Transcript Chapter 28: Anatomy and Physiology of the Renal System

Chapter 28:
Anatomy and Physiology of the
Renal System
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Positioning of the Kidney in the Body
• Retroperitoneal positioning
– Behind and outside the peritoneal cavity
• Adrenal glands on top like a cap
– Removed in a nephrectomy
– Contain vital hormones
• Sit posteriorly about the level of the 12th thoracic to the
3rd lumbar
– Why there is lower back pain with kidney infection
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Circulation to the Kidney: A Very Vascular
Organ
• 25% of blood supply from heart
– Through descending thoracic aorta
– When blood flow diminishes, hormonal
mechanisms come into play to conserve sodium
and water
• Enters through the hilum (pelvis)
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Circulation to the Kidney (cont.)
• Renal artery subdivides into numerous afferent arterioles
• Afferent arterioles form an enclosed capillary network
– Tiny, thin-walled network is called the glomerulus
• Glomerulus filters the blood of byproducts of metabolism,
electrolytes and water
• Blood is returned via the inferior vena cava
• See Figure 28-3.
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The Nephron
• Functional unit of the kidney
– There are around 1 million in each kidney
– This is why a person can lose a kidney and still be
able to function.
• Each nephron has a glomerulus, Bowman’s capsule,
proximal convoluted tubule, loop of Henle, distal
convoluted tubule, and collection duct
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The Glomerulus: A Tough Working
Network
• An enclosed capillary network surrounded by Bowman’s
capsule
• Waste products that are small are filtered into a liquid
substrate called filtrate
– This eventually becomes urine
• Blood and protein cells are too large to filter out
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The Glomerulus (cont.)
• Peritubular capillaries surround the nephron
• Recycled material is returned to the blood through the
renal vein
• Renal vein flows back to the heart via the inferior vena
cava
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Question
What types of cells/products are too large for the
glomerulus to filter normally that are essential to return
to the blood?
A. Urea and nitrogen
B. Calcium and phosphorus
C. Protein and blood
D. Glucose and potassium
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Answer
C. Protein and blood
Rationale: Blood cells and protein particles are too large to
be filtered into the tiny network that makes up the
glomerulus. All of the other answers are reabsorbed and
reused under normal circumstances. When disease or
dysfunction occurs, glucose, blood, and protein may be
liberated into the urine as they pass through the
capillary network of the glomerulus.
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The Tubules
• Proximal convoluted tubule
• Loop of Henle
• Distal convoluted tubule
• Collecting duct
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Proximal Convoluted Tubule
• First portion of tubular system
• Absorbs mostly water and sodium
• Filtrate is hyperosmolar here
• Phosphate, bicarbonate, chloride, potassium, and calcium
are absorbed here
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Loop of Henle
• Descending limb
– Thinner and very permeable to water
– Loop diuretics like furosemide (Lasix) work here
• Ascending limb
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Distal Convoluted Tubule
• Sodium reabsorbed through active transport
• Hydrogen, potassium, and uric acid can be added to the
urine by tubular secretion
• Thiazide diuretics like hydrochlorothiazide (HCTZ) work
here to stop sodium reabsorbtion and promote urinary
excretion
• Antidiuretic hormone (ADH) works here and prevents
reabsorbtion of water
• H+ ions are excreted here in acidosis
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Collecting Ducts
• Usually no electrolyte or water change here
• Antidiuretic hormone (ADH) release works here to
reabsorb water if a person is stimulated by dehydration
or other fluid losses
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Effect of Blood Pressure on the
Glomerulus
• The glomerulus is a high-pressure filtration system.
• The glomerulus is sandwiched between the afferent and
efferent arterioles.
• The relationship between the pressure/blood coming into
the afferent arteriole has a strong regulatory result in
maintaining blood flow to the glomerulus. This process is
called autoregulation.
• Autoregulation tries to maintain a constant blood flow to
the nephron.
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Summary: What the Kidneys Do
• Produce urine
• Filter and excrete waste products
• Maintain water homeostasis and normal BP
• Control electrolyte balance
• Normalize acid-base balance
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Summary (cont.)
• Stimulate the production of red blood cells through
erythropoietin
• Activate vitamin D
• Secrete prostaglandins
• Active the growth hormone
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Urine Formation
• Filter 180 L of blood daily
• Filter entire blood contents about 60 to 70 times a day
• 99% of filtrate is reabsorbed and 1% is excreted
• Urine formation is done by:
– Glomerular filtration
– Plasma oncotic pressure
– Tubular resorption
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GFR: The Key to Determining the Quality
of Kidney Function
• GFR depends upon:
– Glomerular filtration
– Pressure in Bowman’s capsule
– Plasma oncotic pressure
• Maintained as long as mean arterial BP is between 80
and 100 mm Hg
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Question
To compensate for shock, the GFR holds on to sodium and
water.
A. True
B. False
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Answer
B. False
Rationale: The afferent arteriole can dilate and the efferent
arteriole can constrict to maintain autoregulation
normally. Autoregulatory mechanisms fail and other
hormonal factors maintain blood flow to the kidney, like
renin, aldosterone, and antidiuretic hormone.
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GFR Indicator: Serum Creatinine
• Creatinine is a waste product from energy use by muscle
cells
• Levels are normally constant (0.6 to 1.2 mg/dL)
• Smaller particle, so freely excreted
• Increased levels in the blood indicate renal dysfunction
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GFR: Creatinine Clearance
• Also reflects GFR
• Blood and urine sample drawn simultaneously
• Levels decrease with age
• Normally around 75 to 125 mL/min
• A level <100 mL/min indicates decreased urinary
function
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Bowman’s Capsule Pressure
• An increased pressure here will decrease filtration
• Increased pressure resists movement of water/solutes in
the capsule
• Example: Blockage of blood flow with cellular debris
(hemolysis from mismatched blood transfusion) can
cause backpressure, decreasing urinary output
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Plasma Oncotic Pressure
• Mainly controlled by plasma protein albumin
• When there is a decrease in albumin, there is a decrease
in filtration pressure, decrease in GFR, and decrease in
urine output
• Good to monitor albumin, especially in:
– Elderly (nutritional)
– Burn patients (loss through absence of skin)
– Liver patients (can’t produce)
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Hormonal Fluid Regulation by the Kidneys
• Antidiuretic hormone
• Renin-angiotensin system
• Aldosterone
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ADH: The Primary Controller Of
Extracellular Fluid
• Antidiuretic hormone (ADH)
• Secreted by posterior pituitary
• Involves receptors in hypothalamus
• Responsive to increased serum osmolality (concentration)
– Increases permeability of collecting tubule to water
– No electrolytes are involved, just water
• Negative feedback loop; once serum volume is increased, the
process stops
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Question
Which of the following factors can increase the excretion of
ADH in a patient?
A. Fluid overload
B. Decreased temperature
C. Dehydration
D. Calcium channel blockers
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Answer
D. Calcium channel blockers
Rationale: Increased ADH is caused by dehydration or lack
of fluid, not fluid overload. An increased temperature,
not decreased, will cause stimulation of ADH.
Medications like opiates, oral hypoglycemic agents,
some cancer medications, and beta-adrenergic blockers
can stimulate ADH release. Calcium channel blockers
don’t influence ADH.
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Renin-Angiotensin System
• Decrease in GFR
• Secretion of renin
• Renin converts angiotensin I (liver) to angiotensin II
(pulmonary capillary cells)
• Angiotension II is a strong vasoconstrictor that increases
systemic vascular resistance (SVR), increasing BP
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Aldosterone
• ADH stimulates the release of aldosterone in the adrenal
cortex
– In response to decrease blood volume
• Aldosterone increases sodium reabsorption in the distal
tubule
• This increases BP by promoting water and sodium
reabsorption
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The Electrolytes and Kidney Function
• Sodium
• Potassium
• Calcium
• Phosphorus
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Sodium
• Normal is 135 to 145 mEq/L
• Most abundant ECF cation (+ charge)
• Responsible for osmolality of extracellular fluid (ECF)
• Influenced by ADH and aldosterone
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Potassium
• Most abundant intracellular cation
• Responsible for cardiac and nerve electrical transmissions
– Opposite relationship to sodium
• Normal 3.5 to 5.0 mEq/L
• Aldosterone causes an increased K excretion and Na
retention
• Always check the K before giving non-K-sparing diuretics
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Calcium
• Major component in bones and teeth
• Normal is 8.5 to 10.5 mg/dL
• Strong role in:
– Blood clotting
– Muscle contraction
– Nerve impulse transmission
• Role of PTH (parathyroids) and calcitonin (thyroid gland)
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Phosphorus
• 85% bound in bones
• Assists in ATP and acid-base balance
• Regulated by PTH
• Opposite of calcium: when calcium levels are high,
phosphorus levels are low
– Reason why phosphorus is given to decrease high
calcium in renal failure
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Other Hormones
• Calcitrol
– Produced by renal interstitial cells
– Increases intestinal absorption of calcium
• Erythropoietin
– Needed to produce red blood cells
– Stimulated in bleeding, anemia, and hypoxemia
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