Urinary System Anatomy

• Fibrous Capsule, Cortex, the Medulla (medullary pyramids), Renal columns (Bertini), Medullary rays, Papilla, Minor calyces, Major calyces, Renal pelvis, Ureter, Bladder, and Urethra are all integral parts of the urinary system.
• The capsule is thin and durable and has fatty tissue on the outside.
• Hilus is the area where the ureters, veins, arteries, nerves, and lymphatic vessels enter and exit.
• Calyces are formed by the branching of the pelvis and the renal pelvis (pelvis renalis).
• The tips of the medulla pyramids open into minor calyces.
• Renal (medullary) pyramids are located in the medulla
• The cone-shaped tops of the medullary pyramids form the renal papillae
• The bases of the medullary pyramids are adjacent to the inner cortex
• Thin radial extensions extend from the pyramids to the cortex are called medullary lines or Ferrein pyramids.
• Renal columns (Bertini columns) are the cortical tissue between the pyramids.
• A medulla pyramid and the cortex tissue surrounding it form the kidney lobe.
• A Medullar extension, and the cortex tissue associated with it, form the kidney lobule.
• The axis of the lobules is formed by the collecting tubules.
• The kidney is multi-lobed.
• The outer surface of the kidney flattens after birth as it grows.
• The lobes are not very distinguishable from the outside of the kidney.

Histology

• Histologically, the cortex is on the outside of the kidney.
• The medulla is on the inside of the kidney.

Renal Lobe

• A renal lobe consists of a renal pyramid and the overlying area of renal cortex with adjacent tissues of the renal columns.

Nephron Types

• There are two types of nephrons: cortical and juxtamedullary.
• Cortical nephrons are small, renal tubules that have a loop of Henle that does not enter the medulla, ending in the outer cortex, with a short thin part.
• Juxtamedullary nephrons are larger.
• Juxtamedullary nephrons are located in the cortex region close to the medulla.
• The thin part of the loop of Henle in juxtamedullary nephrons extends deep into the medulla of the renal tubules and is quite long.
• The functional unit of the kidney is the nephron.
• There are 1-2 million nephrons in each kidney.

Tubules

• Renal corpuscle, proximal convoluted, and distal convoluted parts are only in the cortex.
• The loop of Henle is located in the medulla of the kidney.

Renal Corpuscle

• Glomerulus (afferent and efferent arterioles)
• Bowman capsule
• Parietal layer : Simple squamous epithelium
• Visceral layer = Podocytes
• Bowman space
• Vascular pole
• Urinary Pole

Glomerular Structure

• The afferent arteriole enters the corpuscle forming a capillary tuft.
• The capillaries join to form the efferent arterioles.
• The endothelium is a single-layered squamous epithelium with windows (pores) between the cells which do not contain a diaphragm.
• Bowman's external capsule is the parietal leaf.
• Bowman's internal capsule is the visceral leaf.

Podocytes

• Flat cells of the visceral leaf are defined as Podocytes.
• Podocyte footlets surround the outer surface of the capillaries.
• Cell cytoplasm extensions form 2nd and 3rd branches and are defined as Pedicels (footlets).
• Large nuclei protrude into the capsule space of the podocytes.
• There are approximately 25nm gaps between the pedicel extensions, and these are filtration gaps.
• Pedicels contain many actin microfilaments and regulate the gap width.

Glomerular Capillaries

• Fenestrated capillaries are present.
• Fenestrae are wider (70-90 nm).
• There is no diaphragm in glomerular capillaries.
• Blood cells and plaques cannot pass through the glomerular capillary pores.

Intraglomerular Mesangial Cells

• Located between capillaries.
• Provide support
• Can contract
• Similar to vascular pericytes and proliferate.
• Control the amount of blood in the glomerulus.
• Perform phagocytosis.
• Phagocytose protein residues accumulated in the glomerular basal lamina during filtration and Igs.
• Synthesize matrix and collagen.
• Secrete prostaglandins, endothelin, IL-1, and platelet-derived growth factor (PDGF).
• If abnormal amounts of proteins and their components accumulate in the basement membrane, mesangial cells can proliferate.

Juxtaglomerular Apparatus

• Arterial pole: afferent and efferent arterioles are present
• It houses the macula densa, juxtaglomerular cells, and extraglomerular mesangial cells (Lacis = Polkissen cells).
• Regulates glomerular blood and filtration rate.

Macula Densa

• Distal tubule touches arterioles.
• Epithelium lengthens in this area and comes together tightly.
• Distal tubule is defined as macula densa.
• The macula densa has ion channels and transporters that are very well developed
• Macula densa cells are sensitive to Na concentration.
• If Na concentration decreases, juxtaglomerular cells are stimulated.

Juxtaglomerular/Granular Cells

• They are cells in the vessel wall that undergo changes.
• Juxtaglomerular cells secrete renin.
• They are more dominant in afferent arterioles and less so in efferent.
• They are adjacent to macula densa cells

Tubules

• There are proximal convoluted tubules
• Proximal straight tubules (form the descending thick part of Henle)
• Thin part of the loop of Henle
• Straight distal tubule part
• Form the ascending thick part of Henle
• Distal convoluted tubules

Proximal Convoluted Tubules

• Found in the cortex and are quite long
• Are simple cuboidal epithelium (pyramidal)
• Have many basal recesses and mitochondria
• Lysosomes are also numerous
• Lateral recesses are prominent
• A brush-like border is present on the apical side

Distal Convoluted Tubules

• In the cortex
• Simple cuboidal epithelium (pyramidal)
• Cells are smaller than proximal convoluted tubules
• Wide lumen
• Microvilli have short basal recesses
• No brush border
• Light staining
• Reabsorb electrolytes via ADH and vasopressin

Loop of Henle

• Descending thick part Lined with simple cuboidal epithelium, no brush border
• Thick parts are located in the medulla and medullary rays
• The thin part is located in the medulla and Lined with simple squamous epithelium
• Located in the medulla and medullary rays
• Lined with cuboidal epithelium, no brush border
• Electrolyte absorption occurs

Collecting Tubules

• Have a single-layered cuboidal shape
• Have two types of cells
• Principal cells are light colored; Na and water reabsorption occur, and K secretion occurs
• Intercalated cells feature numerous mitochondria; H and HCO3 secretion occurs, K reabsorption occurs

Ureter

• Lined with transitional epithelium (urothelium)
• Under the epithelium, the lamina propria
• Smooth muscles are located longitudinally-circularly and longitudinally from the inside to the outside
• The outer longitudinal muscle layer is located only at the distal end of the ureter

Bladder

• T. Mucosa is composed of Uroepithelium (Transitional) and the L.propria
• T. Musularis is composed of three layers including the inner longitudinal, middle circular, and outer longitudinal.
• Dense connective tissue is observed between the muscle layers
• T. Adventitia occurs
• Serosa is only present in the upper region (peritoneum)