Chapter 26: Urine Formation I: Glomerular Filtration and Blood Flow (Discussion 1)

Question 1

Functions of Kidney (4)

Answer 1

Excretion of waste products/foreign chemicals Regulation of: H2O and electrolytes, fluid osmolarity, arterial pressure, acid-base balance Secretion, metabolism and excretion of hormones Gluconeogenesis

Question 2

Kidney Dynamic Range

Answer 2

Capable of increase or decreasing 10x for most electrolytes and water intake in 2-3 days Maintain near constant ECF/ICF fluid volume Maintain concentration of electrolyte

Question 3

Kidney Anatomy

Answer 3

Kidneys on posterior wall of abdomen outside peritoneal cavity

Renal artery and veins supplying blood (high amount of flow for size ~1.1L/min)

Urine transport

• Ureter: transports urine from renal pelvis (kidney) –> bladder
• Bladder: stores urine until excreted out through urethra

Question 4

Nephron (Definition and Structure)

Answer 4

Functional unit of the kidney (urine formation ~1.5L/day)

Complex of vasculature and tubules that exchange solutes and H2O: Filtration occurs from the glomerulus into the bowman’s capsule (BC)

~1 million in each kidney

After 40 y.o. lose about 10% per 10 years (remaining nephrons adapt and compensate)

Question 5

Nephron Blood Supply

Answer 5

Afferent arteriole –> glomerulus (capillary bed) –> efferent arteriole –> peritubular capillaries –> veins

Afferent and efferent arterioles have separate regulation of pressure

Glomerulus has high hydrostatic pressure for capillaries (~60mmHg)

Question 6

Nephron: Filtrate Path

Answer 6

Bowman’s Capsule –> Proximal Tubule –> Loop of Henle –> Distal Tubule –> Connecting Tubule –> Collecting Tubule –> Collecting Duct

Loop of Henle: Thin descending limb –> Thin ascending limb (part way) –> Thick ascending limb

Connecting tubules combine into a collecting tubule which combine into collecting ducts (each collects from ~4000 nephrons)

Question 7

Types of Nephrons

Answer 7

Cortical nephrons:

• glomerulus in outer cortex
• short loop of Henle

Juxtamedullary nephrons:

• Glomerulus deeper in cortex
• long loop of Henle(extends into medulla)
• Vasa recta (specialized peritubular capillaries)
• Important for formation of concentrated urine

Question 8

Urinary Excretion

Answer 8

Urinary Excretion rate (rate of solute excretion) = Filtration rate - Reabsorption rate + Secretion rate

Filtrate fluid is nearly identical to plasma w/o plasma proteins: Ca++ and Fatty acids mostly found bound to plasma proteins –> largely absent from filtrate

1. Filtration only: creatinine
2. Filtration + partial reabsorption: most electrolytes
3. Filtration + total reabsorption: amino acids/glucose
4. Filtration + secretion: acids/bases

Question 9

Glomerular Filtration Rate

Answer 9

Glomerular Filtration Rate (GFR): Amount of plasma that is filtered by glomeruli (~20% renal plasma flow)

Filtered through special capillary membrane (3 layers)

1) Endothelium: fenestrae (holes), neg. charge
2) Basement membrane: collagen/proteoglycan filaments, neg. charges
3) Epithelium: podocytes + slit pores, neg. charges

Question 10

GFR Equation

Answer 10

High GFR is beneficial:

1) Quick removal of waste products
2) Filtering entire body fluid many times/day (~180L/day)

Filterability depends on size (smaller) and charge (+ better)

GFR=Kf (PG –PBC –πG +πBC) G=glomerulus BC=Bowman’scapsule

Normal~GFR=125mL/min=12.5x(60–18–32–0)

Question 11

Factors that increase GFR

Answer 11

Increase P_G

• Increase P_a
• Decrease afferent arteriolar resistance
• Increase efferent arteriolar resistance (but larget increase in efferent arteriolar resistance will lead to an increase in plasma [protein] –> decrease in GFR)

Question 12

Factors that decrease GFR

Answer 12

Strong sympathetic stimulation –> vasoconstriction –> GFR (note: mild sympathetic stimulation will only have a minimal effect). Occurs in:

• Brain ischemia
• Defense reaction (fight/flight)
• Severe Hemorrhage

Question 13

Renal Vasoconstrictors

Answer 13

Increase afferent resistance, decrease GFR

• Epi and norepi
• Endothelin
• Angiotensin II preferentially constricts efferent arteriole –> increase GFR and increase in Na/H₂O reabsorption

Question 14

Renal Vasodilators

Answer 14

Renal blood flow, increase in GFR

• Nitric Oxide
• Prostaglandins (counteract constrictors)
• Bradykinin (counteract constrictors)

Question 15

Pressure diuresis/pressure natriuresis

Answer 15

Changes in renal excretion of H2O/Na+ due to change in arterial pressure

(H₂O and Na+ movement very closely linked throughout filtration and the rest of the kidney tubules)

Question 16

Autoregulation of GFR (Tubuloglomerular Feedback)

Answer 16

↓GFR –> ↓[NaCl] in distal tubule (more absorbed in loop) –> detected at macula densa cells (distal tubule), signals juxtaglomerular cells (aff/eff arterioles)

1. –> afferent dilate –> Increase in GFR
2. –> Release renin from juxtaglomerular cells –> Increase in Angiotensin II –> Angiotensin II –> constricts efferent arterioles –> Increase in GFR

Angiotensin-converting enzyme (ACE) inhibitors used to treat high blood pressure –> Increase in GFR

Question 17

Myogenic Mechanism

Answer 17

Myogenic Mechanism: ↑ Pa stretch arteriolar wall –> ↑Ca2+ influx to muscle –> contract

• Vasoconstriction increases resistance in afferent arteriole and glomerular hydrostatic reduces pressure and RBF
• Prevents excessive ↑in GFR/renal blood flow when Pa ↑

Question 18

Increased GFR/RBF from other factors

Answer 18

• High protein diet –> ↑ in amino acids in plasma –> AAs reabsorbed w/ Na+ –> ↓[Na+] at macula densa –> ↑GFR/renal blood flow
• Diabetes mellitus –> ↑ plasma glucose –> glucose reabsorbed w/ Na+ –> ↓[Na+] at macula densa –> ↑GFR/renal blood flow