Animals accumulate various substances, including nitrogenous wastes (ammonia, urea, uric acid), carbon dioxide, water, and excess ions (Na$^+$, K$^+$, Cl$^-$, phosphate, sulphate). These substances arise from metabolic activities or excess intake and must be removed, either completely or partially, to maintain healthy tissue function.

The process of eliminating these waste materials, particularly nitrogenous wastes, is called excretion.

Major forms of nitrogenous wastes excreted by animals are ammonia, urea, and uric acid. These differ in toxicity and the amount of water required for elimination:

• Ammonia: Most toxic, requires large amounts of water for elimination.
• Uric acid: Least toxic, can be removed with minimal water loss.
• Urea: Intermediate in toxicity and water requirement.

Based on the primary nitrogenous waste product, animals are classified into three types:

• Ammonotelism: Excreting ammonia. Common in many bony fishes, aquatic amphibians, and aquatic insects. Ammonia is highly soluble and usually diffuses across body surfaces or gill surfaces as ammonium ions. Kidneys play minimal role.
• Ureotelism: Excreting urea. Common in mammals, many terrestrial amphibians, and marine fishes. Terrestrial adaptation favors less toxic waste for water conservation. Ammonia produced metabolically is converted to urea in the liver, released into blood, filtered by kidneys, and excreted. Some urea is retained in kidney matrix in some animals to maintain osmolarity.
• Uricotelism: Excreting uric acid. Common in reptiles, birds, land snails, and insects. Uric acid is excreted as pellets or paste, minimizing water loss.

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Excretory structures vary widely across the animal kingdom:

• Most invertebrates have simple tubular structures.
• Vertebrates have complex tubular organs called kidneys.

Examples of excretory structures in invertebrates and some lower chordates:

• Protonephridia (Flame cells): Found in Platyhelminthes (flatworms like Planaria), rotifers, some annelids, and cephalochordates (Amphioxus). Primarily involved in osmoregulation (ionic and fluid volume regulation).
• Nephridia: Tubular excretory structures in earthworms and other annelids. Remove nitrogenous wastes and maintain fluid/ionic balance.
• Malpighian tubules: Excretory structures in most insects (including cockroaches). Help remove nitrogenous wastes and osmoregulation.
• Antennal glands (Green glands): Excretory function in crustaceans like prawns.

Human Excretory System

The human excretory system (urinary system) consists of (Figure 19.1):

• A pair of kidneys.
• One pair of ureters.
• A urinary bladder.
• A urethra.

Kidneys:

• Reddish brown, bean-shaped structures.
• Located between the last thoracic and third lumbar vertebrae, close to the dorsal inner abdominal wall.
• Dimensions (adult human): 10-12 cm length, 5-7 cm width, 2-3 cm thickness. Average weight 120-170 g.
• Hilum: A notch towards the center of the inner concave surface where the ureter, blood vessels, and nerves enter.
• Renal pelvis: Broad funnel-shaped space inside the hilum, with projections called calyces.
• Outer layer: Tough capsule.
• Inside: Two zones – outer cortex and inner medulla.
• Medulla: Divided into a few conical masses called medullary pyramids, which project into the calyces.
• Cortex: Extends between the medullary pyramids as renal columns (Columns of Bertini) (Figure 19.2).

Nephrons: Each kidney contains nearly one million complex tubular structures called nephrons, which are the functional units of the kidney (Figure 19.3).

Each nephron has two parts:

1. Glomerulus: A tuft of capillaries formed by the afferent arteriole (a fine branch of the renal artery). Blood leaves the glomerulus via the efferent arteriole.
2. Renal Tubule: Begins with Bowman's capsule, a double-walled cup-like structure enclosing the glomerulus.
   • Malpighian body (Renal corpuscle): Consists of the glomerulus enclosed within Bowman's capsule (Figure 19.4).
   • Proximal Convoluted Tubule (PCT): Highly coiled network extending from Bowman's capsule.
   • Henle's loop: Hairpin-shaped portion with a descending limb and an ascending limb.
   • Distal Convoluted Tubule (DCT): Another highly coiled region extending from the ascending limb of Henle's loop.

Collecting Duct: 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.

Location of nephron parts:

• Malpighian corpuscle, PCT, and DCT are located in the cortical region of the kidney.
• Loop of Henle dips into the medulla.
  • Cortical nephrons: Have short loops of Henle that extend only slightly into the medulla (majority of nephrons).
  • Juxta medullary nephrons: Have very long loops of Henle that run deep into the medulla.

Vascular network around tubules: The efferent arteriole forms a fine capillary network around the renal tubule called peritubular capillaries. A 'U' shaped vessel called vasa recta runs parallel to Henle's loop; it is formed from the peritubular capillaries and is absent or reduced in cortical nephrons.

Ureters: A pair of tubes that carry urine from the renal pelvis of each kidney down to the urinary bladder.

Urinary bladder: A muscular sac that stores urine temporarily.

Urethra: A tube that carries urine from the urinary bladder to the outside of the body.

Urine Formation

Urine formation is a complex process involving three main steps that occur in different parts of the nephron:

1. Glomerular Filtration (Ultrafiltration):
   • First step in urine formation, carried out by the glomerulus.
   • Blood is filtered from the glomerular capillaries into the lumen of Bowman's capsule.
   • Driving force: Glomerular capillary blood pressure.
   • Filtration membrane: Blood is filtered across three thin layers: endothelium of glomerular capillaries, epithelium of Bowman's capsule (podocytes with filtration slits), and the basement membrane between them.
   • Fine filtration: Almost all plasma constituents, except large proteins, pass into the Bowman's capsule lumen. This non-selective filtration is called ultrafiltration.

   Glomerular Filtration Rate (GFR): The amount of filtrate formed by the kidneys per minute. In a healthy individual, GFR is approximately 125 ml/minute, which equals about 180 liters per day.

   Regulation of GFR: Kidneys have autoregulatory mechanisms. The Juxta Glomerular Apparatus (JGA), a specialized region where the DCT and afferent arteriole meet, plays a key role. A drop in GFR activates JG cells to release renin, which increases glomerular blood flow and restores GFR.

2. Reabsorption:
   • Since 180 liters of filtrate are formed daily but only about 1.5 liters of urine are excreted, nearly 99% of the filtrate must be reabsorbed by the renal tubules.
   • Tubular epithelial cells in different nephron segments reabsorb substances from the filtrate back into the blood.
   • Some substances (glucose, amino acids, Na$^+$) are reabsorbed actively (requiring energy).
   • Nitrogenous wastes are reabsorbed by passive transport.
   • Water reabsorption also occurs passively, especially in the initial segments.
3. Secretion:
   • Tubular cells actively secrete substances like H$^+$, K$^+$, and ammonia into the filtrate.
   • This is an important step for maintaining the ionic balance, pH (acid-base balance), and waste removal from body fluids.

Function Of The Tubules

Different segments of the renal tubule perform specific functions in urine formation (Figure 19.5):

• Proximal Convoluted Tubule (PCT):
  • Lined by simple cuboidal brush border epithelium (increasing surface area).
  • Major site of reabsorption: Nearly all essential nutrients, 70-80% of electrolytes, and 70-80% of water are reabsorbed here.
  • Helps maintain pH and ionic balance: Selective secretion of H$^+$, NH$_3$, and K$^+$ into filtrate; absorption of $\textsf{HCO}_3^-$.
• Henle's Loop:
  • Plays a significant role in concentrating urine and maintaining the high osmolarity of the medullary interstitium.
  • Descending limb: Permeable to water, almost impermeable to electrolytes. Filtrate becomes more concentrated as it moves down due to water loss.
  • Ascending limb: Impermeable to water, permeable to electrolytes (actively or passively transported out). Filtrate becomes diluted as electrolytes move out into the medullary fluid.
• Distal Convoluted Tubule (DCT):
  • Conditional reabsorption of Na$^+$ and water (influenced by hormones).
  • Reabsorption of $\textsf{HCO}_3^-$.
  • Selective secretion of H$^+$, K$^+$, and NH$_3$ to maintain pH and Na$^+$-K$^+$ balance in blood.
• Collecting Duct:
  • Long duct extending from cortex to inner medulla.
  • Allows reabsorption of large amounts of water to produce concentrated urine (influenced by ADH).
  • Allows passage of small amounts of urea into the medullary interstitium, contributing to osmolarity gradient.
  • Selective secretion of H$^+$ and K$^+$ ions helps maintain blood pH and ionic balance.

Mechanism Of Concentration Of The Filtrate

Mammals can produce concentrated urine due to the specialized structure and arrangement of Henle's loop and vasa recta, which operate as a counter current mechanism (Figure 19.6).

Counter current system:

• Flow of filtrate in the descending and ascending limbs of Henle's loop is in opposite directions.
• Flow of blood in the descending and ascending limbs of vasa recta is also in opposite directions (counter current to the flow in Henle's loop).

Mechanism for maintaining medullary osmolarity gradient (increasing from 300 mOsmol/L in cortex to 1200 mOsmol/L in inner medulla):

• NaCl is actively transported out of the thick ascending limb of Henle's loop into the medullary interstitium.
• NaCl is exchanged between the ascending limb of Henle's loop and the descending limb of vasa recta. NaCl is then returned to the interstitium by the ascending portion of vasa recta.
• Small amounts of urea enter the ascending limb of Henle's loop and are transported back to the interstitium by the collecting tubule. This cycle of urea helps maintain the high osmolarity in the inner medulla.

The counter current mechanism, facilitated by the proximity and opposing flows in Henle's loop and vasa recta, maintains this increasing osmolarity gradient in the medullary interstitium.

Concentration of urine: The presence of this high osmolarity gradient in the interstitium outside the collecting duct facilitates the osmotic withdrawal of water from the collecting duct as the filtrate passes through. This results in the production of concentrated urine.

Human kidneys can produce urine nearly four times more concentrated than the initial filtrate.

Regulation Of Kidney Function

Kidney function is regulated by hormonal feedback mechanisms involving the hypothalamus, JGA, and heart.

• Regulation by ADH (Antidiuretic Hormone) or Vasopressin:
  • Osmoreceptors in the body sense changes in blood volume, body fluid volume, and ionic concentration.
  • Excessive fluid loss activates these receptors, stimulating the hypothalamus.
  • Hypothalamus signals the neurohypophysis (posterior pituitary) to release ADH.
  • ADH acts on the later parts of the renal tubule (DCT and collecting duct), increasing their permeability to water. This enhances water reabsorption, preventing excessive water loss (diuresis) and leading to concentrated urine.
  • Increased body fluid volume suppresses osmoreceptors and ADH release.
  • ADH also causes vasoconstriction, increasing blood pressure, which can increase glomerular blood flow and GFR.
• Regulation by Renin-Angiotensin Mechanism:
  • JGA plays a regulatory role. A fall in glomerular blood flow, glomerular blood pressure, or GFR activates JG cells to release renin.
  • Renin converts angiotensinogen in blood to angiotensin I, which is further converted to angiotensin II (a powerful vasoconstrictor).
  • Angiotensin II increases glomerular blood pressure and GFR. It also stimulates the adrenal cortex to release aldosterone.
  • Aldosterone promotes reabsorption of Na$^+$ and water from the DCT and collecting duct. This leads to increased blood volume, increasing blood pressure and GFR.
• Regulation by ANF (Atrial Natriuretic Factor):
  • Released by the atria of the heart in response to increased blood flow (and stretching of atrial walls).
  • ANF causes vasodilation (dilation of blood vessels), decreasing blood pressure.
  • ANF acts as a check on the renin-angiotensin mechanism, countering its vasoconstrictor effects.

Micturition

Urine formed by nephrons is stored in the urinary bladder. The process of releasing urine from the bladder is called micturition.

Mechanism:

• As the bladder fills with urine, its walls stretch.
• Stretch receptors on the bladder walls send signals to the Central Nervous System (CNS).
• The CNS sends motor messages back to the bladder.
• These messages cause the smooth muscles of the bladder to contract and the urethral sphincter to relax simultaneously.
• This leads to the release of urine.

The neural mechanisms causing micturition constitute the micturition reflex. Micturition is a voluntary process (consciously controlled relaxation of the external urethral sphincter).

Characteristics of urine:

• Average volume excreted per day: 1 to 1.5 liters.
• Color: Light yellow.
• Consistency: Watery fluid.
• pH: Slightly acidic (pH $\approx$ 6.0).
• Has a characteristic odor.
• Average urea excreted per day: 25-30 gm.

Urine analysis is a valuable tool for clinical diagnosis. Presence of glucose (glycosuria) and ketone bodies (ketonuria) in urine can indicate diabetes mellitus.

Role Of Other Organs In Excretion

Besides the kidneys, other organs assist in the elimination of excretory wastes:

• Lungs: Remove large amounts of $\textsf{CO}_2$ (approx. 200 mL/minute) and also significant quantities of water vapour daily.
• Liver: The largest gland. Secretes bile, which contains waste substances like bile pigments (bilirubin, biliverdin), cholesterol, degraded steroid hormones, vitamins, and drugs. These substances are released into the digestive tract and eventually eliminated with faeces.
• Skin: Contains sweat glands and sebaceous glands.
  • Sweat glands: Produce sweat, a watery fluid containing NaCl, small amounts of urea, lactic acid, etc. Primary function is cooling, but also aids in waste removal.
  • Sebaceous glands: Eliminate substances like sterols, hydrocarbons, and waxes through sebum secretion, which provides a protective oily skin covering.
• Small amounts of nitrogenous wastes can also be eliminated through saliva.

Disorders Of The Excretory System

Malfunctioning of kidneys or other parts of the excretory system can lead to various disorders:

• Uremia: Accumulation of urea and other nitrogenous wastes in the blood due to kidney malfunction. Highly harmful and can lead to kidney failure.
• Hemodialysis: An artificial method to remove excess urea and wastes from the blood in uremic patients. Blood is drained from an artery, passed through a dialysing unit ("artificial kidney") containing a porous cellophane tube surrounded by dialysing fluid (with composition similar to plasma but without nitrogenous wastes). Wastes diffuse from the blood into the fluid. Cleared blood is returned to the body through a vein.
• Kidney Transplantation: The ultimate solution for acute renal failures. A healthy, functioning kidney from a donor (preferably a close relative to minimize rejection) is surgically placed in the recipient.
• Renal calculi: Formation of stones or insoluble masses of crystallized salts (e.g., oxalates) within the kidney or urinary tract.
• Glomerulonephritis: Inflammation of the glomeruli of the kidney, affecting their filtering function.

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