NCERT Solutions for Class 11 Biology Chapter 16: Excretory Products and Their Elimination

The Glomerular Filtration Rate (GFR) is defined as the volume of filtrate formed by both kidneys per minute.

• It is a key indicator of kidney function.
• In a healthy adult, the GFR is approximately 125 ml/minute.
• This amounts to a staggering 180 litres per day.
• The kidneys have built-in mechanisms to regulate the GFR, primarily through the Juxtaglomerular Apparatus (JGA).

The kidneys have an efficient intrinsic mechanism to regulate the Glomerular Filtration Rate (GFR), known as autoregulation. The primary structure responsible is the Juxtaglomerular Apparatus (JGA).

How it works:

1. Detection of Low GFR: A fall in glomerular blood flow/pressure activates the juxtaglomerular (JG) cells.
2. Release of Renin: The JG cells release a hormone called renin.
3. Activation of Angiotensin: Renin converts plasma protein angiotensinogen into angiotensin I.
4. Conversion to Angiotensin II: Angiotensin I is converted to active angiotensin II by Angiotensin Converting Enzyme (ACE).
5. Effects of Angiotensin II:
   • Vasoconstriction: It constricts the efferent arteriole, increasing glomerular blood pressure and GFR.
   • Aldosterone Release: It activates the adrenal cortex to release Aldosterone, which causes reabsorption of Na+ and water, increasing blood pressure and GFR.

This mechanism is also known as the Renin-Angiotensin-Aldosterone System (RAAS).

(a) Micturition is carried out by a reflex.
➡ True. It's a reflex from the stretching of the urinary bladder, though it can be voluntarily controlled.

(b) ADH helps in water elimination, making the urine hypotonic.
➡ False. ADH promotes water reabsorption, making urine more concentrated (hypertonic).

(c) Protein-free fluid is filtered from blood plasma into the Bowman’s capsule.
➡ True. Large proteins are retained in the blood during ultrafiltration.

(d) Henle’s loop plays an important role in concentrating the urine.
➡ True. The counter-current mechanism in Henle's loop creates the medullary gradient necessary for concentration.

(e) Glucose is actively reabsorbed in the proximal convoluted tubule.
➡ True. 100% of glucose is actively reabsorbed in the PCT in a healthy person.

The counter-current mechanism is a system used to produce concentrated urine and conserve water. It involves the Henle's loop and the vasa recta.

1. Creating the Gradient: The ascending limb of Henle's loop actively transports NaCl into the medullary interstitium, making it highly concentrated (hypertonic), increasing from 300 to 1200 mOsmol/L.
2. Concentrating the Filtrate: As filtrate moves down the water-permeable descending limb, water moves out via osmosis, concentrating the filtrate.
3. Role of Urea: A small amount of urea diffuses from the collecting duct into the medulla, helping maintain the high osmolarity.
4. Maintaining the Gradient: The vasa recta has a counter-current flow that prevents the medullary gradient from being washed away. It picks up solutes and loses water on its way down, and vice-versa on its way up.
5. Final Concentration: The concentrated medullary gradient allows for significant water reabsorption from the collecting duct (under ADH influence), resulting in concentrated urine.

• Liver: It is the site of the urea cycle, converting toxic ammonia to less toxic urea. It also eliminates bile pigments (bilirubin, biliverdin), cholesterol, and degraded hormones through bile.
• Lungs: They are the primary organs for eliminating gaseous waste, mainly carbon dioxide (CO2). They also expel a significant amount of water vapour.
• Skin: Sweat glands excrete water, NaCl, small amounts of urea, and lactic acid. Sebaceous glands eliminate sterols and hydrocarbons through sebum.

Micturition is the process of expelling urine from the urinary bladder. It involves these steps:

1. Filling of Bladder: As urine fills the bladder, its walls stretch.
2. Signal to CNS: Stretch receptors in the bladder wall send signals to the CNS.
3. CNS Response: The CNS sends motor signals causing the bladder's detrusor muscles to contract and the internal urethral sphincter to relax.
4. Urge to Urinate: This creates the sensation or urge to urinate.
5. Voluntary Control: The brain voluntarily controls the external urethral sphincter. When relaxed, urine is expelled. An adult excretes about 1 to 1.5 litres of urine per day.

The JGA is crucial for regulating blood pressure and GFR through the Renin-Angiotensin-Aldosterone System (RAAS).

Significance:

1. GFR Regulation: It detects a drop in GFR or blood pressure.
2. Hormone Secretion: In response, its JG cells secrete the enzyme renin.
3. RAAS Activation: Renin initiates a cascade that forms angiotensin II.
4. Restoration: Angiotensin II restores blood pressure and GFR by constricting efferent arterioles and stimulating aldosterone release (which increases Na+ and water reabsorption).

In essence, the JGA acts as a sensitive sensor ensuring the kidneys filter blood effectively despite blood pressure fluctuations.

(a) A chordate animal which secretes urea as the nitrogenous waste.
➡ Mammals (e.g., humans) and terrestrial amphibians (e.g., frogs).

(b) Parts of the nephron that are situated in the cortical region of the kidney.
➡ Malpighian corpuscle (Glomerulus + Bowman's capsule), Proximal Convoluted Tubule (PCT), and Distal Convoluted Tubule (DCT).

(c) A loop of capillary running parallel to the Henle’s loop.
➡ Vasa recta.

(a) Ascending limb of Henle’s loop is impermeable to water whereas the descending limb is permeable to it.

(b) Reabsorption of water from the distal parts of the tubules is facilitated by the hormone ADH (Anti-diuretic Hormone) or Vasopressin.

(c) Dialysis fluid contains all the constituents as in plasma except nitrogenous wastes (like urea).

(d) A healthy adult human excretes (on an average) 25-30 g of urea/day.

Podocytes are specialized cells found in the inner layer of the Bowman's capsule. They have foot-like projections (pedicels) that wrap around glomerular capillaries.

Function: The gaps between their pedicels form filtration slits, which act as a final barrier, preventing large molecules like proteins from passing into the filtrate.

Urine formation involves three main processes:

1. Glomerular Filtration (Ultrafiltration):
   • Occurs in the Malpighian corpuscle. Blood is filtered under high pressure.
   • Water and small solutes pass through the filtration membrane into Bowman's capsule, while proteins and cells are retained. The resulting fluid is called glomerular filtrate.
2. Tubular Reabsorption:
   • About 99% of the filtrate is reabsorbed back into the blood.
   • PCT reabsorbs essential nutrients (glucose, amino acids), electrolytes, and water.
   • Henle's Loop reabsorbs water and NaCl. DCT and collecting duct perform conditional reabsorption regulated by hormones (ADH, Aldosterone).
3. Tubular Secretion:
   • Waste products (H+, K+, ammonia) and drugs are actively transported from the blood into the filtrate, mainly in the PCT and DCT.
   • This process helps maintain the ionic and acid-base balance (pH) of body fluids.

1. What is the clinical condition of having high urea in the blood called?
➡ The condition is called Uremia.

2. Which part of the nephron is likely damaged, leading to proteinuria?
➡ The glomerulus (Malpighian body) is likely damaged. The filtration membrane is compromised, allowing proteins to leak into the filtrate.

3. What is the procedure suggested by the doctor to clean the patient's blood?
➡ The procedure is hemodialysis (artificial kidney).

4. The dialysis fluid lacks one key component. What is it and why?
➡ It lacks nitrogenous wastes (like urea). This creates a concentration gradient, allowing wastes to diffuse from the patient's blood into the dialysis fluid.

5. Swelling (edema) is caused by fluid retention. Which hormonal imbalance could contribute?
➡ A disrupted RAAS (Renin-Angiotensin-Aldosterone System) can lead to excessive reabsorption of Na+ and water, causing fluid to accumulate in tissues.