HISTOLOGY OF KIDNEYThe kidney made up of a medullary pyramid (actually cone-shaped) and the base of which is enveloped by the cortex.

Nephrons arise in the cortex, loop down into the medulla and return to the cortex. From here they drain into collecting ducts that descend again into the medulla to discharge urine from the apex of the medullary pyramid. The apical part of the pyramid (known as the renal papilla) is enveloped by a funnel-shaped renal pelvis, which represents the dilated proximal part of the ureter.

The human kidney is made up of 10-18 lobes. The renal medulla is made up of multiple medullary pyramids separated by medullary extensions of the cortex. Each renal papilla is surrounded by a branch of the renal pelvis called a calyx; the whole urinary collecting system within the kidney being described as the pelvicalyceal system. The space between the branches of the pelvicalyceal system is filled with fatty supporting tissue and is known as the renal sinus.

The kidney is invested by a tough fibrous capsule, which is surrounded by a thick layer of perinephric fat that is in turn encased in a delicate condensation of connective tissue, known as Gerota's fascia. The fat around the kidney cushions it against trauma.

CORTEX

The renal cortex is easily identified by the presence of renal corpuscles, which are absent in the renal medulla. However, the bulk of the cortex is occupied by the proximal and distal convoluted tubules. The arcuate arteries and veins help to demarcate the cortex from the medulla

The prominent brush border of the PCT is contrasted with the lack of brush border in the DCT.

MEDULLA

The renal medulla consists of closely packed tubules of two types: the loop of Henle and the collecting tubules and ducts as well as the vasa recta

RENAL PAPILLADucts of Bellini DB, the largest of the collecting ducts, converge to drain urine through a number of holes (cribriform area) at the tip of the papilla. At the poles of the human kidney, the papillae are often fused to form complex papillae. The pelvicalyceal system PCS represents the proximal end of the ureter U and as such is lined by typical urinary (transitional) epithelium E. The wall of the pelvis contains smooth muscle SM, continuous with that of the ureter. NEPHRONThe functional and structural unit of the kidney, the nephron, consists of a renal corpuscle (including the glomerulus) and a long folded renal tubule.

The human kidney contains approximately one million nephrons.

Types of nephron

Cortical nephrons (80-85%)

Renal corpuscles lie in the outer portion of the renal cortex Short nephron loops that lie mainly in the cortex and penetrate only into the outer region of the renal medulla Receive their blood supply from peritubular capillaries that arise from efferent arterioles

Juxtamedularry nephrons (15-20%)

Renal corpuscles lie deep in the cortex, close to medulla Long nephron loop that extends into the deepest region of the medulla. Receive their blood supply from peritubular capillaries and from the vasa recta that arise from efferent arterioles

Renal corpuscleThe renal corpuscle is a combination of two structures, Bowman's capsule and the glomerulus.

Bowman's capsule consists of a single layer of flattened squamous epithelial cells resting on a basement membrane; it is derived from the distended, blind end of the renal tubule.

The glomerulus is a globular network of anastomosing capillaries, which invaginates Bowman's capsule.

Thus the capillary loops of the glomerulus are invested by the visceral layer of Bowman's capsule: a highly specialised layer of epithelial cells called podocytes.

A small amount of tissue remains to support the capillary loops and differentiates to form the mesangiumThe visceral layer is reflected around the vascular stalk of the glomerulus to become continuous with the parietal layer that constitutes Bowman's capsule proper.The space between the two layers is known as Bowman's space and is continuous with the lumen of the renal tubule; the parietal epithelium of Bowman's capsule is continuous with the epithelium lining the renal tubule.The afferent arteriole, which supplies the glomerulus, and the efferent arteriole, which drains it, enter and leave the corpuscle at the vascular pole that is usually situated opposite the entrance to the renal tubule, the urinary pole

Renal tubuleThe renal tubule extends from Bowman's capsule to its junction with a collecting duct.

It is up to 55 mm long in humans and is lined by a single layer of epithelial cells.It has a convoluted shape and has four distinct zones :

1. The proximal convoluted tubule (PCT) The longest Most convoluted section of the tubule Responsible for the reabsorption of approximately 65% of the ions and water of the glomerular filtrate

Confined to the renal cortex and make up the greater part of its bulk2. The loop of Henle

Includes the distal straight part of the proximal tubule, the pars recta, the thin descending and ascending limbs, and the thick ascending limb The length varies from short to long depending on the location of the renal corpuscle of the particular nephron.

The final part of the ascending limb makes contact with the afferent arteriole. Because the columnar tubule cells in this region are crowded together, they are known as the macula densa Along side the macula densa, the wall of the afferent arteriole (and sometimes the efferent arteriole) contains modified smooth muscle fibers called juxtaglomerular cells (JG).Together with the macula densa, they constitute the juxtaglomerular apparatus (JGA)The JGA helps regulate blood pressure within the kidney

The main function is to generate a high osmotic pressure in the extracellular fluid of the renal medulla; the mechanism by which this is achieved is known as the counter-current multiplier system

3. The distal convoluted tubule (DCT) Is a continuation of the thick limb of the loop of Henle after its return to the cortex

Shorter and less convoluted than the PCT

Responsible for reabsorption of sodium ions, an active process controlled by the adrenocortical hormone aldosterone. Sodium reabsorption is coupled with the secretion of hydrogen or potassium ions into the DCT, the secretion of hydrogen ions resulting in a net loss of acid from the body.

4. The collecting tubule Is the straight terminal portion of the nephron 2 different types of cells are present :Most are principal cells, which have receptors for both antidiuretichormone (ADH) and aldosterone, two hormones that regulate their functions

A smaller number are intercalated cells, which play a role in the homeostasis of blood pH Several collecting tubules converging to form a collecting duct. The collecting ducts descend through the cortex in parallel bundles called medullary rays Progressively merging in the medulla to form the large ducts of Bellini which open at the tips of the renal papillae to discharge urine into the pelvicalyceal system

The collecting tubules and ducts are not normally permeable to water. However, in the presence of antidiuretic hormone (ADH) secreted by the posterior pituitary, the collecting tubules and ducts become permeable to water

Thus the high osmotic pressure generated by the counter-current multiplier system into the interstitial tissues of the medulla removes water that is returned to the general circulation via the vasa recta

The loops of Henle and ADH thus provide a mechanism for the production of urine that is hypertonic with respect to plasma.