Excretory Products and Their Elimination – Class 11 Biology Notes for NEET

This NEET-focused summary of Class 11 Biology Chapter – Excretory Products and Their Elimination provides clear notes on how the human body eliminates nitrogenous wastes like urea through the kidneys. The excretory system, mainly composed of a pair of kidneys, ureters, urinary bladder, and urethra, plays a crucial role in filtration, reabsorption, and secretion via nephrons. Key concepts include glomerular filtration, urine formation, and the hormonal regulation of kidney function by ADH and aldosterone. The chapter also explains micturition, roles of lungs, liver, and skin in excretion, and major kidney disorders like kidney stones, uremia, and renal failure. Life-saving procedures like dialysis and kidney transplantation are also discussed, making this an essential topic for NEET preparation and CBSE,RBSE, etc. Class 11 Biology in detail.

  • Animals constantly produce certain waste substances like ammonia, urea, uric acid, carbon dioxide, water, and mineral ions such as sodium (Na⁺), potassium (K⁺), chloride (Cl⁻), phosphate, and sulfate. These substances are either generated as a result of metabolic activities or are taken in excess through food and water. It is crucial for the survival and internal balance of animals that these waste materials are eliminated either completely or partially. The process of elimination of these wastes is known as excretion, and it plays a vital role in maintaining homeostasis. Among these, nitrogenous wastes like ammonia, urea, and uric acid are especially important. Ammonia is the most toxic and requires a large quantity of water to be excreted. On the other hand, uric acid is the least toxic and can be removed with minimal water loss, making it suitable for animals living in dry environments.
  • The process of removing ammonia from the body is called ammonotelism. Organisms like many bony fishes, aquatic amphibians, and aquatic insects are ammonotelic, meaning they excrete ammonia as their main nitrogenous waste. Because ammonia is highly soluble in water, it can easily diffuse through the body surface or through gills, especially in fishes, in the form of ammonium ions. In such organisms, kidneys play a very minor role in the removal of ammonia. As animals adapted to terrestrial life, they developed ways to conserve water by converting highly toxic ammonia into less toxic forms such as urea or uric acid. Mammals, many terrestrial amphibians, and marine fishes excrete urea and are known as ureotelic animals. In these animals, ammonia produced during metabolism is converted into urea in the liver, which is then transported through the blood to the kidneys, where it is filtered and finally excreted in the urine. Some animals even retain a small amount of urea in the kidney to maintain osmotic balance in body fluids.
  • Reptiles, birds, insects, and land snails produce uric acid as the main nitrogenous waste. They are known as uricotelic animals. Uric acid is excreted in the form of a thick paste or solid pellet with almost no water loss, which is highly beneficial in dry and water-scarce environments. This adaptation helps them survive in terrestrial ecosystems where water conservation is critical.
  • A broad study of the animal kingdom reveals that different animals have evolved various types of excretory structures to perform the function of waste removal. In invertebrates, these structures are usually simple tubular systems, whereas in vertebrates, the excretory organs are more complex and well-developed, mainly in the form of kidneys. For example, protonephridia or flame cells serve as the excretory organs in organisms like Platyhelminthes (flatworms such as Planaria), rotifers, some annelids, and the cephalochordate Amphioxus. These flame cells are mainly responsible for osmoregulation, which means they help maintain the balance of water and ions in the body.
  • Another common excretory organ is the nephridia, found in annelids such as earthworms. These structures are responsible for removing nitrogenous wastes and also help in regulating the internal fluid and ionic balance. In insects, including cockroaches, the excretory function is performed by Malpighian tubules. These tubules not only eliminate nitrogenous wastes but also play an essential role in osmoregulation. Lastly, antennal glands, also called green glands, are found in crustaceans like prawns and help in the excretion of metabolic wastes.

The human excretory system is made up of a few main parts that work together to remove waste materials like urea and maintain the body’s internal balance. This system includes two kidneys, two ureters, one urinary bladder, and a urethra. The kidneys are the primary filtering organs that clean the blood by removing waste and extra water, forming urine. From each kidney, a thin tube called the ureter carries the urine down to the urinary bladder, where it is stored temporarily. When the bladder becomes full, the urine is passed out of the body through a tube called the urethra. Together, these organs form a complete system that helps the body stay healthy by continuously filtering and eliminating harmful substances from the blood in the form of urine.


  • The kidneys in humans are reddish-brown, bean-shaped organs located deep inside the abdomen, on either side of the spine. They are positioned between the last thoracic vertebra and the third lumbar vertebra, and lie close to the inner back (dorsal) wall of the abdominal cavity. Each adult human kidney is about 10–12 cm long, 5–7 cm wide, and 2–3 cm thick, with an average weight ranging from 120 to 170 grams. On the inner side of each kidney, there is a small notch-like opening called the hilum, which serves as an entry and exit point for the ureter, blood vessels, and nerves. Just inside the hilum is a funnel-shaped cavity known as the renal pelvis, which has cup-like extensions called calyces (singular: calyx) that collect urine.
  • The kidney is protected from the outside by a tough outer covering called the renal capsule. Inside, the kidney has two main internal regions: an outer cortex and an inner medulla. The medulla is made up of cone-shaped structures called medullary pyramids, which project into the calyces. The cortex extends between the medullary pyramids in the form of structures known as renal columns or Columns of Bertini.
  • Each kidney contains about one million nephrons, which are the functional units responsible for urine formation. Each nephron has two main parts — the glomerulus and the renal tubule. The glomerulus is a bunch of tiny capillaries formed from the afferent arteriole, which is a small branch of the renal artery. The filtered blood then leaves the glomerulus through the efferent arteriole. Surrounding the glomerulus is a cup-like structure called the Bowman’s capsule. Together, the glomerulus and Bowman’s capsule form the Malpighian body or renal corpuscle.
  • From Bowman’s capsule, the tubule continues as a highly twisted part known as the proximal convoluted tubule (PCT). After the PCT, the tubule forms a U-shaped loop called Henle’s loop, which has two parts — a descending limb and an ascending limb. After this, the tubule continues as another coiled section called the distal convoluted tubule (DCT). The DCTs from several nephrons join into a straight collecting duct, and many such ducts unite and open into the renal pelvis through the medullary pyramids into the calyces, from where urine finally moves out.
  • The Malpighian corpuscle, PCT, and DCT are all located in the cortex region of the kidney, while the Henle’s loop extends into the medulla. Most nephrons have a short Henle’s loop that barely reaches the medulla — these are called cortical nephrons. Some nephrons, however, have a long Henle’s loop that goes deep into the medulla, and these are known as juxtamedullary nephrons.
  • The efferent arteriole coming out of the glomerulus forms a network of capillaries around the renal tubule, known as peritubular capillaries. A small blood vessel from this network runs parallel to the Henle’s loop and forms a U-shaped structure called the vasa recta. This vasa recta is either absent or very small in cortical nephrons, but it is well-developed in juxtamedullary nephrons, where it helps in maintaining water and salt balance during urine formation.

The process of urine formation in humans is a step-by-step mechanism that occurs inside each nephron of the kidney. This process ensures that waste products and excess substances are removed from the blood while important substances like glucose, water, and salts are retained in the right amounts. Urine is formed through three main steps: glomerular filtration, reabsorption, and secretion. These steps take place in different parts of the nephron and work together to produce clean and concentrated urine. Each step has a specific role in filtering the blood and maintaining the body’s internal balance. Glomerular filtration is the first step, followed by selective reabsorption of useful substances, and finally secretion of additional waste products into the urine before it is passed to the collecting duct.

These steps are detailed explained below there-

  1. The process of urine formation begins with glomerular filtration, which takes place in a part of the nephron called the glomerulus. This is the first step where blood is filtered under high pressure. About 1100 to 1200 ml of blood per minute passes through the kidneys for filtration — this is roughly one-fifth of the total blood pumped out by each side of the heart every minute. The high blood pressure in the glomerular capillaries pushes the blood to filter through three layers — the endothelium of blood vessels, the epithelial lining of Bowman’s capsule, and a thin basement membrane between them. The inner lining of Bowman’s capsule has special cells called podocytes, which are arranged in such a way that they leave tiny gaps called filtration slits or slit pores. These slits allow all parts of the blood plasma (except proteins) to pass through into the Bowman’s capsule. Since the filtration is extremely fine, this process is called ultrafiltration.
  2. The total amount of filtrate produced per minute by both kidneys is known as the glomerular filtration rate (GFR). In a healthy person, GFR is about 125 ml per minute, which means the kidneys filter around 180 litres of fluid per day. However, we only excrete about 1.5 litres of urine per day, so about 99% of this filtrate is reabsorbed back into the blood by the kidney tubules. This step is called reabsorption. The cells lining the nephron tubules actively reabsorb useful substances like glucose, amino acids, and sodium ions (Na⁺) using active transport, while other materials like nitrogenous wastes are absorbed passively. Water is also reabsorbed passively, mainly in the earlier segments of the nephron.
  3. Another important step in urine formation is tubular secretion. Here, the cells of the nephron tubules actively secrete additional substances like hydrogen ions (H⁺), potassium ions (K⁺), and ammonia into the forming urine. This step is vital for maintaining the acid–base balance and the ionic composition of body fluids.
  4. The kidneys have an inbuilt control system to maintain a stable GFR. A structure called the juxtaglomerular apparatus (JGA) helps regulate filtration. It is made up of specialized cells found where the distal convoluted tubule and the afferent arteriole meet. If GFR drops, the JG cells release an enzyme called renin, which helps restore blood flow and filtration pressure, bringing the GFR back to normal.

  • The nephron tubules play a vital role in urine formation and maintaining the body’s internal environment. The Proximal Convoluted Tubule (PCT) is the first part of the nephron tubule, lined with simple cuboidal brush border epithelium to increase surface area for absorption. It reabsorbs almost all essential nutrients and about 70-80% of water and electrolytes. It also helps in maintaining blood pH and ionic balance by selectively secreting hydrogen ions (H⁺) and ammonia (NH₃) into the filtrate while reabsorbing bicarbonate ions (HCO₃⁻).
  • The next part, the Loop of Henle, consists of two limbs with different functions. The descending limb is permeable to water but not to electrolytes, so water is reabsorbed, and the filtrate becomes concentrated. In contrast, the ascending limb is impermeable to water but actively or passively allows transport of electrolytes, making the filtrate dilute as it ascends. This helps maintain the high osmolarity of the medullary interstitial fluid, crucial for water reabsorption.
  • The Distal Convoluted Tubule (DCT) allows conditional reabsorption of sodium ions (Na⁺) and water and selectively secretes hydrogen ions (H⁺), potassium ions (K⁺), and ammonia (NH₃) to maintain pH and sodium-potassium balance in blood.
  • Finally, the Collecting Duct runs from the cortex to the medulla and allows a large amount of water to be reabsorbed, concentrating the urine. It also permits a small amount of urea to pass into the medullary interstitium, contributing to osmolarity. Additionally, it helps regulate blood pH and ionic balance by selectively secreting H⁺ and K⁺ ions. Overall, the tubules ensure the reabsorption of useful substances and the removal of waste, maintaining homeostasis.

  • Humans, especially mammals, have a unique ability to produce highly concentrated urine. This is important because it helps conserve water in the body. The main structures that help in this process are the Henle’s loop and vasa recta, which are parts of the nephron (the basic filtration unit of the kidney). These two structures work together through a process known as the counter current mechanism.
  • In the Henle’s loop, the fluid (called filtrate) flows in opposite directions in its two limbs – descending and ascending. Similarly, the blood in the vasa recta also flows in opposite directions in its two limbs. This opposite flow forms what we call a counter current pattern. The close arrangement of Henle’s loop and vasa recta, along with their opposite flow directions, helps create a gradual increase in concentration (osmolarity) in the kidney’s inner region, called the medullary interstitium. This concentration increases from about 300 mOsm/L in the cortex (outer part of kidney) to 1200 mOsm/L in the inner medulla (innermost region).
  • The substances mainly responsible for creating this concentration gradient are sodium chloride (NaCl) and urea. In the ascending limb of Henle’s loop, NaCl is actively transported out into the surrounding interstitial fluid. This NaCl is then picked up by the descending limb of the vasa recta, and later returned to the interstitium by its ascending limb. Similarly, urea also plays a role. Some amount of urea enters the thin ascending limb of Henle’s loop, and is then transported back to the interstitium by the collecting tubule. This cycling of NaCl and urea helps in maintaining a steep concentration gradient in the medullary region.
  • This whole process, where substances like NaCl and urea are moved around in a special way to increase the osmolarity of the inner medulla, is known as the counter current mechanism. This mechanism is very important because it makes it easy for water to move out of the collecting tubule and back into the body. As a result, the filtrate inside the tubule becomes more concentrated. This is how the kidneys are able to produce urine that is up to four times more concentrated than the original filtrate formed in the nephron.

  • The human body has a smart system to regulate how kidneys function, ensuring that fluid and ion balance is always maintained. This regulation happens through hormonal feedback mechanisms, and the main parts involved are the hypothalamus, the Juxta Glomerular Apparatus (JGA), and even the heart to some extent. Whenever there’s a change in the body’s fluid levels or salt concentration, osmoreceptors in the brain detect it. If the body loses a lot of water (due to sweating, bleeding, or diarrhea), these osmoreceptors get activated. They signal the hypothalamus to release a hormone called ADH (Antidiuretic Hormone) or vasopressin from the neurohypophysis (posterior pituitary). ADH’s main job is to help the kidneys reabsorb water from the latter parts of the nephron (especially the collecting duct), so less water is lost in the urine. This prevents diuresis (excess urine production). On the other hand, if there is already enough fluid in the body, these osmoreceptors are turned off and ADH secretion stops, completing the negative feedback loop.
  • Interestingly, ADH also slightly constricts blood vessels, which causes an increase in blood pressure. A higher blood pressure improves glomerular blood flow, which increases the glomerular filtration rate (GFR) — meaning more blood gets filtered through the kidneys.
  • Another major regulatory system is managed by the Juxta Glomerular Apparatus (JGA). If there’s a drop in blood flow to the glomerulus or a decrease in GFR, the JG cells release an enzyme called renin. Renin starts a chain reaction where it converts a protein in the blood called angiotensinogen into angiotensin I, which then changes into angiotensin II. Angiotensin II is a powerful vasoconstrictor — it narrows blood vessels, raising blood pressure and in turn improving GFR. This hormone also signals the adrenal cortex (a part of the adrenal gland) to release aldosterone. Aldosterone increases the reabsorption of sodium (Na⁺) and water from the distal tubule, further boosting blood volume and pressure, and helping to restore normal GFR. This entire system is called the Renin-Angiotensin Mechanism.
  • Lastly, the heart also plays a role. When there is too much blood flow or fluid, the upper chambers of the heart (atria) release a hormone called Atrial Natriuretic Factor (ANF). ANF causes vasodilation (widening of blood vessels), which helps lower blood pressure. It acts as a natural check against the renin-angiotensin system, preventing the body from holding onto too much salt or water and keeping the system in balance.

  • The process of removing urine from the body is known as micturition. After urine is formed by the nephrons in the kidneys, it travels through the ureters and collects in the urinary bladder. This bladder acts like a storage sac and holds urine until the body is ready to release it. As urine fills the bladder, it stretches the bladder walls. This stretching activates special stretch receptors present in the bladder wall. These receptors then send signals to the central nervous system (CNS), mainly the brain and spinal cord. The CNS, in turn, sends motor signals to the bladder muscles to contract, and at the same time, it causes the urethral sphincter (a ring-like muscle that controls urine flow) to relax. This coordinated action results in the release of urine, which is the process of micturition. The complete neural control involved in this process is called the micturition reflex.
  • In a healthy adult, around 1 to 1.5 litres of urine is excreted every day. Normal urine is usually light yellow, watery, and slightly acidic in nature, with a pH of about 6.0. It also has a characteristic smell. On average, the body gets rid of about 25–30 grams of urea per day through urine, which is a major nitrogenous waste product.
  • However, many health conditions can affect the composition, color, and quantity of urine. That’s why urine tests are commonly used in medical diagnosis. For example, the presence of glucose in urine, a condition known as glycosuria, and the presence of ketone bodies (ketonuria) are both clear signs of diabetes mellitus. In this way, urine analysis becomes a useful tool to detect metabolic disorders and kidney problems in patients.

  • Although kidneys are the main organs responsible for removing waste products from the body, there are a few other organs that also play a significant role in excretion. These include the lungs, liver, skin, and even salivary glands. Each of these helps remove certain types of waste materials in their own unique way.
  • The lungs help us excrete a large amount of carbon dioxide (CO₂) — about 200 mL every minute — which is produced during cellular respiration. Along with CO₂, the lungs also expel water vapor, which is another form of waste. This gas exchange is an essential part of maintaining the body’s acid-base balance and removing gaseous waste.
  • The liver, which is the largest gland in the human body, performs several excretory functions. It breaks down old red blood cells and produces substances like bilirubin and biliverdin, which are waste pigments. It also excretes cholesterol, broken-down steroid hormones, vitamins, and drug residues. These substances are packed into bile, which is secreted into the small intestine and eventually eliminated with feces as part of digestive waste.
  • The skin plays its part in excretion through sweat and sebaceous glands. Sweat glands secrete a watery fluid called sweat, which contains sodium chloride (NaCl), small amounts of urea, lactic acid, and other waste products. While the main function of sweat is to cool down the body, it also helps eliminate some nitrogenous and salt-based wastes. On the other hand, sebaceous glands release an oily substance called sebum, which contains sterols, hydrocarbons, and waxes. Though sebum mainly keeps the skin soft and protects it, it also plays a minor role in excreting oily waste.
  • Interestingly, even our saliva may help in removing tiny amounts of nitrogenous waste, although this is not a primary excretory route. Overall, while the kidneys do most of the work, these additional organs provide important support in keeping the body’s internal environment clean and balanced.

  • The excretory system plays a crucial role in filtering out harmful waste products from the blood. When the kidneys fail to function properly, these waste products, especially urea, start accumulating in the blood. This dangerous condition is known as uremia, and if not treated, it can lead to complete kidney failure, which is a life-threatening situation. To manage this condition, a medical process called hemodialysis is used. In hemodialysis, the patient’s blood is taken out from a suitable artery and passed through an external filtering machine called an artificial kidney or dialysing unit. Before entering the machine, an anticoagulant such as heparin is added to prevent the blood from clotting.
  • Inside the dialyser, the blood flows through thin cellophane tubes that are coiled and surrounded by a special fluid called dialysing fluid. This fluid is designed to have the same composition as blood plasma — but without nitrogenous wastes like urea and creatinine. The porous cellophane membrane of the tubes allows only small molecules to pass based on concentration gradient. Since the dialysing fluid has no nitrogenous waste, these wastes naturally move out from the blood into the fluid, effectively cleaning the blood. Once the blood is purified, it is returned to the body through a vein, after adding anti-heparin to stop bleeding. Hemodialysis has become a life-saving solution for thousands of patients suffering from kidney malfunction around the world.
  • However, the best long-term solution for end-stage kidney failure is kidney transplantation. In this method, a healthy kidney from a donor is surgically transplanted into the patient. Ideally, the donor should be a close relative to reduce the risk of organ rejection by the immune system. Thanks to advances in medical technology and immunosuppressive drugs, the success rate of kidney transplantation has improved greatly in recent years.
  • Apart from uremia and kidney failure, there are other common disorders of the excretory system. One such condition is renal calculi, which refers to the formation of kidney stones — hard masses of crystallised salts like oxalates inside the kidneys. These can block the flow of urine and cause severe pain. Another disorder is glomerulonephritis, which is the inflammation of the glomeruli, the tiny filtering units in the kidneys. This condition affects the filtering ability of the kidneys and can lead to swelling, high blood pressure, and blood in the urine.

The human body continuously produces various waste products such as nitrogen-containing compounds, ions, carbon dioxide, and excess water, which need to be removed to maintain internal balance. The type of nitrogenous waste excreted by an organism — whether it is ammonia, urea, or uric acid — largely depends on its habitat and water availability. To remove these wastes, animals have developed different types of excretory organs, including protonephridia, nephridia, Malpighian tubules, green glands, and in vertebrates, kidneys. These organs not only remove metabolic wastes but also help maintain the acid-base balance and ionic composition of body fluids.

In humans, the excretory system consists of two kidneys, two ureters, a urinary bladder, and a urethra. The basic structural and functional unit of the kidney is the nephron, and each kidney contains over one million nephrons. Every nephron has two main parts — the glomerulus, which is a bundle of capillaries formed from the afferent arteriole, and the renal tubule, which starts with a double-walled Bowman’s capsule. The tubule is divided into the proximal convoluted tubule (PCT), Henle’s loop, and distal convoluted tubule (DCT). The DCTs of several nephrons drain into a collecting duct, and many collecting ducts open into the renal pelvis through the medullary pyramids. Together, the Bowman’s capsule and glomerulus form the renal corpuscle or Malpighian body.

The formation of urine in the nephron involves three major steps: filtration, reabsorption, and secretion. Filtration is a passive, non-selective process that occurs in the glomerulus due to high blood pressure in the capillaries. About 1200 mL of blood is filtered every minute to produce 125 mL of filtrate, a rate known as the glomerular filtration rate (GFR). The juxtaglomerular apparatus (JGA) plays a vital role in regulating GFR. Almost 99% of the filtrate is reabsorbed in various parts of the nephron. The PCT is responsible for the maximum reabsorption and some selective secretion. The loop of Henle mainly helps in maintaining an osmotic concentration gradient (from 300 to 1200 mOsmol/L) in the kidney. The DCT and collecting ducts fine-tune water and electrolyte reabsorption, contributing to osmoregulation. The nephron can also secrete H⁺, K⁺, and NH₃ into the filtrate to regulate the pH and ion balance of the body fluids.

One of the most important mechanisms that helps concentrate urine is the counter current mechanism, which occurs between the descending and ascending limbs of Henle’s loop and the vasa recta (a network of capillaries around the loop). As the filtrate moves down the descending limb, it becomes increasingly concentrated, and as it moves up the ascending limb, it becomes diluted again. This system retains salts and urea in the surrounding tissue fluid (interstitium), which allows water to be reabsorbed in the collecting duct and helps form concentrated urine — up to four times more concentrated than the initial filtrate.

The final urine is temporarily stored in the urinary bladder, and when it becomes full, a signal is sent to the central nervous system (CNS) to initiate micturition (urine release) by relaxing the urethral sphincter and contracting the bladder muscles. Apart from kidneys, other organs like the skin, lungs, and liver also contribute to excretion. The lungs remove CO₂ and water vapour, the liver eliminates waste via bile, and the skin helps excrete salts, urea, and water through sweat and sebum.

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