Anatomy and Function of the Urinary System

• The urinary system maintains the volume and composition of body fluids within normal limits.
• It removes waste products from cellular metabolism and is referred to as the excretory system.
• Other organs like the lungs, skin, liver, and intestines contribute to excretion, but the urinary system has the major role.
• The urinary system regulates fluid volume by controlling water excretion in urine.
• It regulates electrolyte concentrations and maintains normal blood pH.
• Erythropoietin, a hormone, is secreted to control red blood cell production.
• Renin, an enzyme, is secreted to maintain normal blood pressure.

Anatomy of the Urinary System

• Consists of the kidneys, ureters, urinary bladder, and urethra.
• Kidneys form urine.
• Ureters carry urine from kidneys to the urinary bladder.
• The urinary bladder serves as a temporary urine reservoir.
• The urethra is a tubular structure carrying urine from the bladder to the outside.
• Kidneys filter blood, remove wastes, and excrete them into urine.
• Kidneys are located between the twelfth thoracic and third lumbar vertebrae, one on each side of the vertebral column.
• The right kidney is slightly lower due to the liver.
• The kidneys lie behind the parietal peritoneum, making them retroperitoneal and are protected by the lower ribs.
• The renal capsule is a tough, fibrous connective tissue that closely envelops each kidney.
• Adults' kidneys are approximately 3 cm thick, 6 cm wide, and 12 cm long.
• Bean-shaped kidneys have a medial indentation called the hilum or hilus.
• The ureter and renal vein leave the kidney, renal artery enters at the hilum.
• The outer reddish region is the renal cortex which surrounds the darker reddish-brown renal medulla.
• The renal medulla is a series of renal pyramids containing straight tubular structures and blood vessels.
• The renal pelvis, which is continuous with the ureter, is in the kidney's central region.
• The renal pelvis collects urine and has cuplike projections called calyces.
• Minor calyces surround renal papillae and collect urine.
• Several minor calyces converge to form a major calyx, which flows into the renal pelvis and then the ureter.
• Each kidney contains over a million nephrons in the cortex and medulla.
• A nephron consists of a cup-shaped capsule containing capillaries and the glomerulus, and a long renal tube.
• Blood flows into the kidney via the renal artery, branching into capillaries associated with the glomerulus.
• Filtration occurs at Bowman's capsule.
• Filtrate flows through the proximal convoluted tubule (PCT), the loop of Henle, and the distal convoluted tubule (DCT).
• The distal convoluted tubule empties into a collecting duct.
• Collecting ducts gather urine into minor calyces, then major calyces, and finally into the renal pelvis.
• Fluids and solutes are returned to capillaries surrounding the nephron tubule.
• Nephrons have a special part known as the juxtaglomerular apparatus (JGA), which plays a role in controlling blood pressure.
• The JGA is composed of modified smooth muscle cells called juxtaglomerular cells, macula densa cells, and mesengial cells.
• Juxtaglomerular cells secrete renin, while macula densa cells function as osmoreceptors.
• Renin increases systemic blood pressure through the renin-angiotensin system (RAS).
• The renin-angiotensin system regulates blood volume, arterial pressure, and cardiac and vascular function.
• Renal artery hypotension stimulates renin release by juxtaglomerular cells.
• Renin acts on angiotensinogen, a circulating substrate produced by the liver.
• Angiotensinogen undergoes proteolytic cleavage to form angiotensin I.
• Angiotensin I is converted into angiotensin II by angiotensin-converting enzyme (ACE).
• Angiotensin II constricts vessels, increasing systemic vascular resistance and blood pressure.
• It acts on the adrenal cortex to release aldosterone, increasing sodium and fluid reabsorption in the kidneys.
• It stimulates vasopressin or antidiuretic hormone (ADH) release from the posterior pituitary, increasing fluid reabsorption in the kidneys.
• It stimulates thirst centers in the brain.

Kidney Homeostatic Functions

• Include urine formation for maintaining volume, ionic balance, and pH of extracellular fluid.
• Excretion of toxic metabolic by-products like urea, ammonia, and uric acid.
• Hormone synthesis of erythropoietin, renin, and prostaglandins.
• Gluconeogenesis involves producing new glucose from amino acids and fatty acids.
• Nephrons filter 125 ml of body fluid per minute, filtering the entire body fluid component 16 times each day.
• Nephrons produce 180 liters of filtrate daily, with 178.5 liters reabsorbed, and the remaining 1.5 liters forming urine.
• Urine production involves the steps of filtration, reabsorption, and secretion.
• Filtration in the Bowman's capsule from the glomerulus.
• Reabsorption; 60-70% happens in the proximal tubule.
• Secretion mainly occurs in the distal tubule.
• Mechanical filtration of blood plasma occurs in Bowman's capsule, caused by blood plasma being pushed through the membrane's pores.
• The filtration membrane consists of fenestrated endothelium, a basement membrane, and the Bowman's capsule's visceral wall cells (podocytes).
• Podocyte projections (pedicels) surround capillaries, leaving a gap.
• Blood passes through the fenestras of the endothelium and podocyte projections' gaps.
• The glomerulus acts as an ultrafilter, allowing passage of water, electrolytes, and small organic molecules like glucose.
• Filtration pressure is the net pressure from hydrostatic and oncotic pressure in the glomerulus and Bowman's capsule.
• A higher filtration pressure produces more filtrate.
• If filtration pressure equals zero, no filtrate is produced.
• Glomerular and Bowman's capsule oncotic pressure can increase filtration pressure.
• Bowman's capsule hydrostatic pressure and glomerulus oncotic pressure can decrease filtration pressure.
• The hydrostatic pressure of the glomerulus pushes more plasma through the fenestras of the capillary wall.
• The glomerular filtration rate (GFR) characterizes kidney function as the volume of fluid filtered into the Bowman's capsule per unit time.
• Healthy adults have a GFR of about 125 ml/minute.
• From the glomerulus, the filtrate passes into the proximal convoluted tubule (PCT) which is coiled and contains cuboidal epithelium.
• PCT's epithelium are lined by microvilli.
• Microvilli forming a brush border increases surface area.
• The large surface area is important for the uptake of various useful substances that are filtered at the glomerulus like glucose, amino acids, ions, water, and albumin.
• The main reabsorption sites are the proximal convoluted tubules.
• Reabsorption takes place in the loop of Henle, distal convoluted tubules, and collecting tubules.
• Reabsorption mechanisms are diffusion, osmosis, and active transport.
• The loop of Henle consists of a descending and an ascending limb and concentrates urine.
• The loop of Henle forms a concentration gradient to reabsorb water and solutes.
• Sodium and chloride ions are actively transported out of the tubule into the interstitial fluid in the ascending part.
• Water moves out from the tubule by osmosis in the descending limb due to the concentrated interstitial fluid.
• Approximately 178 liters of water are reabsorbed into the bloodstream daily from the proximal tubules.
• The tubules push 99% of the 180 liters of water back into the blood that leaves the body through glomerular filtration using osmosis.
• Glucose (blood sugar) is entirely reabsorbed into the blood from the proximal tubules and back to capillaries.
• Sodium ions and other ions are partially reabsorbed from the renal tubules back into the blood through active transport.
• The amount of sodium reabsorbed varies based on salt intake.
• Increased salt intake decreases sodium reabsorption, increasing sodium excretion in urine.
• The opposite happens with decreased salt intake.
• Secretion moves substances into the distal and collecting tubules from blood in the capillaries.
• Secretion moves substances out of the blood, which are then excreted in urine.
• Substances are secreted through active transport or diffusion.
• Secreted substances include hydrogen ions (H+), potassium ions (K+), ammonia (NH3), and certain drugs.
• Kidney tubule secretion maintains the body's acid-base balance.
• Nitrogen wastes are byproducts of protein metabolism.
• Amino groups are removed from amino acids.
• The amino group (NH2) combines with a hydrogen ion (proton) to form ammonia (NH3) and is converted to urea.
• Urea can be tolerated at higher concentrations and is secreted by amphibians and mammals.
• The antidiuretic hormone (ADH) controls water reabsorption via negative feedback.
• ADH is released from the pituitary gland when fluid levels in the blood drop.
• Signals from the hypothalamus cause the pituitary to release ADH, increasing water reabsorption in the distal convoluted tubules and collecting ducts.
• When too much fluid is present in the blood, sensors in the heart signal the hypothalamus to stop secreting ADH.
• Aldosterone, secreted by the adrenal glands, regulates sodium transfer from the nephron to the blood.
• When sodium levels in the blood decrease, aldosterone causes the transfer of sodium from the nephron to the blood.
• Renin controls aldosterone through the renin-angiotensin system and osmosis moves water into the blood.

Disruption of Kidney Function

• Infection, environmental toxins, and genetic disease can disrupt kidney function.
• Dialysis and kidney transplants can be performed.
• Excess wastes can crystallize in kidney stones that consist of calcium oxalate crystals.
• Kidney stones grow and can cause pain that requires surgery or ultrasound treatments, with some stones being forced into the urethra.

Forms of Diabetes

• Diabetes insipidus (DI) causes frequent urination because the urine is diluted.
• Deficiency of ADH, or kidneys being insensitive to ADH can lead to DI.
• Symptoms can be excessive thirst and urination.
• DI should not be confused with diabetes mellitus, which is from insulin deficiency or resistance causing high blood glucose.
• Diabetes mellitus (DM) is of two types, type 1 and type 2 with similar signs and symptoms, such as excessive thirst and frequent urination.
• Type 1 and Type 2 result from insulin resistance.
• The urinary bladder is a muscular, distensible sac that stores urine before urination and dimensions change, can hold a liter of urine.
• The inner lining of the urinary bladder is a mucous membrane of transitional epithelium that is continuous with the ureters.
• When the bladder is empty, mucosa has folds called rugae that allow the bladder to expand as it fills.
• Submucosa helps supports mucous membrane.
• The next layer is the muscularis, which is made up of smooth muscle and contraction expels urine from the bladder.
• The outer layer of the bladder wall is parietal peritoneum, and in other regions, the outer layer is fibrous connective tissue.
• Three openings in the floor of the urinary bladder form a triangular area is the trigone.
• The ureter is the two openings that form the base of the trigone.
• Small flaps of mucosa cover the ureter.
• The third opening is the apex of the trigone leading into the urethra, and a band of muscle encircles the opening to form the internal urethral sphincter.
• The internal urethral sphincter provides involuntary control, and a circular band of skeletal muscle forms the external urethral sphincter, which is under voluntary control.
• Urethra in females is about 2.5-3 cm long, while in males it is about 18-20 cm long.
• The micturition reflex coordinates urination or micturation and bladder stretch stimulates the urinary wall.
• Sensory fibers send signals to the brain, creating awareness of pressure, where the urge to urinate starts at around 200ml of urine in the bladder.
• When stretch receptors are stimulated to urination, the internal and external sphincters need to be open.
• External sphincter is voluntarily controlled.
• After voiding, less than 10 ml of urine should be left in the bladder.
• Incontinence is the inability to control urination and can be caused by trauma to the external sphincters.
• The normal amount of Urine is 1.5 Liters a day, and a yellow appearance due to bilirubin, normally clear and sterile without any viruses or bacteria.
• Urine generally has a acid pH of 5-6.5 that is dependant on you diet.
• Eating more meat results in more acidic urine, while eating more vegetables.
• There should only be 1-2 blood cells in in healthy urine.
• Blood in the urine mean you have diabetes.