Excretory Products and their Elimination

Introduction

Excretory Products and their elimination form a critical aspect of animal physiology, encompassing the intricate mechanisms by which organisms rid themselves of waste materials to maintain internal equilibrium. As metabolic processes fuel life, they concurrently generate various waste substances, including nitrogenous compounds like ammonia, urea, and uric acid, as well as ions such as Na+, K+, Cl, SO42-, and HPO4 etc. The accumulation of these byproducts can disrupt the delicate balance within an organism’s internal environment. To counteract this, animals have evolved diverse excretory strategies and specialized structures, such as kidneys, nephridia, and Malpighian tubules, tailored to their habitat and evolutionary demands.

Animals and their modes of excretion

  • The process of excretion involves the elimination of waste products generated by metabolic activities, thereby maintaining the internal balance of the organism.
  • Different animals have evolved various strategies for excretion, often influenced by factors such as habitat, diet, and water availability. Solubility and toxicity of nitrogenous product(s) determine the mode of excretion in animals.
  • Osmoregulation: The process by which organisms control the balance of water and solutes in their bodies to maintain stable internal conditions despite changes in their surroundings.
  • Order of toxicity: Ammonia > Urea > Uric Acid
  • Amount of water required to dissolve: Ammonia > Urea > Uric Acid

Ammonotelism

Some aquatic organisms, including bony fishes, aquatic insects, and amphibians, are ammonotelic. They excrete toxic ammonia directly across their body surfaces or through gill membranes. Ammonia requires a substantial amount of water for elimination. This strategy is efficient in water-rich environments.

Ureotelism

Terrestrial animals, particularly mammals and certain amphibians, have adopted ureotelism. They convert ammonia, a highly toxic waste, into less toxic urea in the liver. Urea requires less water for elimination, making it ideal for animals living in environments where water conservation is crucial.

Uricotelism

Reptiles, birds, and some insects are uricotelic. They excrete waste as uric acid, which is relatively insoluble and forms a semi-solid paste. This mode of excretion minimizes water loss, a vital adaptation for animals in arid environments.

Organisms and their excretory structure

Organism Group

Excretory Organ

Examples

Fish

Gills

Bony fishes, cartilaginous fishes

Amphibians

Skin and Kidneys

Frogs, salamanders

Insects

Malpighian tubules

Cockroaches, grasshoppers, insects

Reptiles

Lobed kidney/Cloaca

Snakes, lizards, turtles

Birds

Kidney

Eagles, sparrows, penguins

Mammals

Kidneys

Humans, dogs, cats, whales

Annelids

Nephridia

Earthworms, leech

Platyhelminthes

Flame Cells or Protonephridia

Planaria, flatworms

Cephalochordates

Notochordal Canal System

Amphioxus

Crustaceans

Antennal or green glands

Prawns, crabs

Human Excretory System

  • Humans have a pair of bean-shaped kidneys in the retroperitoneal cavity against the posterior abdominal wall.
  • It lies between the last thoracic (T12) and third lumbar (L3) vertebra.
  • Size of kidney (in adults): l x b x h = 10-12 x 5-7 x 2-3 cm.
  • Weight = 120-170 gm (approx.)
  • The right kidney is slightly lower than the left one. It is due to the presence of the liver on the right side.
  • The kidney is encapsulated by a tough layer of collagen fibres and other connective tissue components. This layer is called the renal capsule.
  • The kidney is divided into two regions, the outer cortex and the inner medulla.
  • The side of the kidney facing each other has a notch called Hilum. This is the space through which blood vessels, nerves, and ureter enter and exit.
  • Inner to the hilum, a broader funnel-shaped region called the renal pelvis is present. It has projections called the renal calyx.
Figure: Schematic representation of L.S of human kidney
  • In the medulla, there are 8-10 conical masses present in each kidney called the medullary or renal pyramids. It opens in the renal pelvis through the renal calyx. The space between two adjacent renal pyramids is called the renal column or Columns of Bertini.
  • The renal pelvis opens into the ureter which empties urine into the urinary bladder.
  • Each kidney contains 0.8-1 million nephrons which are the functional units of the kidney.
  • Each nephron consists of a tuft of capillaries formed by the afferent arterioles called the Glomerulus and a long tubular part called the Renal tubule in which the filtered fluid is converted into urine.
  • The renal tubule forms a double-walled cup-like structure surrounding the glomerulus called Bowman’s capsule.
  • Bowman’s capsule along with the Glomerulus is called the Malpighian body (MB) or renal corpuscle.
  • The tubule proximal to the Malpighian body is highly convoluted and called the proximal convoluted tubule (PCT). It is followed by a ‘U’ shaped structure called Henle’s loop and then again, a convoluted structure called distal convoluted tubule (DCT).
  • The Henle’s loop has a descending limb and an ascending limb. The upper portions of the descending and ascending limbs are wider and the lower portions are narrower.
  • The efferent arteriole from the Malpighian body forms a peritubular network around the Henle’s loop called the vasa recta.
  • It runs in the opposite direction to Henle’s loop forming a countercurrent mechanism.
  • The MB, PCT, and DCT lie in the cortical region of the kidney. The Henle’s loop, in the majority of the nephrons, resides only a little into the medulla. Such nephrons are called medullary nephrons. In other nephrons, it goes deep into the medulla, such nephrons are called Juxta medullary nephrons.
  • At the junction of the end of the thick ascending limb of Henle’s loop (DCT) and afferent arteriole, specialized cells form a plaque called macula densa (of juxta glomerular apparatus, JGA). The JGA controls the function of the nephron. It regulates the nephron’s activity by secreting renin.

The Process of Urine formation

  • The urine formation takes place by three main processes:

(1). Ultrafiltration,

(2). Reabsorption, and

(3). Secretion.

  • Excretion = Ultrafiltration – Reabsorption + Secretion
  • The ultrafiltration takes place through three layers, the endothelium of glomerular blood vessels, the epithelium of Bowman’s capsule and a basement membrane between them.
  • The epithelial layer of Bowman’s capsule is formed by the podocytes which are intricately arranged to form small slits between them through which filtration takes place.
  • In ultrafiltration, all the constituents of plasma are filtered except the proteins.
  • Ultrafiltrate = plasma – proteins.
  • The rate of ultrafiltration is called glomerular filtration rate (GFR). It is 180L/day or 125mL/min. This is the amount of filtrate.
  • Around 1100-1200 mL, blood flows through the glomeruli every minute. As, plasma constitutes 55% of blood, 605-660 mL of plasma is filtered every minute.
  • Reabsorption and secretion take place in Henle’s loop. Glucose, amino acids, and Na+ in the filtrate are reabsorbed actively whereas the nitrogenous wastes and H2O are absorbed passively.
  • H+, K+ and ammonia in the filtrate are secreted to maintain ionic and acid-base balance.

Tubular functions

  • Different segments of the nephron are specialized to reabsorb and secrete specific substances to fine-tune the process of urine formation.

Portion of Nephron

Type of cells

Functions

PCT

Cuboidal cells with extensive microvilli

Na+, Cl, H2O, HCO3 ions, glucose, amino acids, small peptides, and Urea, reabsorption

H+, K+ and NH3 secretion

Descending Henle’s loop

Squamous epithelium

H2O reabsorption

Ascending Henle’s loop

Squamous epithelium

Na+, Cl, K+, Ca2+, Mg2+‑, HCO3absorbed H+, K+, NH3 secretion

DCT

Cuboidal epithelium

Na+, H2O, CO3absorption H+, K+, NH3 secretion

Collecting duct (CD)

Flattened cells, squamous to cuboidal (in cortical CD)

Cuboidal cells, (in medullary CD)

Columnar cells (CD with larger diameter)

H2O, absorption

H+, K+ secretion

Properties of normal urine

  • Specific gravity: 1.002 to 1.028 g/ml.
  • Color: Pale yellow to amber (due to the presence of a pigment called urochrome).
  • Odor: Slightly ammonia-like.
  • Transparency: Clear.
  • Volume: 1-2 L/day

Regulation of urine formation: Renin-angiotensin mechanism

  1. Decreased Blood Pressure or Blood Volume
  2. Kidneys Release Renin from released from JGA
  3. Renin Converts Angiotensinogen (from the Liver) to Angiotensin I
  4. Angiotensin I is Converted into Angiotensin II by angiotensin-converting enzyme (ACE) in the vascular epithelium, particularly in the lungs
  5. Angiotensin II:
  •   Causes Constriction of Blood Vessels (Vasoconstriction)
  • Stimulates Release of Aldosterone from Adrenal Glands
  • Promotes Thirst

       6. Aldosterone:

  • Acts on Kidneys
  • Increases Sodium Reabsorption
  • Increases Water Reabsorption
  1. Increased Blood Volume and Blood Pressure
  2. Re-established Blood Pressure and Normalizes Urine Formation

Regulation of urine formation: Anti Diuretic Hormone (ADH) or Vasopressin

  1. Osmoreceptors in the Hypothalamus Detect Increased Blood Osmolarity
  2. Hypothalamus Stimulates Posterior Pituitary Gland to Release ADH
  3. ADH Acts on Kidneys:
  •  Increases Permeability of Collecting Ducts
  • Increases Water Reabsorption
  1. More Water is Reabsorbed from Collecting Ducts into the Blood
  2. Decreased Blood Osmolarity
  3. Negative Feedback Inhibits Further ADH Release

References:

  • Biology: Text Book for Class Xi. (2006).
  • Hall, J. E., PhD. (2015). Guyton and Hall Textbook of Medical Physiology. Elsevier Health Sciences.
  • Widmaier, E. P., Raff, H., & Strang, K. T. (2003). Vander et Al’s Human Physiology: With OLC Bind-In Card. McGraw-Hill Science, Engineering & Mathematics

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