Urinary Tract Anatomy and Function

Urinary System

• The kidney is a compound tubular gland that performs many functions.
• The kidney is composed of stroma and parenchyma, with the stroma consisting of a fibrous capsule and interstitial connective tissue.

Histology of Kidney

• The kidney has an outer cortex and an inner medulla.
• The outer cortex is reddish and granular, and is subdivided into three regions: cortex proper, columns of Bertini, and labyrinth.
• The inner medulla is greyish and contains medullary pyramids and medullary rays.
• A renal lobe consists of a medullary pyramid and the overlying cortex proper.
• A renal lobule is a medullary ray and the corresponding cortical tissue.

Uriniferous Tubules

• The uriniferous tubules are composed of nephrons and collecting tubules.
• There are approximately 1-4 million nephrons in each kidney.
• Nephrons are of two types: cortical nephrons with short loops of Henle and juxtamedullary nephrons with long loops of Henle.

The Nephron

• A nephron is formed of four components: Malpighian renal corpuscle, proximal convoluted tubule, loop of Henle, and distal convoluted tubule.
• The Malpighian renal corpuscle is a rounded body in the cortex, approximately 200 µm in diameter, with two poles: urinary pole and vascular pole.

Malpighian Renal Corpuscle

• The Malpighian renal corpuscle is formed of three components: Bowman's capsule, glomerular capillaries, and mesangial cells.
• Bowman's capsule is a double-walled cup-shaped epithelial capsule, formed of outer parietal layer and inner visceral layer (podocytes).
• The capsular space is between the two layers.
• Podocytes are modified squamous cells in the visceral layer of Bowman's capsule, separated from glomerular capillaries by subpodocytic space.

Podocytes

• Podocytes are formed of a cell body and major cytoplasmic processes that give rise to minor processes and terminate by feet processes firmly implanted on the basement membrane of capillaries.
• The gaps between the implanted feet are called filtration slits (40 nm) and are closed by slit diaphragms.
• The functions of podocytes are filtration and formation of the renal blood barrier, and renewal of the glomerular basement membrane.

Glomerulus

• The glomerulus is formed of 50 tufts of blood capillaries that arise from the afferent arteriole and drain into the efferent arteriole.
• The glomerular endothelium is fenestrated, and the glomerular basement membrane is thick.

Mesangial Cells

• Mesangial cells are branched cells between glomerular capillaries, attached to their walls by processes.
• The functions of mesangial cells are support, renewal of glomerular basement membrane, and change to macrophages to phagocytose foreign bodies and old basement membrane.

Renal Blood Barrier

• The renal blood barrier is a barrier between blood in glomerular capillaries and glomerular filtrate in the capsular space.
• The renal blood barrier has three components: glomerular endothelium, fused basement membrane of glomerular endothelium and podocytes, and filtration slit diaphragms.
• The ultrastructure of the renal blood barrier shows feet processes of podocytes, glomerular basement membrane, and slit diaphragms.