CBSE Class 11 Biology Revision Notes Chapter 19

CBSE Class 11 Biology Revision Notes Chapter 19 – Excretory Products and their Elimination

CBSE Class 11 Chapter 19 Biology consists of the study of Excretory Products and their Elimination. In this chapter, students learn about the process of excretion and how the whole process occurs in the human body. For an in-depth and thorough explanation of the concepts of this chapter, students can refer to the CBSE Revision Notes of Class 11 prepared by Extramarks. These notes summarise all the important concepts covered in the chapter in a clear and concise manner to help students with their preparation and last-minute revisions.

Study Revision Notes For Class 11 Biology Chapter 19 – Excretory Products and Their Elimination

Different metabolic reactions that occur in the body lead to the formation of various harmful substances, which need to be excreted. These harmful substances include urea, ammonia, uric acid, water, ions, carbon dioxide, etc.

There are three types of nitrogenous wastes in higher animals: ammonia, urea and uric acid. Out of these, ammonia is the most dangerous and harmful nitrogenous waste.

Animals that expel ammonia as nitrogenous waste are known as ammonotelic animals. These animals include aquatic amphibians and most bony fishes. Ammonia is exonerated via diffusion. Organisms that excrete urea as nitrogenous waste are known as Ureotelic organisms. Amphibians and mammals are some examples of such organisms.

Organisms that excrete uric acid as nitrogenous waste are known as Uricotelic organisms. Examples are reptiles and birds.

Excretory Organs are Found in Many Organisms

Different organisms have different excretory structures. For excretion, paramecium and amoeba have contractile vacuoles. In sponges, the canal system is the term used for excretory systems. The sponges use a Canal system for excretion. The excretory function of coelenteron is found in Hydra. Platyhelminthes contain flame cells. Annelids, like earthworms, contain nephridia. A green-coloured excretory gland is there in Prawns. Malpighian tubules form the excretory system in insects. 

The Excretory System Of Humans

The excretory system in humans consists of two kidneys, a urinary bladder, two ureters and a urethra. 

Kidneys have a bean-like shape and are situated in the abdominal cavity. In humans, the right kidney is a little bit lower than the left kidney. Kidneys have a concave inner surface while the other surface is convex. The hilum is a notch found near the centre of the kidney’s inner concave surface. The blood vessels, ureters and nerves enter through the hilum. The funnel-shaped space inside the hilum is the renal pelvis which has projections known as calyces. The outer and inner sections are the two sections of the kidneys. The cortex is the outer part, and the inner part is called the medulla. The medulla is divided into conical masses called medullary pyramids. 

The nephron is the structural and functional unit of the kidneys. The renal corpuscle and renal tubule are the two major parts of the nephron.

A tuft of capillaries forms a glomerulus. The afferent arteriole transports the blood into the glomerulus. Bowman’s capsule is a cup-shaped structure that holds Glomerulus together. The Bowman’s Capsule and glomerulus are collectively known as the Renal corpuscle or Malpighian body.

Types of Nephrons

Two types of nephrons are Cortical nephrons and Medullary Nephrons:

Cortical Nephrons: When the small portion of the loop of Henle is found in the medulla due to its short length, Cortical Nephrons are formed.

Medullary Nephrons: When the loop of Henle becomes long and spreads into the medulla, Medullary Nephrons are formed.

Urine Production

Glomerular filtration, tubular reabsorption and secretion are the three major steps in urine production.

Glomerular filtration starts as soon as the blood enters the glomerulus through the afferent arteriole. Proteins exit through the efferent arteriole, and water and nitrogenous waste enter the glomerulus and blood cells. The kidneys filter approximately 1100 ml to 1200 ml of blood per minute. Blood is filtered through three different layers due to the glomerular capillary blood pressure. The endothelium is the first layer that surrounds the glomerular blood vessels. Bowman’s capsule epithelium is the second layer, with a basement layer separating the two layers. Epithelial cells of the Bowman’s capsule are called Podocytes arranged in an intricate pattern to leave some tiny spaces called slit pores or filtration slits. The amount of filtrate produced by the kidneys in a minute is called the glomerular filtration rate.

Tubular reabsorption: The process of absorbing the necessary molecules such as amino acids, glucose, or ions like sodium ions, etc., is known as Tubular reabsorption. Some of the substances are absorbed passively, while others are absorbed actively. Amino acids and glucose are actively absorbed, while water is absorbed passively.

Urine Secretion: Secretion is the final step in the formation of urine. Hydrogen ions, potassium ions and ammonia are released to maintain the ionic and acidic balance of the body fluids.

Functions Of Tubules

Proximal Convoluted Tubules (PCT): Simple cuboidal brush border epithelium lines the proximal convoluted tubules (PCT). A larger surface area for reabsorption is provided by such epithelium. In PCT, a majority of the electrolytes and water are reabsorbed. It helps maintain the pH and ionic balance of body fluids through the secretion of hydrogen ions, potassium ions and ammonium ions into the filtrate.

Henle’s Loop: Henle’s loop helps maintain the fluid osmolarity. In the ascending limb, reabsorption is very little. It is selectively permeable to electrolytes. The descending limb absorbs a majority of the water, concentrating the filtrate. The descending limb is nearly impermeable to all electrolytes. As a result, different parts of Henle’s loop absorb differently.

Functions of Tubules of Nephron 

Distal Convoluted Tubule (DCT): The distal convoluted tubule absorbs water, sodium ions and bicarbonate ions to maintain the fluid’s ionic balance while excreting potassium and hydrogen ions.

Collecting Ducts: The collecting duct reabsorbs a large amount of water to concentrate the urine. The collecting duct also secretes hydrogen and potassium ions. It helps keep the blood’s ionic balance and pH stable.

Kidney Function Regulation

The hormones released by the juxtaglomerular apparatus (JGA), hypothalamus and heart are involved in kidney regulation. Any change in ionic balance or blood volume activates the body’s osmoreceptors. Due to this, the hypothalamus produces an antidiuretic hormone (ADH)/ vasopressin. This helps in water reabsorption from the tubules. This increases blood volume and turns off the osmoreceptors via a negative feedback mechanism.

The juxtaglomerular apparatus is activated during a drop in glomerular blood pressure. An enzyme called Renin is released by Juxtaglomerular cells that converts angiotensinogen in the blood to angiotensin I, which is then converted to angiotensin II. Because angiotensin II is a potent vasoconstrictor, it increases glomerular blood pressure. Angiotensin II also stimulates the adrenal cortex to produce aldosterone, which raises sodium ions and water reabsorption from the distal tubules. This procedure is called the renin-angiotensin-aldosterone system (RAAS).

RAAS (Renin-Angiotensin-Aldosterone System) regulates kidney functions

Atrial natriuretic factor (ANF), a polypeptide hormone, is produced by the heart and acts as a vasodilator, thus lowering blood pressure. The renin-angiotensin system functions as a negative feedback mechanism.

Micturition: The process of release or excretion of urine outside the body is known as Micturition. The CNS controls it via a variety of neural mechanisms.

Other Organs in Excretion: The kidneys, lungs, liver and skin work together to excrete wastes such as urea, carbon dioxide, toxic substances, etc.

Excretory System Dysfunctions:

Uremia: Uremia: Accumulation of urea in the blood due to kidney failure is called Uremia.

Acute renal failure: Acute renal failure is the condition when one or both kidneys fail to filtrate the urine and cannot function for various reasons.

Renal calculi: Renal calculi are insoluble kidney stones caused by the accumulation of insoluble crystals like oxalates.

Glomerulonephritis: It is an inflammation of the Glomerulus.

Hemodialysis can remove excess urea from the blood in cases of kidney failure. Blood is removed from the body via cellophane tubules and dialysed against an isotonic liquid to remove wastes before being pumped back into the body.

Overview of Class 11 Excretory Products and Their Elimination Notes

NCERT Class 11 Biology Chapter 19 notes include the following topics.

1. An Introduction to Excretory Products And Excretory Organs

Class 11 Biology Chapter 19 notes let students learn about various excretory products and their elimination process.

In our bodies, various metabolic reactions result in excretory products. These products include substances such as ammonia, carbon dioxide, urea, uric acid, water, etc. Ammonia is one of the most harmful and dangerous nitrogenous wastes. Animals are classified as ammonotelic, uricotelic and ureotelic based on their excretory products. Mammals are ureotelic as they excrete urea.

Different organisms have different excretory structures, such as canal systems in sponges, contractile vacuoles in paramecium, Malpighian tubules in insects, and flame cells in Platyhelminthes.

2. Human Excretory System

• The excretory system in humans consists of kidneys which are bean-shaped organs situated in the abdominal cavity. The blood vessels, ureters and nerves enter the kidneys via the hilum. Kidneys are divided into two sections which are the outer cortex and inner medulla. Conical masses exist in the medulla, referred to as medullary pyramids.
• The functional and structural unit of the kidneys is known as nephrons. There are two main parts of nephrons: one is the renal corpuscle, and the second is the renal tubule.
• The glomerulus with the bowman’s capsule forms the renal corpuscle or Malpighian body. A glomerulus is a group of blood capillaries. The afferent arteriole brings blood into it, and the efferent arteriole gets the blood out of it. Bowman’s capsule is a cup-shaped structure that covers the glomerulus.
• The renal tubules consist of the Proximal Convoluted tubule (PCT) extending to form Henle’s loop, which consists of ascending and descending limbs. The ascending limb forms the Distal Convoluted Tubules (DCT), which connect to the collecting duct. 

In this topic, students will also learn about cortical nephrons and medullary nephrons in detail.

3. Mechanism of Urine Formation

This section of CBSE Class 11 Biology Chapter 19 notes covers the steps involved in urine formation: 

• Glomerular Filtration: The blood that enters the glomerulus is filtered, and all the nitrogenous wastes and water move into the glomerulus. The kidneys filter about 1.2 litres of blood per minute. The filtrate rate produced by the kidneys in a minute is called Glomerular Filtration Rate (GFR).

• Reabsorption: In this step, all the molecules and ions that the body may require, such as glucose, amino acids, etc., are absorbed actively or passively. 
• Secretion: This step involves the secretion of potassium and hydrogen ions and ammonia into the tubules to maintain the ionic balance of the body fluids.

4. The Function of The Tubules

In this part of the notes of Class 11 Biology Chapter 19, students will study the functions of the renal tubules and their structure:

• PCT (Proximal Convoluted Tubule): PCT enables most electrolytes and water reabsorption. It is lined with brush border epithelium, which raises the surface area for reabsorption. It also secretes potassium, ammonium and hydrogen ions into the filtrate to maintain the body’s ionic balance. 

• Loop of Henle: The loop of Henle helps maintain the osmolarity of the body fluids. Its two limbs are descending and ascending. The ascending limb is permeable to electrolytes but impermeable to electrolytes, and therefore very less reabsorption of water occurs here. 

• Distal Convoluted Tubule (DCT): In DCT, the reabsorption of water, bicarbonate ions and sodium ions occur while potassium and sodium are secreted to maintain the ionic balance of the fluid.
• Collecting Duct: Water is reabsorbed in the collecting duct to concentrate the urine, and secretion of ions such as hydrogen and potassium also occurs here. It is responsible for maintaining the pH and ionic balance of the blood.

5. Kidney Function Regulation

In this part of the Class 11 Biology chapter, 19 notes students get the details of the regulatory mechanisms involved in the kidney’s proper functioning. 

• Hypothalamus: Due to the disruption in the blood volume or ionic balance, osmoreceptors get activated, stimulating the hypothalamus. Antidiuretic Hormone (ADH) is secreted by the hypothalamus, which reabsorbs the water, raising blood volume. The same osmoreceptors are closed through the negative feedback mechanism as the blood volume increases.

• RAAS (Renin Angiotensin Aldosterone System): When there is a decrease in glomerular blood pressure, it activates the JuxtaGlomerular Apparatus (JGA). The JGA cells then release the enzyme renin, which helps convert angiotensinogen to angiotensin I. Angiotensin I is converted into angiotensin II, a vasoconstrictor. As a result, the glomerular blood pressure increases, and then this angiotensin II acts on the adrenal cortex to release a hormone called aldosterone. Aldosterone increases the absorption of water and sodium ions from the DCT. 
• Heart: Atrial Natriuretic Factor (ANF) is a hormone secreted by the heart, decreasing blood pressure by increasing vasodilation. It acts as negative feedback to the RAAS mechanism.

6. Micturition

In this section, students understand Micturition. It is the process by which urine is released from the body. The Central Nervous System or CNS regulates this process of Micturition.

7. Role of Other Organs in Excretion

Other organs like skin, lungs and liver also play an important role in excreting toxic substances from the body. All this has been explained in chapter 19 of Biology Class 11 notes.

8. Disorders of the Excretory System 

In this part, the various excretory system disorders are covered. Uremia is a disorder in which excessive urea accumulation occurs, resulting in the malfunctioning of kidneys. Acute renal failure takes place in one or both the kidneys, which fail the kidneys to function.