The Urinary System

Function• Maintain the consistency of fluids in the body• Similar to a water purification plant

Facts

• Kidneys filter 200 liters of fluid every day• Removes metabolic waste, ions, toxins from

the bloodstream• Help regulate blood pressure and secretes

erythropoietin

Organs• Kidneys• Ureters• Urinary

bladder• Urethra

Kidney Location

• Superior lumbar region• T12 to L3• Receive some protection from the

lower ribs• Right kidney is crowded by liver so it

lies slightly lower than left kidney• Weighs 150g (0.33 lbs)• 12cm long, 6cm wide, 3cm thick

(about size of large bar of soap)

Internal Anatomy

• Three regions:– Cortex– Medulla• Contains renal

pyramids

– Pelvis• Collects urine to

send down ureter to bladder

Blood Supply• Under normal conditions, the renal arteries

deliver approximately ¼ of blood supply to the kidneys every minute

Nephrons• Each kidney contains over one million tiny

processing units called nephrons• They carry out the process of making

urine• Each consists of:

Glomerulus – mass of capillariesBowman’s capsule – structure that collects filtrate from glomerulus

Glomerulus

• Endothelium is very porous • Large amounts of solute-rich, protein

free fluid pass from the blood to the Bowman’s capsule• This fluid (or filtrate) is raw material

that is processed by the renal tubules to form urine

Renal Tubule• Filtrate leaves the glomerulus

and travels through a set of small tubes = tubules

• About 3 cm long• Three parts:

1) Proximal convoluted tubule (closer to glomerulus)

2) Loop of Henle3) Distal convoluted tubule

Collecting Ducts• Receive filtrate from many nephrons• Give renal pyramids striped appearance

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Quiz

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Kidney Physiology• Of the approximately 1000mL of blood that

passes through the glomeruli each minute, 650mL is plasma

• About of 120mL of plasma is forced into the renal tubules

• Equivalent to filtering entire blood plasma 60 times a day = 47 gallons

• Kidneys consume 20-25% of all oxygen used by body at rest

Filtrate vs. Urine

• Once filtrate has moved through the collecting ducts, it has lost most of its water, nutrients and essential ions

• What remains is called urine and contains mostly metabolic wastes

• About 1.5L of the 180L of filtrate leaves the body as urine

• Rest is returned to circulation

Urine Formation

Three processes:1) Glomerular

filtration2) Tubular

reabsorption3) Tubular secretion

Glomerular Filtration

• For the most part, filtration is a passive, nonselective process in which fluids and solutes are forced through a membrane by hydrostatic pressure

• Very efficient because of high permeability and high blood pressure

• 55mm Hg in contrast to other capillaries in body at 18mm Hg

Tubular Reabsorption

• Our total blood volume is filtered into the renal tubules about every 45 minutes

• If some was not reclaimed, all of our plasma would be drained away within an hour

• This reclaiming process is called tubular reabsorption

Tubular Reabsorption• All organic nutrients (glucose, amino acids) are completely

reabsorbed but the reabsorption of water and ions are regulated by hormones

• Which hormone regulates the reabsorption of water by adjusting the permeability of the collecting ducts? Antidiuretic hormone

• Reabsorption of: –Cations (Na+, K+, Mg2+, Ca2+)–Anions (HCO3

-, Cl-)

HCO3- is bicarbonate; where did that come from?

Small Intestine

Tubular Secretion• Disposing of certain drugs (penicillin, phenobarbital – seizures and

anxiety) • Eliminates urea– CO(NH2)2

– How body eliminates nitrogen• Eliminates uric acid– C5H4N4O3

– Uric acid is a product of the breakdown of purine nucleotides. Which ones are purines?

adenine (A) and guanine (G)– High blood concentrations of uric acid can lead to a type of

arthritis known as gout

Gout

GOUT

Formation of Dilute Urine

• Simple process because no water is absorbed in the renal tubules

Formation of Concentrated Urine

• Antidiuretic hormone (ADH) inhibits urine output by increasing reabsorption of water• During maximum ADH secretion, 99% of water

is reabsorbed and returned to blood

Diuretics

• Chemicals that increase urinary output– Alcohol inhibits ADH

release – Caffeine, drugs for

hypertension and edema from congestive heart failure inhibit sodium ion reabsorption which inhibits water reabsorption

Characteristics of Urine

• Generally clear and pale to deep yellow in color• Color results from the body’s destruction of

hemoglobin and creation of by-product bilirubin• More water = pale urine, and vice versa• Pink or brown urine can result from foods like

beets or rhubarb or due to bile pigments or blood in urine

• Cloudy urine may indicate infection of some part of urinary tract

Odor– Fresh urine has a slight odor– Urine that is allowed to stand

develops an ammonia odor due to bacteria

– Some foods or drugs can create unusual odor

– In persons with diabetes, urine can smell fruity when glucose is high

Characteristics of Urine

Chemical Composition• 95% of urine is water• 5% is solutes – Urea • breakdown of amino

acids– Creatinine• Helps regenerate ATP,

more in skeletal muscles– Uric acid• Breakdown of purines

Kidney Stones

• Caused by Ca, Mg, or uric acid salts crystallizing in kidney

• Most are smaller than 5mm so they pass without problems

• Larger crystals can obstruct ureter• Prevention includes adequate hydration• Treatment includes shock wave lithotripsy

(using ultrasound to break up stones)

Pyelonephritis

• Infection of entire kidney• In females, usually caused

by E. coli bacteria from anal area into urinary tract

• Severe cases can result in abscesses that cause kidney to fill with pus

• If left untreated, kidneys can be damaged

Renal Failure• Causes:–Repeated damage from infections–Physical injury to kidneys–Crush injury to skeletal muscles (releases myoglobin

that clogs renal tubules)–Heavy metal poisoning (Hg or Pb)–Organic solvent poisoning (paint thinner, acetone,

dry-cleaning fluid, etc.)– Inadequate blood supply (can occur in

arteriosclerosis)

Physiology of Renal Failure

• Filtrate formation decreases or stops• Nitrogen waste builds up• Blood pH becomes more acidic

Hemodialysis• Artificial kidney• Patient’s blood is passed through membrane tubing

that is permeable and wastes diffuse out of blood• Usually done 3-4 times per week and each session

takes 4-8 hours• Life threatening kidney failure only becomes

obvious after about 75% of renal function has been lost

• Only permanent solution is kidney transplant

Ureters

• Tubes that take urine from each kidney to bladder

• Each begins at L2• Stretching of ureters

causes peristalsis to push urine down

Urinary Bladder

• Smooth, collapsible, muscular sac that temporarily stores urine

• A moderately full bladder holds about 500mL of urine (2 cups)

Urethra• Thin walled muscular tube that drains

urine from the bladder to outside the body

• Has sphincters:Internal – involuntary sphincter keeps urethra closed when urine is not being passedExternal – voluntary sphincter that relaxes during voiding of urine

Urinary Tract Infections

• Overall, 40% of all women get UTIs –Urethra is only 3-4 cm long–Proximal to anal opening–Improper toilet habits–Sexual intercourse can cause vaginal

bacteria to move into urethra

Incontinence• Inability to control urination– Infants have not learned to control external sphincter– In older adults, the sphincter malfunctions

Crash Course

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