Excretory products and their Elimination Question Answers: NCERT Class 11 Biology

The Henle’s loop and vasa recta play a significant role in the formation of concentrated urine. The flow of filtrate in two limbs, i.e. ascending limb and descending limb of Henle’s loop is in opposite directions and thus forms a counter current. The flow of blood in the two limbs of vasa recta is also in counter current pattern. This mechanism of counter current helps to maintain concentration gradient in the medullary interstitium. Presence of such interstitial gradient help in an easy passage of water from collecting tubule thereby concentrating the filtrate. Human kidney can produce urine nearly 4 times concentrated than the initial filtrate which is initially formed.

Glycosuria and ketonuria are two metabolic disorders that can be diagnosed by analysis of urine.

The main processes are filtration, reabsorption, secretion and concentration.

1. Urinary excretion = Glomerular filtration – Tubular reabsorption + tubular secretion
2. Dialysis fluid = Plasms – nitrogenous waste

NaCl and urea exit from the tubules in order to maintain a concentration gradient in the medullary interstitium. 

1. Kidney: Urea
2. Lungs: CO₂ and H₂O
3. Liver: Bilirubin, cholesterol, degraded steroid hormones, vitamins and drugs.
4. Skin: Sweat and sebum

Nephron shows the tubular reabsorption and tubular secretion. And briefly shows in the diagram:

Diagram:

Micturition is the process of the passage of the urine out of the urinary bladder. It is controlled by brain thus it is a voluntary. It involves following steps:

• Urinary bladder filled with urine and get flattened.
• Stretch receptors are present on the walls of bladder and they send signal to CNS.
• And then motor message is send to smooth muscles in urinary bladder by CNS.
• Muscles of urinary bladder contracts and urine rushes out of the bladder and sphincter of urethra becomes relaxed which results in flow of urine to the outside.

Disorder of excretory system:

1. Renal failure: It is also known as kidney failure in which both kidney stops working. This can be managed only by transplant of kidney.
2. Uremia: There is high concentration of non- protein nitrogen like urea, uric acid etc.
3. Renal calculi: Stone or insoluble mass of crystalized salt are formed in the kidney in renal calculi.
4. Glomerulonephritis: Because of some infection or some other underlying disease inflammation of glomerulus occurs, which is known as Glomerulonephritis.

Tubular secretion is also an important step in urine formation. It helps in the maintenance of ionic and acid- base balance of body fluids. During the urine formation substances like K⁺ H⁺ and ammonia are secreted by tubular cells into the filtrate.

In PCT: Selective secretion of Hydrogen ions, ammonia and potassium take place. PCT helps to maintain the pH and ionic balance of the body fluids.

In DCT: Selective secretion of Hydrogen ions, NH₃ and potassium. It also maintains the pH and sodium- potassium balance in blood.

In Collecting Duct: There is selective secretion of hydrogen and potassium ions. Collecting duct help in maintaining the balance of pH in blood.   

The Henle’s loop has descending and ascending limbs.

The ascending limb of Henle’s loop is impermeable to water but permeable to electrolytes. Thus, there is minimum reabsorption in ascending limb and the filtrate gets diluted as it moves up the ascending limb of Henle’s loop.

The descending limb of Henle’s loop is impermeable to electrolytes and permeable to water. The filtrate gets concentrated as it moves down the descending limb of Henle’s loop.

Kidneys are reddish brown bean shaped organ situated between last thoracic and third lumbar vertebra. Each kidney has a notch on its inner side called hilum through which ureter, blood vessels and nerved enter. Each kidney is 10- 12 cm long, 5- 7 cm wide and 2- 3 cm thick. It weighs around 120- 170g.

• Inside the kidney there are two zone- outer cortex and inner medulla.

1. Medulla: The medulla is also divided into a few conical masses called medullary pyramids which projects into the calyceas.
2. Cortex: It extends between medullary pyramids and renal column or columns of Bertini.

• Renal pelvis: This is a broad funnel shaped space present inside the hilum.
• Each kidney contains about a million nephrons which is known as functional unit of kidney. Each nephron has two parts: the glomerulus and renal tubules.

In the case of kidney failure, urea is removed from the body of the patient by the process called Hemodialysis. The main function of the excretory organ kidney is to remove toxic substances from the body and a hemodialysis unit is called an artificial kidney because it is responsible for performing the same functions that a kidney perform.

The process of reabsorption of some molecules like glucose, ion and certain amino acids from the filtrate as they passes through the neuron is called selective reabsorption. Osmoreceptors in the body are activated by the changes in blood volume, body fluid volume and ionic concentration. An excessive loss of fluid activates these receptors which stimulates the hypothalamus to release ADH or vasopressin which facilitates water reabsorption from tubule and prevent diuresis. ANF and ADH are the hormones which monitor the function of the kidney.

Glomerular filtration rate is the amount of glomerular filtrate formed in all the nephrons of both the kidneys per minute. In a healthy individual, it is about 125 mL/minute. Glomerular filtrate contains glucose, amino acids, sodium, potassium, urea, uric acid, ketone bodies, and large amounts of water.

Juxtaglomerular apparatus (JGA) is a complex structure made up of a few cells of glomerulus, distal tubule, and afferent and efferent arterioles. It is located in a specialised region of a nephron, wherein the afferent arteriole and the distal convoluted tubule (DLT) come in direct contact with each other.

The juxtaglomerular apparatus contains specialised cells of the afferent arteriole known as juxtaglomerular cells. These cells contain the enzyme renin that can sense blood pressure. When glomerular blood flow (or glomerular blood pressure or glomerular filtration rate) decreases, it activates juxtaglomerular cells to release renin.

Renin converts the angiotensinogen in blood into angiotensin I and further into angiotensin II. Angiotensin II is a powerful vasoconstrictor that increases the glomerular blood pressure and filtration rate. Angiotensin II also stimulates the adrenal cortex of the adrenal gland to produce aldosterone. Aldosterone increases the rate of absorption of sodium ions and water from the distal convoluted tubule and the collecting duct. This also leads to an increase in blood pressure and glomerular filtration rate. This mechanism, known as renin-angiotensin mechanism, ultimately leads to an increased blood pressure.

(a) Amphioxus is an example of a chordate that has flame cells as excretory structures. Flame cell is a type of excretory and osmoregulatory system.

(b) The cortical portions projecting between the medullary pyramids in the human kidney are the columns of Bertini. They represent the cortical tissues present within the medulla.

(c) A loop of capillary that runs parallel to Henle’s loop is known as vasa rectae. Vasa rectae, along with Henle’s loop, helps in maintaining a concentration gradient in the medullary interstitium.

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

(b) Reabsorption of water from distal parts of the tubules is facilitated by the hormone vasopressin.

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

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

The mechanism by which the kidney regulates the glomerular filtration rate is autoregulative. It is carried out by the juxtaglomerular apparatus. Juxtaglomerular apparatus is a microscopic structure located between the vascular pole of the renal corpuscle and the returning distal convoluted tubule of the same nephron.

It plays a role in regulating the renal blood flow and glomerular filtration rate. When there is a fall in the glomerular filtration rate, it activates the juxtaglomerular cells to release renin. This stimulates the glomerular blood flow, thereby bringing the GFR back to normal. Renin brings the GFR back to normal by the activation of the renin-angiotensin mechanism.

(a) True

(b) False

(c) True

(d) True

(e) True

The counter current mechanism operating inside the kidney is the main adaptation for the conservation of water. There are two counter current mechanisms inside the kidneys. They are Henle’s loop and vasa rectae.

Henle’s loop is a U-shaped part of the nephron. Blood flows in the two limbs of the tube in opposite directions and this gives rise to counter currents. The Vasa recta is an efferent arteriole, which forms a capillary network around the tubules inside the renal medulla. It runs parallel to Henley’s loop and is U-shaped. Blood flows in opposite directions in the two limbs of vasa recta. As a result, blood entering the renal medulla in the descending limb comes in close contact with the outgoing blood in the ascending limb.

The osmolarity increases from 300 mOsmolL ^-1 in the cortex to 1200 mOsmolL ^-1 in the inner medulla by counter current mechanism. It helps in maintaining the concentration gradient, which in turn helps in easy movement of water from collecting tubules. The gradient is a result of the movement of NaCl and urea.

Liver, lungs, and skin also play an important role in the process of excretion.

Role of the liver:

Liver is the largest gland in vertebrates. It helps in the excretion of cholesterol, steroid hormones, vitamins, drugs, and other waste materials through bile. Urea is formed in the liver by the ornithine cycle. Ammonia – a toxic substance – is quickly changed into urea in the liver and then eliminated from the body. Liver also changes the decomposed haemoglobin pigment into bile pigments called bilirubin and biliverdin.

Role of the lungs:

Lungs help in the removing waste materials such as carbon dioxide from the body.

Role of the skin:

Skin has many glands which help in excreting waste products through pores. It has two types of glands – sweat and sebaceous glands.

Sweat glands are highly vascular and tubular glands that separate the waste products from the blood and excrete them in the form of sweat. Sweat excretes excess salt and water from the body.

Sebaceous glands are branched glands that secrete an oily secretion called sebum.

Micturition is the process by which the urine from the urinary bladder is excreted. As the urine accumulates, the muscular walls of the bladder expand. The walls stimulate the sensory nerves in the bladder, setting up a reflex action. This reflex stimulates the urge to pass out urine. To discharge urine, the urethral sphincter relaxes and the smooth muscles of the bladder contract. This forces the urine out from the bladder. An adult human excretes about 1 – 1.5 litres of urine per day.

Osmoregulation is a homeostatic mechanism that regulates the optimum concentration of water and salts in the tissues and body fluids. It maintains the internal environment of the body by water and ionic concentration.

Terrestrial animals are either ureotelic or uricotelic, and not ammonotelic. This is because of the following two main reasons:

(a) Ammonia is highly toxic in nature. Therefore, it needs to be converted into a less toxic form such as urea or uric acid.

(b) Terrestrial animals need to conserve water. Since ammonia is soluble in water, it cannot be eliminated continuously. Hence, it is converted into urea or uric acid. These forms are less toxic and also insoluble in water. This helps terrestrial animals conserve water.