Flashcards in Lecture 16: Micturition and Glomerular Filtration Deck (21):
Describe the Micturition Reflex
See Slide 4
* Superimposed micturition contractions begin to appear as bladder fills.
* Sensory signals from bladder stretch receptors:
- Conducted to sacral region of spinal cord via pelvic nerves
- Conducted reflexively back to bladder via parasympathetic nerves
* Reflex contractions relax spontaneously when bladder is only partially filled.
* Once initiated, the micturition is self-regenerative.
* The self-regenerative reflex fatigues after a few seconds and the bladder relaxes.
* As bladder continues to fill, micturition reflexes occur more often and are more powerful.
* When micturition reflex is powerful enough, it causes a second reflex:
* Passes through pudendal nerves to inhibit external sphincter.
* Higher brain centers (in pons) keep micturition partially inhibited except when micturition is desired.
* When it is time to urinate, the cortical centers can facilitate the sacral micturition centers to help initiate a micturition reflex and at the same time inhibit the external urinary sphincter so that urination can occur.
- See Slide 7-9
Describe the function of nephrons
* Get rid of waste materials:
- Urea, creatinine, uric acid, bilirubin
* Regulate water and electrolyte balance
* Regulate body fluid osmolarity
* Regulate arterial pressure:
- Long term:
-- Excrete variable amounts of sodium ion and water
- Short term:
-- Secrete hormones and vasoactive factors such as renin
* Regulate acid-base balance:
- Excrete acids and regulate body fluid buffer stores
- Eliminate sulfuric and phosphoric acids (from protein metabolism)
* Secretion, metabolism, and excretion of hormones:
- Active form of vitamin D 12
Describe the three processes that determine the rates at which different substances are excreted in the urine
Know the mathematical expression of the urinary excretion rate
Urinary Excretion Rate =
Filtration Rate ─ Reabsorpon Rate + Secretion Rate
- See Slide 14-17
Filtration is the first step in urine formation. Components of the glomerular filtrate:
Filtration fraction = GFR/Renal plasma flow
- Fraction of renal plasma flow that is filtered ≈ 0.2 (i.e., 20% of plasma flowing through kidney is filtered.)
- Concentration of most substances except for proteins is the same in the filtrate and the plasma.
- Some low-molecular weight substances are not freely filtered because they are partially bound to proteins.
See Slide 22
What are the three layers of the filtration barrier?
- With fenestrae and negative charges
* Basement membrane
- With collagen and proteoglycan fibers and strong negative charges
- With negative charges
Describe the glomeruler filtration rate (GFR)
* Determined by:
- Balance of hydrostatic and colloid osmotic forces acting across capillary membrane
- Capillary filtration coefficient
-- Product of permeability and filtering surface area of capillaries (K1)
8 GFR = 125 ml/min = 180 L/day
* Water has a filterability of 1.0.
* Albumin molecules (6 nm) are slightly smaller than the filtration pores (8 nm) but have negative charges.
1. What are some of the diseases that lower glomerular capillary filtration coefficient?
2. Define minimal change nephropathy
3. Define hydronephrosis
1. Chronic uncontrolled hypertension and diabetes mellitus
2. Loss of negative charges on the basement membrane
3. Distension and dilation of renal pelvis and calyces
What is the glomerular filtration rate equation?
GFR = K1x Net filtration pressure:
GFR = K1x (Pg− Pb− πg+ πb)
- Pg= glomerular hydrostatic pressure = 60 mm Hg
- Pb= Bowman’s capsule hydrostatic pressure = 18 mm Hg
- πg= glomerular capillary colloid osmotic pressure = 32 mm Hg
- πb= colloid osmotic pressure of Bowman’s capsule = 0
K1 = Capillary filtration coefficient
= Product of permeability and filtering surface area of capillaries
So GFR would equal 10.
K1 = GFR/Net filtration pressure
What does raising and lowering the K1 do to the GFR
- Raising K1 raises GFR
- Lowering K1 lowers GFR
* Factors that influence glomerular capillary colloid osmotic pressure:
- Arterial plasma colloid osmotic pressure
* Filtration fraction Factors that increase glomerular colloid osmotic pressure:
- Increasing filtration fraction
What are variables that determine glomerular hydrostatic pressure
* Arterial pressure:
- Increase → ↑PG→ ↑GFR
* Afferent arteriolar resistance:
- Increase → ↓PG→ ↓ GFR
* Efferent arteriolar resistance:
- Increase → ↑PG→ ↑ GFR (slightly)
List factors that determine renal blood flow
What is the renal blood flow formula?
* Kidneys have 7X the blood flow of the brain but only 2X the oxygen consumption of the brain.
* Much of the oxygen consumed by the kidneys is related to the high rate of active sodium reabsorption.
* Tubular sodium reabsorption is closely related to GFR and rate of sodium filtered.
Renal Blood Flow = (Renal artery pressure − Renal vein pressure)/(Total vascular resistance)
Describe nervous regulation of GFR
* All blood vessels of the kidney are richly innervated by sympathetic system.
* Strong activation of renal sympathetic nerves:
- Constrict renal arterioles
- Decrease renal blood flow and GFR
* Moderate sympathetic activation has little effect.
List mechanisms controlling GFR consistency
* Sympathetic system (see above)
- Norepinephrine and epinephrine (from adrenal medulla):
-- Parallel the sympathetic system
- Angiotensin II:
- Endothelial-derived NO
- Prostaglandins and bradykinin
Describe the role of endothelin in controlling GFR
* Released by damaged vascular endothelial cells of the kidneys and other tissues.
- May contribute to renal vasoconstriction leading to reduced GFR
- May contribute to hemostasis when a blood vessel is severed.
- Plasma levels increase in certain disease states associated with vascular injury:
-- Toxemia of pregnancy
-- Acute renal failure
-- Chronic uremia
Describe the role of Angiotensin II in controlling GFR
* Preferentially constricts efferent arterioles
* Formed usually in situations associated with decreased arterial pressure or volume depletion.
- Effects on the efferent arterioles will help to increase GFR
* Afferent arterioles seem to be protected against the effects of angiotensin II.
- Due to release of prostaglandins and nitric oxide which are vasodilators
Describe the role of nitric oxide in controlling GFR
- Derived from endothelial cells
- Basic level helps maintain renal vasodilation
Describe the role of bradykinin and prostaglandins in GFR
* Vasodilators that may offset effects of sympathetic and angiotensin II vasoconstrictor effects (esp. on afferent arterioles)
* Autoregulation refers to the relative constancy of GFR and renal blood flow.
* Primary function is to:
- Maintain a relatively constant GFR
- Allow precise control of renal excretion of water and solutes.
- Prevent relatively large changes in GFR and renal excretion that would otherwise occur with changes in blood pressure.
* Normal GFR = 180 L/day
* Tubular reabsorption = 178.5 L/day
* Normal daily fluid excretion = 1.5 L/day
Without autoregulation, a slight increase in blood pressure could increase GFR up to 225 L/day.
* This would increase urine flow to 46.5 L/day.
Describe the tuberoglomerular feedback system and the juxtaglomerular complex
* Tubuloglomerular feedback mechanism for autoregulation:
- Two components:
-- An afferent arteriolar feedback mechanism
-- An efferent arteriolar feedback mechanism
* Juxtaglomerular complex:
- Macula densain distal tubule
- Juxtaglomerular cells in afferent and efferent arterioles
- See Slide 42