Excretory Products and their Elimination Class 11 Biology Chapter 16 Notes

Excretory Products and their Elimination Class 11 Biology Chapter 16 Notes

Excretory Structures in the Animal Kingdom

A survey of the animal kingdom reveals a diverse range of excretory structures adapted to suit the specific needs of different organisms. In most invertebrates, these structures are relatively simple tubular forms, whereas vertebrates possess complex tubular organs known as kidneys. Let’s explore some of these excretory structures found in various animal groups:

1. Protonephridia or Flame Cells:

• Found in: Platyhelminthes (Flatworms, e.g., Planaria), rotifers, some annelids, and the cephalochordate Amphioxus.
• Function: Protonephridia are primarily responsible for ionic and fluid volume regulation, a process known as osmoregulation. They help in maintaining the internal balance of ions and fluids within the organism.

2. Nephridia:

• Found in: Earthworms and other annelids.
• Function: Nephridia serve to remove nitrogenous waste products from the body and contribute to the maintenance of a proper fluid and ionic balance. These structures play a crucial role in keeping the internal environment of the organism stable.

3. Malpighian Tubules:

• Found in: Most insects, including cockroaches.
• Function: Malpighian tubules are the primary excretory structures in insects. They play a crucial role in eliminating nitrogenous waste products and are also involved in osmoregulation. Insects face unique challenges due to their exoskeletons, making Malpighian tubules an essential component of their excretory system.

4. Antennal Glands or Green Glands:

• Found in: Crustaceans, such as prawns.
• Function: Antennal glands, also known as green glands, serve as the excretory organs in crustaceans. They are responsible for eliminating waste products and contributing to osmoregulation. These glands are particularly important in the context of aquatic life and are named for their greenish color.

The Human Excretory System

The human excretory system is a complex arrangement of organs designed to remove waste products and regulate the body’s internal environment. This system consists of several vital components:

1. Kidneys:
   • Location: The kidneys are reddish-brown, bean-shaped organs located between the levels of the last thoracic and the third lumbar vertebra, near the dorsal inner wall of the abdominal cavity.
   • Dimensions: Each adult human kidney measures approximately 10-12 cm in length, 5-7 cm in width, and 2-3 cm in thickness, with an average weight ranging from 120 to 170 g.
   • Structure: The inner concave surface of the kidney has a notch called the hilum, through which the ureter, blood vessels, and nerves enter. Inside the kidney, there are two distinct zones, an outer cortex, and an inner medulla. The medulla contains conical masses known as medullary pyramids, which project into structures called calyces. The cortex extends between these medullary pyramids in the form of renal columns referred to as Columns of Bertini.
2. Nephrons:
   • Functional Units: Each kidney houses nearly one million nephrons, which are the functional units of the excretory system.
   • Structure: Each nephron comprises two main parts, the glomerulus and the renal tubule.
     • Glomerulus: This is a tuft of capillaries formed by the afferent arteriole, a fine branch of the renal artery. Blood from the glomerulus is carried away by an efferent arteriole.
     • Renal Tubule: The renal tubule begins with Bowman’s capsule, a double-walled, cup-like structure that encloses the glomerulus. Together, the glomerulus and Bowman’s capsule are called the Malpighian body or renal corpuscle. The tubule continues with a highly coiled network known as the proximal convoluted tubule (PCT). It then features a hairpin-shaped Henle’s loop with a descending and an ascending limb. The ascending limb leads to another highly coiled tubular region called the distal convoluted tubule (DCT). Many DCTs from different nephrons open into a straight tube called the collecting duct, which ultimately converges and opens into the renal pelvis through the medullary pyramids in the calyces.
3. Nephron Location:
   • In the Kidney: The Malpighian corpuscle, PCT, and DCT of the nephron are situated in the cortical region of the kidney. However, the loop of Henle extends into the medulla. There are two main types of nephrons:
     • Cortical Nephrons: In most nephrons, the loop of Henle is relatively short and extends only a small distance into the medulla. These are known as cortical nephrons.
     • Juxta-medullary Nephrons: Some nephrons have a long loop of Henle that runs deep into the medulla. These are referred to as juxta-medullary nephrons.
4. Peritubular Capillaries and Vasa Recta:
   • Peritubular Capillaries: The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule, known as the peritubular capillaries.
   • Vasa Recta: A small vessel from this network runs parallel to Henle’s loop, forming a ‘U’ shaped structure called vasa recta. In cortical nephrons, vasa recta is absent or highly reduced.

The human excretory system, with its intricate nephron structures and the role of the kidneys in filtering and regulating the body’s internal environment, is crucial for maintaining overall health and homeostasis.

Urine Formation: Processes in the Nephron

Urine formation is a vital process in the human body, involving three key processes: glomerular filtration, reabsorption, and secretion, all of which occur at different segments of the nephron within the kidney.

1. Glomerular Filtration:

• Location: This initial step takes place in the glomerulus, a capillary network within Bowman’s capsule.
• Process: The glomerular capillary blood pressure forces blood to be filtered through three layers: the endothelium of glomerular blood vessels, the epithelium of Bowman’s capsule, and a basement membrane separating these two layers.
• Filtration Slits: Epithelial cells in Bowman’s capsule, known as podocytes, create intricate structures with minute spaces called filtration slits or slit pores.
• Ultra Filtration: The filtration is so precise that nearly all the constituents of plasma, except for proteins, pass into the lumen of Bowman’s capsule. This process is often referred to as ultrafiltration.
• Glomerular Filtration Rate (GFR): The amount of filtrate formed by the kidneys per minute is termed the glomerular filtration rate. In a healthy individual, GFR is approximately 125 ml/minute, which translates to a staggering 180 liters per day.

2. Regulation of GFR:

• Juxtaglomerular Apparatus (JGA): The kidneys possess mechanisms for the regulation of GFR, one of which is the juxtaglomerular apparatus. The JGA is a specialized region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole at their point of contact.
• Renin Release: A decrease in GFR can stimulate the JG cells to release renin, which, in turn, increases glomerular blood flow and restores GFR to normal levels.

3. Reabsorption:

• Location: Reabsorption occurs along different segments of the renal tubules within the nephron.
• Process: This step involves the movement of various substances from the filtrate back into the bloodstream. Reabsorption can occur via active or passive mechanisms, depending on the substance.
• Examples: Substances like glucose, amino acids, and sodium ions (Na+) are actively reabsorbed, while nitrogenous waste products are absorbed passively. Water reabsorption, especially in the initial nephron segments, is primarily passive.

4. Tubular Secretion:

• Location: Tubular secretion takes place in the renal tubules, specifically during urine formation.
• Process: During this phase, tubular cells secrete substances such as hydrogen ions (H+), potassium ions (K+), and ammonia into the filtrate.
• Importance: Tubular secretion is crucial for maintaining the ionic and acid-base balance of body fluids.

It’s worth noting that the remarkable difference between the volume of filtrate formed per day (180 liters) and the amount of urine excreted (1.5 liters) underscores the necessity of reabsorption processes, ensuring that nearly 99% of the filtrate is reabsorbed to maintain bodily homeostasis. The combined processes of glomerular filtration, reabsorption, and tubular secretion are essential for the formation of urine and the regulation of the body’s internal environment.

Functions of Renal Tubules in the Nephron

The renal tubules within the nephron of the kidney play distinct roles in the process of urine formation. Here are the functions of each tubule segment:

1. Proximal Convoluted Tubule (PCT):

• Epithelium: The PCT is lined by simple cuboidal brush border epithelium, which increases the surface area for reabsorption.
• Functions:
  • Reabsorption: Nearly all essential nutrients and a significant portion (70-80%) of electrolytes and water are reabsorbed in the PCT. This includes glucose, amino acids, and various ions.
  • pH and Ionic Balance: The PCT contributes to maintaining the pH and ionic balance of body fluids. It selectively secretes hydrogen ions (H+) and ammonia into the filtrate and absorbs bicarbonate ions (HCO3–).

2. Henle’s Loop:

• Reabsorption: Reabsorption in Henle’s loop is minimal in its ascending limb. However, the descending limb of the loop is permeable to water but almost impermeable to electrolytes, concentrating the filtrate as it moves down. The ascending limb is impermeable to water but allows active or passive transport of electrolytes, diluting the filtrate as it moves upward.
• Osmolarity Maintenance: Henle’s loop plays a crucial role in maintaining the high osmolarity of the medullary interstitial fluid.

3. Distal Convoluted Tubule (DCT):

• Conditional Reabsorption: The DCT is responsible for conditional reabsorption of sodium ions (Na+) and water.
• pH and Ionic Balance: It can also reabsorb bicarbonate ions (HCO3–) and selectively secrete hydrogen ions (H+) and potassium ions, as well as ammonia (NH3), to maintain the pH and sodium-potassium balance in the blood.

4. Collecting Duct:

• Length and Reabsorption: The collecting duct extends from the cortex of the kidney to the inner parts of the medulla. This segment allows for substantial reabsorption of water, producing a concentrated urine.
• Osmolarity Maintenance: The collecting duct also allows the passage of small amounts of urea into the medullary interstitium to maintain osmolarity.
• pH and Ionic Balance: It plays a role in the maintenance of the pH and ionic balance of blood by selectively secreting hydrogen ions (H+) and potassium ions (K+).

Mechanism of Filtrate Concentration in the Kidneys

Mammals, including humans, have the remarkable ability to produce a concentrated urine, which is crucial for maintaining water balance and conserving body fluids. The mechanism of concentration involves the interaction of Henle’s loop and the vasa recta, which creates a counter-current system. Here’s how this mechanism works:

1. Counter-Current System:
   • In the nephron, the flow of filtrate in the two limbs of Henle’s loop (the descending and ascending limbs) is in opposite directions, forming a counter-current.
   • Similarly, the flow of blood through the two limbs of the vasa recta (the peritubular capillaries associated with the loop of Henle) also operates in a counter-current pattern.
   • The proximity of Henle’s loop and the vasa recta, along with their counter-current flows, is crucial for creating a concentrated environment in the medullary interstitium.
2. Osmolarity Gradient:
   • Through the counter-current system, a concentration gradient is established within the medullary interstitium. This gradient increases from about 300 mOsmol/L in the cortex to approximately 1200 mOsmol/L in the inner medulla.
   • This osmolarity gradient is primarily created by the movement of sodium chloride (NaCl) and urea.
3. Counter-Current Mechanism:
   • NaCl Transport: NaCl is actively transported out of the ascending limb of Henle’s loop, creating a higher concentration in the medullary interstitium. This NaCl is exchanged with the descending limb of the vasa recta, further contributing to the interstitial concentration.
   • Urea Transport: Small amounts of urea enter the thin segment of the ascending limb of Henle’s loop and are transported back to the interstitium by the collecting tubule.
   • The movement of these substances, facilitated by the unique arrangement of Henle’s loop and vasa recta, is termed the counter-current mechanism.
4. Concentration Gradient’s Role:
   • The presence of the concentration gradient in the medullary interstitium allows for an easy passage of water from the collecting tubule, thus concentrating the filtrate, ultimately forming urine.
   • Human kidneys can produce urine nearly four times more concentrated than the initial filtrate formed due to this concentration mechanism.

Regulation of Kidney Function

• Hormonal feedback mechanisms
  • Hypothalamus – releases antidiuretic hormone (ADH) in response to changes in blood volume, body fluid volume, and ionic concentration.
  • ADH – promotes water reabsorption from the distal tubules of the kidney, preventing diuresis.
  • Osmoreceptors – detect changes in blood osmolarity and stimulate or inhibit ADH release.
• Juxtaglomerular apparatus (JGA)
  • Renin – released by JG cells in response to a decrease in glomerular blood flow, glomerular blood pressure, or GFR.
  • Angiotensinogen – converted to angiotensin I by renin.
  • Angiotensin I – converted to angiotensin II by angiotensin-converting enzyme (ACE).
  • Angiotensin II – powerful vasoconstrictor that increases glomerular blood pressure and GFR.
  • Aldosterone – released by the adrenal cortex in response to angiotensin II.
  • Aldosterone – promotes Na+ and water reabsorption from the distal tubules of the kidney.
• Atrial natriuretic factor (ANF)
  • Released by the atria of the heart in response to increased blood volume.
  • Causes vasodilation and decreases blood pressure.
  • Inhibits renin release and aldosterone secretion.

Overall effect of hormonal regulation

• To maintain blood volume, body fluid volume, and ionic concentration within a narrow range.
• To ensure that the kidneys are functioning optimally.

Micturition: The Process of Urine Elimination

Micturition, commonly known as urination or voiding, is the process by which urine formed by the nephrons in the kidneys is eliminated from the body. This process is controlled by neural mechanisms and involves several steps:

1. Urinary Bladder: Urine from the nephrons is transported to the urinary bladder, where it is stored until the bladder reaches a certain level of distension due to the accumulation of urine.
2. Stretch Receptors: Stretch receptors located in the walls of the urinary bladder are sensitive to changes in bladder volume. As the bladder fills with urine, these receptors are activated.
3. Central Nervous System (CNS): The signals from the stretch receptors are transmitted to the central nervous system (CNS), which includes the brain and spinal cord. The CNS processes these signals and initiates a micturition reflex.
4. Motor Messages: The CNS sends motor messages back to the bladder and the urethral sphincter, a ring of smooth muscle that controls the opening of the urethra.
5. Bladder Contraction: In response to the motor messages from the CNS, the smooth muscles in the bladder wall contract. This contraction increases the pressure inside the bladder, which is essential for expelling urine.
6. Urethral Sphincter Relaxation: Simultaneously, the CNS signals the urethral sphincter to relax, allowing the passage of urine from the bladder to the urethra.
7. Urination: With the bladder muscles contracting and the urethral sphincter relaxed, urine is expelled from the bladder through the urethra and out of the body. This is the actual process of urination or micturition.
8. Control: In adult humans, the process of micturition is typically under voluntary control, meaning that the individual can choose when and where to initiate urination.

On average, an adult human excretes approximately 1 to 1.5 liters of urine per day. The characteristics of urine, such as its color (light yellow), acidity (pH around 6.0), and odor, can vary under different conditions. Medical professionals analyze urine to diagnose various metabolic disorders and kidney malfunctions. For example, the presence of glucose (glycosuria) and ketone bodies (ketonuria) in urine can be indicative of diabetes mellitus.

Role of Other Organs in Excretion

While the kidneys are the primary organs responsible for excretion, several other organs also play a role in eliminating waste products from the body. These additional organs include the lungs, liver, and skin:

1. Lungs:
   • Elimination of Carbon Dioxide (CO2): The lungs are vital for the removal of carbon dioxide, a waste product of cellular respiration. Approximately 200 milliliters of CO2 are removed from the body per minute.
   • Water Elimination: Lungs also assist in the elimination of significant quantities of water vapor during respiration.
2. Liver:
   • Bile Secretion: The liver is the largest gland in the human body and plays a central role in metabolism. It secretes bile, which is stored in the gallbladder. Bile contains various substances, including bilirubin, biliverdin, cholesterol, degraded steroid hormones, vitamins, and drugs.
   • Digestive Waste Elimination: Most of the substances present in bile, along with the waste products, are eventually eliminated from the body through the digestive system.
3. Skin:
   • Sweat Glands: Sweat glands in the skin produce sweat, which is primarily composed of water, sodium chloride (NaCl), small amounts of urea, lactic acid, and other substances. While the primary function of sweat is to regulate body temperature by facilitating cooling, it also helps in the removal of some waste products.
   • Sebaceous Glands: Sebaceous glands in the skin produce sebum, an oily substance that contains sterols, hydrocarbons, and waxes. Sebum provides a protective oily covering for the skin and hair.
4. Saliva:
   • Small Elimination of Nitrogenous Wastes: While not a primary excretory organ, saliva, produced by the salivary glands, can eliminate small amounts of nitrogenous waste products. This elimination through saliva is a minor pathway for the excretion of waste substances.

Disorders of the Excretory System

The excretory system, including the kidneys, can be susceptible to various disorders and conditions. Here are some common disorders of the excretory system:

1. Uremia:
   • Description: Uremia is a condition characterized by the accumulation of urea and other waste products in the blood due to kidney dysfunction.
   • Consequences: Uremia can have severe and harmful effects on the body, potentially leading to kidney failure and affecting various other organ systems.
   • Treatment: Hemodialysis is a common treatment method for removing waste products from the blood in individuals with kidney failure. During hemodialysis, blood is circulated through a dialysis machine (artificial kidney) where it is filtered and returned to the body.
2. Kidney Transplantation:
   • Description: Kidney transplantation is a procedure performed to replace a non-functioning kidney with a functioning one.
   • Indications: It is typically used to correct acute renal failure, end-stage kidney disease, or other severe kidney conditions.
   • Procedure: A kidney from a donor, often a close relative, is transplanted into the recipient to restore kidney function. Advances in transplantation procedures have improved success rates.
3. Renal Calculi:
   • Description: Renal calculi, commonly known as kidney stones, are solid masses formed within the kidney. These stones are typically composed of crystallized salts, such as oxalates.
   • Consequences: Kidney stones can be painful and may obstruct the urinary tract, causing discomfort and possible complications.
   • Treatment: Treatment options for kidney stones may include dietary changes, medications to help pass the stones, or medical procedures like lithotripsy (shock wave therapy) or surgery.
4. Glomerulonephritis:
   • Description: Glomerulonephritis is an inflammatory condition affecting the glomeruli of the kidneys. The glomeruli are the filtering units responsible for removing waste and excess substances from the blood.
   • Consequences: Inflammation of the glomeruli can impair kidney function, leading to proteinuria (the presence of excess proteins in the urine), hematuria (blood in the urine), and impaired filtration.
   • Treatment: Management of glomerulonephritis may include addressing the underlying cause, medications to control inflammation, and measures to protect kidney function.

These are just a few examples of the many disorders and conditions that can affect the excretory system and, in particular, the kidneys. Proper diagnosis, treatment, and management are crucial in addressing these issues and maintaining overall health.