Human Physiology: Urinary System Quiz

Urinary (Renal) System - Nephron Physiology and Urination

• The urinary system, also known as the renal system, is responsible for urine production and excretion.
• The nephron is the functional unit of the kidney.
• Nephron processes include filtration, reabsorption, secretion, and excretion.
• Excretion = Filtration - Reabsorption + Secretion

Nephron locations of water and ion reabsorption

• The concentration of ions in the cortex is consistent (~300 mmol/L).
• There is an ion concentration gradient in the medulla, with higher concentrations at the distal end (~1200 mmol/L), decreasing towards the top (~400 mmol/L).
• The gradient is actively maintained to maximize water reabsorption at specific nephron sections.

Nephron locations of water and ion reabsorption (continued)

• Salt, water, and glucose are passively reabsorbed in the cortex by the proximal convoluted tubule (PCT) (60-80%).
• The medullary part maximizes passive water reabsorption driven by the concentration gradient.
• The ascending and descending limbs of the loop of Henle play a vital role in creating the medulla's high osmolarity.
• Active ion transport in the ascending limb maintains the gradient.

Medullary Osmotic Gradient

• The medulla's high osmolarity plays a critical role in maximizing water reabsorption, supporting dehydration survival.
• The descending limb is permeable to water, not solutes.
• The ascending limb is impermeable to water, permeable to solutes.

Countercurrent Multiplication

• The nephron loops use a countercurrent multiplication mechanism.
• Tubular fluids flow in opposite directions creating a concentration gradient.
• This gradient maximizes water reabsorption, leading to concentrated urine.

Variable Reabsorption from the Loop of Henle - Countercurrent Multiplier

• Thin descending limb is permeable to water, impermeable to solutes, and solute concentration increases.
• Thick ascending limb is impermeable to water, selectively permeable to Na+ and Cl-, and solute concentration decreases.
• Active ion transport (Na+ and Cl−) in the thick ascending limb helps maintain the medullary osmotic gradient.

Nephron Loop - Descending vs Ascending Limb

• The descending limb is permeable to water but not to solutes; water moves from the tubular fluid to the peritubular fluid by osmosis.
• The ascending limb actively transports Na+ and Cl− out of the tubular fluid, maintaining the medullary osmotic gradient.

Putting it All Together - Counter Current Multiplier

• Most water and solutes absorbed from the PCT in the cortex are redistributed back to the circulation from afferent arterioles.
• Water absorbed in the medulla is via the descending limb and redistributed back to circulation via the vasa recta of the peritubular capillaries.
• Tubular fluid is concentrated because water is removed creating a concentrated urine.

Nephron loop - Concentrates Urine and Maximizes Water Reabsorption

• Water is variably reabsorbed along the distal convoluted tubule (DCT) and collecting duct in relation to blood pressure.
• The papillary region of the collecting duct helps maintain the medullary ion gradient.
• Typical urea concentration in the papillary duct is 450 mols/L.

Urine Volume and Concentration

• Urine volume and concentration can be regulated to manage blood pressure hormonally.
• Obligatory water reabsorption occurs in the PCT and descending limb.
• Facultative water reabsorption occurs in the DCT and collecting duct.

Urine volume without ADH (antidiuretic hormone)

• No water is reabsorbed in the DCT and collecting tubule.
• No facultative water reabsorption occurs.

Urine volume with ADH

• ADH (antidiuretic hormone) allows water channels (aquaporins) to form in the apical plasma membranes of DCT and collecting tubule cells.
• Water permeability of the last tubular segments is increased; increasing water reabsorption.

Normal Urine Volume and Concentration

• Normal urine volume is about 1200 mL/day.
• Normal urine has an osmotic concentration of 1000 mOsm/L.
• Values differ among individuals.
• Kidneys adapt their function to maintain homeostasis.

Renal Function

• Kidneys filter wastes from blood and maintain homeostasis.
• Impairment causes reduced urine production, elevated blood pressure, anemia, and central nervous system problems.

Chronic Renal Failure

• Kidney function deteriorates gradually.
• Progression can be slowed, but the condition is irreversible.
• Minimizing urine volume through restricted fluid, salt, and protein intake is a management technique.
• Acidosis is a common problem with renal failure.

Acute Renal Failure

• Kidneys may be impaired for weeks following rapid deterioration of function.
• It can be caused by exposure to toxic drugs, infection, renal ischemia, or blockage.

Renal Failure - Chronic Treatment: Dialysis

• Dialysis is a process of passive diffusion across a selectively permeable membrane.
• Hemodialysis uses an artificial membrane to mimic kidney function and regulate blood composition.
• Shunts connect blood vessels with the dialysis machine allowing efficient filtration of blood and regulation of solute concentrations.

Renal Failure - Transplant

• Kidney transplant is the only real cure for severe renal failure.
• Patient survival is typically >90% at two years.
• Close relatives are preferred donors.
• Immunosuppressive drugs are necessary to reduce transplant rejection.

Urinary tract (Post Kidney Functions)

• The urinary tract, including the ureters, bladder, and urethra, encompasses urine transport, storage, and elimination.
• Imaging with a pyelogram—an X-ray of the urinary tract—can visualize the tract after a radiopaque dye is administered intravenously.

Urinary tract Anatomy (Ureters)

• Paired muscular tubes, approximately 30 cm long, connecting the kidneys to the urinary bladder.
• Retroperitoneal and attached to the posterior abdominal wall.

Urinary tract Anatomy (Urinary Bladder)

• A hollow muscular organ that holds urine.
• Rugae (folds in the bladder lining) allow the bladder to expand.
• Ureteral orifices are slit-like structures in the bladder that prevent urine back-flow.

Urinary tract Anatomy (Urethra)

• The urethra extends from the urinary bladder to the body exterior.
• Different lengths and functions differentiate male and female urethras.

The ureters, urinary bladder, and urethra functions

• Ureters conduct urine from the kidneys to the bladder.
• The bladder stores urine until urination occurs.
• The urethra carries urine out of the body.

Conduction and storage of urine (Wall of the urinary bladder)

• Mucosa, submucosa, and muscularis layers constitute the urinary bladder wall.
• The muscularis layer is made of three layers of smooth muscle (collectively detrusor muscle).

Conduction and storage of urine (Wall of the urethra)

• The urethra's wall has a thick, elastic lamina propria (a layer of connective tissue).
• Longitudinal mucous membrane folds are present.
• Mucin-secreting cells are found within epithelial pockets.
• Lined with stratified epithelium.

Urinary reflexes coordinate Urine storage & Urination

• Micturation reflexes coordinate urine storage and urination.
• These reflexes involve afferent sensory and efferent motor responses involving two anatomical regions (spinal reflex and pontine storage center).
• The spinal reflex involves the stretch receptors, sympathetic stimulation to the bladder's detrusor muscle, and internal urethral sphincter contraction to hold urine.
• The pontine storage center, within the brain stem, controls conscious bladder emptying.

Clinical Module: Urinary disorders

• Changes in urine volume and appearance can indicate underlying urinary disorders.
• Polyuria is excessive urine production.
• Oliguria is reduced urine production (50-500 mL/day).
• Anuria is severely reduced urine production (<50 mL/day).
• Urine analysis can identify urinary disorders by assessing characteristics such as volume, appearance, and chemical composition.

Other signs of Urinary disorders

• Urinary disorders can present with varying symptoms, including changes in frequency (increased urgency or frequency), incontinence (difficulty controlling urination), pain in the superior pubic region or lumbar region, and urinary retention (difficulty urinating).

Test your knowledge on the urinary system and its complex functions with this engaging quiz. Explore topics such as the micturition reflex, urinary disorders, and nephron functions. Perfect for students of human physiology or anyone interested in understanding how our body manages waste.