Urinary System Overview

Functions of the Urinary System

• Filters salts and wastes from the blood.
• Maintains electrolyte and water concentrations.
• Regulates pH and body fluid volume.
• Controls red blood cell production and blood pressure.

Organs of the Urinary System

• Kidneys filter blood, produce urine, and maintain pH and electrolyte balance.
• Ureters transport urine from kidneys to bladder.
• Urinary bladder stores urine.
• Urethra conveys urine outside the body.

The Kidney

• A reddish-brown, bean-shaped organ, 12 cm long, enclosed in a tough, fibrous capsule.
• Kidneys are retroperitoneal, located behind the parietal peritoneum.
• They lie along vertebral column sides, between the 12th thoracic and 3rd lumbar vertebrae.
• The right is slightly inferior to the left.
• Connective and adipose tissue holds them in place.
• Renal hilum is a medial depression leading to the renal sinus, where blood vessels, nerves, and the ureter enter or exit.
• The renal sinus contains the renal pelvis, subdivided into major and minor calyces.
• Renal pelvis is the urine-collecting funnel, which connects to the ureter.
• Small renal papillae project into each minor calyx to deliver urine.
• The kidneys consist of the renal medulla (inner region) and renal cortex (outer region).
• Renal medulla contains cone-shaped renal pyramids.
• Renal papillae are the tips of the pyramids.
• A striated appearance is due to long, microscopic tubules.
• Renal cortex surrounds the renal medulla.
• Renal columns are extensions of the cortex that dip between the renal pyramids.
• Granular appearance results from renal corpuscles, parts of the nephrons.

Kidney Functions

• Regulate the volume, composition, and pH of body fluids.
• Remove metabolic wastes from the blood to form urine.
• Secretory functions control red blood cell formation by secreting erythropoietin.
• They regulate blood pressure and volume by secreting the enzyme renin.
• They activate vitamin D to form calcitriol (activated vitamin D), which promotes calcium absorption in the small intestines.

Renal Blood Supply

• The abdominal aorta gives rise to the renal arteries, which supply blood to the kidneys.
• Renal arteries receive 25% of cardiac output.
• Renal arteries branch into segmental, interlobar, arcuate, and cortical radiate arteries, then afferent arterioles, leading to nephrons.
• The afferent arteriole gives rise to a capillary cluster, a glomerulus.
• Blood not filtered by the glomerulus enters the efferent arteriole, and continues into the peritubular capillaries.
• Venous blood returns through vessels corresponding to arterial pathways.
• The renal vein joins the inferior vena cava in the abdominal cavity.

Nephron Structure

• A nephron is the functional unit of the kidney that can produce urine independently.
• Approximately one million nephrons are in each kidney.
• It consists of a renal corpuscle and a renal tubule.
• The renal corpuscle is a filtration structure in the renal cortex for the first step of urine formation.
• It consists of the glomerulus (capillary cluster) and a glomerular capsule, a cup-shaped sac that receives filtrate.
• The glomerular capsule is the expanded proximal end of the renal tubule.
• The renal tubule consists of the glomerular capsule, proximal convoluted tubule, nephron loop (U-shaped with descending and ascending limbs), and distal convoluted tubule.
• Several distal convoluted tubules join to become a collecting duct.
• The collecting duct passes from the renal cortex into the renal medulla, emptying into a minor calyx at the papilla, and is not a nephron part.

Types of Nephrons

• Cortical Nephrons account for about 85% of nephrons.
• They're located almost completely in the renal cortex.
• Peritubular capillaries reabsorb most filtrate.
• Juxtamedullary Nephrons account for about 15% of nephrons.
• Long nephron loops extend deep into renal medulla.
• Vasa recta determine urine concentration.
• Peritubular capillaries and vasa recta receive blood from efferent arterioles.

Juxtaglomerular Apparatus (JGA)

• JGA consists of the macula densa and juxtaglomerular cells.
• The macula densa is found at the contact point between the afferent and efferent arterioles and the ascending limb of nephron loop.
• Epithelial cells monitor NaCl concentration in filtrate.
• Juxtaglomerular cells are in the afferent arteriole wall, near the glomerulus.
• Large smooth muscle cells monitor blood pressure.
• JGA monitors blood pressure and water & electrolyte balance, and secretes renin.

Urine Formation

• Involves glomerular filtration, tubular reabsorption, and tubular secretion.
• Glomerular filtration is the urine formation first step.
• Glomerulus filters water/small substances from blood into the glomerular capsule as glomerular filtrate.
• Filtrate then moves to renal tubules.
• Produces approx 180 liters fluid per day.
• Tubular reabsorption is a process transportating filtered substances from the filtrate blood to peritubular capillaries.
• Most useful substances are returned by this process to the blood.
• Tubular secretion: additional wastes from the peritubular capillaries are moved to the renal tubule.
• Amount of a substance excreted in urine is calculated by "Amount filtered by glomerulus - amount reabsorbed by renal tubule + amount secreted by renal tubule".

Glomerular Filtration

• The urine formation first step.
• Involves filtration of water and small dissolved substances from the glomerular capillaries into the glomerular capsule.
• Glomerular capillaries have many fenestrae (openings), so they're more permeable than other capillaries.
• Cells called podocytes prevent large plasma proteins from being filtered out.
• Glomerular filtrate is similar to interstitial fluid: mainly water, electrolytes, small nutrient molecules, and small waste, however lack plasma proteins.
• Glomerular hydrostatic pressure is the main force responsible for filtering substances through the glomerular capillary wall, and favors filtration.
• Smaller diameter of the efferent arteriole compared to the afferent arteriole raises the blood pressure in the glomerulus.
• Colloid osmotic pressure is caused by plasma proteins in the glomerulus; it opposes filtration.
• Hydrostatic pressure in the glomerular capsule also opposes filtration.
• The net filtration pressure (NFP) forcing substances out of the glomerulus sums the above forces.
• Filtration will occur as along as positive NFP, which is typical under normal circumstances.
• Glomerular filtration rate (GFR) averages 120-125 mL/min, or 180 L/day.
• Almost all filtrate is reabsorbed into the blood; with only a small percentage excreted in the urine.
• The GFR proportional to net filtration pressure (NFP), so factors affecting NFP also alter GFR.
• Therefore, glomerular hydrostatic pressure, glomerular plasma osmotic pressure, and hydrostatic pressure glomerular capsule affect GFR.
• When the afferent arteriole constricts, net filtration pressure, and hence GFR, decreases.
• When the efferent arteriole constricts, net filtration pressure, so GFR, increases.
• When osmotic pressure glomerular plasma is high, the GFR decreases.
• When hydrostatic pressure inside the glomerular capsule is high, then GFR also decreases.
• Glomerular filtration rate (GFR) is relatively constant.
• GFR increases in times of excess body fluid and decreases in times of fluid deficiency.
• Sympathetic impulses lower GFR. Constriction and dilation in afferent and efferent arterioles will help maintain blood pressure, GFR, and blood volume.
• Renin-angiotensin regulates GFR, which regulates sodium and potassium excretion:
• Juxtaglomerular cells secrete renin in response to stimuli indicating decreased blood pressure.
• Afferent arteriole cells sense drop in blood pressure. Sympathetic stimulation occurs.
• Macula densa cells sense decreased CI-, K+, or Na+ ions at the end ascending limb nephron loop.
• Renin converts the plasma protein, angiotensinogen, to angiotensin 1.
• Angiotensin 1 is converted to angiotensin 2 by the enzyme ACE (angiotensin converting enzyme).
• Angiotensin 2 is a vasoconstrictor of the efferent arteriole which increases NFP and GFR.
• Angiotensin 2 increases aldosterone for reabsorption of Na+ ions, so then water reabsorption follows.
• In tubular reabsorption; useful filtered substances return to the blood, by transporting them tubular fluid back into the peritubular capillaries
• Most reabsorption occurs in the proximal convoluted tubule (PCT), although it occurs throughout the renal tubule.
• PCT microvilli improve inrease surface area for reabsorption.
• Some substances are reabsorbed via protein the limited of transport capacity, so excessive amounts, exceeding transport maximum (Tm), excreted into urine.
• Glucose and amino acids reabsorbed by active transport, osmosis for water, and endocytosis small proteins.
• Other reabsorbed substances by active transport w/ limited transport capacities creatine, lactic, citric, uric acid, ascorbic acid, phosphate, sulfate, calcium, potassium, and sodium ions .
• If a substance exceeds its renal plasma threshold, then the excess appears in the urine.
• Sodium and water reabsorption transports 70% of Nation in proximal convoluted tubule through active transport.
• As Nat ions are reabsorbed, negatively charged ions (CI and HCO3) through passive transport.
• As Nations are reabsorbed, water follows by osmosis.
• Substances remaining in the filtrate, and are not reabsorbed into the blood, become more concentrated as water removes from filtrate.
• Tubular fluid volume greatly decreases due to reabsorption quantity.
• Reabsorption of Nations and H₂O continues all along the renal tubule, towards the collecting duct.
• Almost all Nat and HO that are filtered by the glomerulus are reabsorbed before they are excreted in urine.
• Tubular secretion reverses tubular reabsorption.
• 20% of the plasma is filtered by the glomerulus, while 80% is transported to the efferent arteriole and peritubular capillaries
• Tubular secretions occurs for substances from the plasma to peritubular capillaries into the fluid of the renal tubule.
• Wastes and large molecules combine to in urine formation through tubular secretion.
• Through Active transport mechanisms, excess H+ and K+ ions, some organic comp such as histamine and penicillin, into the renal tubule are moved.
• Through H⁺ occurs, the regulation of pH of body fluids is important.
• Hormones control of Na+ and H₂O in the distal convoluted tubule and collecting duct.
• Provides the adjustment in urine composition occurs. Aldosterone promotes in adrenal lands: distal convoluted tubule reabsorption of Na+ ions (H₂O also) and secretion of K* ion.
• Aldosterone secretes in response to increase in K* Normally: the distal convoluted tubule and collecting duct are impermeable to water, unless the antidiuretic hormone (ADH).
• ADH enhances to these portion of the renal tubules to the water, also means the water will get reabsorb
• The H2o will get decrease and stimulates secretion of ADH.
• Urea as byproduct is involved in the amino acid metabolism.
• After filtration of glomercular, 40-60% of urea returns to normal. Urine excrete the rest of urea.
• Uric acids, byproduct of nucleic acid, they reabsorb it through out active transport. Very small will secret into the tubules of renal.
• 95% water in the component for the urine. ( also contains urea, uric acid, amino acids, and electrolytes traces). There from 0.6- 25 liters per day that volume produce.
• Protein, ketones, glucose, and blood cells, they are normal to not be found in the urine so a sign might indicate the the disease
• Concentration of some substances in the urine will derive as combination a glomerulate of filtration, transportation of reabsorption, and transportation secretion.

Urine Formation

• After forming in the nephrons, urine passes from the collecting ducts, and transports tips of the renal pyramids ( the renal papillae)
• Urine now transfers, into the minor next to the major calyces. The next continuing step from renal pelvis, that will store the urn into each and ever kidney. Now the renal will get narrower. That narrowing and renal of kidneys that where ureter will get a empties. Storage happen in the unitary bladder for the urine. Than the body releases urn out through the urethra.

Ureters

• Musucular tube transporting urn form kidney to the urinary bladder.
• The begginings start as a form shape funnel from renal pelvis. There of the compose layers of the ureter:
• inner musous layer: of consisting transipitional eputhelelium, the muscularis coat is more muscular and the fibrous coat is more outter.
• Transportation of urine through the musuclar by peristalalitic will moves of the urinary bladder. Therefore the angle where the uerreters will enters.

Unninary Balder

• That hollow part of the muscular that lies, that will be in the pelvic.
• Urine can get store and excrete through utherhra. Also consists in
• Floor of the bladder of the internal: a trignon-like from opening the erreters, and also uthera's.
• Wall in area of the unitery of the ballder, that covers inner mucosa the for, which is transfer by epithuem , submusoca, more muscluar will create the derture muscles, and in the serous out . Part of the destrusr will surroud to the neck.

The Utethral Sphincater

• The transporation through the Utherthra where uthereal landds , inner mucosa is the many of mucleus.
• The long where the feamelas , 4 cm in long the openeign which is clatiouus and the vagianl

Micturition

• Urine releasing through the uniarty bladder.
• Detursor of the muscles with goes contract reflex.
• Internal of urethral that also the sphinxter, that wil lundertgo into relzations. and trigger of streching, where bladder.