Urinary System: Anatomy and Functions

• The urinary system is described

Functions

• The kidneys eliminate waste through urine
• Waste includes nitrogenous wastes, toxins, drugs and excess ions
• Regulatory functions include production of renin to maintain blood pressure
• Production of erythropoietin to stimulate red blood cell production
• Conversion of vitamin D to its active form

Organs

• Organs include the kidneys, ureters, urinary bladder, and urethra

Kidneys

• Kidneys sit against the dorsal body wall in a retroperitoneal position behind the parietal peritoneum
• They are situated at the level of the T12 to L3 vertebrae
• The right kidney sits slightly lower than the left due to the position of the liver

Kidney Structure

• An adult kidney is about 12cm (5") long and 6cm (2.5") wide
• The renal hilum is a medial indentation where structures like the ureters, renal blood vessels, and nerves enter or exit the kidney
• An adrenal gland sits atop each kidney
• Kidneys are enclosed by three protective layers
• The fibrous capsule encloses each kidney
• The perirenal fat capsule surrounds the kidney and cushions against blows
• The renal fascia is the most superficial layer that anchors the kidney and adrenal gland to the surrounding structures
• Three regions are seen in a longitudinal section
• The renal cortex is the outer region
• The renal medulla is the deeper region that contains triangular regions of tissue called renal (medullary) pyramids
• Renal columns are extensions of cortex-like material that separate the pyramids
• The renal pelvis is a medial region that is a flat, funnel-shaped tube
• Calyces form cup-shaped "drains" that enclose the renal pyramids that collect urine and send it to the renal pelvis, onto the ureter, and to the urinary bladder for storage

Blood Supply

• One-quarter of the total blood supply of the body passes through the kidneys each minute
• The renal artery provides each kidney with an arterial blood supply and it divides into segmental arteries, then interlobar arteries, then arcuate arteries and finally cortical radiate arteries
• Venous blood flows from cortical radiate veins, then to arcuate veins, then to interlobar veins, then to the renal vein
• There are no segmental veins, the renal vein returns blood to the inferior vena cava.

Nephron

• The nephron is the structural and functional unit of the kidney
• Each kidney contains over a million nephrons
• Each nephron consists of two main structures

Renal Corpuscle

• The renal corpuscle consists of the glomerulus and the glomerular (Bowman's) capsule
• The glomerulus is a knot of capillaries made of podocytes
• Podocytes make up the inner (visceral) layer of the glomerular capsule
• The foot processes cling to the glomerulus
• Filtration slits create a porous membrane, which is ideal for filtration
• The Glomerulus (Bowman’s) capsule surrounds the glomerulus; it is the first part of the renal tubule

Renal Tubule

• The renal tubule extends from the glomerular capsule and ends when it empties into the collecting duct
• From the glomerular capsule, the sections of the renal tubule include proximal convoluted tubules (PCT), the nephron loop (loop of Henle), and distal convoluted tubule (DCT)

Nephron Types

• Cortical nephrons are located entirely in the cortex, and most nephrons are cortical
• Juxtamedullary nephrons are found at the cortex-medulla junction and its nephron loop dips deep into the medulla
• Collecting ducts collect urine from both types of nephrons (either cortical or juxtamedullary) through the renal pyramids, to the calyces, and then to the renal pelvis

Capillary Beds

• Two capillary beds associated with each nephron
• The glomerulus is fed and drained by arterioles
• The afferent arteriole arises from a cortical radiate artery and feeds the glomerulus
• The efferent arteriole receives blood that has passed through the glomerulus
• Glomeruli are specialized for filtration, high pressure forces fluid and solutes out of the blood and into the glomerular capsule
• Peritubular capillary beds arise from the efferent arteriole of the glomerulus
• They are low-pressure, porous capillaries and adapted for absorption instead of filtration
• Peritubular capillaries cling close to the renal tubule to receive solutes and water from tubule cells and drain into the interlobar veins

Urine Formation

• Urine formation is the result of three processes: glomerular filtration, tubular reabsorption, and tubular secretion

Glomerular Filtration

• The glomerulus acts as a filter
• Filtration is a nonselective passive process
• Water and solutes smaller than proteins are forced through glomerular capillary walls
• Proteins and blood cells are too large to pass through the filtration membrane
• Once in the capsule, fluid is called filtrate, this filtrate leaves via the renal tubule
• Filtrate will be formed if systemic blood pressure is normal, if arterial blood pressure is too low, filtrate formation stops because the glomerular pressure will be too low to form filtrate

Tubular Reabsorption

• The peritubular capillaries reabsorb useful substances from the renal tubule cells
• Reabsorbed substances include water, glucose, amino acids, and ions
• Some reabsorption is passive, most active which means using ATP
• Most reabsorption occurs in the proximal convoluted tubule

Tubular Secretion

• Reabsorption in reverse, some materials move from the blood of the peritubular capillaries into the renal tubules to be eliminated in filtrate
• Materials secreted include hydrogen and potassium ions, and creatinine (produced by muscles from the breakdown of creatine)
• Secretion is important for getting rid of substances not already in the filtrate, removing drugs and excess ions, and maintaining acid-base balance of blood
• Materials left in the renal tubule move toward the ureter

Nitrogenous Wastes

• Nitrogenous waste products are poorly reabsorbed, if at all, tending to remain in the filtrate and are excreted from the body in the urine
• Nitrogenous wastes include urea (the product of protein breakdown), uric acid (results from nucleic acid metabolism), and creatinine (associated with creatine metabolism in muscles)
• In 24 hours, about 1.0 to 1.8 liters of urine are produced

Urine and Filtrate

• Filtrate contains everything that blood plasma does, (except proteins)
• Urine is what remains after the filtrate has lost most of its water, nutrients, and necessary ions through reabsorption
• Urine contains nitrogenous wastes and substances that are not needed

Urine Characteristics

• Urine is clear and pale to deep yellow
• Yellow color is normal due to the pigment urochrome (from the destruction of hemoglobin by the liver) and solutes, dilute urine is a pale, straw-color
• Urine is sterile at the time of formation
• It is slightly aromatic but smells like ammonia with time
• Urine is slightly acidic (pH of 6)
• The specific gravity of urine ranges from 1.001 to 1.035
• Solutes normally found in urine include sodium and potassium ions, urea, uric acid, creatinine, ammonia, and bicarbonate ions
• Solutes not normally found in urine include glucose, blood proteins, red blood cells, hemoglobin, WBCs (pus), and bile

Other Organs of the urinary system

• Ureters
• Urinary Bladder
• Urethra