Living organisms produce wastes through metabolic activities and other means like ingestion. These accumulated wastes like ammonia, urea, uric acid, carbon dioxide, and ions like e Na +, K+, Cl–, phosphate, sulphate, etc., need to be eliminated partially or totally from time to time. In this topic, we will learn more about the mechanisms that facilitate the process of excretion. In this blog, we will discuss Class 11 Excretory Products and their Elimination. This topic of Class 11 NCERT Biology will mostly emphasize nitrogenous wastes as they are the commonly eliminated wastes.
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The chapter on excretory products and their elimination talks about the process of ammonotelism. The process of elimination of ammonia is called ammonotelism. The organisms which excrete ammonia are mentioned below.
- Ammonotelic: Bony fishes, aquatic amphibians and aquatic insects.
- Ureotelic (excrete urea): Bony fishes, aquatic amphibians and aquatic insects
- Uricotelic (uric acid): Reptiles, birds, land snails and insects
|Flatworms, some annelids and cephalochordates.||Protonephridia or flame cells.|
|Earthworms and annelids||Nephridia|
|Insects including cockroaches||Insects including cockroaches|
Human Excretory System
According to the chapter on excretory products and their elimination, the human excretory system consists of:
- A pair of Kidneys
- A pair of ureters
- A urinary bladder
- A urethra
Kidneys are situated between the last thoracic and third lumbar vertebra levels close to the dorsal inner wall of the abdominal cavity. They are bean-shaped and reddish-brown in colour. They are 10-12 cm long, 5-7 cm wide, 2-3 cm thick with an average weight of 120- 170 g. Through a notch called the hilum, the ureter, blood vessels, and nerves enter the kidney. The kidney can be divided into 2 layers: the outer cortex and an inner medulla.
- Columns of Bertini when the cortex extends between the medullary pyramids in the form of renal columns, it is called Columns of Bertini.
- Nephrons are the structural unit of the kidney and a kidney has an average of 1 million nephrons. It consists of a glomerulus and a renal tubule.
- A double-walled cup-like structure at the beginning of the renal tubule is called Bowman’s capsule.
- The Glomerulus and the Bowman’s capsule are together called the Malpighian bodies.
- The Proximal Convoluted Tubule is the continuation that forms a highly coiled structure. Henle’s loop is a continuation of the part of the tubule that has an ascending and descending limb.
- The distal convoluted tubule is an ascending limb that continues as another highly coiled tubular region
- Vasa rectum is a minute u shaped vessel that runs parallel to Henle’s loop.
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One of the important topics of Class 11 Excretory Products and their Elimination is Urine Formation. It involves 3 main processes namely:
This involves filtration of blood which is carried out by the glomerulus. An average of 1100-1200 ml of blood is filtered by the kidneys per minute. This process is also called ultrafiltration as almost all the constituents except proteins are filtered onto the lumen of Bowman’s capsule. Glomerular filtration rate (GFR) in a healthy human being is approximately 125 ml/minute, i.e., 180 litres per day.
Approximately 1.5 litres of urine is released every day but the filtrate formed is 180 litres. This means a significant amount of filtrate is reabsorbed in the renal tubule. various substances like glucose, amino acids, Na+ , etc., in the filtrate are reabsorbed actively whereas the nitrogenous wastes are absorbed by passive transport.
- The tubular cells secrete substances like H+ , K + and ammonia into the filtrate during urine formation. This helps with the maintenance of ionic and acid base balance of body fluids.
Function of Tubules
Next, the chapter on class 11 excretory products and their elimination mentions the important functions of the tubules. They are mentioned below.
Proximal Convoluted Tubule (PCT)
The important features of the proximal convoluted tubule are mentioned below.
- It is lined by simple cuboidal brush cells on the border. This increases the surface area of the for reabsorption.
- 70-80 percent of electrolytes and water are reabsorbed by this segment.
- Helps to maintain the pH and ionic balance of the body fluids through selective secretion of hydrogen ions, ammonia and potassium ions into the filtrate.
According to excretory products and their elimination class 11, the important features of the Henle’s loop are mentioned below.
- Minimum reabsorption in its ascending limb. Maintains high osmolarity of medullary interstitial fluid.
- Descending loop – permeable to water but impermeable to electrolytes. Ascending loop – is impermeable to water but allows transport of electrolytes.
- As the concentrated filtrate passes upward, it gets diluted due to the passage of electrolytes to the medullary fluid.
Distal Convoluted Tubule (DCT)
- There is conditional reabsorption of Na+ and water.
- Capable of reabsorption of HCO3 – and selective secretion of hydrogen and potassium ions and NH3. Maintains the pH and sodium-potassium balance.
- It extends from the cortex of the kidney to the inner parts of the medulla.
- Water is reabsorbed from this region to produce a concentrated urine.
- Maintains osmolarity by allowing the passage of small amounts of urea into the medullary interstitium.
- Selectively secrets H+ and K+ ions and maintains ph and ionic balance of the body.
Mechanism of Concentration of Filtrate
Moving ahead in the chapter of excretory products and their elimination, we look at the mechanism of concentration of filtrate. The important features of the same are mentioned below.
- The Henle’s loop and vasa recta play an important role in concentration of urine.
- A countercurrent is formed when the filtrate flows in opposite directions of the 2 limbs of Henle’s loop and vasa recta.
- The countercurrent mechanism helps to maintain a concentration gradient in the medullary interstitium.
- Interstitial gradient helps in an easy flow of water from the collecting tubule hence concentrating the filtrate (urine).
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Role of Kidney Function
Kidney function plays a pertinent role in excretory products and their elimination. It is regulated by hormonal feedback mechanisms involving the hypothalamus, JGA and the heart. Osmoreceptors are activated in the body by changes in blood volume, body fluid and ion concentration. This stimulates the hypothalamus to release ADH or vasopressin. As the glomerular blood pressure decreases, the JG cells are activated to release renin. It converts angiotensinogen to angiotensin I and II .This, inturn, heightened the glomerular blood pressure and released aldosterone to increase Na+ ions and water absorption.
The expulsion of urine from the urinary bladder is called micturition. It is an integral part of the chapter excretory products and their elimination. The neural mechanism that governs it is called the micturition reflex. Till a voluntary signal is given by the CNS, the urine is stored in the urinary bladder. This causes the contraction of smooth muscles of the bladder and a relaxation of the urethral sphincter, causing urine releasee. An adult human excretes up to 1.5 litres of urine per day.
Disorders of the Excretory System
Towards the end of the chapter on excretory products and their elimination, we find out about the disorders of the excretory system. They are as follows:
- Uremia – accumulation of urea in blood due to malfunctioning of kidneys. This can lead to kidney failures. This problem can be remedied with the help of haemodialysis.
- Renal failure – the condition where the glomerular filtration stops and both the kidney’s stop working is called Renal failure. In case of acute failures, kidney transplant is the only option available.
- Renal calculi – when stones or insoluble salts are formed within the kidney, the condition is called renal calculi.
- Glomerulonephritis (Bright’s Disease) – is the condition where there is inflammation of glomeruli in the kidneys. It occurs due to the entry of RBCs or proteins.
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