Anatomy and Function of the Urinary System

• The urinary system, also known as the renal system, includes the kidneys, ureters, urinary bladder, and urethra.

Components and Function

• The kidneys (2) filter, cleanse, and regulate blood and body fluids.
• The ureter is a tube that carries urine from each kidney to the urinary bladder, where it is stored.
• The urethra transports urine from the bladder to the external environment for elimination, called micturition.

Urine Composition

• Urine is a non-toxic and aseptic waste byproduct of blood cleansing in healthy individuals.
• It typically contains 95% water, 2.5% urea, and 2.5% mineral salt mixtures, enzymes, and trace wastes.

Kidney Function

• The kidneys are major excretory organs that filter and cleanse blood and maintain homeostasis of blood and body fluids.
• They are located retroperitoneally, behind the peritoneal cavity, and are sealed to the posterior abdominal wall by an adipose capsule.

Kidney Location

• The kidneys are in the retroperitoneal cavity.
• They are tethered to the posterior abdominal wall from T11 to L3, but not in the peritoneal cavity.
• The kidneys are small organs, about 5 inches long, 2-3 inches wide, and 1½ inches thick, weighing 140-160 grams each.
• They receive 20-25% of the total cardiac output.

Basic Functions of the Urinary System

• Regulation of Blood and Extracellular Fluid Volume: Kidneys regulate and maintain blood volume, related to Mean Arterial Pressure (MAP).
• Regulation of Osmolarity: Osmolarity of blood plasma is controlled by regulating water excreted in urine, normally ranging from 295 to 310 mOsM.
• Maintenance of Ion Balance in Blood: Appropriate levels of essential ions and minerals are crucial for health, impacting systems like nervous, cardiac, and skeletal muscle systems, like Na+, K+, Cl-, Ca2+, H+, Mg2+, and PO4.
• Homeostatic Regulation of pH in Body Fluids: Selective secretion of H+ (hydrogen ions) regulates acidic conditions, and HCO3- (bicarbonate ions) regulates alkaline conditions, primarily in the distal convoluted tubule.
• Filters Wastes and Metabolic Products from Blood and Excretes them from the Body: The urinary system removes metabolic waste, like urea, uric acid, and creatinine, which are harmful if accumulated.
• Excretion of Foreign Substances from the Blood: The kidneys and liver eliminate toxic, unnatural substances, like drugs, pesticides, preservatives, and genetically modified food substances.
• Kidneys produce renin and erythropoietin:
  • Renin: An enzyme/hormone from juxtaglomerular cells, triggered by increased blood osmolarity to conserve water.
  • Erythropoietin: Released from juxtaglomerular cells when oxygen levels are low in the blood, stimulates red blood cell formation.

External Anatomy of the Kidneys

• The kidneys are surrounded by three capsules.
  • Renal Capsule: A thin, tough coat of dense irregular connective tissue that encloses the kidney, maintaining shape and protecting from trauma.
  • Adipose Capsule: A fat capsule external to the renal capsule that cushions, holds the kidneys in place, and insulates them.
  • Renal Fascia: Dense irregular connective tissue external to the adipose capsule that binds the kidneys to the posterior abdominal wall and peritoneum.
• Para-renal fat: Located externally and posterior to the renal fascia which functions as padding for protection.

Regions of the Kidney

• Renal Cortex: Outer region, lighter in color, contains renal corpuscles and extends inward as renal columns.
• Renal Medulla: Below the cortex, divided into pyramid-shaped structures called renal pyramids.
• Renal Pyramids: Made of collecting ducts arranged in parallel, located in the renal medulla.
• Renal Papillae: The tips of the renal pyramid, proximal to the minor calyx, the point where filtrate becomes urine.
• Renal Columns: Situated between renal pyramids, allow passage of blood vessels and anchor the renal cortex to the medulla.
• Minor Calyx: A cup-shaped tube receives urine from the collecting ducts, with one for each pyramid.
• Major Calyx: Formed by the merging of several minor calyces, collecting urine from them.
• Renal Pelvis: A flat, funnel-shaped tube that is the expanded superior part of the ureter formed by the junction of all major calyces.

Structures

• Renal Ureter: Becomes the ureter upon leaving the renal pelvis that carries urine to the urinary bladder.
• Renal Artery and Vein: Two blood vessels through the renal hilus responsible for delivering and returning blood to cleanse it.
• Renal Sinus: Inner compartment containing fat, stabilizing the renal pelvis, vessels, and nerves. Fat pads protect these structures.

The Nephron

• Functional unit of the whole organ which is the kidneys
• Responsible for filtration of blood and removing wastes
• Before leaving the kidneys, the nephrons give 99% of what was filtered back to the body

Renal Corpuscle

• Structure that aids with filtration of blood Main components of the renal corpuscle:
• Glomerulus
• Bowan's Capsule
• Bowman's space

Renal Processes

• Filtration: Water and solutes smaller than proteins pass from glomerulus capillaries into Bowman's capsule.
• Reabsorption: Filtrate moves into tubules to recover nutrients, water, and essential ions, returning them to blood; what remains is urine.
• Secretion: Wastes still in the capillaries surrounding renal tubules (not filtered by glomerulus) secreted into the filtrate which most prominently occurs in the Distal Convoluted Tubule (DCT).
• Excretion: Elimination of waste product urine from the body, temporarily stored in the bladder, that is normally aseptic.

Types of Nephrons

• Cortical Nephrons: 85% of nephrons that have renal corpuscle located high in the renal cortex and have shorter loops of Henle
• Juxtamedullary Nephrons: 15% of nephrons that have long loops of Henle which dip deep into the renal medulla.

Renal Tubule Regions

There are 3:

• Proximal Convoluted Tubule (PCT): Coiled portion of the renal tubules immediately proximal to the Bowman's capsule, and is active in reabsorption and secretion. Walls are cuboidal epithelial cells with microvilli.
• Loop of Henle: A U-shaped loop in the medulla important for maintaining a salty medulla and water conservation, and has thick and thin segments.
• Distal Convoluted Tubule (DCT): The last part of the renal tubules is also convoluted and coiled, loops back to the renal corpuscle between arterioles. Near the DCT is the macula densa that auto-regulates the kidneys. Lined with cuboidal epithelium

Structure of the Urinary Tract

• Glomerular capsule -> proximal convoluted tubule -> loop of Henle -> distal convoluted tubule -> collecting duct -> renal papilla -> minor calyx -> major calyx -> renal pelvis -> ureter -> bladder -> urethra -> out of body.
• Ureters: Paired tubes are 10 inches long that transports urine from the kidney to the urinary bladder.
• Urinary Bladder: Located posterior to pubic symphysis for storage and expulsion of urine.
• Urethra: tube that transports urine from the bladder to the outside of the body.

Blood Flow in the Kidneys

• The kidneys receive about 20-25% of cardiac output.

Blood Vessels for Renal Circulation

• Abdominal Aorta: Oxygenated blood from the left ventricle to the aorta.
• Renal a.: Blood is delivered to the renal arteries from the abdominal aorta.
• Segmental a.: Each renal artery divides into five segmental arteries that enter the renal hilus approaching a kidney.
• Interlobar a.: The segmental arteries then divide that lie in the renal columns between the medullary pyramids.
• Arcuate a.: At the medulla-cortex junction, where these create an arc over the bases of the renal pyramids from interlobar arteries.
• Cortical radiate a.: Radiate outward up into the renal cortex that is where the arcuate arteries radiate from. Afferent arteriole: Incoming blood from arterioles to the glomerulus.
• Glomerulus: Capillaries produce a filtrate, a small fraction of which becomes urine.
• Efferent arteriole: Receives blood from the glomerulus.
  • Peritubular Capillaries: Associated with cortical nephrons.
  • Vasa Recta Capillaries: Associated with juxtamedullary nephrons.
• Peritubular capillaries arise from efferent arterioles draining cortical glomeruli. They thread around renal tubules, absorbing solutes and water.
• Vasa recta capillaries arise from efferent arterioles draining juxtamedullary glomeruli. They are part of the urine-concentrating mechanism.
• Veins drain blood in reverse of the arteries.
  • Interlobular v.v.
  • Arcuate veins
  • Interlobar veins
  • Renal v.
  • Inferior vena cava