Ch. 14: The Urinary System

Something to Think About

• Functions of urinary system and anatomy of kidney

• Urine formation

• Renal function tests

• Urine storage and elimination

Functions and Structures• Excretory

– Filters wastes from bloodstream– water

• Endocrine– Renin– Erythropoietin

• Vitamin D3 metabolism

• Urine storage

• Kidneys, ureters, urinary bladder, urethra

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Nitrogenous Wastes

• Waste – any substance that is useless to the body or present in excess of the body’s needs

• Metabolic waste – waste substance produced by the body

• Urea formation– proteins amino acids NH2 removed forms

ammonia, liver converts to urea

• Uric acid– product of nucleic acid catabolism

• Creatinine– product of creatine phosphate catabolism

• Blood urea nitrogen (BUN) – expression of the level of nitrogenous waste in the blood

– normal concentration of blood urea is 10 – 20 mg/dl– azotemia – elevated BUN

• indicates renal insufficiency– uremia – syndrome of diarrhea, vomiting, dyspnea, and

cardiac arrhythmia stemming from the toxicity of nitrogenous waste

• treatment – hemodialysis or organ transplant

Figure 23.2

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H

H HN

O

NH2

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CH3

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CreatinineUric acid

UreaAmmonia

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HN

Kidney• Renal cortex

• Renal medulla

• Renal pelvis

• Functional unit– Nephron

• Renal plexusleechestherapy.com

Nephrons

• Afferent arteriole

• Efferent arteriole

• Two capillary beds– Glomerulus– Peritubular – Vasa recta

Nephron• PCT

• Descending limb

• Loop of Henle

• Ascending Limb

• DCT

Urine Formation

• Filtration

• Tubular reabsorption

• Tubular secretion

• Urine in collecting ducts

Glomerulus

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Glomerular Filtration• Higher pressure than other capillaries

– Blood hydrostatic pressure (BHP) 60 mm Hg

• Filtration membrane– Fenestrated endothelium– Basement membrane– Podocytes

• Filtration slits

• Filtrate (into glomerular capsule)– Essentially everything in plasma except proteins

Glomerular Filtration• Net filtration pressure

– 60 – 18 (cap. press.) – 32 (COP) = 10 mm Hg out

• Glomerular filtration rate (GFR)– 12.5 ml/min for every 1 mm Hg (Kf)– GFR = NFP x Kf

• Regulation– Renal autoregulation

• Myogenic mechanism• Tubuloglomerular feedback

– Juxtaglomerular apparatus

Renin-Angiotensin-Aldosterone Mechanism• Sympathetic stimulation

• Renin secreted by juxtaglomerular cells if BP drops dramatically

• Renin converts angiotensinogen, a blood protein, into angiotensin I

• In lungs and kidneys, angiotensin-converting enzyme (ACE) converts angiotensin I to angiotensin II, the active hormone– works in several ways to restore

fluid volume and BP

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Liver

Kidney

Kidney

Lungs

Hypothalamus

Renin

Aldosterone

Drop in bloodpressure

Angiotensinogen(453 amino acids long)

Angiotensin I(10 amino acids long)

Angiotensin-convertingenzyme (ACE)

Angiotensin II(8 amino acids long)

Cardiovascularsystem

Vasoconstriction

Thirst anddrinking

Elevated bloodpressure

Sodium andwater retention

Adrenalcortex

Figure 23.15

Tubular Reabsorption• Transepithelial process

– Luminal and basolateral membranes– Endothelium

• Paracellular route– Through tight junctions between cells

• Na+ reabsorption is key

• Most of the filtrate is reabsorbed at the PCT

Tubular Reabsorption

• Transport Maximum– Tm– Max rate

of reabsorbti-on for any solute

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Tubular Secretion

• Unneeded substances secreted into the filtrate– PCT, nephron loop

• Waste removal– Urea, uric acid, ammonia, K+, drugs

• Acid-base balance– H+ and bicarbonate ions

• What remains in collecting duct essentially urine

Nephron Loop

• Primary function is to generate a salinity gradient

• Functions in the formation of concentrated urine

• Cotransport of Na+, K+, an Cl- in thick segment– All pumped out basolateral membrane– K+ back into cell via Na-K pump, then out into tube– NaCl remains in ECF

23-17

Countercurrent Multiplier of Nephron Loop

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The more salt thatis pumped out of theascending limb, thesaltier the ECF is inthe renal medulla.

Na+

K+

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The saltier the fluid in theascending limb, the moresalt the tubule pumps intothe ECF.

The more water that leavesthe descending limb, thesaltier the fluid is thatremains in the tubule.

H2O

H2O

H2O

The higher the osmolarityof the ECF, the more waterleaves the descending limbby osmosis.

More salt is continuallyadded by the PCT.

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Figure 23.20

DCT and CD• More reabsorption

• Two cell types– Principal cells

• Salt and water balance• Hormone receptors

– Intercalated cells• Acid-base balance

• Hormones– Aldosterone, ANP, ADH, PTH

Maintenance of Osmolarity in Renal Medulla

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Medulla

Cortex

Nephron loop

Key

Collecting duct Vasa recta

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Active transport

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Osmolarity ofECF(mOsm/L)

Na+

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Na+

K+

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Na+

K+

Cl–H2O

H2O

H2O

H2O

Diffusion througha membrane channel

Figure 23.21

Urine Formation

• Countercurrent mechanism

• Dilute urine

• Concentrated urine– ADH

Composition and Properties of Urine• Urinalysis – the examination of the physical and chemical properties of

urine

• Appearance - clear, almost colorless to deep amber - yellow color due to urochrome pigment from breakdown of hemoglobin (RBCs) – other colors from foods, drugs or diseases

– cloudiness or blood could suggest urinary tract infection, trauma or stones– pyuria – pus in the urine– hematuria – blood in urine due to urinary tract infection, trauma, or kidney stones

• Odor - bacteria degrade urea to ammonia, some foods impart aroma

• Specific gravity - compared to distilled water• density of urine ranges from 1.001 -1.028

• Osmolarity - (blood = 300 mOsm/L) • ranges from 50 mOsm/L to 1,200 mOsm/L in dehydrated person

• pH - range: 4.5 to 8.2, usually 6.0 (mildly acidic)

• Chemical composition: 95% water, 5% solutes– normal to find - urea, NaCl, KCl, creatinine, uric acid, phosphates, sulfates, traces of

calcium, magnesium, and sometimes bicarbonate, urochrome and a trace of bilirubin– abnormal to find – glucose, free hemoglobin, albumin, ketones, bile pigments

Urine Volume

• normal volume for average adult - 1 to 2 L/day• polyuria - output in excess of 2 L/day• oliguria – output of less than 500 mL/day• anuria - 0 to 100 mL/day

– low output from kidney disease, dehydration, circulatory shock, prostate enlargement

– low urine output of less than 400 mL/day, the body cannot maintain a safe, low concentration of waste in the plasma

Diabetes• Diabetes – any metabolic disorder resulting in

chronic polyuria

• Four forms of diabetes– Diabetes mellitus type 1, type 2, and gestational diabetes

• high concentration of glucose in renal tubule• glucose opposes the osmotic reabsorption of water• more water passes in urine (osmotic diuresis)• glycosuria – glucose in the urine

– Diabetes insipidus• ADH hyposecretion causing not enough water to be reabsorbed

in the collecting duct• more water passes in urine

Diuretics• Diuretics – any chemical that increases urine volume

– Increase GFR• caffeine dilates the afferent arteriole

– Reduce tubular reabsorption of water• alcohol inhibits ADH secretion

– Act on nephron loop (loop diuretic) - inhibit Na+ - K+ - Cl- symport• impairs countercurrent multiplier reducing the osmotic gradient in the

renal medulla• collecting duct unable to reabsorb as much water as usual

• Commonly used to treat hypertension and congestive heart failure

Ureters, Bladder, Urethra• Ureters

– Transport urine to bladder– Peristalsis

• Urinary bladder– Urine storage– Distensible– Can store ~1L

• Urethra– Drains urine from body– Two sphincters

• Internal – smooth muscle• External – skeletal muscle

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Voiding Urine• Between acts of urination, the bladder is filling

– detrusor muscle relaxes– urethral sphincters are tightly closed

• accomplished by sympathetic pathway from upper lumbar spinal cord• postganglionic fibers travel through the hypogastric nerve to the detrusor

muscle (relax) and internal urethral sphincter (excite)– somatic motor fibers from upper sacral spinal cord through pudendal

nerve to supply the external sphincter give us voluntary control

• Micturition – the act of urinating

• Micturition reflex - spinal reflex that partly controls urination– Involuntary

• Voluntary control– Micturition center in pons

Voiding Urine – Micturition Reflex• urge to urinate usually arises at an inconvenient time

– one must suppress it– stretch receptors fatigue and stop firing

• as bladder tension increases– signals return with increasing frequency and persistence

• there are times when the bladder is not full enough to trigger the micturition reflex but one wishes to ‘go’ anyway– Valsalva maneuver used to compress bladder – excites stretch receptors early getting the reflex started

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Stretch receptors

From pons

Pelvic nerve

Urethra

S2

S3

S4

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Involuntary micturition reflex

Stretch receptors detect fillingof bladder, transmit afferentsignals to spinal cord.

Signals return to bladder fromspinal cord segments S2 and S3via parasympathetic fibers inpelvic nerve.

Efferent signals excitedetrusor muscle.

Efferent signals relax internalurethral sphincter. Urine isinvoluntarily voided if notinhibited by brain.

Voluntary control

For voluntary control, micturitioncenter in pons receives signalsfrom stretch receptors.

If it is timely to urinate,pons returns signals tospinal interneurons thatexcite detrusor and relaxinternal urethral sphincter.Urine is voided.

If it is untimely to urinate,signals from pons excitespinal interneurons thatkeep external urethralsphincter contracted. Urineis retained in bladder.

If it is timely to urinate, signalsfrom pons cease and externalurethral sphincter relaxes. Urineis voided.

Sacral segmentsof spinal cord

To pons

Motorfiber

Sensoryfiber

Fullurinary bladder

Para-sympatheticganglion inbladder wall

Somatic motor fiberof pudendal nerveExternal urethral

sphincter (voluntary)

Internal urethralsphincter (involuntary)

Motor fibers todetrusor muscle

Figure 23.24

Renal Insufficiency & Hemodialysis• Renal insufficiency – a state in which the kidneys cannot maintain homeostasis

due to extensive destruction of their nephrons

• Causes of nephron destruction– hypertension, chronic kidney infections, trauma, prolonged ischemia and hypoxia,

poisoning by heavy metals or solvents, blockage of renal tubules in transfusion reaction, atherosclerosis, or glomerulonephritis

• Nephrons can regenerate and restore kidney function after short-term injuries– others nephrons hypertrophy to compensate for lost kidney function

• Survival with one-third of one kidney possible

• When 75% of nephrons are lost and urine output of 30 mL/hr is insufficient (normal 50 -60 mL/hr) to maintain homeostasis– causes azotemia, acidosis, and uremia develops, also anemia

• Hemodialysis – procedure for artificially clearing wastes from the blood– wastes leave bloodstream and enter the dialysis fluid as blood flows through a semipermeable

cellophane tube; also removes excess body water