Kidneys, The Human Excretory Organ

Excretion is the process of removal or elimination of waste products from the body through lungs, skin, liver and kidney. This system involves in excretion process by removal of metabolic waste. Human body consist of two kidney pairs. Kidneys are the primary organ of body which takes part for the removal of wastes in the form of urine. Human excretory system divided into,

  • 2 kidneys
  • 2 ureter
  • 1 Urinary bladder
  • 1 Urethra


  1. Kidneys are of two pairs, bean like shape
  2. Located at posterior side of the spine one on each side
  3. Kidney divided into three regions
    • Renal cortex: It is outer layer; it is the part where blood is filtered
    • Renal medulla: It is middle layer, contain capillary ducts which receives filtrate to the pelvis
    • Renal pelvis: Pelvis is a hollow cavity, this layer carry urine from kidneys and send to the ureter


Nephrons are the filtering unit of kidneys. Approximately one million nephrons present in kidney. Nephrons are present in each medulla and cortex of the kidneys. Nephrons surrounds a network of blood vessels. Parts of Nephron: Each nephron divided into six parts are following.

      1. Glomerulus: It is a network of nerve ending; small blood capillaries ball like structure present at the end of kidney’s tubule. Function:  It is responsible for blood filtration and formation of urine
      2. Bowman’s Capsule: It is a thin double membranous capsule, Sac round structure located at the starting of the tubule.
      3. Proximal tubule: This tube like structure made up of ciliated epithelial cells. Function: This tube is reabsorbing glucose, water and peptides from the tubule and get back to the blood
      4. Loop of Henle: This is responsible to form concentration of urine by the help of water and sodium chloride absorption. This divided into two ascending loop of Henle and descending loop of Henle
      5. Distal tubule: This tube receive filtrate from thick ascending loop of Henle, present in kidney cortex region. Function: This tube is responsible for maintain the pH of urine through re-absorption of calcium, sodium and chloride.
      6. Collecting duct: It is long, twisted and last part that receive filtrate from and move towards pelvis and ureter also referred as renal collecting tubule.


  • It is a thin, delicate muscular tube extends from renal pelvis
  • Ureter size usually in human 25 – 30cm long
  • Receives urine from kidneys and empty urine in urinary bladder
  • Valves are present which prevent the backflow of urine known as ureterovesical valve
  • Ureter posteriorly runs on the lateral walls of the pelvis

Urinary Bladder

  1. Pouch like structure, covered by smooth muscles
  2. Urinary bladder stores urine until it is excreted from the body
  3. Micturition is the process controlled by the brain, bladder closing opening and expel out urine are interlinked through brain, bladder sphincters and bladder muscles
  4. In human male location of bladder base present between rectum and pubic region


  • It forms a connection of urinary bladder to outside the body
  • Urinary bladder extrudes a tube, this tube varies in male and female.
  • In man this passage is common for urine and sperm

Functions of kidney

  1. Main function is filtration of blood and eliminate cellular waste from body
  2. At a time 20% blood store in kidneys that’s why person can survive with one kidney. If failure of one kidney so the workload moves towards the second kidney by increasing its size.
  3. It also maintains homeostasis of body by removal of extra water, vitamins and minerals. If these are in excess amount so kidney detects and maintain body equilibrium.
  4. It also controls pH of water, responsible for secretion of erythropoietin and activate vitamin D production in skin.
  5. Responsible for extracellular volume maintenance
  6. Remove toxic substances from the body like urea, ammonia and uric acid
  7. Maintain ionic balance in extracellular fluid

Urine and its Composition

Urine is clear, liquid waste containing water soluble substances like urea, salt etc, is excreted by the kidneys through urination process.

Characteristics of Urine

  • Physical Characteristics: Physical characteristics of urine are listed below
  • Urine Color: Urine color may vary or depending on person’s water intake and diet, normal color is amber yellow. If you take plenty of water so concentration becomes change and color change to transparent
  • Turbidity: Normal fresh urine should be clear or slight turbid. If turbidity increase it indicates some urinary tract infections or blockage
  • Smell (odor): Odor of fresh urine is slightly mild, in some patients like diabetic patient have fruity smell due to presence of ketone bodies, old age patient has change urine odor. This smell is an indicator of person health
  • pH: Normal urine pH between 4.6 – 8, must vary person to person depend on diet
  • Density: The specific gravity or density of fresh urine between 0.001-0.035

Chemical Characteristics of Urine

  • Urea: 3 g/L.
  • Chloride: 87 g/L.
  • Sodium: 17 g/L.
  • Potassium: 750 g/L.
  • Creatinine: 670 g/L.
  • Other dissolved ions, inorganic and organic compounds (proteins, hormones, metabolites).

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