The kidneys are compound tubular concerned with excretion of urine. It consists of excretory or uriniferous tubules, which produce urine, conducting tubules, which convey urine form the kidney to the renal pelvis, and blood vessels. The kidneys are embedded in fat and each kidney is covered by a capsule of dense collagenous fibres and a elastic fibres.
The glandular part consists of peripheral cortex and an inner Medulla. The Medulla is in the form of renal pyramids whose bases are in contact with the cortex and apices form the renal papillae. The glandular part surrounds a cavity adjacent to the hilus called the renal sinus in which lies the renal pelvis, the dilated origin of ureter.
The renal pelvis divides into primary divisions or Major calyces, which in turn divide into minor calyces. Each minor calyx receives a renal paila and the tip of the papilla shows small openings called the area cribrosa.
The cortex forms the outer zone but cortical tissue also projects down into the medulla between the bases of the renal pyramids as the renal columns of Bertini.
The cortical region is subdivided into a pars convoluta or cortical labyrinth containing convoluted tubules and glomeruli and a pars radiata or medullary rays, which are columns of straight tubules, which radiate out ward from the medulla.
The parenchyma of the kidney consists of uriniferous tubules and related blood vessels and collecting tubules between which are a scanty amount of interstitial tissue.
The structural and functional unit of the uriniferous tubule is called a nephron. Each nephron begins as a spherical expansion known as Bowman’s capsule, which encloses a tuft of capillaries – the glomerulus. The Bowman’s capsule and the glomerulus together form the Renal or Malphigian corpuscle. These are distributed in the pars convoluta of the cortex.
Apart from the Bowmans’ capsule the nephron consists of the following segments;
- Proximal convoluted tubule – a highly tortuous tube in the cortical labyrinth, which passes down into the medulla as,
- the Descending limb of Henle which extends for varying distances into the medulla and then forms,
- the loop of Henle after which it passes up into the cortex as,
- the Ascending limb of Henle. This reaches the renal corpuscle of its nephron, attaches itself to its vascular pole and is then continued as the ,
- Distal convoluted tubule in the cortical labyrinth. Here ends the nephron or secretory segment of the renal tubule and opens into arched collecting tubule in the cortical labyrinth. These arched collecting tubules open into straight collecting tubules, located in the pars radiata of the cortex, which pass down the medulla and reach the apex of the renal pyramid and form the papillary ducts of Bellini, which open through the area cribrosa of renal papilla into a minor calyx.
