidney:
- Remove nitrogenous wastes such as urea, uric acid, and creatinine.
- Maintain water and electrolyte balance (osmoregulation).
- Regulate blood pressure through hormones like renin.
- Produce erythropoietin, which stimulates red blood cell production.
Urine Formation – Filtration, Reabsorption & Secretion
Urine formation is a complex process that involves three main steps: glomerular filtration, reabsorption, and secretion, occurring in different parts of the nephron.
Blood enters the kidneys through the renal artery. Nephrons filter blood in the glomerulus of the kidney.
Glomerular Filtration
The first step in urine formation is the filtration of blood by the glomerulus, a process known as glomerular filtration.
The rate of filtrate formation by the kidneys is referred to as the glomerular filtration rate (GFR).
In a healthy individual, GFR is approximately 125 mL/min, or about 180 liters per day.
The kidneys regulate GFR efficiently through mechanisms like the juxtaglomerular apparatus (JGA).
Reabsorption
Of the 180 liters of filtrate formed daily, only 1.5 liters of urine is excreted, indicating that nearly 99% of the filtrate is reabsorbed by the renal tubules.
After glomerular filtration, filtrate passes into the renal tubule. Where useful substances like glucose, amino acids, and water are reabsorbed into the blood.
Reabsorption is carried out by the tubular epithelial cells in different segments of the nephron, either through active or passive mechanisms:
- Active Reabsorption: Substances like glucose, amino acids, and Na⁺ are reabsorbed actively.
- Passive Reabsorption: Nitrogenous wastes and water are reabsorbed through passive transport.
- Water Reabsorption: Occurs passively in the initial segments of the nephron.
Tubular Secretion
During urine formation, tubular cells also secrete substances like H⁺, K⁺, and ammonia into the filtrate. Tubular secretion is crucial for maintaining the ionic balance and the acid-base balance of body fluids.
Summary of Processes
Glomerular Filtration: Blood is filtered to produce ultrafiltrate (~180 liters/day).
Reabsorption: Nearly 99% of the filtrate is reabsorbed, ensuring essential substances are retained.
Secretion: Tubular cells add waste products and ions to the filtrate to maintain homeostasis.
Excretion: The final urine, containing water, urea, and other wastes, is collected in the renal pelvis, transported via ureters, stored in the bladder, and expelled through the urethra.
Together, these processes ensure the kidneys effectively filter waste while maintaining the body’s fluid and electrolyte balance.
Regulation of Kidney Function
The functioning of the kidneys is intricately regulated by hormonal feedback mechanisms involving the hypothalamus, the juxtaglomerular apparatus (JGA), and, to a certain extent, the heart.
Role of Osmoreceptors and ADH
Osmoreceptors in the body are sensitive to changes in blood volume, body fluid levels, and ionic concentration.
When there is excessive fluid loss from the body, these receptors are activated, stimulating the hypothalamus to release antidiuretic hormone (ADH) (also called vasopressin) from the neurohypophysis.
ADH Function:
Enhances water reabsorption in the latter parts of the renal tubule, reducing urine output (antidiuresis).
- Has a vasoconstrictory effect on blood vessels, increasing blood pressure.
- Increased blood pressure improves glomerular blood flow and enhances the glomerular filtration rate (GFR).
Conversely, an increase in body fluid volume deactivates osmoreceptors, suppressing ADH release, completing the feedback mechanism.
Role of the Juxtaglomerular Apparatus (JGA)
The JGA plays a vital role in regulating kidney function, particularly in response to changes in glomerular blood flow, blood pressure, or Glomerular Filtration Rate (GFR):
Activation of Renin Release:
- A decrease in glomerular blood flow, blood pressure, or GFR activates the juxtaglomerular (JG) cells to release renin.
- Renin catalyzes the conversion of angiotensinogen (a protein in blood) to angiotensin I, which is further converted to angiotensin II by the action of enzymes.
Effects of Angiotensin II:
- Vasoconstriction: Angiotensin II is a potent vasoconstrictor, increasing glomerular blood pressure and, consequently, GFR.
- Stimulation of Aldosterone Release: Angiotensin II stimulates the adrenal cortex to secrete aldosterone, which promotes the reabsorption of Na⁺ and water in the distal parts of the renal tubule. This increases blood volume, blood pressure, and GFR.
This feedback system is collectively referred to as the Renin-Angiotensin Mechanism.
Role of the Heart: Atrial Natriuretic Factor (ANF)
When blood flow to the atria of the heart increases, the atrial walls release Atrial Natriuretic Factor (ANF).
Effects of ANF:
- Causes vasodilation (widening of blood vessels), which reduces blood pressure.
- Acts as a counter-regulatory mechanism to the renin-angiotensin system, preventing excessive increases in blood pressure.
Summary of Regulatory Mechanisms
ADH Mechanism: Regulates water retention and blood pressure through osmoreceptor activation.
Renin-Angiotensin Mechanism: Manages blood pressure and GFR through vasoconstriction and aldosterone-mediated fluid retention.
ANF Mechanism: Counters high blood pressure by promoting vasodilation and reducing fluid retention.
These mechanisms work together to ensure optimal kidney function and maintain homeostasis in the body.
Ureters
Tubes that transport urine from the kidneys to the urinary bladder.
Structure:
- Thin, muscular tubes approximately 25–30 cm long.
Function:
- Use peristaltic movements to push urine to the bladder.
Urinary Bladder
Structure:
- A hollow, elastic organ located in the pelvic cavity.
- Can store about 400–600 ml of urine.
Function:
- Temporarily stores urine before excretion.
- Muscles of the bladder contract during urination to release urine through the urethra.
Urethra
A tube that carries urine from the bladder to the exterior of the body.
Structure:
- Longer in males (~20 cm) and serves dual functions (urine and semen transport).
- Shorter in females (~4 cm).
Function:
- Excretes urine from the body.
- Excretion Process
Role of Other Organs in Excretion
While the kidneys are the primary excretory organs, other organs like the lungs, liver, skin, and even the gastrointestinal tract play vital roles in waste elimination.
Lungs:
– The lungs eliminate large amounts of carbon dioxide (approximately 200 mL/min) and significant quantities of water vapor during exhalation.
– This process is essential for maintaining the acid-base balance in the body.
Liver:
Urea Formation:
– The liver plays a critical role in the detoxification of ammonia, a byproduct of protein metabolism, through the urea cycle (or ornithine cycle).
– Ammonia is converted into urea, which is less toxic and water-soluble, and then transported to the kidneys for excretion.
Bile Secretion:
– The liver produces bile, which contains waste substances like bilirubin, biliverdin, cholesterol, degraded steroid hormones, and drugs.
– These substances are excreted along with feces through the intestines.
Skin:
Sweat Glands:
– Sweat glands excrete water, sodium chloride (NaCl), urea, lactic acid, and other waste products.
– While the primary function of sweating is to regulate body temperature, it also aids in waste elimination.
Sebaceous Glands:
– Sebaceous glands secrete sebum, which contains sterols, hydrocarbons, and waxes. This oily secretion helps protect the skin while excreting certain lipid-soluble wastes.
Salivary Glands:
– Small amounts of nitrogenous wastes are excreted in saliva, though this is a minor pathway for waste elimination.
Gastrointestinal Tract:
– The gastrointestinal tract excretes undigested food residues and metabolic wastes, such as bile pigments, through feces.
– Some heavy metals and waste substances are also excreted via the digestive system.
Tears:
The lacrimal glands (tear glands) can remove small amounts of waste, such as salts.
Hair and Nails:
Certain waste products like heavy metals and keratinized materials can be expelled through hair and nail growth.
Together, these organs complement the kidneys by ensuring efficient waste removal and maintaining homeostasis in the body.
Associated Disorders
Kidney Stones:
Hard deposits of minerals and salts in the kidneys.
Urinary Tract Infections (UTIs):
Infections in the urethra, bladder, or kidneys.
Chronic Kidney Disease (CKD):
Gradual loss of kidney function.
Dialysis:
When the kidneys fail due to infection or injury, waste products accumulate in the blood, causing life-threatening conditions. In such cases, dialysis, an artificial process of filtering blood, is necessary for survival.
Other Important Facts
Composition of Urine: Urine is composed of approximately 96% water, 2% urea, and 2% uric acid.
pH of Urine: The average pH value of human urine is 6.0 (mildly acidic). However, it can range from 4.5 to 8.0, depending on various conditions such as diet and health.
Color of Urine: The light yellow color of urine is due to the presence of a pigment called urochrome.
Color of Feces: The characteristic color of feces is attributed to bilirubin, a component of bile salts.
Kidney Stones: Kidney stones are primarily composed of calcium oxalate.
Urea Production and Filtration: Urea is produced in the liver and is filtered (separated from the blood) by the kidneys.
Ammonia-like Smell in Urine: The ammonia-like smell in urine is caused by the conversion of urea into ammonia by bacteria. If urine remains in the bladder for a prolonged period, this conversion can occur, leading to the distinctive smell.
Prelims PYQs on Human Excretory System UPSC & Other Exams
Coming Soon.
Courtesy eMock Test – https://emocktest.in/
FAQs on Human Excretory System UPSC & Other Exams
Coming Soon.
Human Digestive System UPSC
Biology Notes UPSC
Science UPSC. Biology Notes.