Urinary System

• The Urinary System unit aims to enable students to interpret anatomy and physiology facts, understand kidney structures and functions, and perform relevant physical exams.

Anatomy and Physiology of the Kidney

• Kidneys are located on either side of the vertebral column with the superior border at the 12th thoracic vertebrae, lying retroperitoneal behind the peritoneum.
• The Costovertebral Angle (CVA) is on the posterior side.
• Connective and adipose tissue hold and protect the kidneys.
• 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.
• The primary function is to regulate the volume, composition, and pH of body fluids via several means.
• The elimination of metabolic wastes from the blood accomplishes the function of balancing the organism's internal conditions.
• Regulation of red blood cell (RBC) production, regulation of blood pressure, and regulation of calcium absorption are among these means.
• Regulation of RBC uses erythropoietin, and calcium absorption activates vitamin D.

Renal Blood Vessels

• Renal arteries originate from the abdominal aorta and branch multiple times before reaching the nephron.
• Afferent arterioles are the final branches carrying blood to the nephron.
• Efferent arterioles carry blood away from the nephron, eventually becoming peritubular capillaries.
• The renal veins return blood to the inferior vena cava.

Nephrons

• Nephrons are the functional units of the kidney.
• The two main components are the Renal Corpuscle and the Renal Tubule.
• The renal corpuscle contains the glomerulus and Bowman's capsule.
• The Glomerulus comprises afferent and efferent arterioles and filters blood within the nephron.
• Bowman's capsule (glomerular capsule) is a glove-like structure over the glomerulus that represents an expansion of the renal tubule where filtrate collects.
• The Renal Tubule parts include the Proximal Convoluted Tubule (PCT) and the Nephron Loop.
• The Proximal Convoluted Tubule (PCT) is the coiled section of the renal tubule exiting the glomerulus.
• The Nephron Loop has a descending limb dipping toward the renal pelvis and an ascending limb returning toward the corpuscle.
• The Distal Convoluted Tubule (DCT) is straighter, located after the loop.
• The collecting duct receives several distal convoluted tubules and is technically not part of the nephron.

Juxtaglomerular Apparatus

• An apparatus including the macula densa and juxtaglomerular cells.
• The Macula Densa has epithelial cells in the DCT sensitive to Na+, K+, and Cl- levels, with decreased levels stimulating renin secretion.
• Juxtaglomerular cells are in the wall of the afferent arteriole near its attachment to the glomerulus and act as baroreceptors sensitive to blood pressure; key for renin secretion.

Urine Formation

• Glomerular filtration involves the glomerulus filtering water and small dissolved particles, maintaining filtration pressure via hydrostatic blood pressure.
• GFR (Glomerular Filtration Rate) is affected by Afferent Arteriole Pressure via vasoconstriction which decreases GFR, resulting in less blood flow to the glomerulus.
• Vasodilation of afferent arteriole pressure increases Glomerular Filtration Rate (GFR).
• Efferent Arteriole Pressure can increase GFR via vasoconstriction which causes the same amount of blood coming into glomerulus to come out slowly.
• Colloid osmotic pressure affects GFR with decreased plasma protein levels increasing GFR, and increased levels decreasing it.
• Changes in the glomerulus surface area (obstructions, damage, renal disease) can affect GFR.
• Therefore Net filtration = force favoring filtration - forces opposing filtration.
• Filtration pressure is normally positive but can be affected by disease conditions and certain medications, such as diabetes mellitus, hypertension, lupus erythematosus, and NSAIDs.
• Most filtrate is reabsorbed due to colloid osmotic pressure of the plasma.
• Tubular reabsorption involves the movement of water, electrolytes, and glucose, and its location is important when considering hypertension medications.
• The Proximal Convoluted Tubule reabsorbs about 70% of the filtered Na+, other ions, and water.
• About 97-99% of the filtered material is reabsorbed by the time the filtrate goes through the loop and Distal Convoluted Tubule.
• Tubular secretion introduces H+ and toxins from blood directly into the tubule.

Renin-Angiotensin-Aldosterone System

• A system which has a great effect on the glomerular filtration rate.
• Juxtaglomerular cells constantly monitor blood pressure; renin is released if blood pressure drops.
• Renin reacts with angiotensinogen in a circulating plasma cycle.
• Stored and released in the liver, where Renin and angiotensinogen result in the production of angiotensin I.
• Angiotensin I is converted to angiotensin II by angiotensin converting enzyme (ACE), which is manufactured in the lungs.
• Angiotensin II works at the adrenal gland to secrete aldosterone, which stimulates sodium reabsorption in the distal convoluted tubule.
• Resulting ultimately in increased blood pressure through conserving sodium via the distal convoluted tubule (DCT).
• Two common medications work through this mechanism to control hypertension, ACE inhibitors, and angiotensin receptor blockers (ARBs).
• A common side effect of ACE inhibitors is a cough.

Effects of ADH and Caffeine on Urine Formation

• Anti-diuretic hormone (ADH) is secreted by the posterior pituitary gland to control body water reabsorption.
• ADH has an effect on the distal convoluted tubule where ADH secretion increases water reabsorption, raising BP.
• The DCT and collecting duct are relatively impermeable to water when ADH is not present.
• ADH causes the urine to become more concentrated, and inadequate secretion can cause Diabetes insipidus.
• Grain alcohol blocks ADH release from the Adenohypophysis.
• Caffeine inhibits proximal tubular sodium reabsorption resulting in less Na+ reabsorbed equating to more Na+ and water in the renal tubule.

Urine Composition

• The kidneys receive roughly 25% of cardiac output while a person is at rest.
• The Glomerular filtration rate is approximately 180 liters per day (45 gallons), the majority of fluid through the kidneys is reabsorbed.
• Water makes up approximately 95% of urine with small amounts of urea/electrolytes.
• Composition varies based on dietary activities and disease.
• Urea gives urine its typical color and is a by-product of protein catabolism, and the relative concentration of solutes in the urine reflects its specific gravity.
• Kidney failure is indicated by Normal urine output = 50-60 mls/hr, 30< mls/hr.

Elimination of Urine

• After forming in the nephron, the urine drains from the collecting ducts sequentially into the minor calyces, major calyces, renal pelvis, and ipsilateral ureter.
• Ureters are muscular tubes, about 25cm long, that have peristaltic waves to move urine from the renal pelvis to the urinary bladder.
• Common cause of pain and obstruction from such stones stems from precipitation of solutes out of solution resulting in Renal stones, and renal stones that are calcium-oxalate.
• The Urinary bladder is a hollow, distensible muscular organ within the pelvic cavity.
• A triangular region on the floor of the bladder called the trigone has 3 openings that have two ureters coming in and one urethra going out.
• Cystitis is bladder inflammation most often caused by bacterial infection but also certain drugs, radiation therapy, potential irritants and even anxiety.
• The Urethra tube conveys urine from the bladder to the body, and inflammation commonly results from sexually transmitted/infections.
• Females have ≈ 4 cm, and males have ≈ 20 cm.
• Micturition is also called voiding, or urination, a reflex that causes urine to leave the bladder when the urinary bladder distends with urine.
• Stretch receptors in the bladder stimulate the micturition reflex in the sacral nerves.
• Micturition is often accompanied by a sense of urgency, and the detrusor muscle within the bladder contracts, increasing internal pressure and intensifying contractions and resulting in exerting force on the internal sphincter causing it to open.
• Another reflex allows the external urethral sphincter (under conscious control) to relax and urine is ultimately excreted through the urethra.

Urinary System Physical Exam

• A limited exam includes Costovertebral Angle (CVA) tenderness.
• A practitioner will place palm flat over CVA and strike with other fist, repeating bilaterally.
• Assess for pain.
• Pain is elicited due to inflammation of the renal capsule in patients with suspected renal colic or pyelonephritis.
• Palpation of the kidneys is extremely difficult to elicit findings and is often not performed in such an exam, thus the most evaluation stems from laboratory testing.
• Urinalysis in disorders block, will perform UA dipstick.
• This also includes Metabolic panels comprised of BUN (Blood Urea Nitrogen) and Cr (Creatinine).

Quiz Questions for Urinary System 101