Functions of the Kidney

Kidney Functions

• The kidney has four main functions: regulation of total body water, maintenance of extracellular fluid, excretion of waste products of metabolism, and regulation of blood pressure.

Kidney Structure

• The kidney consists of an outer dark-stained cortex and an inner pale medulla.
• The structural and functional unit of the kidney is the nephron.

Nephron Structure

• Each nephron consists of:
• Bowman's capsule
• Proximal convoluted tubule
• Loop of Henle (with descending and ascending segments)
• Distal convoluted tubule

Renal Corpuscles

• The proximal end of each nephron is expanded to form Bowman's capsule, which is indented by a globular tuft of capillaries called the glomerulus.
• Each renal corpuscle has two poles: a vascular pole and a urinary pole.

Bowman's Capsule

• Bowman's capsule is a double-walled cup formed of parietal and visceral layers, separated by Bowman's space.
• The parietal layer is lined by a single layer of squamous epithelium.
• The visceral layer dips inward between capillary lobules and is lined by podocytes.

Podocytes

• Podocytes are stellate in shape and give many primary and secondary processes (foot processes or pedicles).
• The pedicles make intimate contact with the basement membrane of blood capillaries, with narrow spaces or filtration slits (25 nm) between them.

Filtration Barrier

• The filtration barrier consists of:
• Fenestrated endothelium
• Basement membrane
• Filtration slit membrane
• This barrier is present between the blood circulating in capillaries and the filtrate in the capsular spaces.

Proximal Convoluted Tubules (PCT) vs. Distal Convoluted Tubules (DCT)

• PCT:
• Lined with simple cuboidal epithelium
• Has brush borders
• Cytoplasm is granular and deeply acidophilic
• Basal part of cytoplasm shows vertical acidophilic striations
• Cell borders are ill-defined
• Only 3-5 nuclei appear in a cross-section
• DCT:
• Lined with shorter cuboidal cells
• No brush border
• Less granular and less acidophilic cytoplasm
• Less basal striations
• Cell borders are more distinguishable
• 5-8 nuclei appear in a cross-section

Juxtaglomerular Apparatus

• Modifications in the wall of the afferent arteriole and in the portion of the distal convoluted tubule that lies in contact with the renal corpuscle.
• Components:
• Epitheloid cells (Juxtaglomerular cells) in the afferent arteriole
• Macula densa cells in the distal convoluted tubule
• Extra-glomerular mesangial cells
• Function: controls blood pressure