Renal System Overview

Regulates homeostasis through fluid and salt levels via filtration, reabsorption, and secretion.
Regulates blood pressure through Renin production.
Stimulates red blood cell production via Erythropoietin secreted by peritubular fibrocytes.

Two Kidneys: responsible for filtration and reabsorption.
Two Ureters: conduct urine from kidneys to bladder.
One Urinary Bladder: stores urine.
One Urethra: excretes urine.
Blood flow to kidneys is approximately 1.2-1.3 liters per minute, filtering all blood every 4-5 minutes.

Each kidney contains roughly 1 million nephrons, a constant number that doesn't increase after birth and cannot be replaced if damaged.
Composed of the renal corpuscle and renal tubules (proximal convoluted tubule, loop of Henle, distal convoluted tubule, and collecting duct).

Approximately 200 µm in diameter.
Consists of the glomerulus, a tuft of capillaries, and the glomerular (Bowman's) capsule.
The glomerular capsule is a double-walled epithelial capsule surrounding the glomerulus, forming the urinary space between the parietal and visceral layers.
The urinary space receives the fluid filtered through the capillary wall and visceral layer.
The vascular pole, where the afferent arteriole enters and the efferent arteriole leaves, marks the point where the afferent arteriole divides into 2-5 primary branches, forming the glomerulus.
The urinary pole is where the proximal convoluted tubule begins.

The parietal layer is composed of simple squamous epithelium supported by a basal lamina and a thin layer of reticular fibers, transitioning to simple cuboidal or low columnar epithelium at the urinary pole.
The visceral layer is composed of podocytes with primary processes that give rise to numerous secondary processes (pedicels) that embrace glomerular capillaries.
Secondary processes interdigitate, forming filtration slits bridged by diaphragms.
Podocytes contain bundles of actin filaments, enabling their contractile capacity.
The basement membrane, between fenestrated endothelial cells of glomerular capillaries and podocytes, acts as the filtration barrier.

Fenestrated capillaries.
Underlying basement membrane.
Slits formed by criss-crossing secondary processes of podocytes surrounding capillaries.

Specialized cells around blood vessels in the kidneys.
Extraglomerular mesangial cells (lacis cells) are light-staining cells found outside the glomerulus, forming part of the juxtaglomerular apparatus.
Intraglomerular mesangial cells are specialized pericytes among glomerular capillaries within a renal corpuscle, contributing to filtration, phagocytosis, and structural support.

Starts at the urinary pole of the renal corpuscle, where the squamous epithelium of the parietal layer of Bowman's capsule transitions to the cuboidal (low columnar) epithelium of the PCT.
Longer than the distal convoluted tubule, frequently seen near renal corpuscles in the renal cortex.
PCT cells have acidophilic cytoplasm due to numerous elongated mitochondria.
The apical cell region features abundant microvilli, forming a brush border.

U-shaped structure composed of a thin descending limb and thick ascending limb.
Lined with simple cuboidal epithelium.
The thick descending limb suddenly narrows in the outer medulla, forming the thin descending limb with a wide lumen due to squamous epithelial cells.

Lined with simple cuboidal epithelium, differentiating from PCT by lacking a brush border, apical canaliculi, and having smaller cells.
More nuclei are visible in DCT compared to PCT.
DCT cells have elaborate basal membrane invaginations and associated mitochondria, indicative of their ion-transporting function.
The juxtaglomerular region, where DCT contacts the vascular pole of the renal corpuscle, features columnar cells with closely packed nuclei, forming the macula densa.
The macula densa cells are sensitive to ionic content and water volume of the tubular fluid.

Urine passes from the DCT to collecting tubules, which join to form larger, straight collecting ducts.
Collecting ducts approach the tips of medullary pyramids.
Smaller collecting tubules are lined with cuboidal epithelium, becoming columnar as they penetrate deeper into the medulla.
The collecting duct reaches a diameter of 200 µm near the tips of medullary pyramids.

Composed of the macula densa of the DCT, extraglomerular mesangial cells, and juxtaglomerular cells of the afferent arteriole.
Juxtaglomerular (JG) cells, modified smooth muscle cells in the tunica media of the afferent arteriole adjacent to the renal corpuscle, contain secretory granules of Renin.
JG cells act as chemoreceptors and baroreceptors, secreting Renin in response to a decrease in blood pressure.
Renin converts angiotensinogen to angiotensin I, further processed to angiotensin II, a potent vasoconstrictor.
Angiotensin II stimulates aldosterone release from the suprarenal glands, increasing sodium and water reabsorption from the DCT.

Includes the calyces, renal pelvis, ureter, urinary bladder, and urethra.
Basic structure of these components is the same:
- Mucosa: lined with transitional epithelium (unique to the urinary system) which is impermeable to urine components.
- Lamina propria: primarily composed of connective tissue.
- Muscularis: usually consists of three layers of smooth muscle cells: inner longitudinal, middle circular, and outer longitudinal.