15. Excretion and Osmoregulation part 02- Excretory system in human being

 

15. Excretion and Osmoregulation part 02- Excretory system in human being


Excretory system in human being :

Kidney : 


  • A pair of shaped kidneys are present on either side of from 12th thoracic to 3rd Lumbar vertebra. 
  • Kidneys are present behind peritoneum  Hence are called Retroperitoneal.
  • Dimensions of each kidney are 10 x 5 x 2 cms. 
  • Average weight is 170 g in males and 135 g in female.
  •  Outer surface is convex  and inner is concave. 
  • Notch on the inner concave surface is called Renal artery enters and renal vein as well as ureter leave the kidney through hilus.
  • Each kidney has almost 1 million functional units called Nephrons.

Ureters : 

  • A pair of ureters arise from pelvis of each kidney. 
  • Each ureter is a long muscular tube 25- 30 cm. in length. 
  • Ureters open into urinary bladder by separate openings, which are not guarded by valves. 
  • They pass obliquely through the wall of urinary bladder.
  • This helps in prevention of backflow  of urine due to compression of ureters while bladder is filled.


Urethra :
  •  It is a tube like structure arising from urinary bladder and opening to the exterior of the body. 
  • There are Two urethral sphincters between urinary bladder and urethra.

a. Internal sphincter : 


  • Made up of smooth muscles, involuntary in nature.


b. External sphincter : 

  • Made up of skeletal  muscles, voluntary in nature.
  • If this valve is not functioning properly during inflammation of bladder, it can lead to kidney infection.


Urinary bladder :

  •  It is a median distensible  sac. 
  • A hollow muscular organ, the bladder is situated in pelvic cavity posterior to public symphysis. 
  • At the base of the urinary bladder there is a small inverted triangular area called Trigone. 
  • At the apex of this triangle is opening of urethra.
  • At the two points of the base of the triangle are openings of ureters. Urinary bladder is covered externally by peritoneum. 
  • Inner to peritoneum is muscular layer. 
  • It is formed by detrusor muscles which consist of three layers of smooth muscles. 
  1. Longitudenal
  2. circular
  3. longitudenal respectively. 
  • Innermost layer is made up of transitional epithelium . It helps bladder to stretch.



  • Kidneys bring about separation and elimination of nitrogenous waste, excess water and toxic substances from the body. 
  • They maintain homeostasis by way of their role in osmoregulation and regulation of pH of body fluids. 
  • They produce calcitriol and renin. 
  • Erythropoietin secreted by kidneys is essential for production of RBCs. 
  • Ureters transport urine from renal pelvis to urinary bladder. 
  • Urinary bladder is a temporary storage organ for urine. 
  • It helps to expel urine (micturition). 
  • Urethra is a passage way for discharging urine from body. 
  • In males, it acts as urinogenital organ.


Micturition :

  • Average capacity of urinary bladder is 700 ml. 
  • When urinary bladder is almost half filled, stretch receptors in urinary bladder transmit impulses to spinal cord. This initiates conscious desire to expel urine. 
  • Micturition reflex center of spinal cord transmit impulses to the wall of urinary bladder and internal urethral sphincter.
  • Bladder muscles contract and muscles of internal urethral sphincter relax. 
  • Then external sphincter receives impulses from conscious centre of brain and relaxes. This leads to expellation/ elimination of urine from bladder.
  • Infants up to 2 years of age show lack of voluntary control over micturition. This is because neurons to the external sphincter muscles are not developed.



  • Each kidney is covered by 3 layers of tissue. 
  • Outermost Renal fascia is made up of thin layer of fibrous connective tissue. 
  • It anchors the kidney to abdominal wall as well as surrounding tissue.
  • Middle layer is a mass of fatty tissue called adipose capsule Protects kidneys by shock absorption. 
  • Innermost layer, renal capsule is a smooth transparent fibrous membrane that is continuous with outer layer of ureters. 
  • It acts as a barrier against spread of infections in kidney. L.S. of kidney shows two distinct regions within capsule.


  • Histologically, kidney is divisible into two regions as renal cortex and renal medulla. 
  • Renal cortex is outer / peripheral, red coloured and granular region. 
  • Cortex contains Malpighian bodies, convoluted tubules and blood vessels. 
  • Medulla is inner region of kidney with pale red colour and striated appearance.
  • Medulla mainly consists of Loops of Henle and collecting ducts. 
  • All these are arranged in conical manner to form renal pyramids.
  • Cortex extends in medulla as columns of Bertini /renal columns between pyramids. 
  • Narrow tipof pyramid is called as renal papilla.




  • There are several pyramids. 
  • Renal papilla open into minor calyx.
  • Minor calyces merge together to form major calyces and major calyces unite together to form renalpelvis. 
  • Renal pelvis (renal sinus) is funnelshaped area in the region of medulla of kidney. 
  • Renal pelvis continues as ureter which leaves kidney through hilus.


Nephrology 

  • It is branch of biology that deals with structure, function and disorders of male and female urinary system.


Nephron : 


  • Nephrons are structural and functional units of kidney. 
  • Each nephron consists of about 4 - 6 cm long thin-walled tube- ‘renal tubule’ and a bunch of capillaries-‘glomerulus’. 
  • Wall of renal tubule is made up of single layer of epithelial cells.
  • Its proximal end is wide, blind, cup-like, called as Bowman’s capsule. 
  • Distal end is open. It is divisible into Bowman’s capsule, neck, proximal convoluted tubule (PCT), Loop of Henle (LoH), distal convoluted tubule (DCT) and collecting tubule (CT).
  • Glomerulus is present in the cuplike cavity of Bowman’s capsule and both are collectively known as renal corpuscle or Malpighian body.
  • Each Malpighian body is about 200μm in diameter and consists of a Bowman’s capsule and glomerulus.
Glomerulus : 



  • Glomerulus is a bunch of fine blood capillaries lying in the cup of Bowman’s capsule. 
  • A small terminal branch of renal artery called as afferent arteriole enters the cup cavity and undergoes extensive fine branching to form network of several capillaries. This bunch is called as glomerulus. 
  • Capillary wall is fenestrated. 
  • All capillaries reunite and form an efferent arteriole that leaves the cup cavity.
  • Diameter of afferent is greater than efferent arteriole to create a high hydrostatic pressure in glomerulus. It is important for ultrafiltration.

Bowman's capsule : 


  • It is a cup-like structure having double wall. 
  • Both walls are composed of squamous epithelium. 
  • Outer wall is called as parietal and inner wall is called as visceral wall. 
  • There is a space called as capsular space/ urinary space in between two walls.
  • Visceral wall consists of special type of squamous cells called podocytes having a foot-like pedicel.
  • These podocytes are in close contact with the walls of capillaries of glomerulus. 
  • There are small slits called as filtration slits in between adjacent podocytes. 
  • Parietal wall is thin consisting of simple squamous epithelium. 
  • It continues into neck. 
  • The wall of neck is made up of ciliated epithelium. Lumen of neck is called urinary pole. 
  • Neck leads to proximal convoluted tubule.
Proximal Convoluted Tubule : 

  • This is highly coiled part of nephron. 
  • It is lined by cuboidal cells with brush border (microvilli) and surrounded by peritubular capillaries. 
  • It is place of selective reabsorption. 
  • Due to convolutions (coiling), filtrate flows slowly and remains in the PCT for longer duration. This ensures the reabsorption of maximum amount of useful molecules.

Loop of Henle : 

  • This is ‘U’ shaped tube consisting of descending and ascending limb.
  • Descending limb is thin walled and permeable to water. 
  • It is lined with simple squamous epithelium. 
  • Ascending limb is thick walled and impermeable to water. 
  •  It is lined with simple cuboidal epithelium.
  • LoH is surrounded by capillaries called vasa recta. 
  • Its function is to operate counter current system - a mechanism for osmoregulation. 
  • Regulation of salt and water balance in body is called as osmoregulation.
  • Ascending limb of Henle’s loop leads to DCT.

Distal convoluted tubule: 

  • This is another coiled part of nephron. 
  • Its wall consists of simple cuboidal epithelium. 
  • DCT performs tubular secretion / augmentation / active secretion in which, wastes are taken up from surrounding capillaries and secreted into passing urine.
  • DCT helps in water reabsorption and regulation of pH of body fluids.

Collecting tubule: 


  • This is a short, straight part of DCT. 
  • Collecting tubule reabsorbs water and secretes protons. 
  • Collecting tubule opens to collecting duct.
Types of nephrons:
 1. Cortical nephrons
2. Juxtamedullary nephrons

Cortical nephrons: 

  • with shorter loop of Henle which extend very little in medulla. 
  • Most of the nephrons are cortical nephrons.

Juxtamedullary nephrons :

  • Few nephrons have longer loop of Henle that runs deep into medulla. These are called Juxtamedullary nephrons.



  • A small branch of efferent arteriole forms peritubular capillary network around DCT, PCT and Henle’s loop of cortical nephrons 
  • Efferent arteriole also forms loop-shaped vasa recta around Henle’s loop of juxtamedullary nephrons.
  • Nephrons are responsible for elimination of waste and osmoregulation. 
  • Hence are richly supplied with blood.


Juxta Glomerular Apparatus :

  • Some smooth muscle cells of the wall of afferent arteriole are modified in such a way that their sarcoplasm is granular. These cells are called ‘juxtaglomerular (JG) cells.
  • In each nephron, beginning part of DCT makes contact with the afferent arteriole of same nephron.
  •  Cells in the wall of DCT in this region are packed more densely than those in other region of DCT. This is called macula densa.
  • Macula densa and the JG cells together form Juxta Glomerular Apparatus (JGA) .
  • JGA plays important role in blood pressure regulation within kidney.

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