Lecture 15: Micturition and glomerular filtration-Exam 3 Flashcards
Function of nephrons
Get rid of waste materials such as urea, uric acid, creatinine and bilirubin
Regulate water and electrolyte balance
Regulate body fluid osmolarity
Regulate arterial pressure long term by excretion of variable amounts of sodium ions and water, short term by secretion of hormones and vasoactive factors such as renin
Regulate acid base balance by eliminating acids such as sulfuric acid and phosphoric acid and regulate body fluid buffers stores
Secretion, metabolism and excretion of hormones such as erythropoietin and active form of vitamin D
Gluconeogenesis
Processes that determine the rate at which different substances are excreted in the urine
Filtration: First step in urine formation
Reabsorption
Secretion
Urinary excretion rate
= Filtration rate-Reabsorption rate+ secretion rate
Component of the glomerular filtrate
Ions,glucose, water and ions
Concentration of most substances except for proteins is the same in the filtrate as it is in the plasma
Some small molecules not filtered due to being proteins bound
Filtration fraction
Fraction of renal plasma flow filtered
=GFR/Renal plasma flow
Layers of filtration barrier
Endothelium with fenestrae and negative charges
Basement membrane with collagen and proteoglycan fibers and strong negative charges
Podocytes with negative charges
Glomerular Filtration Rate (GFR)
Determined by balance of hydrostatic and colloid osmotic forces acting across capillaries membranes
GFR= 125 mL/min=180 mL/day
GFR= K1 x net filtration rate
K1= coefficient filtration rate
Coefficient filtration rate K1
Product of permeability and filtering surface area of capillaries
Increase of K1—> Increase of GFR
Decrease of K1 –> Decrease of GFR
Net filtration rate
=Pg-Pb-Pig+Pib
Pg=glomerular hydrostatic pressure=60 mm Hg
Pb=Bowman’s capsule hydrostatic pressure= 18 mm Hg
Pig=glomerular capillary colloid osmotic pressure= 32 mm Hg
Pib=Bowman’s capsule colloid osmotic pressure = 0 mm Hg
Diseases that lower coefficient filtration rate
Chronic uncontrolled hypertension
Diabetes mellitus
Minimal change nephropathy: loss of negative charges on basement membrane
Hydronephrosis: distention and dilation of renal pelvis and calyces
Factors that influence glomerular capillary colloid osmotic pressure
Arterial plasma colloid osmotic pressure
Filtration fraction: Increase causes increase glomerular capillary colloid osmotic pressure
Variables that determine glomerular hydrostatic pressure
Arterial pressure: increase causes increased Pg which increases GFR
Afferent arteriole resistance: increase causes decreased Pg which decreases GFR
Efferent arteriole resistance: Increase causes increased Pg which slightly increases GFR
Factors that determine renal blood flow
Blood flow in kidney 7 times more than blood flow in brain
Oxygen consumption in kidney 2 times more than oxygen consumption in brain
Much of oxygen consumption due to high rate of active sodium reabsorption
Tubular sodium reabsorption closely related to GFR and rate of sodium filtered
Renal blood flow
=(Renal artery pressure- Renal vein pressure)/Total vascular resistance
Nervous regulation of GFR
All blood vessels of kidney richly innervated by sympathetic system
Strong activation of renal sympathetic nerves constricts renal arterioles and decreases renal blood flow and GFR
Moderate sympathetic activation has little effect
Mechanisms controlling GFR consistency
Sympathetic
Hormones: Norepinephrine and epinephrine, angiotensin II, endothelial derived nitric oxide, prostanglandin and bradykinin
Effects of endothelin
Released by damage endothelial cells of kidney and other tissues
Contribute to vasoconstriction which reduces GFR
Contribute to hemostasis when blood vessel is severed
Plasma levels increase in certain diseases state associated with vascular injury ( toxemia of pregnancy, acute renal failure, chronic uremia)
Effects of angiotensin II
Preferentially constricts efferent arterioles
Formed usually in situations with decreased arterial pressure and/or volume depletion
Afferent arterioles protect against effects
Effects of nitric oxide
Derived from endothelial cells
Basic level helps maintain renal vasodilation
Effects of prostaglandin and bradykinin
Protect afferent arterioles against effects of sympathetic and angiotensin II
Autoregulation
Refers to relative constancy of GFR and renal blood flow
Primary function of autoregulation
Maintain a relatively constant GFR
Allow precise control of renal excretion of water and solutes
Prevent relatively changes in GFR and renal excretion that would otherwise occurs with changes in blood pressure
Tubuloglomerular feedback mechanisms
Afferent arterioles feedback mechanisms
Efferent arterioles feedback mechanisms
Juxtaglomerular (JG) complex feedback mechanisms
Macula densa in distal tubule
JG cells in afferent and efferent arterioles
Autoregulation feedback
⬇️AP->⬇️ Pg->⬇️ GFR-> slow flow rate in loop of Henle ->⬆️ reabsorption of NaCl in ascending limb->⬇️ NaCl in macula densa->⬇️ resistance in afferent arterioles(1)
⬇️ NaCl in macula densa->JG cells -> ⬆️ renin->⬆️ angiotensin II->⬆️resistance in efferent arterioles (2)
1 and 2–> ⬆️ glomerular hydrostatic pressure (Pg)–>⬆️GFR
Micturition reflex
Superimposed micturition complex appears as bladder fills
Sensory signals from bladder stretch receptors conducted to the sacral region of spinal cord via pelvic nerves and conducted reflexively back to bladder via parasympathetic nerves
Micturition stops as bladder is partially filled
Once initiated reflex is self regenerative and fatigue after a few seconds and bladder relaxes
As bladder filled reflex occurs more often and more powerful
When powerful enough reflex causes secondary reflex
Second reflex conducted to pudendal nerves and inhibit external urinary sphincter
Higher brain centers in pons inhibit micturition unless desired
when it is time to urinate, brain centers help micturition centers to facilitate initiation of reflex and also inhibit external urinary sphincter and urination occurs
