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Autoregulation Feedback
High GFR, Rapid flow of filtrate in renal tubules, Sensed by macula densa, Paracrine Secretion, Constriction of afferent arteriole, Reduced GFR
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Glomerular Blood Pressure: Sympathetic Regulation
- Sympathetic nervous system and adrenal epinephrine constrict afferent arterioles during exercise and when blood pressure drops.
- Reduces GFR and urine output
- Redirects blood to heart, brain, skeletal muscles
- In extreme athlete, it can lead to hypoxia in the kidneys, and proteinuria and hematuria
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Glomerular Blood Pressure: Endocrine Regulation
- Renin secreted by juxtaglomerular cells when blood pressure drops (systemic or local), renal artery blockage occurs, or the sympathetic nervous system is activated
- Renin converts angiotensinogen to angiotensin 1
- Lungs and kidneys convert angiotensin I to angiotensin II (with angiotensin-converting enzyme)
- Angiotensin affects constriction
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Angiotensin II
- Promotes vasoconstriction throughout the body
- Except: lowers blood pressure in peritubular capillaries to enhance reabsorption of NaCl and water
- Stimulates aldosterone and ADH secretion
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Tubular Reabsorption in the PCT
- Proximal convoluted tubule reabsorbs about 65% of glomerular filtrate
- Reabsorbs almost all glucose, amino acids, electrolytes through special channels and ion pumps
- Water is reabsorbed through aquaporins
- As water leaves, it pulls urea and electrolytes with it
- 40-60% of urea reabsorbed, creatinine remains in filtrate
- Most reabsorbed water and solutes diffuse into peritubular capillaries
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Glycosuria
- A limited number of solutes can be reabsorbed in tubules
- Limited number of transport proteins
- Blood glucose over 220 mg/dl exceeds maximum transport, so glucose appears in urine
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Tubular Secretion in the PCT
- Renal tube extracts chemicals from capillary blood and secretes them into tubular fluid
- Necessary because filtration doesn't remove everything from blood (and reabsorption adds some things back to blood)
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Waste Removal
- Urea, uric acid, bile acids, ammonia, catecholamines, prostaglandins, some creatinine
- Clears pollutants, morphine, penicillin, aspirin, other drugs
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Acid Base Balance
Secretion of H+ and bicarbonate regulates pH of body fluids
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Reabsorption in the Loop of Henle
- Loop of Henle generates salt gradient that helps collecting duct concentrate urine and conserve water
- Thick segment reabsorbs about 25% of the sodium, potassium, and chloride that we in glomerular filtrate
- Impermeable to water
- Thin segment reabsorbs about 15% of water that was in glomerular filtrate
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Reabsorption in the DCT and Collecting Duct
- Fluid arriving in distal convoluting tubule still contains 20% of water and 7% of salts from filtrate
- Amounts of water/salt reabsorbed regulated by hormones
- Aldosterone, ADH, parathyroid hormone, atrial natriuretic peptide
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Effects of Aldosterone On the DCT/CD
- Aldosterone secreted when blood Na+ concentration falls or blood pressure drops
- Stimulates reabsorption of NaCl and water
- Reduces urine volume
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Effects of ADH on the DCT/CD
- ADH secretion during dehydration
- Stimualtes reabsorption of water
- Reduces urine volume
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Effects of Parathyroid Hormone on the DCT
- Parathyroid hormone secreted when blood calcium levels are low
- Acts on thick segment and DCT to increase calcium reabsorption
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Effects of Atrial Natriuretic Peptide (ANP) on the DCT
- Secreted by atrial myocardium when blood pressure is high
- Dilates afferent arteriole and constricts efferent arteriole to increase GFR
- Inhibits renin, aldosterone, and ADH secretion
- Inhibits NaCl reabsorption
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Reabsorption and Secretion Review
- PCT reabsorbs 65% of filtrate and returns it to peritubular capillaries
- Nephron loop reabsorbs 25% of filtrate
- DCT reabsorbs NaCl and water
- Tubule extract (secrete) drugs, wastes from peritubular capillaries into tubular fluid
- Collecting duct conserves water
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Water Conservation
- Collecting duct (CD) reabsorbs water and concentrates urine up to 4 times
- Medullary part of CD is more permeable to water than NaCl
- Medulla fluid is very salty, so water leaves
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Countercurrent Multiplier of Loop of Henle
- Pumping ions out of the thick segment of the loop increases osmolarity of medulla
- Increased osmolarity of medulla causes lots of water conservation
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Countercurrent Exchange
- Need blood supply to medulla, but don't want blood to carry away salt
- Vasa recta exchanges salt and water to maintain osmolarity of medulla
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Hypotonic Urine
- Urine has a low concentration of solutes
- Typically from water diuresis (drinking lots of water)
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Hypertonic Urine
- Urine has high concentration of solutes
- Dehydration
- High blood osmolarity causes ADH release, more water is reabsorbed, urine more concentrated
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Urinalysis
- Provides a very good "snapshot" of the physiological state of the body
- Examination of physical and chemical properties of urine
- Appearance: clear to deep amber; yellow color due to breakdown of hemoglobin. Cloudiness, pus or blood suggests some kind of infection or trauma
- Odor: due to breakdown of urea. Abnormal odor can indicate diabetes mellitus, UTI, or other condition
- Specific gravity: reflects solute concentration
- Osmolarity: reflects hydration state of body
- pH: reflects pH regulation in tissues
- Chemical composition: 95% water, 5% solutes
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Urine Volume
Typically average adult produces 1-2 L/day
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Polyuria
- Excess of 2 L/day
- Symptom of diabetes
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Oliguria
- Less than 500 ml/day
- Can result from kidney disease, dehydration, prostate enlargement
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Anuria
- 0-100 ml/day
- Body can't get rid of waste
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Renal Insufficiency and Hemodialysis
- Kidneys can't maintain homeostasis because of nephron destruction
- When >75% of nephrons are lost
- Hemodialysis: procedure for clearing wastes from blood
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Diabetes
Any metabolic disorder resulting in chronic polyuria
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Diabetes Mellitus
- Type 1, type 2, and gestational
- High concentration of glucose in blood; opposes the reabsorption of water; glycosuria
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Diabetes Insipidus
ADH hyposecretion, so not enough water is reabsorbed in collecting duct; risk of dehydration
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Diuretics
- Any chemical that increases urine volume
- Some increase GFR
- Caffein dilates afferent arteriole
- Some reduce tubular reabsorption of water
- Alcohol inhibits ADH secretion
- Commonly used to treat hypertension
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Ureters
- Muscular tubes about 25 cm long
- Enter on posterior surface of bladder
- Valve prevents urine from going back into ureter when bladder contracts
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Adventitia
Connective tissue connecting ureter to surround tissue
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Muscularis
Smooth muscle that contracts in peristaltic waves
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Mucosa
Transitional epithelium that can contract and expand
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Urinary Bladder
Muscular sac on the floor of pelvic cavity, posterior to pubic symphysis
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Detrusor Muscle
Three layers of smooth muscle
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Rugae
Wrinkles in the mucosa of a relaxed bladder
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Trigone
Triangle formed by urethra and ureters
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Urinary Tract Infection (UTI)
- Cystitis: infection of urinary bladder
- Especially common in females due to short urethra
- Frequently triggered by sexual intercourse
- Can spread up ureter
- Pyelitis: infection of renal pelvis
- Pyelonephritis: infection that reaches nephrons
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Renal Calculus
- Kidney stones
- Hard granule of calcium phosphate, calcium oxalate, uric acid, or struvite
- Form in renal pelvis
- Large stones can block renal pelvis or ureter, which causes pressure buildup in kidney and destroys nephrons
- Causes: hypercalcemia, dehydration, pH imbalances, frequent UTIs, enlarged prostate
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Female Urethra
- 3-4 cm long
- Bound to anterior wall of vagina
- External urethral orifice between vaginal orifice and clitoris
- Internal urethral sphincter: detrusor muscle thickening that is involuntarily controlled
- External urethral sphincter: where urethra passes through pelvis floor, under voluntary control
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Male Urethra
- Approximately 18 cm long
- Prostate urethra passes through prostate
- Membranous urethra passes through floor of pelvic cavity
- Spongy (penile) urethra passes through penis
- Same two sphincters as female
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Voiding Urine
- As bladder fills, detrusor muscles relax, urethral sphincters closed
- Urination occurs when bladder contracts and both sphincters open
- Micturition: act of urinating
- Micturition Reflex: spinal reflex that partly controls urination
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