Excretory System

The excretory system removes nitrogenous wastes such as ammonia, urea and uric acid, along with excess water and ions, from the body. This page explains ammonotelism, ureotelism, uricotelism and the excretory structures of different animal groups. It then describes the human excretory system: the kidneys, their internal structure and the nephron with its blood vessels. It follows the NCERT Class 11 chapter Excretory Products and their Elimination. NEET often asks match-the-list questions on animals and their wastes or excretory organs, and on the parts of the kidney and nephron.

1. Excretory Products

• Animals accumulate ammonia, urea, uric acid, carbon dioxide, water and ions such as $\mathrm{N}{\mathrm{a}}^{+}$, ${\mathrm{K}}^{+}$, $\mathrm{C}{\mathrm{l}}^{-}$, phosphate and sulphate.
• These substances build up through metabolic activities or by other means, such as excess ingestion.
• They have to be removed from the body, either totally or partially.
• ★ Exam imp Ammonia, urea and uric acid are the major forms of nitrogenous wastes excreted by animals.
• The kind of nitrogenous waste an animal forms, and how it excretes it, depends mainly on its habitat, that is, on the availability of water.

2. Ammonotelism, Ureotelism and Uricotelism

2.1 Ammonotelism

• Ammonotelism: the process of excreting ammonia. Animals that do this are ammonotelic.
• ★ Exam imp Examples: many bony fishes, aquatic amphibians and aquatic insects.
• Ammonia is readily soluble in water.
• So it is generally excreted by diffusion across body surfaces, or through gill surfaces in fishes, as ammonium ions $\mathrm{N}{\mathrm{H}}_4^{+}$.
• Kidneys do not play any significant role in its removal.

2.2 Ureotelism

• Life on land required animals to conserve water.
• This terrestrial adaptation led to the production of less toxic nitrogenous wastes, urea and uric acid.
• Ureotelic animals: animals that mainly excrete urea.
• ★ Exam imp Examples: mammals, many terrestrial amphibians and marine fishes.
• In these animals, ammonia produced by metabolism is converted into urea in the liver.
• Urea is released into the blood, which is filtered by the kidneys, and urea is excreted.
• Some urea may be retained in the kidney matrix of some of these animals, to maintain a desired osmolarity.

2.3 Uricotelism

• Uricotelic animals: animals that excrete nitrogenous wastes as uric acid.
• ★ Exam imp Examples: reptiles, birds, land snails and insects.
• Uric acid is excreted in the form of a pellet or paste, with a minimum loss of water.

Examples to Remember

3. Excretory Structures in Animals

• Animals show a variety of excretory structures.
• Most invertebrates have simple tubular excretory structures.
• Vertebrates have complex tubular organs called kidneys.
• ★ Exam imp Protonephridia (flame cells) occur in Platyhelminthes (flatworms, e.g., Planaria), rotifers, some annelids and the cephalochordate Amphioxus.
• Protonephridia are mainly concerned with ionic and fluid volume regulation, that is, osmoregulation.
• Nephridia: tubular excretory structures of earthworms and other annelids. They remove nitrogenous wastes and maintain fluid and ionic balance.
• Malpighian tubules: excretory structures of most insects, including cockroaches. They remove nitrogenous wastes and help in osmoregulation.
• Antennal glands or green glands: perform the excretory function in crustaceans such as prawns.
• These organs not only eliminate nitrogenous wastes but also help maintain the ionic and acid-base balance of body fluids.

Examples to Remember

4. Human Excretory System

• ★ Exam imp The human excretory system consists of a pair of kidneys, one pair of ureters, a urinary bladder and a urethra.
• Kidneys: reddish brown, bean-shaped structures.
• ★ Exam imp They lie between the levels of the last thoracic and the third lumbar vertebra.
• They lie close to the dorsal inner wall of the abdominal cavity.
• Each adult kidney measures 10-12 cm in length, 5-7 cm in width and 2-3 cm in thickness.
• Its average weight is 120-170 g.

5. Structure of the Kidney

• ★ Exam imp Hilum: a notch towards the centre of the inner concave surface of the kidney. The ureter, blood vessels and nerves enter through it.
• Renal pelvis: a broad, funnel-shaped space inner to the hilum.
• The renal pelvis has projections called calyces (singular: calyx).
• The outer layer of the kidney is a tough capsule.
• Inside the kidney are two zones: an outer cortex and an inner medulla.
• The medulla is divided into a few conical masses, the medullary pyramids, which project into the calyces.
• ★ Exam imp The cortex extends in between the medullary pyramids as renal columns called Columns of Bertini.

1. Outermost: the tough renal capsule.
2. Below it: the cortex, which also forms the Columns of Bertini.
3. Inner zone: the medulla, divided into medullary pyramids.
4. The pyramids project into the calyces.
5. The calyces open into the renal pelvis, which lies inner to the hilum.

6. The Nephron

6.1 Parts of a nephron

• ★ Exam imp Each kidney has nearly one million complex tubular structures called nephrons.
• Nephrons are the functional units of the kidney.
• Each nephron has two parts: the glomerulus and the renal tubule.
• ★ Exam imp Glomerulus: a tuft of capillaries formed by the afferent arteriole, a fine branch of the renal artery.
• Blood from the glomerulus is carried away by an efferent arteriole.

6.2 The renal tubule

• The renal tubule begins with a double-walled, cup-like structure called Bowman's capsule, which encloses the glomerulus.
• ★ Exam imp The glomerulus along with Bowman's capsule is called the Malpighian body or renal corpuscle.

• The tubule continues as a highly coiled network, the proximal convoluted tubule (PCT).
• Next comes the hairpin-shaped Henle's loop, which has a descending and an ascending limb.
• The ascending limb continues as another highly coiled region, the distal convoluted tubule (DCT).
• The DCTs of many nephrons open into a straight tube called the collecting duct.
• Many collecting ducts converge and open into the renal pelvis through the medullary pyramids in the calyces.

Path of the filtrate from the nephron to the renal pelvis

1. Bowman's capsule, around the glomerulus
2. Proximal convoluted tubule (PCT)
3. Henle's loop: descending limb, then ascending limb
4. Distal convoluted tubule (DCT)
5. Collecting duct, shared by many nephrons
6. Renal pelvis, through the medullary pyramids in the calyces

6.3 Location of the parts: cortical and juxta medullary nephrons

• ★ Exam imp The Malpighian corpuscle, PCT and DCT lie in the cortical region of the kidney.
• The loop of Henle dips into the medulla.
• In the majority of nephrons, the loop of Henle is too short and extends only a little into the medulla. These are cortical nephrons.
• In some nephrons, the loop of Henle is very long and runs deep into the medulla. These are juxta medullary nephrons.

6.4 Blood vessels around the tubule

• The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule: the peritubular capillaries.
• A minute vessel of this network runs parallel to Henle's loop, forming the U-shaped vasa recta.
• ★ Exam imp The vasa recta is absent or highly reduced in cortical nephrons.

Path of blood through a nephron

1. Renal artery
2. Afferent arteriole, a fine branch of the renal artery
3. Glomerulus, the tuft of capillaries inside Bowman's capsule
4. Efferent arteriole
5. Peritubular capillaries around the tubule, including the vasa recta along Henle's loop

7. Exam Essentials

Pairs to Match

Numbers to Remember

• Kidney length 10-12 cm, width 5-7 cm, thickness 2-3 cm (adult).
• Average weight of a kidney: 120-170 g.
• Position: between the last thoracic and the third lumbar vertebra.
• Nephrons per kidney: nearly one million.
• Human excretory organs: 2 kidneys, 2 ureters, 1 urinary bladder, 1 urethra.
• Parts of a nephron: 2 (glomerulus and renal tubule); limbs of Henle's loop: 2.

8. Quick Revision

• Wastes to remove: ammonia, urea, uric acid, carbon dioxide, water and ions such as $\mathrm{N}{\mathrm{a}}^{+}$, ${\mathrm{K}}^{+}$, $\mathrm{C}{\mathrm{l}}^{-}$, phosphate and sulphate.
• Ammonia is the most toxic and needs much water; uric acid is the least toxic and needs the least.
• Ammonotelic: many bony fishes, aquatic amphibians, aquatic insects; ammonia leaves by diffusion or through gills as $\mathrm{N}{\mathrm{H}}_4^{+}$.
• Ureotelic: mammals, many terrestrial amphibians, marine fishes; urea forms in the liver and is filtered by the kidneys.
• Uricotelic: reptiles, birds, land snails, insects; uric acid leaves as a pellet or paste.
• Flame cells: flatworms, rotifers, some annelids, Amphioxus (osmoregulation). Nephridia: annelids. Malpighian tubules: most insects. Green glands: crustaceans.
• Human excretory system: 2 kidneys, 2 ureters, urinary bladder, urethra.
• Kidneys: reddish brown, bean-shaped, last thoracic to third lumbar vertebra; 10-12 cm long, 5-7 cm wide, 2-3 cm thick; 120-170 g.
• Hilum leads to the renal pelvis with calyces; capsule, cortex and medulla (medullary pyramids).
• Columns of Bertini: cortex extending between the medullary pyramids.
• Nearly one million nephrons per kidney; nephron = glomerulus + renal tubule.
• Bowman's capsule + glomerulus = Malpighian body; tubule: PCT, Henle's loop, DCT, then collecting duct.
• Corpuscle, PCT and DCT in the cortex; Henle's loop in the medulla.
• Cortical nephrons: short loop, vasa recta reduced; juxta medullary nephrons: long loop.
• Efferent arteriole forms the peritubular capillaries and the U-shaped vasa recta.

9. Solved Examples

10. Practice Questions

Common Mistakes to Avoid