Excretion is defined as the process of removal of toxic waste products of metabolism, not only of nitrogenous waste products but also of carbon dioxide and bile pigments.

The excretory system:

It includes organs involved in the removal of waste products and the process or physiology of excretion.

1. Kidney:

• 2 kidneys are on either side of vertebral column along the posterior abdominal wall in the lumbar region of our body.
• Right one lies slightly lower than the left side due to the space occupied by liver.
• 11×6x3 cm3 dimension, weight is 150gm in male and 135gm in female.
• Reddish brown and bean shaped.
• Covered by tough fibrous renal capsule.
• Renal capsule is covered by adipose capsule( adipose tissue)
• This is again covered by renal fascia (fibrous membrane).
• These structures protect the kidney from injury
• Each kidney has a concave inner and convex outer surface.
• Concave surface has opening called renal hilum through which blood vessels and ureter enter or leave the kidney.

Internal structures of kidney:

Can be studied in longitudinal section,

1. Capsule: it is the structure covering the kidney.

• Made up of fibrous connective tissue.
  2. Cortex: Outer 1/3^rd of the kidney

• Light pink in colour.
• Contains Bowman’s capsule and proximal and distal part of nephron.
  3. Medulla: Inner 2/3^rd of the kidney.

• Darker in colour
• Contains loop of Henle and collecting tubes.
• Divided into conical medullary pyramids which lead to minor and major calyces.
• The substance of cortex extends between the pyramids and forming the renal columns of Bertini.
  
  4. Pelvis: funnel shaped structure formed by the union of major calyces towards the hilum.

• Pelvis leads to the ureter.

Microscopic structure of the kidney:

• Each kidney is composed of 1 to 2 million nephrons or uriniferous tubules.
• Nephron is the structural and functional unit of kidney.
• They are held together in connective tissues.

Structure of nephron:

• Nephron consists of a long and coiled tube closed at one end and other end opening into the collecting tubule.
• Closed end forms malpighian body situated in cortex
• Contains cup shaped Bowman’s capsule and network of capillaries called glomerulus.
• Bowman’s capsule is cup like and double walled.
• Lined by squamous epithelium.
• In the depression lies network of capillaries called glomerulus.
• Afferent arteriole enters and efferent arteriole leaves the Bowman’s capsule.
• Afferent arteriole has wider lumen than the efferent arteriole.
• There is capsular space between the inner and outer lining of the Bowman’s capsule.
• There is only 2 cell layer thick barrier between the blood of the glomerular capillaries and the fluid in the capsular space.
• Bowman’s capsule leads to the tubules.
• Lined with ciliated epithelium.’
• First part of tubule is convoluted and lies in cortex of kidney called proximal convoluted tubule.
• It extends to ‘U’ shaped loop of Henle, lies in medulla.
• Loop of Henle has a thin descending limb, thin ascending limb and a thick ascending limb.
• It leads to distal convoluted tubule, lies in cortex and open into the collecting duct.
• Collecting ducts finally open into renal pelvis conveying urine to ureter.
• The efferent arteriole divides into capillaries and envelops whole tubule system before draining into the renal vein.

1. Ureter:

• There are two ureters arising from the pelvis of kidney.
• It is thick walled, muscular tube of about 25cm in length.
• They run down on either side of vertebral column along the posterior abdominal wall to open into the urinary bledder.

1. Urinary Bladder:

• It is a hollow muscular organ lies in the pelvic cavity.
• It has strong muscular wall  made up of Detrusor muscle which has ability to didtend.
• Bladder has 3 openings, 2 for ureters and 1 for urethra, forming a triangle called trigone.
• The opening of bladder to the urethra is guarded by internal sphincters made up of smooth muscles (involuntary in nature).
• Urinary bladder serves as a temporary storage organ for urine

1. Urethra:

• It is the outlet through which urine flows out of the body.
• It is thick walled muscular tube.
• In male, it is long (about 18cm).
• In female, it is short(about 4 cm).
• It is guarded by the external sphincter(voluntary in nature ) in the opening to exterior.