Download The Urinary Physiology Chapter 17

The Urinary Physiology
Chapter 17
• Chapter Outline
• Structure of Nephron and Renal Corpuscle
• Structure of Kidney
• 3 Basic Renal Processes: Glomerular Filtration, Tubular Reabsorption, Tubular Secretion.
• Glomerular Filtration Rate (GFR) and its Regulation
• Regulation of Ion / Water Balance – Role of ADH, Role of Renin – Angiotensin-2
• Regulation of H+ Ion Balance – Acidosis / Alkalosis
• Diuretics – drugs that decrease Na+ absorption or increase urine output
• Kidney Disease – Uremia, Chronic Kidney Failure
• Hemodialysis – artificial removal of waste products and maintenance of electrolyte / acid balance.
• Functions of kidney
• Regulation of water, electrolyte and pH balance
• Removal of waste materials from blood into urine; this includes Nitrogenous wastes Urea, NH3, Uric
acid
• Removal of foreign chemicals, including drugs
• Gluconeogenesis: production of glucose from glycerol and amino acids
• Secretion of Hormones:
• Erythropoietin - ↑erythrocytes #
• Renin – promotes formation of Angiotensin
• 1,25-Dihydroxyvitamin D - ↑ calcium absorption
• Nephrons
• Nephron has renal corpuscle formed of Glomerulus (bunch of capillaries) and Bowman’s capsule
• Visceral layer of Capsule has Podocytes having filtration slits between their foot processes
• Bowman’s space is present between visceral and parietal layers.
• Bowman’s space  Proximal convoluted tubule  descending limb  thin part of Ascending limb
 thick part of Ascending limb  distal convoluted tubule  cortical collecting duct  medullary
collecting duct    pelvis  ureter
• Nephrons
Nephrons are units of kidney function
Cortical nephrons
Medullary Nephrons
1. Present in cortex
1. Present in cortex
2. No loop of Henle or very small one
2. No loop of Henle or very small one
3. Efferent arterioles form peritubular capillaries 3. Efferent arterioles form peritubular capillaries
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Filtration
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Takes place in renal corpuscles
Afferent Arteriole  glomerular capillaries  fenestrae (holes in wall)  basement membrane
Filtration Slits (between foot processes of Podocytes)  Bowman space  proximal tubule
Water and solutes move out due to blood pressure in capillaries
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• Kidney
Each kidney has 1 million Nephrons = functional units of kidney
Cortex is outer part having renal corpuscles
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Medulla is inner part having loop of Henle’s and collecting ducts
Medullary collecting ducts  medullary pyramid  minor calyx  major calyx  pelvis  ureter
• Juxtaglomerular Apparatus
Macula Densa are a patch of cells in thick ascending loop of Henle passing between afferent and
efferent arterioles.
At the same point, secretory Juxtaglomerular cells, lie in afferent arteriole
Macula Densa + Juxtaglomerular cells = Juxtaglomerular Apparatus = JGA.
JGA secretes Renin an enzyme.
• Glomerular Filtration Rate
Glomerular Filtration Rate = GFR
Glomerular capillary blood pressure = PGC = 60mm/Hg
Fluid pressure in Bowman’s space = PBS = 15mm/Hg
Osmotic pressure due to plasma protein = πGC= 29mm/Hg
GFR = PGC – PBS – πGC
GFR = 60 – (15 + 29) = 16mm/Hg
• Glomerular filtration refers to the movement of fluid and solutes from the glomerular
capillaries into Bowman’s space.
• Tubular secretion refers to the secretion of solutes from the peritubular capillaries into the
tubules.
• Tubular reabsorption refers to the movement of materials from the filtrate in the tubules into
the peritubular capillaries.
• Regulation of Water Balance
• Water absorption is coupled to Na+ absorption.
• More Na+ absorption means more water absorption.
• More water in blood means ↑ blood pressure; loss of water in large quantity means ↓ blood
pressure.
• Role of ADH = Vasopressin
• Vasopressin is a peptide hormone with rapid action
• It makes cells of collecting duct embed Aquaporin proteins in their luminal side. Collecting ducts
reabsorb a lot of water from urine to release small amount of hyperosmotic urine.
• In absence of vasopressin no aquaporins embedded and kidneys excrete large amount of
hypoosmotic urine = diuresis, a condition called Diabetes insipidus
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• The Countercurrent Multiplier System
Loop of Henle’s and Collecting ducts of juxtamedullary nephrons form the countercurrent
multiplier system
Vasa recta the long loops of capillaries around them form the 2nd component of this system.
Isoosmotic = 300mOsm; Countercurrent = fluid flows in opposite direction in ascending and
descending limbs of loop of Henle and Vasa recta; Concentration Multiplier = from cortex to
deep inside medulla the osmolarity of interstial fluid increases gradually from 100  300  600
 900  1200mOsm (4 X isoosmotic)
Vasa recta removes the water and ions absorbed from filtrate/urine and help to maintain
multiplier system
The system helps the kidneys to excrete Hypoerosmotic urine
• Renin-Angiotensin System
Liver  angiotensinogen in plasma
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JGA  renin (limiting factor)
Renin converts angiotensinogen angiotensin 1
ACE = Angiotensin Converting Enzyme lines endothelium of capillaries and converts angiotensin
1  angiotensin 2 (active hormone)
Angiotensin 2 stimulates adrenal cortex to secrete aldosterone; it also causes vasoconstriction;
both effects lead to ↑ blood pressure.
Salt depletion  ↑renin secretion  ↑ plasma renin  ↑ plasma Angiotensin 1  ↑ plasma
Angiotensin 2  ↑ Aldosterone secretion  ↑ plasma aldosterone  modifies gene
expression to increase Na+-K+ pumps, channels for Na+ and K+ ions ↑ Na+ absorption and ↑
water absorption  ↑ blood pressure
• Atrial Natriuretic Peptide
Natrium = Na = Sodium
Natriuretic = greater excretion of Na in urine
↑ blood volume expands atria and stimulate them to secrete ANP in blood
ANP
↑ Na excretion by inhibiting reabsorption of Na
↑ GFR that leads to greater excretion of Na
Inhibits aldosterone secretion
• Acid-Base Balance
Kidneys secrete H+ in filtrate to reabsorb HCO3CO2 + H2O  H2CO3  H+ + HCO3Kidneys generate bicarbonate ions by different means to regulate acid base balance
Kidneys and lungs help to maintain pH of interstial fluid and blood is around 7.4
H3PO4 H2PO4-  HPO4- 2 PO4-3 is the most important buffer to resist changes in pH.
H2CO3 H+ + HCO3-  CO3-2
• Acidosis / Alkalosis
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Acidosis is the increase of H concentration of arterial plasma.
Alkalosis is the decrease of H+ concentration of arterial plasma.
Respiratory Acidosis/ alkalosis is due to change in alveolar ventilation – change to eliminate CO2
Metabolic acidosis/alkalosis includes all other situations than respiratory reasons.
• Diuretics
Diruretics are drugs that decrease Na+ reabsorption; increase the volume of urine passed out;
decrease blood volume; decrease blood pressure.
Homeostasis attains stability of total-body sodium mass and extra cellular volume.
Diuretics are used to treat retention of salt/water by kidney, for example in congestive heart
failure.
Diuretic drugs are also used in hypertension.
• Kidney Diseases
Kidney Stones are accumulation of mineral deposits
Chronic Kidney Failure develops over the years and is asymptomatic. Untreated hypertension
and diabetes can cause it.
Kidney failure leads to Uremia.
Hemodialysis – patients are put on hemodialysis to remove excess metabolic wastes like urea
and restore electrolyte / acid balance.
13% Americans are suffering from it.
Accounts for 25% of total Medicare Expenditure.