Urinary System

Urinary System Functions

1) Regulate water & electrolyte balance

2) Regulate blood pH

3) Remove waste from blood

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Nephron = functional unit of the urinary system

Filtrate = in capsule

Tubular Fluid = in PCT to DCT

Urine = in collecting duct & beyond

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Cortical Nephrons = about 85%•Most reabsorption & secretion

Juxtamedulary Nephrons = 15%•Produce concentrated urine

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Urine Formation

1) Glomerular Filtration

2) Tubular Reabsorption

& Secretion

3) Water Reabsorption

(Conservation)

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Glomerular Filtration

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Filtration (Endothelial-Capsular) Membrane•Fenestrated Endothelium•Basement Membrane•Filtration Slits

Prevents passage of

cells & most proteins •Plasma = 7% protein•Filtrate = 0.03% protein

Ca++, Fe++, T4 hormone

Bound to proteins

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Blood hydrostatic pressure (BHP) is much higher than normal capillaries

About 60 mm Hg vs 15 mmHg

Because

Afferent arteriole (inlet)

is larger than

efferent arteriole (outlet)

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Capsular hydrostatic pressure (CP)

is about 18 mm Hg vs about 0 mm Hg

for interstitial fluid

Why?

Net Filtration Pressure (NFP)

Glomerular Filtration Rate (GFR) =

amount of filtrate formed by BOTH kidneys per minute

Normal is 12.5 ml for each 1 mm Hg NFP for men & 10.5 ml for women

For NFP = 10 mm Hg

GFR = 125 ml/min

or 180 L/day

What would happen if glomerular hydrostatic pressure dropped

to 50 mm Hg ?

What would be the effect on GFR?

GFR Control1) Autoregulation = self regulation, no

neural or endocrine

Myogenic Mechanism = smooth muscle contracts when stretched

How will smooth muscle of afferent arteriole react if BP decreases? If it increases?

What effect will dilation of afferent arteriole have on GFR? Effect of constriction?

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Juxtaglomerular cells =

Smooth muscle in afferent arteriole, that secretes renin

Macula densa = receptor cells in wall of DCT

Mesangial cells = specialized contractile cells that may also ??

Juxtaglomerular Apparatus

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GFR Control1) Autoregulation: Tubuloglomerular Feedback = Juxtaglomerular apparatus monitors fluid entering DCT and adjusts

GFR to maintain

homeostasis

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If GFR is too high macula densa detects the change & sends chemical signal to Juxtaglomerular cells, which constrict the afferent arteriole, and thereby reduce GFR

Juxtaglomerular Apparatus

GFR Control2) Sympathetic Nervous System

In response to strenuous exercise the sympathetic N.S. redirects blood to heart, brain and skeletal muscle, and away from organs like kidneys by constricting the afferent arterioles

What effect will this have on GFR?

GFR Control3) Renin-Angiotensin Mechanism

When BP drops, sympathetic N.S. triggers release of renin by juxtaglomerular cells

Renin converts a plasma protein, angiotensinogen into angiotensin I

Angiotensin-converting enzyme (ACE) from lungs & kidneys converts angiotensin I into angiotensin II, which is a hormone with many powerful effects

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Angiotensin II Effects

•Systemic Vasoconstriction

•Constricts afferent & efferent arterioles

•Stimulates NaCl & water reabsorption by kidneys

•Stimulates aldosterone release by adrenal cortex

•Stimulates antidiuretic hormone (ADH) release

•Stimulates thirst center in hypothalamus

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Urine Formation

1) Glomerular Filtration

2) Tubular Reabsorption

& Secretion

3) Water Reabsorption

(Conservation)

Proximal Convoluted Tubule (PCT)• Longest & most coiled of portion of tubule• Simple cuboidal epithelium with microvilli• Many mitochondria provide ATP for high

amount of active transport

Tubular Reabsorption = movement of water and solutes from tubular fluid to blood

Tubular Secretion = movement of substances from blood into tubular fluid

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PCT Sodium (Na+) reabsorption = 60 -70% of total through secondary active transport

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PCT Chloride (Cl-) reabsorption = 60 -70% of total mostly by following sodium (electrical attraction)

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PCT Bicarbonate (HCO3-) reabsorption? = most

of total returned to blood by “slight of hand trick” & carbonic anhydrase

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PCT Nutrient (glucose, amino acids) reabsorption = Normally 100% returned via cotransport with Na+

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PCT Nitrogenous waste reabsorption = About half of urea diffuses out of tubular fluid, uric acid diffuses out, NOT creatinine (too large)

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PCT Water reabsorption = 60 -70% of total through osmosis, basically by following solutes, called obligatory water reabsorption, (no control/choice)

Uptake by Peritubular Capillaries

Water = Osmosis

•Build up of interstitial H2O from PCT means greater hydrostatic pressure•Narrow efferent arteriole means low BHP in peritubular capillaries, about 8 mm Hg•Trapped proteins (not filtered with water) means higher colloidal osmotic pressure (COP)

Solutes = Solvent Drag

Water “drags” solutes into capillary

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Transport Maximum (Tm) = maximum rate of reabsorption that is reached when all carrier proteins are saturated

Transport maximum determines the renal threshold, or the plasma concentration at which a specific compound or ion will begin appearing in the urine

For example, renal threshold

for glucose is about 220 mg/dL

Untreated diabetes mellitus

may be 400 mg/dL

Nephron Loop

Thin, descending limb = water permeable

Reabsorbs about 15% of water, now about 80% of total

Thick, ascending limb = water impermeable

Reabsorbs about 25% of Na+ and Cl-, now about 90% of total

So tubular fluid becomes more dilute

Leaves excess Na+ and Cl- in peritubular fluid (interstitial fluid around tubules)

Osmolarity

Osmolarity = amount of dissolved particles in one liter of solution

Normal range of physiological osmolarity is measured in milliosmoles per liter (mOsm/L)

Blood plasma, interstitial fluid, and

intracellular fluid (cytosol) measure about 300 mOsm/L

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Countercurrent Multiplier= establishes salinity gradient

in renal medulla

Distal Convoluted Tubule (DCT) & Collecting Duct

Fluid arriving at DCT contains about 20% of water & 10% of salts (mostly NaCl)

Reabsorption of remaining water and salts is variable and under the control of hormones

This allows for the regulation of water and salt balances

Aldosterone = “salt-retaining hormone”

Secreted by adrenal cortex, triggered by;

• Low blood Na+

• Elevated K+

• Low BP Renin Angiotensin II

Principle cells with aldosterone receptors in

• Ascending limb of nephron loop

• Distal convoluted tubule

• Collecting duct (cortical part only)

Aldosterone binds nuclear receptors & activates transcription of gene for Na+/K+ pump

In 10 to 30 minutes effect of pumps at work in membrane is seen

Na+/K+ pump in tubular cell basal membrane

Moves Na+ out = Na+ reabsorptionMoves K+ in= K+ secretion

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Atrial Natriuretic Peptide (ANP)

Secreted by atrial myocardium in response to high blood pressure

Dilates afferent & constricts efferent arterioles• Inhibits renin and aldosterone secretion• Inhibits ADH secretion and action of ADH on

kidneys• Inhibits NaCl reabsorption by collecting ducts

Reduces blood volume

which reduces blood pressure

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Urine Formation

1) Glomerular Filtration

2) Tubular Reabsorption

& Secretion

3) Water Reabsorption

(Conservation)

Antidiuretic Hormone (ADH)

Secreted by posterior pituitary, triggered by;

Dehydration & rising blood osmolarity

Causes more water to be reabsorbed in collecting duct, so less water lost in urine

Relies on high osmolarity in extracellular fluid of inner medulla

Total = 1200 mOsm/L

• About 750 mOsm/L from NaCl

• About 450 from urea (papillary region)

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ADH causes cells in collecting duct to synthesize aquaporins (water-channel proteins) & install them in plasma membrane

Resulting in more water being able to pass through the tubule wall

What determines the highest osmolarity possible for urine?

Renal ClearanceRenal Clearance = volume of blood plasma from

which a particular waste is completely removed in one minute

Glomerular filtration of waste (125 ml/min)

PLUS amount added by tubular secreation

MINUS amount removed by tubular reabsorption

Renal Clearance

Inulin = 125 ml/min

Urea = 60 ml/min

Creatinine = 140 ml/min

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Conduction of Urine

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Conduction of Urine

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