� Each individual normally has two kidneys, located lateraly on either side of vertebral column levels between the last thoracic and third lumber vertebra close to the dorsal inner wall of the abdominal cavity.
� In humans right kidney is at slightly lower level than left kidney.
� Dorsal surface of the kidney is attached to the dorsal abdominal wall, so only its ventral surface is covered by visceral peritoneum, therefore this type of kidney is called retro-peritoneal kidney or extra peritoneal kidndey.
� Mammalian kidneys are bean shaped, reddish brown coloured with tough fibrous connective tissue covering capsule.
� Each kidney measures 10-12 cm in length, 5-7 cm in breadth and 2-3 cm in thickness, weighing about 120-170 gm in an adult. Lateral surfaces of kidney are convex while medial surfaces are concave.
� On the concave margins of the kidney longitudinal opening called Hilum (Hilus renalis) is present. Through this, renal artery and nerve
enter while renal vein and ureter leave the kidney.
� The Hilum leads to a funnel shaped space called the renal pelvis.
The kidney tissue surrounding the pelvis is arranged in an outer renal cortex and inner renal medulla. The renal medulla forms conical pyramid shaped masses which project into the renal pelvis. These are called as medullary pyramids or renal pyramids (8 to 12 in humans, while only one pyramid is present in kidney of rabbit)
� The cortex extends in between the medullary pyramids as renal columns called columns of Bertini.
� Each kidney has nearly one million complex tubular structures called Nephrons which are the functional units.
- These nephrons are arranged in a radiating fashion within the renal pyramids.
- Urine produced by each nephron empties into collecting duct.
- The collecting duct passes through a papilla into the renal calyx (Pleural- calyces).
- The renal calyces drain urine in the central cavity of renal pelvis.

� Urine passes from the pelvis into the ureter. Both the ureters open through separate oblique openings into the urinary bladder. The obliquity of the openings prevent the backflow of urine.
� Externally, the bladder is lined by detrusor muscle, it is involuntary in nature while internally the bladder is lined by transitional epithelium or urothelium. This epithelium has great capacity to expand so that large volume of urine can be stored. Opening of urinary bladder is controlled by sphincters made of circular muscle. In human two sphincters are present. Inner= Internal sphincter (made up of involuntary muscle) Outer= External sphincter (Voluntary muscle). These normally remain contracted and during micturition these relax to release urine.
Nephron--> Collecting duct --> Papilla --> Renal calyx --> Renal pelvis -->Ureters --> Urinary bladder --> Urethra
� Urinary bladder opens into a membranous duel called Urethra.
� The urethra leads to end of the penis in males and into the vulva in females. In males the urethra has three parts, prostatic, membranous & penile urethra respectively. ln Females both sphincters are present in membranous urethra.
� The process of release of urine is called micturition and the neural mechanism causing it is called micturition reflex. This reflex is initiated when interoceptors present in the wall of urinary bladder, get stimulated by the tension created due to stretching of bladder wall as the bladder gradually fills with urine brought into it by the ureters.
� In response, the stretch receptors on the walls of the bladder send signals to the CNS. The CNS passes on motor messages to initiate the contraction of smooth muscles of the bladder and simultaneous relaxation of urethral sphincter causing the release of urine.

� Nephron is the structural and functional unit of kidney. It is an epithelial tube which is about 3 em long and 20-60 micro-mt in diameter.

A nephron can be divided into following regions :-
1. Bowman's capsule : At the proximal or closed end the nephron is expanded and curved inwardly to form a double walled cup shaped Bowman's capsule. Within the Bowman's capsule a network or tuft of capillaries is present, it is called Glomerulus. It is formed by the afferent arteriole (a fine branch of Renal artery). Blood from the glomerulus is carried away by an efferent arteriole.
� Malpighian body : Glomerulus and its surrounding Bowman's capsule together forms Malpighian body or
Renal corpuscle.
� The outer wall of Bowmans capsule is composed of flattened squamous cells.
The inner, invaginated wall that lines the concavity of Bowmans capsule is composed of a special type of cells called Podocytes. Which are arranged in an intricate so as to leave same minute spaces called filtration slits or slit pores.
- These cells bear finger like projections which are coiled around the capillaries of glomerulus.
- The Bowmans capsule is followed by a short neck part lined by ciliated epithelium.

2. Proximal convoluted tubule (PCT) : The epithelial cells of this region are specialised for transport of salts and other substances from the lumen to the interstitial fluid. It is lined by simple cuboidal brush border epithelium.
- The membranes of these cells facing the tubule lumen has numerous microvilli (finger like projections or Brush-Borders) which increase the surface area. Near its basolateral surface, the mitochodria are concentrated, to allow reabsorption of salts by active transport.

3. Loop of Henle : It starts after the proximal convoluted tubule, It ends before the distal convoluted tubule. This
hairpin like loop has a descending limb, followed by an ascending limb.
(a) Descending limb :
� Its upper part -
- constitutes thick segment
- has the same diameter as PCT
-is also lined by simple cuboidal epithelium
� Its lower part -
- constitutes thin segment
- is lined by flat squamous cells.
(b) Ascending limb :
This part too has a thin segment first which xvidens abruptly at medullary zone to form thick segment. Lower thin segment is lined by simple squamous epithelium while the upper thick segment is lined by cuboidal epithelium.

4. Distal convaluted tubule (DCT) :
- The ascending limb of henle's loop merges into distal convoluted tubule. This is lined by cuboidal epithelial cells with a few microvilli.
- The DCT of different nephrons open into a straight tube called collecting duct.
- Collecting ducts (present in medullary pyramids) are long tubules which traverse through the medulla in the
pyramids. In the papilla of the medullary pyramid, Several adjacent collecting ducts converge to open into a
common short and thick duct of Bellini (present in papilla of medulla).
- All ducts of Bellini then open at the tip of the papillae into the pelvis. ,JGA is a special sensitive region formed by cellular modifications in the distal convoluted tubule and the afferent arteriole a1 the location of their contact.
Renal cortex : The malpighian corpuscle, PCT & DCT of the nephrons are located here.
Renal medulla: Loop of henle, collecting duct and ducts of Bellini are found in this region.
The efferent arteriole emerging from the glomerulus forms a fine capillary network around the renal tubule
called peritubular capillaries. A minute vessel of this network runs parellel to the long Henle's loop forming a ''U-shaped'' Vasa recta.

- Juxtaglomerular apparatus :Juxtaglomerular cells+ Macula densa + Lacis / Polkissen.
- Macula densa : The cells of OCT epithelium in contact with the arteriolar wall are denser then other epithelial cells. These are collectively called as macula densa.
- Juxtaglomerular cells : The smooth muscle cells of the wall of both arterioles in contact with DCT epithelium are swollen and contain dark granules of inactive renin. These are called juxtaglomerular cells.

Each kidney receives its blood supply by a single renal artery frorri dorsal aorta, and is drained off by a sinle renal vein, which opens in the inferior vena cava.
As the artery enters into the medulla after traversing through the hilum, it divides into a number of branches.
These branches enter into renal cortex through the columns of Bertini and subdivide into afferent arterioles which form glomerular capillaries. These capillaries are drained, not by venules, but by efferent arterioles.

In Cortical nephrons : The efferent arterioles break up into dense Peritubular network of capillaries around their tubules.
In Juxtamedullary nephrons :
� In this a minute vessel of peritubular capillary network runs parallel to the Henle's loop forming a "U' shaped
Vasa recta.
� This vasa recta dips into the medulla or its pyramids along side the loops of Henle.
� Both peritubular capillaries of cortical nephrons and vasa recta of juxtamedullary nephrons lead into venules which join and rejoin to form small and large veins, all of which ultimately join to form renal vein.