Excretory Products and Their Elimination

The blood comprises numerous toxic products produced by cellular metabolism and intake of foreign substances. These need to be excreted regularly and timely to avoid contracting health issues. The excretory system is significant for this purpose as it excretes the liquid waste out of the body. The waste is obtained by blood filtration, which also undergoes osmotic regulation. The method varies among vertebrates and invertebrates.

Table of Contents:

Excretory Organs in Different Animals
Excretory Products
Factors Determining Excretory Product in Organisms
The Function of Components of Kidney

Excretory Organs in Different Animals

The variety in excretory organs is seen as follows:

Protonephridia: Also known as flame cells, these structures are present in Platyhelminthes, annelids, rotifers and Cephalochordata. The organs are responsible for osmoregulation, where ion content and fluid volume are maintained for the body.
Nephridia: These are tubular-shaped structures in Earthworms and annelids. These organisms excrete their nitrogenous wastes while performing osmoregulation.
Malpighian Tubules: The malpighian tubules are present in insects such as cockroaches, tardigrades and myriapods for excretion and osmoregulation.
Antennal or green glands: The crustaceans like prawns, lobsters, crawfishes, barnacles and others have such glands for excretion.
Kidney: All vertebrates, including humans, have a pair of kidneys to perform the same function of excretion as others.
Others: The other significant excretory forms are carbon dioxide from the lungs, toxins from the liver and water, heat and some amount of urea from the skin.

Excretory Products

The excretory products among animals include:

Amino acids: These are obtained from protein breakdown in animals like molluscs, echinoderms and others. The process of ammonia secretion is called aminotelism or ammonotelic excretion.
Ammonia: These are often directly obtained from protein breakdown, where amino acids are deaminated to form ammonia and keto.
Urea: It is formed in land residents and aquatic animals. This form of excretion is less toxic than ammonia. Also, it is less soluble in water and allows more water conservation in the animals. The process is called ureotelic excretion or ureotelic.
Uric Acid: It is seen in animals with comparatively least contact with water, such as among birds, insects, reptiles, mammals and residents of arid conditions. Conserving the maximum amount of water, it is insoluble in water and also non-toxic to the carriers. It is excreted in solid form as a pellet or paste. The process of excreting uric acid is called uricotelism or uricotelic excretion.

Factors Determining Excretory Product in Organisms

The above types saw similarities in producing nitrogenous wastes, but they differed due to varying ability to conserve water. The animals differed in their habitat as well. Hence, there are two main factors contributing to the type of excretory products produced:

Habitat: Aquatic organisms secrete water-soluble waste, which is highly toxic. It can not be stored in the body. In contrast, land and flight animals secrete a non-toxic form of waste that is safe to be stored for a considerable time.
Water: The waste products in water-rich environments are different from dry conditions. The products contributing to more water conservation are preferred in the latter conditions.

The Function of Components of Kidney

Different parts of the kidney play important roles in urine formation:

Renal Cortex: The outermost part of the kidney comprises renal corpuscles. The latter further consists of glomeruli and Bowman’s capsule. It is important for filtration.
Renal Medulla: Present beneath the renal cortex, it comprises renal pyramids that include tubules associated with reabsorption and urine concentration.
Glomerulus: It is the capillary network receiving blood from the body.
Bowman’s capsule: It receives the filtrate from the glomerulus.
Proximal Convoluted Tubule (PCT): It reabsorbs water, glucose and amino acids from the filtrate.
Loop of Henle: Comprising descending and ascending limbs, it is significant for water and electrolyte reabsorption.
Distal Convoluted Tubule (DCT): Further electrolyte, ion and pH concentration takes place.
Collecting Duct: It reabsorbs water in accordance with the endocrine system.
Renal Pelvis: It collects the urine from collecting ducts and transfers it to the ureter for further transport into the urinary bladder.

Practice Problems

Q1. What purpose does the afferent artery serve in the glomerulus?

a. Transport oxygenated blood
b. Transport deoxygenated blood
c. Transport toxin-carrying blood
d. Both a and c

Ans. d. Both a and c

The afferent artery brings oxygenated blood for filtration and transports blood containing toxins to the glomerulus.

Q2. How do amoebas excrete their excretory product?

a. Flame cells
b. Nephridia
c. Canal system
d. Contractile vacuoles

Ans. d. Contractile vacuoles

Unicellular organisms like Amoeba and Paramecium use contractile vacuoles for excretion.

Q3. Which is the outer and inner part of the kidney?

a. Medulla and cortex
b. Cortex and medulla
c. Ascending and descending limb of the loop of Henle
d. Descending and ascending limb of the loop of Henle

Ans. b. Cortex and medulla

The renal cortex is outside, while the medulla is on the inner part of the kidney.

Frequently Asked Questions

Q1. What is hilum?
Answer: Hilum is the point of entrance of the ureter, nerves and blood vessels. It is present on the inner concave surface of the kidney at the centre.

Q2. What are the two components of a nephron?
Answer: The renal corpuscle and renal tubule are the two components of the nephron.

Q3. What are the types of nephrons?
Answer: There are two types of nephrons, cortical and medullary.