The Urinary System: Components and Functions

• The urinary system consists of the kidneys, ureters, urinary bladder, and urethra.

Components of the Urinary System

• Kidneys: Organs that produce plasma ultrafiltrate, erythropoietin, and renin.
• Ureters: Transport urine from the kidneys to the bladder.
• Bladder: Serves as a temporary storage for urine before excretion.
• Urethra: Conducts urine from the bladder to the exterior of the body.

Functions of the Urinary System

• Production of urine.
• Plasma filtration.
• Regulation of hydroelectrolytic balance.
• Regulation of acid-base balance.
• Endocrine function includes the production of renin and erythropoietin.
• Controls arterial blood pressure.
• Controls volemia (blood volume).

Kidney Shape and Size

• Bean-shaped, tubular gland.
• Adult human kidneys are approximately 10-12 cm long, 5 cm wide, and 4-5 cm thick.
• They have a concave region called the hilum, where vascular elements, nerves, and the ureter enter and exit.
• Each kidney weighs approximately 150 grams.

Kidney Location

• Located behind the peritoneum in the posterior and superior part of the abdomen, on each side of the lumbar vertebrae.
• The right kidney is slightly lower than the left by about 1-2 cm.

External Structure of the Kidney

• The kidney is covered by a thin capsule, consisting mostly of irregularly dense connective tissue.
• The fibroconnective tissue capsule can be easily separated from the underlying parenchyma.

Internal Structure of the Kidney

• The renal parenchyma is divided into the cortex and the medulla.
• The cortex is dark reddish-brown with a granular appearance due to the presence of renal corpuscles (Malpighi's corpuscles).

Calyces

• There are typically 2 to 3 major calyces and 8 to 12 minor calyces.
• Each minor calyx encloses a conical projection of the renal parenchyma called the papilla.

Medulla and Renal Papilla

• The medulla is composed of pyramid-shaped structures.
• The base of each pyramid is oriented towards the cortex, from which medullary rays emerge.
• The apex is termed the renal papilla.

Uriniferous Tubules

• The structural unit of the kidney is the uriniferous tubule, a highly convoluted structure consisting of two parts: the nephron and the collecting tubule.
• A single collecting tubule drains several nephrons.
• Multiple collecting tubules unite deep within the medulla, forming larger collecting ducts.

Nephron

• Each kidney contains 1 to 2 million nephrons.
• This is a long tube lined with epithelium, beginning blindly and terminating by joining an excretory duct.
• Each nephron has various structural segments.

Parts of a Nephron

• Renal corpuscle (of Malpighi).
• Proximal tubule (with both convoluted and straight segments).
• Loop of Henle.
• Distal tubule (with straight and convoluted sections).

Types of Nephrons

• Long (Juxtamedullary).
• Short (Cortical).
• Intermediate (Mediocortical).

Renal Corpuscle (of Malpighi)

• Measures 150 to 250 microns in diameter.
• Components include the Bowman's capsule and the glomerulus.

Bowman's Capsule

• A double-walled, cup-like expansion of the nephron's terminal end.

Bowman's Capsule Layers

• Parietal layer: Simple squamous epithelium.
• Visceral layer: Surrounds the glomerulus and consists of podocytes.

Glomerulus

• A group of blood capillaries.
• The renal corpuscle possess a vascular pole, where afferent and efferent arterioles enter and exit, and a urinary pole, continuous with the proximal convoluted tubule (PCT).

Visceral Layer of Bowman's Capsule

• Closely envelops the glomerulus and comprises specialized epithelial cells known as podocytes.
• These cells exhibit multiple primary processes that extend outwards like octopus tentacles or starfish arms.
• Also exhibit numerous secondary processes or pedicels, which adhere to the exterior surface of the glomerular basal laminae.

Pedicels

• They interdigitate in a complex manner, creating a complex system of filtration slits or slit pores between them.
• This arrangement (pores) allows for the kidneys to filter large areas of fluid.

Parietal Layer of Bowman's Capsule

• Separated from the visceral layer by Bowman's space.
• It is lined by a simple squamous epithelium.

Glomerulus as a Tuft of Capillaries

• It originates from the division of the afferent arteriole.
• These capillaries are type II fenestrated.
• Both the outer endothelial cells and underlying podocytes are separated by a basal lamina.

Epithelial Cells and Mesangial Cells

• The visceral epithelium surrounds the glomerulus, creating intercapillary spaces close to the afferent and efferent arterioles.
• These spaces, which contain basal laminae not coated by endothelium, are occupied by mesangial cells.
• The function of the mesangial cells is phagocytosis and removal of filtration residue.

Juxtaglomerular Apparatus Components

• Juxtaglomerular cells.
• Macula densa.
• Lacis cells (extraglomerular mesangial cells).

Juxtaglomerular Cells

• When the afferent arteriole approaches the glomerulus, the smooth muscle cells in its middle layer differentiate into epithelioid-like juxtaglomerular cells.
• The nuclei of juxtaglomerular cells become rounded.
• Their cytoplasm contains granules of the hormone renin.

Juxtaglomerular Cell Function

• Renin acts on angiotensinogen (a plasma globulin), converting it into angiotensin I.
• Angiotensin I is then converted to angiotensin II by ACE (produced in the lung), which is a potent vasoconstrictor.
• The macula densa detects changes in sodium levels and stimulates YG cells to produce renin.
• Angiotensin II prompts the adrenal cortex to generate aldosterone, which acts on collecting tubules and distal tubules, to increase sodium and chlorine reabsorption and hence water.

Proximal Tubule

• Approximately 14 mm in length with a diameter of 50-60 microns. Each has a coiled and straight portion which goes into the Loop of Henle.
• It features a high simple cuboidal epithelium with high, densely packed microvilli.
• The cells have a grand, spherical base, or centrally placed nuclei.

Proximal Tubule Function

• An amount of filtrate between 80-85% gets reabsorbed through a sodium potassium pump.
• Glucose, amino acids, proteins, bicarbonates and vitamins also reabsorbed here.

Loop of Henle

• Short nephrons display brief narrow sections in the proximal portion of the tubule on the descending limb of Henle's loop.
• The epithelium lining changes rapidly at the interface of the proximal convoluted tubule. The lumen's width is lowered to 12-15 μm at the switch from Cubico to flat simple.
• This plays an important role for producing hypertonic urine.

Distal Tubule

• Shorter that the proximal and not coiled.
• There are three portions: straight portion, macula densa, and convoluted section.
• The cells are cuboidal without a striated border.
• Nuclei are away from the basal lamina and stick out into the lumen of the tubule.

Collecting Tubules

• The distal tubules continue with the arched collecting tubules which continue with the straight ones
• The cells are pale in color with dark, spherical nuclei and some small structures.

Excretory Ducts Structures

• Major calyces.
• Minor calyces.
• Renal pelvis.
• Ureters.
• Bladder.
• Urethra.
• The walls possess the same composition, with three layers except for those that compose the urethra: mucosa, muscle, and serosa (or adventitia in certain regions).

Ureters

• A duct that measures 25 cm in length, found in the abdominal cavity, behind the peritoneum.
• There is heightened thickness, with the thickness of the renal pelvis is less than that of the ureter.

Ureter Histological Layers

• Mucosa: Has transitional epithelium. The number of cell layers gradually increases from 2-3 in the pelvis to 4-5 in the ureter. These cells have the outer plasma membrane which is thickened. The lamina propria consists of areolar tissue along with considerable elastic fibers.
• Muscle layer that is muscular, consisting of an external circular layer and an internal longitudinal layer.
• Adventitia: Consists of fibroelastic tissues that envelope structures. It forms the outer layer.

Bladder Structure

• Basically same as ureter structure with epithelium covering is far thicker.
• Beneath the mucosa lies a discontinuous muscularis mucosae. Within the lamina propria, there are a number of diminutive mucosecretorial glands.
• Arranged in 3 intercalated muscle layers: an internal longitudinal, middle circular, and external longitudinal.
• The superior part of the bladder is covered by the peritoneum.

Urethra

• Terminal excretory duct communicates with the exterior.
• In males measure 15-20 centimeters. Divided into three regions: the prostate urethra, membranous urethra, and spongy urethra.
• The epithelium lining the prostate urethra the transitions to pseudostratified or stratified morphology elsewhere. It consists of uneven surface covered by tiny pits or fossae what develop to those Littre glands here.
• In females measure is 3-4 centimeters. The muscle is made up of two smooth layer that are outside. There is a striated sphincter muscle near the orifice.