Urinary System

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?

5 mm is the limit of stones which can pass on their own

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 Physiology

Filtration

• 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

Other Hormonal Effects