Renal I PT 3

• Lecture on Renal structure and function
• The goal of the lecture is to understand how the nephron operates to produce concentrated and dilute urine
• It is also about understanding what role hormones play in this process

Medullary Osmotic Gradient

• Osmolality of interstitial fluid increases from 300 - 1200 mOsm from the cortex to the medulla

Osmolality Definition

• Osmolality details the concentration of solutes in a fluid

Osmolality Calculation

• Osmolality = Amount of solutes / Amount of fluid

Osmolality Examples

• High osmolality means high solute concentration and less fluid, resulting in concentrated urine
• Low osmolality means low solute concentration and more fluid, resulting in diluted urine

Countercurrent Multiplication

• Countercurrent multiplication establishes the body's medullary osmotic gradient
• Filtrate flows via countercurrent through two parallel sections of nephron loop
• The descending limb is permeable to water, but not salt, where only water leaves
• The ascending limb is impermeable to water but pumps out salt
• Filtrate is more concentrated as water leaves descending loop
• Filtrate is more diluted as sodium and chloride are pumped out of ascending loop

Urea Recycling

• Urea is a common waste product from breakdown of proteins that contributes to osmotic pressure
• Urea concentration increases along the nephron with high concentration in the collecting duct
• This urea diffuses out of the collecting duct into the medulla which contributes to the medullary osmotic gradient
• It is then recycled back into the nephron loop up to 6 times

Vasa Recta

• Vasa recta acts as a countercurrent exchanger, exchanging H2O and solutes
• As blood moves down, H2O will leave and solutes will enter vasa recta
• As blood moves up, solutes leave and H2O will enter the vasa recta, resulting in a 1:1 ratio
• There is no net loss or gain of water or solutes
• The medullary gradient is maintained

Osmotic Gradient Importance

• Nephron loops create gradient
• Urea recycling contributes to the gradient
• Vasa recta preserves and maintains the gradient
• Collecting ducts use & allows the kidneys to vary urine concentration dramatically
• The Kidneys regulate body water levels
• Without the gradient, urine would always be diluted and large in volume because the body would not he able to conserve H2O

ADH Concentrated Urine

• Antidiuretic Hormone (ADH) results in anti-urinating and less water, and is as a result of dehydration
• This results in an increase in the number of aquaporin water channels in collecting duct
• There is also increased H2O reabsorption from collective duct back into blood
• A small volume of concentrated urine is peed out
• The body is reabsorbing water

ADH Dilute Urine

• Antidiuretic Hormone (ADH) decreases with overhydration
• There are a lack of water channels in the collecting duct
• The H2O stays in the collecting duct and large volumes of dilute urine are created

ANP Dilute Urine

• Atrial Natriuretic Peptide (ANP) is released because of high blood pressure
• ANP is released from the atrium of the heart
• ANP suppresses ADH release
• ANP acts directly on the collecting duct to decrease Na+ which results in the decrease reabsorption of water
• This increased volume of dilute urine brings down blood volume and blood pressure

Micturition

• Micturition is urination
• This involves emptying the urinary bladder through the mechanism of action
• Micturition involves the contraction of the detrusor muscle which is parasympathetic
• This involves relaxation of the internal sphincter which is parasympathetic
• Also relaxation of the external sphincter which is somatic allows for conscious control

Diuretics

• Diuretics enhance urinary output
• Alcohol inhibits ADH
• Caffeine promotes renal vasodilation, which increases GFR
• Pharmaceuticals inhibit Na+ reabsorption