NCERT Solutions For Class 11 Biology Chapter 19 Excretory Products and Their Elimination

(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 hormone antidiuretic hormone or vasopressin.

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

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

Terrestrial animals are generally either ureotelic or uricotelic because it is an adaptation to conserve water. In these animals, water soluble ammonia is converted into less soluble urea and uric acid. These are then filtered by the kidney and excreted out with urine. This requires minimum water; thus the animal conserves water.

Osmoregulation is a process by which organisms maintain homeostasis of water content in their body. It protects body fluids from becoming too dilute or too concentrated. For example, large amount of water goes into the nephrons from blood during the process of glomerular filtration. This water is reabsorbed by medullary interstitium and transferred again to blood in vasa recta thus, maintaining water homeostasis in blood.

Glomerular Filtration Rate (GFR) is the amount of filtrate formed by the kidneys per minute. It is measured in ml/min. In a healthy person, GFR is approximately 125 ml/min that is equal to 180 litres per day.

(a) Amphioxus

(b) Columns of Bertini

(c) Vasa recta

Micturition is the process of releasing of urine from urinary bladder. Urine formed by kidney is carried to urinary bladder where it is stored. When urinary bladder is filled with urine, it exerts the pressure on bladder wall. Pressure activates receptors present in bladder wall that send signals to central nervous system (CNS). The CNS sends the signal for the contraction of smooth muscles of the bladder and simultaneous relaxation of the urethral sphincter causing the release of urine.

An adult human excretes about 1 to 1.5 litres of urine per day. The urine is light yellow coloured watery fluid which is slightly acidic with a characteristic odour.

Kidneys have a microscopic structure called juxta glomerular apparatus (JGA) between an afferent arteriole and distal convoluted tubule of the same nephron that is required for the maintenance of uniform Glomerular Filtration Rate (GFR). Whenever there is a fall in GFR, cells of juxta glomerular apparatus release a hormone called renin which travels into the bloodstream of glomerulus where it converts angiotensinogen into angiotensin I. Angiotensin I further gets converted to angiotensin II. Angiotensin helps in constriction of smooth muscle cells of the blood vessel that increases the blood pressure. Increased blood pressure results in an increase in glomerular filtration rate. Angiotensin also stimulates the secretion of aldosterone from adrenal glands. Aldosterone increases the absorption of sodium ions and water which increases blood pressure and brings the GFR back to normal. Thus, juxta glomerular apparatus plays a complex regulatory role in the functioning of the kidney.

In addition to kidneys, liver, lungs and skin also play an important role in excretion of waste materials from the body.

Liver: It is the largest gland of the human body. Liver converts toxic ammonia into less toxic urea; thus carry out the detoxification role. It helps in the excretion of bile containing substances like bilirubin, biliverdin which are produced by the decomposition of haemoglobin pigment. Then bilirubin, biliverdin, urea along with other waste like cholesterol, degraded steroid hormones, vitamins and drugs are excreted by the liver. Finally, these substances are thrown out of the body along with digestive wastes.

Role of Lungs: Lungs diffuse out a large amount of gaseous waste like CO₂ (18 litres/day) as part of normal respiration along with the significant quantity of water.

Role of Skin: Skin contains sweat and sebaceous glands which help in the excretion of many substances. The sweat produced by sweat glands secrete NaCl, small amounts of urea, lactic acid, etc., along with water. Sebaceous glands help in excretion by elimination of certain sterols, hydrocarbons and waxes through sebum.

The transport of substances facilitated by a special arrangement of Henle’s loop of nephron and vasa recta is called counter current mechanism.

When fluid flows in these two tubes which are anti-parallel to each other, it forms a counter current. This helps mammals to produce concentrated urine. There are two counter current systems in human kidneys-

• Henle’s loop consists of ascending and descending limbs. Fluid flows in the two limbs in opposite directions making a counter current system.
• Second counter current system is made by vasa recta. Blood flows in the two limbs of vasa recta in anti-parallel direction.

Henle’s loop and vasa recta are positioned next to each other. Descending limb of Henle’s loop is anti-parallel to ascending limb of vasa recta and ascending limb of Henle’s loop is anti-parallel to descending limb of vasa recta. Counter current system and proximity of the loops help to increase the osmolarity (solute concentration) in inner medullary interstitium i.e. from 300mOsmolL^-1 in the cortex to about 1200mOsmolL^-1 in the inner medulla. Increased solute concentration in medullary interstitium causes water to come out of collecting duct making the urine concentrated. This system has the ability to concentrate urine upto four times depending upon the need.

(a) True

(b) False

(c) True

(d) True

(e) True

Kidneys maintain a uniform Glomerular Filtration Rate (GFR) with the help of the autoregulatory mechanism. Kidneys have a microscopic structure called juxta glomerular apparatus (JGA) located between an afferent arteriole and distal convoluted tube of the same nephron that is required for this auto-regulation. Whenever there is a fall in glomerular blood flow/glomerular blood pressure/GFR, cells of juxta glomerular apparatus release a hormone called renin which converts angiotensinogen in the blood to angiotensin I and further to angiotensin II.

• Angiotensin II is a strong vasoconstrictor. It increases the glomerular blood pressure and blood flow and thereby, increases the GFR.
• Angiotensin II also activates the release of Aldosterone which causes reabsorption of Na⁺ and water from the distal parts of the tubule. This also causes an increase in blood pressure and GFR.