Functions of the Kidney

• Regulation of extracellular fluids

• Concentrations of waste, salts, regulating pH, etc.

• Formation of urine

• Regulation of plasma volume (blood pressure)

Urine Characteristics

• The more solutes = darker urine

• Measured in specific gravity

• Measuring solutes:

• PURE water: 1.000

• Dilute urine: 1.001

• Concentrated urine: 1.035

Urine Characteristics

• Urine may become dilute due to:

• Excess drinking, diuretics, renal failure

• Urine may become concentrated due to:

• Inadequate fluid intake, fever, pyelonephritis

Abnormal solutes signal problems:

• Glucose • due to excess sugar intake

or diabetes • Proteins

• due to physical exertion, pregnancy, glomerulonephritis, hypertension

• Pus • urinary tract infection

• rbc's • bleeding in the urinary tract

• Bile pigments • liver disease (hepatitis)

Urinary System Anatomy

• Location, Size

• Hilum

• Renal capsule

• Adipose capsule

The Kidney Kidney in cross section

Kidney Blood Supply

• Renal artery ! many vessels branching and becoming smaller in diameter ! Afferent arterioles

• Efferent arterioles ! many vessels joining and becoming larger in diameter ! renal vein.

Ureters

Tubes which run from the hilum of the kidney to the posterior aspect of the bladder.

Is it just gravity that moves urine to the bladder?

Urinary Bladder

• Bladder consists of 3 layers of smooth muscle

• Mucosa is transitional epithelium.

• Empty bladder: 5 - 7.5 cm long

• Moderately full: 12.5 cm long and holds 500 ml

• Urinary bladder plasticity

Urethra

• Carries urine from the bladder to the outside of the body.

• Internal urethral sphincter: at bladder / urethra junction.

• External urethral sphincter: as urethra passes through pelvic floor

• Is this voluntary or involuntary?

urethral challenges...

Seriously...

• How much excretion?

• Obligatory water loss = 400 ml / day

• Maximum? What do you think?

• How is this regulated?

Urinary System II Renal Physiology

Meet the Nephron!

Nephron Anatomy• Functional unit of the

kidney

• Made of tubular elements and vascular elements interwoven together

• Fluid in the tubular elements is called FILTRATE, while fluid in the vascular elements is called BLOOD

• Afferent arteriole ! Glomerulus ! efferent arteriole

• is there something wrong here?

Nephron Anatomy: Vascular Elements

• Renal corpuscle

• Glomerulus + Bowman's capsule

• Proximal tubule

• Loop of Henle

• Distal tubule

• Collecting duct (not part of the nephron)

Nephron Anatomy: Tubular Elements

Nephron PhysiologyFiltration

• Movement of fluid from blood to nephron lumen

• Once in the lumen, what is it? Where is it?

• Specific or non-specific?

• What drives it?

• Why does the blood get filtered?

• Haven’t we done this before?

Filtration in the Nephron

Exchange is now from one tube to another!

so there’s another gradient in play

Glomerular Filtration Rate

• (GFR) = amount of fluid filtering into Bowman's capsule per unit time

• = 115 ml/ min in women and 125 ml/ min in men

• Influenced by:

• Net filtration pressure

• Available surface area

Regulation of GFR

• Extrinsic regulation: ANS

• Intrinsic regulation: Autoregulation

• Intrinsic regulation: Tubuloglomeruler feedback

• What does this tell you?

Reabsorption

• Returning fluid from the lumen to the blood

• 180L of plasma is filtered, but 99% of that (178.2L) is reabsorbed

• WHY?

Reabsorption may be:

• Active: Primary, secondary active transport

• Passive: diffusion, leak channels, facilitated diffusion carriers

Secretion

• Transporting substances from the blood to lumen

• Depends mostly on membrane transport systems

Excretion

• Urine is very different from the filtrate entering the nephron

• What is enriched? What is removed?

Journey Through the Tubulesand TWO BIG QUESTIONS:

How can the kidneys produce urine that is hypertonic to the blood being filtered?

and

How is urine volume / osmolarity regulated as hydration levels change?

How can solutes be concentrated?Doesn’t water always follow the solute?

Sunset Cliffs at low tide

Let’s Take a Trip…

Proximal Tubule

• Filtrate coming in is isoosmotic w/ plasma (300 mOsm)

• Ion reabsorption: pumps for Na+ and K+

• Cl- ions follow Na+ through channels

• Osmosis: water reabsorption

• So... is peritubular blood isoosmotic? What’s the point?

• Regulation here?

Loop of Henle

• Reabsorption of 20% of salts and water, regardless of hydration state

• Regulation here?

• Ascending limb:

• Na+ reabsorption via pumps; Cl- follows passively.

• What about water?

Loop of Henle

Walls of the ascending limb are not permeable to water.

Descending Limb

• Impermeable to salt

• Permeable to water; water is reabsorbed via osmosis out of the descending limb in response to the hypertonic tissue fluid

• Countercurrent multiplier effect