Urine Formation Notes

Urine Formation

• Urea

• water

• uric acid

• creatine

• electrolytes

• small metabolites

Urine

• waste fluid produced by nephrons in the kidneys

Urea

• by product of protein metabolism

uric acid

 by product of nucleic acid metabolism

creatine

• by product of muscle activity

Nephron

• made of microscopic tubular structures

• surrounded by blood vessels

• located in both the cortex and medulla

Renal Corpuscle

• glomerulus

• bowman’s capsule

Renal tubules

• proximal tubules

• loop of Henle

• distal tubules

3 Main Steps

• glomerular filtration: initial filtration of blood plasma

• tubular reabsorption: blood capillaries reabsorbed important molecules

• tubular secretion: final filtration of blood and secretion of waste, drugs, and toxins

Afferent Arteriole (Glomerular filtration)

•  conveys blood into renal corpuscle

• larger than efferent arteriole

• changes size to control filtrate rate and pressure

Glomerulus (Glomerular filtration)

• coiled network of capillaries in corpuscule

• extremely permeable/ leaky

Efferent Arteriole (Glomerular filtration)

• conveys blood out of renal corpuscle

Bowman’s Capsule (Glomerular filtration)

• thin, epithelial capsule surrounding glomerulus

• captures filtrate to convey out of renal corpuscle

Glomerular filtration

• Hydrostatic (water) pressure in blood plasma forces substances into capsule.

• filtrate = solution filtered out of blood

Low Blood Pressure

Afferent Arteriole Widens

High Blood Pressure

Afferent Arteriole Narrows

What gets filtered

Ions, Water, Small Molecules are filtered

What does not get filtered

Cells and Large Proteins not filtered

Glomerular Filtration Rates

• 120 mL every minute

• 180 L every 24 hours

• entire blood plasma in circulation filtered 60 times a day

• most filtrate reabsorbed in renal tubules

Tubular Reabsorption

• process of returning biological important molecules back into blood

Tubular Secretion

• filtration of peritubular capillaries into renal tubules for excretion as urine

Happens in Renal Tubules

• proximal tubules

• nephron loop (loop of Henle)

• distal tubules

Peritubular capillaries

• blood vessels surrounding tubules

Collecting Duct

• transport urine from nephron

Tubular Reabsorption

• biologically important molecules are transported back into blood (peritubular capillaries)

  • water, glucose, amino acids, electrolytes

• ~99% are reabsorbed

Where Tubular Reabsorption Happens

• happens in all parts of renal tubules

• proximal tubule = most active area of reabsorption

Diabetes (Tubular Reabsorption)

• glucose present in urine

• too much in blood stream

• renal tubules CANNOT remove all glucose

Where Tubular Secretion Occurs

Happens in all part of renal tubules

Secretes from peritubular capillaries to renal tubules:

• Urea

• Hormones

• ions

• Drugs

What’s found in urine (Tubular Secretion)

• drugs and toxins found in urine

  • antibiotics and other prescription drugs

  • hormones

  • drugs: alcohol, marijuana, opiates, cocaine

Regulation of Urine Production

• ADH (anti-diuretic hormone) 

  • vasopressin

  • controls amount of water secreted in urine

  • hypothalamus: senses drop in blood volume and blood pressure

  • pituitary gland: secretes ADH into blood

Low ADH concentration

• distal tubule and collecting duct  impermeable to water 

• removes excess water

• diluted urine

High ADH Concentration levels

• distal tubules to collecting duct very permeable 

• water reabsorbed into capillaries 

• preserves water during dehydration

• concentrated urine

Diuretics

chemical that increases urine production.

Caffeine

• binds to surface of cells in nephron

  • increases blood flow into glomerulus

  • slows tubular reabsorption

Alcohol

• blocks ADH production by brain

  • causes excess urination

  • leads to dehydration and hangovers