These waste products can be nitrogenous (containing nitrogen), such as ammonia, urea, and uric acid, or non-nitrogenous, such as carbon dioxide, excess water, and salts.
Nitrogenous Wastes:
- Ammonia: The most toxic form of nitrogenous waste, requiring a large amount of water for elimination. Excreted by aquatic animals like bony fishes, aquatic amphibians, and aquatic insects.
- Urea: Less toxic than ammonia, requires less water for elimination.
- Uric Acid: The least toxic form, requires the least amount of water for elimination. Excreted by birds, reptiles, and insects.
Excretory Organs:
- Protonephridia: Found in flatworms, they are simple tubular structures that filter waste products from the body.
- Nephridia: Found in earthworms and other annelids, they are paired organs that filter waste products and maintain fluid balance.
- Malpighian Tubules: Found in insects, they are thin, branching tubes that collect waste products from the hemolymph.
- Green Glands: Found in crustaceans, they filter waste products from the blood.
- Kidneys: The primary excretory organs in vertebrates, responsible for filtering waste products from the blood and producing urine.
Human Excretory System:
- Kidneys: A pair of bean-shaped organs located on either side of the spine.
- Urinary Bladder: A muscular sac that stores urine.
- Urethra: A tube that carries urine from the bladder out of the body.
The process of urine formation involves three main steps:
- Glomerular Filtration: Blood is filtered through the glomerulus, a network of capillaries in the nephron.
- Tubular Reabsorption: Useful substances like glucose, amino acids, and water are reabsorbed from the tubules back into the blood.
- Tubular Secretion: Waste products and excess ions are secreted from the blood into the tubules.
The final urine is then transported through the ureters to the urinary bladder and eventually eliminated from the body through the urethra.
Factors affecting urine formation:
- Blood pressure: Higher blood pressure leads to increased filtration rate.
- Hormones: Hormones like ADH (antidiuretic hormone) regulate water reabsorption in the tubules.
- Diet: The amount and type of food and fluids consumed can affect urine composition.
- Exercise: Increased physical activity can lead to increased urine production.
Exercise
1. Define Glomerular Filtration Rate (GFR)
Ans : It represents the volume of fluid filtered through the glomeruli (tiny blood vessels) in the kidneys per unit time. A normal GFR is typically considered to be between 90 and 120 milliliters per minute (mL/min).
2. Explain the autoregulatory mechanism of GFR
Ans :
Autoregulation of GFR is a mechanism by which the kidneys maintain a relatively constant glomerular filtration rate (GFR) despite fluctuations in systemic blood pressure.
This is crucial for ensuring that the kidneys can effectively filter waste products from the blood and maintain fluid balance.
The autoregulatory mechanism involves the juxtaglomerular apparatus (JGA), a specialized structure located near the glomerulus. The JGA consists of three components:
- Juxtaglomerular cells: These cells are located in the walls of the afferent arteriole, the blood vessel that supplies blood to the glomerulus. They secrete renin, an enzyme that activates the renin-angiotensin-aldosterone system (RAAS).
- Macula densa: A group of specialized cells in the distal convoluted tubule that monitors the sodium chloride concentration in the tubular fluid.
- Mesangial cells: These cells surround the glomerulus and play a role in regulating glomerular blood flow.
3. Indicate whether the following statements are true or false :
(a) Micturition is carried out by a reflex.
(b) ADH helps in water elimination, making the urine hypotonic.
(c) Protein-free fluid is filtered from blood plasma into the Bowman’s capsule.
(d) Henle’s loop plays an important role in concentrating the urine.
(e) Glucose is actively reabsorbed in the proximal convoluted tubule
Ans :
(a) True (b) False (c) True (d) True (e) True
4. Give a brief account of the counter current mechanism.
Ans :
The countercurrent mechanism is a specialized system in the kidneys that allows for the production of highly concentrated urine. It involves the interaction of the Henle’s loop and the vasa recta, two structures located in the renal medulla.
Key features of the countercurrent mechanism:
- Countercurrent Flow: The filtrate flowing through the Henle’s loop and the blood flowing through the vasa recta move in opposite directions, creating a countercurrent system.
- Osmolarity Gradient: The descending limb of the Henle’s loop is permeable to water but not to salts, while the ascending limb is permeable to salts but not to water. This creates an increasing osmolarity gradient (salt concentration) from the cortex to the medulla of the kidney.
- Water Reabsorption: As the filtrate descends down the descending limb of the Henle’s loop, water is drawn out into the surrounding interstitial fluid due to the higher osmolarity in the medulla.
- Salt Reabsorption: In the ascending limb of the Henle’s loop, salts are actively transported out of the filtrate and into the interstitial fluid, further increasing the osmolarity of the medulla.
- Urea Recycling: Urea, a waste product, is recycled back into the descending limb of the Henle’s loop, contributing to the high osmolarity of the medulla.
- Concentrated Urine: The countercurrent mechanism allows for the reabsorption of a large amount of water from the collecting duct, resulting in the production of concentrated urine.
5. Describe the role of liver, lungs and skin in excretion.
Ans :
Excretory Organs and Their Roles
While the kidneys are the primary organs responsible for excretion, other organs also play a significant role in eliminating waste products from the body.
Liver
- Role: The liver plays a crucial role in excreting nitrogenous waste products. It converts toxic ammonia into less toxic urea, which is then transported to the kidneys for elimination.
- Bile Pigments: The liver also produces bile pigments, such as bilirubin, which are excreted through the digestive system.
Lungs
- Role: The lungs are primarily responsible for the elimination of carbon dioxide, a waste product of cellular respiration. During exhalation, carbon dioxide is expelled from the body through the lungs.
Skin
- Role: The skin plays a minor role in excretion through the process of perspiration. Sweat contains water, salts, and urea, which are eliminated from the body through the sweat glands.
- Sebaceous Glands: The sebaceous glands in the skin secrete sebum, a fatty substance that helps to waterproof the skin and remove waste products.
6. Explain micturition
Ans :
Micturition, or urination, is the process of releasing urine from the bladder, involving a coordinated interaction between muscles and nerves.
Here’s a breakdown of the micturition process:
- Urine Accumulation: As urine is produced by the kidneys, it accumulates in the urinary bladder.
- Stretch Receptors: When the bladder fills to a certain point, stretch receptors in the bladder wall are activated.
- Neural Signals: These stretch receptors send signals to the spinal cord and brain, indicating that the bladder is full and needs to be emptied.
- Micturition Reflex: The spinal cord coordinates the micturition reflex, involving the contraction of the detrusor muscle (the muscular wall of the bladder) and the relaxation of the internal and external urethral sphincters.
- Expulsion of Urine: The contraction of the detrusor muscle pushes urine out of the bladder, while the relaxation of the urethral sphincters allows it to pass through the urethra and out of the body.
7. Match the items of column I with those of column II.
Column I Column II
(a) Ammonotelism (i)Birds
(b) Bowman’s capsule (ii)Water reabsorption
(c) Micturition (iii)Bony fish
(d) Uricotelism (iv)Urinary bladder
(e) ADH (v)Renal tubule
Ans :
(a) Ammonotelism – (iii) Bony fish
(b) Bowman’s capsule – (v) Renal tubule
(c) Micturition – (iv) Urinary bladder
(d) Uricotelism – (i) Birds
(e) ADH – (ii) Water reabsorption
8. What is meant by the term osmoregulation?
Ans :
Osmoregulation is the process by which organisms maintain a constant internal water and salt balance. It is essential for survival as it helps to regulate cellular functions, blood pressure, and other vital processes.
Mechanisms of Osmoregulation:
Different organisms have evolved various mechanisms to regulate their internal water and salt balance. Some key mechanisms include:
- Osmoreceptors: These specialized cells detect changes in the body’s fluid concentration.
- Hormones: Hormones like antidiuretic hormone (ADH) and aldosterone play crucial roles in regulating water and salt balance.
- Kidneys: The kidneys are the primary organs involved in osmoregulation. They filter blood, reabsorb necessary substances, and excrete waste products.
- Behavioral Adaptations: Some organisms, such as marine fish and desert animals, have developed behavioral adaptations to conserve water, such as drinking seawater or reducing water loss through their skin.
9. Terrestrial animals are generally either ureotelic or uricotelic, not ammonotelic, why ?
Ans :
Terrestrial animals are predominantly ureotelic or uricotelic, rather than ammonotelic, due to the challenges associated with excreting ammonia in land-based environments.
- Ammonia Toxicity: Ammonia is highly toxic and requires a significant amount of water for its elimination. Terrestrial environments often have limited water availability, making it impractical for most land-dwelling animals to excrete ammonia.
- Energy Cost: Converting ammonia into urea or uric acid requires energy. However, the energy cost of producing urea is generally lower than that of producing uric acid.
- Adaptation to Terrestrial Life: Over evolutionary time, terrestrial animals have adapted their excretory systems to produce less toxic nitrogenous waste products. This has allowed them to thrive in environments with limited water resources.
10. What is the significance of juxta glomerular apparatus (JGA) in kidney function?
Ans :
The juxtaglomerular apparatus (JGA) is a specialized structure located near the glomerulus in the nephron, the functional unit of the kidney. It plays a crucial role in regulating glomerular hemodynamics (blood flow through the glomerulus) and renin release.
Key functions of the JGA:
- Renin Secretion: Juxtaglomerular cells in the afferent arteriole secrete renin, an enzyme that initiates the renin-angiotensin-aldosterone system (RAAS).
- Blood Pressure Regulation: The RAAS system helps to regulate blood pressure by increasing water and sodium reabsorption in the kidneys.
- Glomerular Filtration Rate (GFR) Regulation: The JGA also helps to regulate GFR, ensuring that the kidneys filter an appropriate amount of blood.
- Macula Densa Function: The macula densa, a group of specialized cells in the distal convoluted tubule, monitors the sodium chloride concentration in the tubular fluid. This information helps to regulate renin secretion and GFR
11. Name the following:
(a) A chordate animal having flame cells as excretory structures
(b) Cortical portions projecting between the medullary pyramids in the human kidney
(c) A loop of capillary running parallel to the Henle’s loop.
Ans :
(a) Amphioxus
(b) Columns of Bertini are the cortical portions projecting between the medullary pyramids in the human kidney.
(c) Vasa recta is a loop of capillary running parallel to the Henle’s loop.
12. Fill in the gaps :
(a) Ascending limb of Henle’s loop is _______ to water whereas the descending limb is _______ to it.
(b) Reabsorption of water from distal parts of the tubules is facilitated by hormone _______.
(c) Dialysis fluid contain all the constituents as in plasma except _______.
(d) A healthy adult human excretes (on an average) _______ gm of urea/day.
Ans :
(a) Impermeable , Permeable
(b) Antidiuretic hormone (ADH).
(c) Urea.
(d) 25-30
