Lecture 15: Micturition and Glomerular Filtration Flashcards
Micturition Reflex
Contraction begins to appear as bladder fills; relax spontaneously when bladder is partially filled; self-regenerative - causes a 2nd reflex when powerful enough
Where do sensory signals from bladder stretch receptors conduct?
Sacral region of spinal cord via pelvic nerves and back to bladder via parasympathetic nerves
How do adults keep micturition inhibited (unless desired)?
Via higher brain centers in the pons
Describe the reflex process that occurs during urination
Cortical centers facilitate sacral micturition centers to initiate micturition reflex; simultaneously, the external urinary sphincter is inhibited so that urination can occur
What are the functions of nephrons?
Get rid of waste materials, regulate H2O/electrolyte balance, regulate body fluid osmolarity, regulate arterial pressure, regulate acid-base balance, secretion/metabolism/excretion of hormones, gluconeogenesis
What are the three processes that determine the rates at which different substances are excreted in the urine?
Filtration, Reabsorption, Secretion
Urinary excretion rate
Filtration rate - Reabsorption rate + Secretion rate
What is the first step in urine formation?
Filtration
Filtration fraction
GFR/Renal plasma flow
How much of the plasma flowing through the kidneys is filtered?
20%; Filtration fraction = 0.2
What two factors influence how substances are filtered?
Molecular size and charge; larger negative molecules are repelled
What are the three layers of the filtration barrier?
Endothelium (fenestrae, negative charges), Basement membrane (colagen, proteoglycan fibers, negative charges), Podocytes (filtration slits, negative charges)
List the 2 major determinants of Glomerular Filtration Rate
Balance of hydrostatic and colloid osmotic forces acting across capillary membranes (Starling Forces), Capillary filtration coefficient (product of permeability and filtering surface area of capillaries)
Average GFR
125 ml/min = 180 L/day
Capillary filtration coefficient
K1 = permeability x filtering surface area of capillaries; depends on leakiness of pores
Minimal change nephropathy
Loss of negative charges on the basement membrane
Hydronephrosis
Distension and dilation of renal pelvis and calyces
What diseases lower glomerular capillary filtration coefficient?
Chronic uncontrolled hypertension, diabetes mellitus
Express GFR in terms of Starling Forces
GFR = K1 x Net Filtration Pressure
How does K1 affect GFR?
Increasing K1 raises GFR, decreasing K1 lowers GFR
What effect does increasing arterial pressure have on hydrostatic pressure and GFR?
Increases glomerular hydrostatic pressure, Increases GFR
What effect does increasing afferent arteriolar resistance have on hydrostatic pressure and GFR?
Decreases glomerular hydrostatic pressure, Decreases GFR
What effect does increasing efferent arteriolar resistance have on hydrostatic pressure and GFR?
Increases glomerular hydrostatic pressure, Increases GFR
Factors that determine renal blood flow
O2 consumption related to high rate of sodium reabsorption, tubular sodium reabsorption (closely related to GFR)
Renal Blood Flow
(Renal artery pressure - Renal vein pressure)/Total vascular resistance
What effect does the sympathetic system have on GFR?
Strong activation: constricts renal arterioles, decreases renal blood flow and GFR; Moderate activation: little effect
Norepinephrine/epinephrine
Parallel the sympathetic system (constrict renal arterioles, decrease GFR)
Endothelin
Released by damaged vascular endothelial cells; may contribute to renal vasoconstriction and reduced GFR
Angiotensin II
Formed in situations associated with decreased arterial pressure/volume depletion. Preferentially constricts efferent arterioles, increases GFR; Afferent arterioles protected against effects
Nitric Oxide
Derived from endothelial cells; Maintains renal vasodilation
Prostaglandins and bradykinin
Vasodilators that may offset effects of sympathetic system and angiotensin
Autoregulation
Maintains relatively constant GFR, allows precise control of renal excretion of water/solutes, prevents large changes in GFR/excretion that would normally occur with BP changes
Normal daily fluid excretion
1.5 L/day
What are the two components of the tubuloglomerular feedback mechanism for autoregulation?
Afferent arteriolar feedback mechanism, efferent arteriolar feedback mechanism
JG complex
Macula densa in distal tubule, JG cells in afferent and efferent arterials (close together)
How does JG complex participate in autoregulation?
Lower GFR = slow flow rate in loop of Henle = more reabsorption of Na+ and Cl- and loweded [NaCl] at macula densa = dilated afferent arterioles = increased renin release from JG cells = increased angiotensin II = increased efferent arteriolar resistance = INCREASED GFR
