Urinary System: Anatomy and Function

• The urinary system consists of a pair of kidneys and a urinary tract and functions as organs of excretion.

Kidney Function

• Kidneys filter blood to remove metabolic waste products and modify the resulting fluid.
• Kidneys regulate fluid and electrolyte balance and control osmolarity.
• Kidneys regulate acid-base balance and blood pH, influencing blood pressure.
• Kidneys regulate red blood cell production by releasing erythropoietin.
• Kidneys detoxify substances in the blood, activating Vitamin D and produce new glucose.

Urinary Tract

• The urinary tract consists of a pair of ureters, the urinary bladder, and a single urethra.
• Urine exits the kidneys through the ureters, located on the posterior body wall, and empties into the urinary bladder.
• The urinary bladder is located on the floor of the pelvic cavity and stores urine before it exits the body through the urethra.

Internal Kidney Anatomy

• The kidney has three distinct regions: the outer renal cortex, the middle renal medulla, and the inner renal pelvis.
• The renal cortex and renal medulla together form the urine-forming portion of the kidney.
• The renal pelvis alongside its associated structures serve to drain urine formed in the cortex and medulla.
• The renal cortex is reddish-brown due to its rich blood supply.
• 90-95% of all kidney blood vessels are in the renal cortex.
• Renal columns are extensions of the renal cortex that pass through the renal medulla, housing branches of the renal artery.
• Over one million nephrons, functioning as the filtering apparatus, are located within the cortex and medulla of each kidney.
• Nephrons consist of a globe-shaped renal corpuscle in the renal cortex and a long renal tubule, mostly in the cortex.
• Cone-shaped renal pyramids reside within the renal medulla, separated by renal columns.

Kidney Blood Supply

• The kidney contains a unique capillary bed system where arterioles both feed and drain capillaries, usually the function of a venule.
• Each interlobular artery leads to afferent arterioles, which feed the ball-shaped capillary bed called the glomerulus.
• The glomerulus and its capillaries are part of the renal corpuscle of the nephron, draining into the efferent arteriole.
• The efferent arteriole feeds into a second capillary bed, known as the peritubular capillaries.

Nephron and Collecting System

• The nephron itself has two main divisions: the renal corpuscle and the renal tubule
• The renal corpuscle is responsible for blood filtration.
• The renal corpuscle consists of the glomerulus and the glomerular capsule (Bowman’s capsule), forming a double-layered outer sheath of epithelial tissue.
• The glomerulus is a group of looping fenestrated capillaries, highly permeable to allow filtration.
• The glomerular capsule is lined by an outer parietal and an inner visceral layer.
• The visceral layer consists of modified epithelial cells called podocytes which form filtration slits around glomerular capillaries.
• The capsular space is a hollow region between the parietal and visceral layers, continuous with entrance to the renal tubule lumen.
• Podocytes and fenestrated glomerular capillaries together form a complex membrane filtering the blood.
• Filtered fluid (filtrate) exits the glomerular capillaries into the capsular space and proceeds to the renal tubule lumen.
• The newly formed filtrate enters the renal tubule for further modification.
• The renal tubule consists of three structurally and functionally distinct regions: the proximal tubule, nephron loop, and distal tubule.
• The proximal tubule is the first and longest segment, featuring both straight and coiled sections made of simple cuboidal epithelial cells with microvilli.
• Microvilli are densely packed in the tubule lumen to form a brush border, greatly increasing the surface area.
• The nephron loop (loop of Henle) is the next segment, consisting of a descending and an ascending limb and dips into the renal medulla.
• The descending limb, also known as the thin descending limb, is made of simple squamous epithelial cells and travels toward the renal medulla.
• Some nephrons may have a thin ascending limb at the bend, composed of thin simple squamous epithelial cells, instead of thicker simple cuboidal cells.
• The distal tubule, the last segment, presents both straight and convoluted sections and is made of simple cuboidal epithelium without a brush border.
• Several distal tubules converge to form the collecting system.
• Filtrate from several distal tubules enters the cortical collecting duct, which reside in the renal cortex.
• Reaching the renal medulla, the cortical collecting duct transitions into the medullary collecting duct.
• Multiple medullary collecting ducts merge to form a papillary duct.
• Once filtrate enters the papillary duct, it is known as urine instead of filtrate.
• Urine exits the papilla of the renal pyramid into the minor calyx.

Types of Nephrons

• Kidneys contain two structurally and functionally distinct nephrons – cortical and juxtamedullary.
• Distinction is made based on nephron loops and the organization of peritubular capillaries.
• Cortical nephrons make up about 80% of nephrons in kidneys and are located in the renal cortex.
• They have renal corpuscles in the outer renal cortex and short nephron loops that barely enter the renal medulla.
• Peritubular capillaries supply blood to the loops of cortical nephrons indirectly through exchanging materials through interstitial fluid.
• Juxtamedullary nephrons are less common.
• Renal corpuscles are located near the boundary between the renal cortex and medulla, with long nephron loops.
• The cortical part of the nephron is surrounded by peritubular capillary branches neighboring cortical nephrons.
• The nephron loop is surrounded by a ladder-like network of capillaries (vasa recta) that arise from the efferent arteriole.
• The unique capillary structural arrangement allows juxtamedullary nephrons to control urine volume and concentration.

Kidney Stones

• Nephrolithiasis refers to the formation of renal calculi (kidney stones), consisting of crystalline structures of calcium oxalate salts.
• Renal calculi form when concentrations of ions (like sodium, hydrogen, and uric acid) are present in higher than normal amounts in the filtrate, known as supersaturation.
• Crystals form in the nephron loop, distal tubule, or collecting system.
• Most crystals pass unnoticed into the urine, but sometimes, they adhere to the epithelium of tubules, forming seed crystals.
• Stones develop when the crystals remain in the collecting system leading to blockages in the calyces, renal pelvis, and ureter.
• Risk factors include dehydration and a diet high in fat, animal protein, and salt and obesity.
• The most common symptom is severe pain (renal colic) radiating from the lumbar region to the pubic region, often accompanied by blood in urine, sweating, nausea, and vomiting.
• Diagnosis is achieved through computed tomography (CT) scanning or intravenous pyelogram (IVP) to visualize the stone.