Excretory System: Excretory Organs And Types Of Wastes In Animals

What Is Excretion?

Excretion is the process by which an organism rids itself of waste products accruing from metabolic activities. Such wastes, if eliminated, can be toxic, perturbing normal physiological functions. Excretion helps in the maintenance of homeostasis—that is, a stable internal environment from which cellular activities can be carried out.

Excretion is a specialised process accomplished via particular organs and routes in various organisms. It regulates water and salt amounts, eliminates nitrogenous waste products, and excretes other byproducts of metabolism. This important function underlies general health and vitality.

Excretory Systems in Organisms (Overview)

The excretory systems have evolved differently in the plant and animal kingdoms. Typically in plants, waste products are stored in vacuoles or excreted by the process of leaf fall or through transpiration. Invertebrates make use of several structures like nephridia or Malpighian tubules for excretion. More developed systems and organs like kidneys perform the function of filtering blood/lymph and forming urine.

Types Of Excretory Wastes

Excretory wastes can be categorised as per their source and composition. The major types are nitrogenous wastes and other metabolic wastes like carbon dioxide and salts.

Nitrogenous Wastes

The 3 major nitrogenous wastes are:

• Ammonia

• Urea

• Uric Acid

Other Metabolic Wastes

Other metabolic wastes include:

• Carbon dioxide

• Water

• Salts

Excretory Organs in Different Organisms

Methods of excretion differ considerably among different groups of organisms and reflect their evolutionary adaptations and habitats.

Protists (Amoeba, Paramecium)

• Contractile vacuole: This specialised organelle helps remove excess water and waste products from the cell to maintain its osmotic balance.

• Diffusion through plasma membrane: Waste substances, such as ammonia, directly diffuse through the plasma membrane to the exterior.

Porifera & Cnidaria

• Diffusion: Waste products, including ammonia, diffuse directly through the body surface into surrounding water.

Platyhelminthes

• Flame cells: Kidney-type cells that filter and excrete waste products through a system of tubules.

Annelids

• Nephridia: Tubular excretory organs that filter the fluid of the coelom and discharge waste through external openings.

Arthropods

• Malpighian tubules: These are tubules that absorb waste products from the hemolymph and discharge these into the digestive tract to get rid of them.

Molluscs

• Nephridia (modification from Annelids): Mollusks have annelid-like nephridia modified to their body and believe in filtering wastes directly from blood and coelomic fluid.

Fish

• Gills: Ammonia is eliminated directly into the water.

• Kidneys: Filter blood to produce urine which is eliminated through the urinary route.

Amphibians

• Skin: Waste products, for example, ammonia and carbon dioxide, can be excreted directly into the surroundings through moist skin.

• Kidneys: Produce urine that is excreted from the body by the urinary system.

Reptiles

• Kidneys: Forms uric acid, which is excreted as a paste to conserve water.

Birds

• Kidneys: Forms uric acid, which is excreted along with the faeces via the cloaca.

• Cloaca: A common chamber and outlet into which the intestinal, urinary and reproductive tracts open.

Mammals

• Kidneys: The kidneys filter wastes and urine, which are then stored in the urinary bladder and excreted through the urethra.

Human Excretory System (Organs & Structure)

The human excretory system is well-developed and efficient in human beings. It filters wastes from the blood; and excretes them so that the homeostasis of the body stays intact.

Kidneys

Kidneys are two bean-shaped organs that filter blood, removing waste products from it and certain excessive materials to form urine.

Ureters

They are tubes that carry urine from the kidneys to the urinary bladder.

Urinary bladder

A muscular sac designed for storing urine for excretion at will.

Urethra

The final tube through which urine gets expelled out of the organism.

Mechanism Of Urine Formation

The mechanism of filtration includes 3 major steps:

Filtration

Filtration of blood to form a filtrate containing water, glucose, salts, and waste products in the glomerulus.

Reabsorption

Essential substances like amounts of glucose, water, and salts are reabsorbed from the filtrate back into the blood.

Secretion

More waste products are secreted into the filtrate from the blood. Thereafter, the makeup of the urine is finally determined.

Regulation Of Urine Formation

The regulation of urine formation includes:

Hormonal Control

ADH, or antidiuretic hormone, increases the rate of water reabsorption into the kidneys, while aldosterone regulates the quantity of sodium and potassium.

Feedback Mechanisms

The body uses feedback mechanisms to adjust the production of urine according to its water hydration status, blood pressure, and overall electrolyte balance.

Disorders Of The Human Excretory System

The disorders of the human excretory system includes:

Kidney Stones

These are rigid aggregations of minerals and salts that form in the kidneys and cause pain and problems related to urination.

Urinary Tract Infections (UTI)

Infections of any part affecting the urinary tract. The common causative agents are bacteria.

Chronic Kidney Disease (CKD)

A long-term condition characterised by gradual loss of kidney function.

Excretion NEET MCQs (With Answers & Explanations)

Important questions asked in NEET from this topic are:

• Excretion in different organisms

• Mechanism and regulation of urine formation

Practice Questions for NEET

Q1. Protonephridia or flame cells are the excretory structure in

1. Porifera

2. Ctenophora

3. Arthropoda

4. Platyhelminthes

Correct answer: 4) Platyhelminthes

Explanation:

In Platyhelminthes, such as Planaria, the excretory structures are called protonephridia or flame cells. These cells are primarily involved in osmoregulation—the regulation of ionic balance and fluid volume within the organism. Protonephridia consist of a network of tubules with specialized cells that contain cilia, which create a flame-like appearance. The cilia help in the movement of fluid through the tubules, where waste materials are filtered out and excess water is excreted. This system plays a crucial role in maintaining the internal environment of the flatworm, helping it adapt to different osmotic conditions.

Hence, the correct answer is option 4) Platyhelminthes.

Q2. Osmoregulators are

1. Animals whose internal osmolarity changes in relation to their external environment.

2. Animals that excrete urea

3. Animals can maintain their internal osmolarity at a particular level regardless of the external environment.

4. Animals whose osmolality is not fixed

Correct answer: 4) Animals can maintain their internal osmolarity at a particular level regardless of the external environment.

Explanation:

Osmoregulatory: Animals who can maintain their internal osmolarity at a particular level regardless of the external environment. This remarkable adaptation allows them to thrive in various external environments, whether exposed to freshwater, saline water, or fluctuating conditions. By efficiently controlling the movement of water and solutes, osmoregulatory animals can maintain homeostasis and ensure their physiological processes function optimally, regardless of external osmotic pressures.

Hence, the correct option is 3) Animals who can maintain their internal osmolarity at a particular level regardless of the external environment.

Q3. Coxal glands are found in

1. Insects

2. Crustaceans

3. Molluscs

4. Arachnids

Correct answer: 4) Arachnids

Explanation:

Excretory Structures in Animals -

• Protonephridia or flame cells are the excretory structures in Platyhelminthes (Flatworms, e.g., Planaria), rotifers, some annelids, and the cephalochordate – Amphioxus.
• Protonephridia are primarily concerned with ionic and fluid volume regulation, i.e., osmoregulation.
• Nephridia are the tubular excretory structures of earthworms and other annelids.
• Nephridia helps to remove nitrogenous wastes and maintain a fluid and ionic balance.
• Malpighian tubules are the excretory structures of most of the insects including cockroaches.
• Malpighian tubules help in the removal of nitrogenous wastes and osmoregulation.
• Coxal glands are found in arachnids for excretion
• Antennal glands or green glands perform the excretory function in crustaceans like prawns.
• Mollusks show Organs of Bojanus and Keber’s organs for excretion
• Neural glands are present in urochordates such as Herdmania for excretion.

Hence, the correct answer is option 4) arachnids.

Also Read:

• MCQ Practice on Human Excretory System

• Micturition

• Counter Current Mechanism

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