KIDNEY

Location: bean-shaped organs on either side of the spinal column in the retroperitoneal tissue of the posterior abdominal cavity

Function:

• Conservation of water, essential electrolytes, and metabolites, and removal of certain waste products of metabolism from the body
• Synthesis and secretion of erythropoietin, renin, and hydroxylation of Vitamin D

Anatomic features

• Cortex, the outer part, consists of renal corpuscles, along with the convoluted (distal and proximal) and straight (distal and proximal) tubules of the nephron, the collecting tubules, and extensive vascular supply. There are medullary rays (of Ferrein) projecting from the medulla. They contain straight tubules (distal and proximal) and straight collecting tubules
• Medulla, the inner part, consists of straight (distal and proximal) tubules, collecting ducts, and a special capillary network. The tubules because of differences in length collectively form a number of conical structures called pyramids. There are renal columns (of Bertini). They represent cortical tissue contained within the medulla
• The apical portion of each pyramid, papilla, projects into a minor calyx. It is a branch of the two or three major calyces that, in turn, are major divisions of the renal pelvis
• Nephron, the functional unit of the kidney. The renal corpuscle, the beginning part, consists of glomerulus, a tuft of capillaries, and the renal or Bowman's capsule. The glomerular capillaries are supplied by an afferent arteriole and are drained by an efferent arteriole. The tubular parts of nephron are proximal thick segment, thin segment, and distal thick segment
• Nephron connects to the collecting tubule, often through a connecting tubule, thus forming the uriniferous tubule
• Interstitial tissue, the loose connective tissue, surrounds the components of the nephrons, the ducts, and the blood and lymphatic vessels. In the cortex, the fibroblasts predominate. In the medulla, the principal interstitial cells resemble myofibroblaststs

CAPSULE

Location: cover the kidney surface and sinus

Function: resistance of volume and pressure variations

Composition:

• Outer layer of fibroblasts and collagen fibers
• Inner layer of myofibroblasts

RENAL (MALPIGHIAN) CORPUSCLE

Function: blood flowing through capillaries undergoes a filtration process to produce the initial urine filtrate

Composition:

• Glomerulus, a tuft of fenestrated capillaries
• Mesangial cells, as pericytes, are enclosed by basal lamina of the glomerular capillaries
• Renal or Bowman's capsule, double-layered epithelial cup with Bowman's space between layers
  • Visceral layer of Bowman's capsule is constituted by podocytes with processes and pedicles (foot processes), and thick basal lamina
  • Parietal layer of Bowman's capsule is a simple squamous epithelium

Location: terminal portion of the distal thick segment of the nephron directly adjacent to the afferent and efferent arterioles

Function: renin synthesis, storage, and releasing into the blood. Regulates blood pressure, composition, and volume

Composition:

• Macula densa cells, modified epithelial cells of distal segment wall
• Juxtaglomerular cells, modified smooth muscle cells of afferent arteriole
• Extraglomerular mesangial cells

PROXIMAL THICK SEGMENT

Function: the initial and major site of reabsorption of amino acids, sugars, polypeptides, and peptides

Composition:

• Proximal convoluted tubule is formed by simple cuboidal epithelium. The cells have surface specializations associated with absorption and fluid transport

• Proximal straight tubule, thick descending limb of the loop of Henle. It is constituted by simple cuboidal epithelium

THIN SEGMENT

Function: a part of the countercurrent exchange system that functions in concentrating the urine. The descending portion is permeable, permitting free passage or equilibration of salt and water between the lumen of the nephron and the peritubular connective tissue. The ascending portion is impearmeable to water

Composition: simple squamous epithelium with basal lamina

Distal Thick Segment

Composition:

• Thick, straight ascending tubule, the third part of loop of Henle, is composed by simple cuboidal epithelium

• Distal convoluted tubule is composed by simple cuboidal epithelium

Function: reabsorption of water under antidiuretic hormone regulation

Composition: simple squamous, cuboidal, and columnar epithelium. The cells gradually become taller as ducts pass from the outer to inner medulla and become columnar in the region of the renal papilla

Cells:

• Light or collecting duct (CD) cells with single cilium
• Dark or intercalated (IC) cells