Urinary System: Nephron Loop and Interstitial Fluid

• Urinary System - Part 2

Interstitial Fluid Concentration Gradient and the Nephron Loop

• Descending limb is permeable to water but impermeable to salts.
• Water moves from the tubular fluid to the interstitial fluid.
• Salts are retained in the tubular fluid, which becomes more concentrated.
• Ascending limb is impermeable to water but permeable to salts, pumping salts out.
• The more concentrated the salts are, the more is pumped out at the beginning of the ascending limb.
• The less concentrated the salts are, the less is pumped out at the end of the ascending limb.

Countercurrent Exchange System

• The system helps maintain the concentration gradient in the nephron loop.
• Water diffuses out of vasa recta capillaries by osmosis
• Salt in the interstitial fluid enters the vasa recta via diffusion.
• Salt concentration increases in the vasa recta.
• The vasa recta runs along the descending limb of the nephron.
• Gradients are reversed, with salt diffusing out and water diffusing in.

Characteristics of Urine

• Urine is sterile unless contaminated with microbes in the kidney or urinary tract.
• Urine is composed of 95% water and 5% solutes. The solutes consist of salts, nitrogenous wastes, some hormones, drugs, and ketone bodies.
• Volume: An average of 1 to 2 liters of urine are produced per day. Variations depend on factors like fluid intake, blood pressure, temperature, diuretics, diabetes, and other fluid excretion.
• The normal pH range is between 4.5 and 8.0.
• Higher protein or wheat intake leads to a more acidic urine. A diet high in fruits and vegetables leads to less acidic urine.
• Urine pH is influenced by metabolism and infection.
• Specific gravity is slightly higher than water due to the presence of solutes.
• Urine color ranges from almost clear to dark yellow, depending on the concentration of urobilin.
• The normal smell of fresh urine is urinoid.

Urinary Tract

• The urinary tract consists of the ureters, urinary bladder, and urethra.

Ureters

• The ureters originate at the renal pelvis in each kidney.
• Muscularis layers contract to propel urine towards the urinary bladder.

Urinary Bladder

• The urinary bladder is an expandable, muscular urine reservoir.
• It has two ureter openings and one urethral opening.
• A triangular area called the trigone maintains the bladder's shape as it expands, acting as a funnel toward the urethra.
• Contains mucosal folds (rugae) to allow for greater distension.
• The bladder has three layers of muscularis, collectively called the Detrusor muscle.

Female and Male Urethra

• The female urethra is shorter than the male urethra.

Micturition Reflex

• The micturition (urination) reflex occurs when the volume of urine in the bladder is between 200 to 300 mL.
• Bladder distension activates baroreceptors, which activates bladder wall.
• Visceral sensory neurons are signaled by baroreceptors and stimulate the micturition center in the pons.
• The micturition center alters nerve signals down the spinal cord through pelvic splanchnic nerves.
• Parasympathetic stimulation causes the detrusor muscles to contract, leading to internal urethral relaxation.

Bladder Storage Reflex

• Continuous sympathetic stimulation: this causes detrusor relaxation to accommodate urine.
• Contraction of the internal urethral sphincter is stimulated so that urine is retained in the bladder.
• The external urethral sphincter is continuously stimulated by the pudendal nerve to remain contracted.