Urinary 201

Anatomy and Physiology of the Kidney

• Kidneys lie on either side of the vertebral column.
• The superior border is located at the 12th thoracic vertebrae.
• The Costovertebral angle (CVA) is on the posterior side.
• Kidneys are retroperitoneal, located behind the peritoneum.
• Connective and adipose tissue hold the kidneys in place and protect them.
• Major anatomical landmarks include the hilum and renal pelvis.
  • The hilum is a concave region where vessels, nerves, and ureters enter and leave the kidney.
  • The renal pelvis is a funnel-shaped collecting area in the medial body of the kidney.
• The two major divisions of the kidneys are the renal medulla and the renal cortex.
  • The renal medulla houses the collecting ducts of renal tubules.
  • The renal cortex houses the main filtering units of the kidneys.
• Main function is to regulate the volume, composition, and pH of body fluids.
  • This is achieved through removing and excreting metabolic wastes from the blood.
  • Controlling the rate of red blood cell (RBC) production via erythropoietin.
  • Regulation of blood pressure.
  • Regulation of calcium absorption by activating vitamin D.
• Renal blood vessels consist of renal arteries which arise from the abdominal aorta and branch several times.
  • The final branches are afferent arterioles, which carry blood to the nephron.
  • Efferent arterioles carry blood away from the nephron.
  • These become peritubular capillaries.
  • Venous return occurs from the renal veins into the inferior vena cava.

Nephrons

• Nephrons are the functional units of the kidney.
• Two components of nephrons: renal corpuscle and renal tubule.
• The renal corpuscle contains the glomerulus and Bowman's capsule (glomerular capsule).
  • The glomerulus is comprised of afferent and efferent arterioles and acts as a filtering unit within the nephron.
  • Bowman's capsule (glomerular capsule) is a glove-like structure over the glomerulus that collects filtrate.
• The renal tubule consists of the proximal convoluted tubule (PCT), nephron loop, and distal convoluted tubule (DCT).
  • The PCT is the highly coiled, proximal portion of the renal tubule leaving the glomerulus.
  • The nephron loop has a descending limb (PCT dips down toward renal pelvis) and an ascending limb (returns toward corpuscle).
  • The Distal convoluted tubule (DCT) is a shorter and straighter coiled tubule after the loop.

Juxtaglomerular Apparatus

• Includes the macula densa and juxtaglomerular cells.
• The macula densa consists of epithelial cells in the DCT.
  • These cells are sensitive to [Na+], [K+], and [Cl-] in the DCT.
  • Decreasing levels of these electrolytes stimulate renin secretion.
• Juxtaglomerular cells are located in the wall of the afferent arteriole.
  • They are near the attachment to the glomerulus.
  • They are baroreceptors sensitive to blood pressure.
  • Act as a key structure for renin secretion.

Urine Formation

• Glomerular filtration occurs when the glomerulus filters out water and small dissolved particles.
• Filtration pressure is maintained via hydrostatic blood pressure.
  • Filtration is affected by extreme changes in blood pressure.
  • Afferent arteriole has a larger diameter than the efferent arteriole, allowing blood to back up in the glomerulus.
• Several factors affect glomerular filtration rate (GFR)
  • Afferent arteriole pressure.
  • Vasoconstriction results in decreased GFR, decreasing blood flow to the glomerulus.
  • Low blood volume will result in decreased filtration and an increase in blood volume.
  • Vasodilation increases GFR.
  • Efferent arteriole pressure.
  • Vasoconstriction here results in increased GFR.
  • The same amount of blood is coming into the glomerulus but is slower to come out.
  • Colloid Osmotic Pressure.
  • Decreased plasma protein in vessels (aka colloid pressure) increases GFR by decreasing fluid pulled back into the vessels.
  • Increase in plasma protein pressure decreases GFR as more fluid is pulled back into the vessels)
  • Change in surface area of glomerulus.
  • Obstructions or damage to the glomerulus can affect GFR.
  • Renal disease, overuse of NSAIDs, scarring, etc can affect GFR.
• Net filtration = force favoring filtration - the forces opposing filtration.
• Filtration pressure is normally positive but various disease conditions and medications can affect it.
  • NSAIDs, diabetes mellitus, hypertension and lupus erythematous.
• Most filtrate is reabsorbed because of colloid osmotic pressure of the plasma.
  • Tubular reabsorption involves water, electrolytes, and glucose.
  • Location is important when considering where certain HTN medications have their effects.
  • The PCT reabsorbs 70% of filtered Na+, other ions, and water.
  • By the time the filtrate goes through the loop and DCT, 97-99% is reabsorbed.
• Tubular secretion involves H+ and toxins from the blood directly into the tubule.

Renin-Angiotensin-Aldosterone System

• Greatly affects the glomerular filtration rate.
• Juxtaglomerular cells monitor BP, if BP drops→ renin is secreted.
• Renin reacts with a circulating plasma protein angiotensinogen.
  • Stored and released in the liver Renin + angiotensinogen → angiotensin I.
• Angiotensin I is converted to angiotensin II by ACE in the lungs.
• Angiotensin II works at the adrenal gland to secrete aldosterone.
• Aldosterone stimulates sodium reabsorption in the distal convoluted tubule.
  • This conserves sodium and water increasing circulatory volume → rise in BP.
• ACE inhibitors and angiotensin receptor blockers (ARBs) are common medications working to control hypertension.
• A common side effect of ACE inhibitors is a cough.

Effects of ADH and Caffeine on Urine Formation

• Anti-diuretic hormone (ADH):
  • Secreted by the posterior pituitary gland.
  • ADH controls water reabsorption at DCT and collecting duct by increasing ADH secretion.
  • Greater water reabsorption raises BP.
  • Without ADH, the DCT/collecting duct is relatively impermeable to water.
  • Urine becomes more concentrated.
  • Inadequate secretion of ADH →Diabetes Insipidus
• Grain alcohol blocks the release of ADH from the posterior pituitary.
• Caffeine inhibits proximal tubular sodium reabsorption.
  • Less Na+ reabsorbed resulting in = more Na+ and water in the renal tubule.

Urine Composition

• At rest, the kidneys receive ≈ 25% of cardiac output.
• Glomerular filtration occurs at ≈ 180 liters a day (45 gallons).
• The majority of fluid through the kidneys is reabsorbed.
• ≈ 95% of urine is water with small amounts of urea/electrolytes.
• Composition varies based on dietary activities and disease.
• Urea gives urine its typical color as a by-product of protein catabolism.
• Urine specific gravity reflects the relative concentration of solutes in the urine.
• Normal urine output is 50-60 mls/hr, < 30 mls/hr may indicate kidney failure.

Elimination of Urine

• After formation in the nephron urine drains from the collecting ducts.
• Goes to the minor calyces → major calyces → renal pelvis → ipsilateral ureter.
• Ureters are Muscular tubes ≈ 25cm long Peristaltic waves move urine from renal pelvis to urinary bladder.
• Renal stones (nephrolithiasis) are a common cause of pain and obstruction.
• This is caused by the precipitation of solutes out of solution; the majority of stones are calcium-oxalate.
• The Urinary bladder is a hollow, distensible muscular organ within the pelvic cavity.
• The trigone, a triangular region on the floor of the bladder, has 3 openings includes two ureters coming in and one urethra going out.
• Cystitis is bladder inflammation, often caused by bacterial infection, certain drugs, radiation therapy, potential irritants, and even anxiety.
• The urethra is a tube conveying urine from the bladder to the outside of the body.
  • Females: ≈ 4 cm long
  • Males: ≈ 20 cm long
• Urethritis is inflammation of the urethra, often caused by from sexually transmitted diseases/infections

Micturition

• The voiding or urination reflex causes urine to leave the bladder.
• The urinary bladder distends with urine.
• Stretch receptors in the bladder stimulate micturition reflex in the sacral nerves.
• Detrusor muscle within the bladder contracts and is accompanied by a sense of urgency.
• Internal pressure increases, contractions intensify and force the internal sphincter open.
• Another reflex allows the external urethral sphincter (under conscious control) to relax and urine is then excreted through the urethra.

Urinary System Physical Exam

• The physical exam is limited to costovertebral angle tenderness.
• Palpation of the kidneys is extremely difficult to elicit findings and is not performed in this course.
• Most evaluation is through laboratory testing, including:
  • Urinalysis as a will perform UA dipstick in disorders block.
  • Metabolic panels which measure BUN (Blood Urea Nitrogen) and Cr (Creatinine).
• Costovertebral angle tenderness is measured by:
  • Placing palm flat over CVA and strike with other fist; repeat bilaterally.
  • It's important to assess for pain.
  • In patients with suspected renal colic or pyelonephritis pain is elicited due to inflammation of the renal capsule.

The kidneys regulate body fluids by removing metabolic wastes from the blood. The major anatomical landmarks include the hilum and renal pelvis. The two divisions of the kidneys are the renal medulla and the renal cortex. The renal cortex houses the main filtering units of the kidneys.