Kinesiology 132 Systems Physiology II PDF

Kinesiology 132 Systems Physiology II Urinary System – UR1 Today’s topics: Urinary System – overview Glomerular Filtration – process Glomerular Filtration Rate – renal autoregulation, neural regulation Urinary (UR) System – overview Focus of course: Glomerular filtration: moving from gl...

Kinesiology 132 Systems Physiology II Urinary System – UR1 Today’s topics: Urinary System – overview Glomerular Filtration – process Glomerular Filtration Rate – renal autoregulation, neural regulation Urinary (UR) System – overview Focus of course: Glomerular filtration: moving from glomerulus (blood) to capsular space (filtrate). Tubular reabsorption: moving from renal tubule (filtrate) to peritubular capillaries (blood). Tubular secretion: moving from peritubular capillaries (blood) to renal tubule (filtrate). Urinary excretion: elimination from Urinary body by a combination of above 3 excretion processes: Excreted = Filtered + Secreted – Reabsorbed Goals: regulation Homeostasis – water, ions, acidity, blood volume and pressure. Glomerular Filtration Principle of filtration – use pressure to move fluids/solutes through a membrane. Filtration creates filtrate (fluids/solutes leaving blood and entering capsular space/renal tubule). Filtration fraction: percentage (~20%). Glomerular filtration rate (GFR): volume per unit time (~125 mL/min or ~150 – 180 L/day). Glomerulus or glomerular capillaries: More efficient filter than other capillary beds (150-180 L/day glomerulus vs. 4 L/day systemic capillaries): Larger surface area. Very permeable (with pores; 45x “leakier” than typical systemic capillaries). Higher glomerulus blood pressure (~ 60 Glomerular Filtration Strong regulatory mechanisms to maintain a relatively constant GFR (homeostasis). mL/min Issues if: GFR too high Not enough time for reabsorption and too much goes to urinary excretion. Valuable fluid/solutes lost. GFR too low Nearly all reabsorbed and too little goes to urinary excretion. Some wastes, foreign substances, and excesses not adequately eliminated.  Glomerular Filtration – Starling forces 4 Starling forces: Blood hydrostatic pressure (PGC) ~60 mmHg Filtrate hydrostatic pressure (PCS) ~15 mmHg πCS Blood osmotic pressure (πGC) ~29 mmHg Filtrate osmotic pressure (π CS) ~0 mmHg PCS PG Favour filtration: PGC and πCS (glomerulus πCG to capsular space). Oppose filtration: PCS and πGC (capsular C space to glomerulus). Combined as a net filtration pressure (NFP): GLOMERULU NFP = (PGC + π CS) – (PCS + πGC) S In healthy, favour filtration (glomerulus to CAPSULAR capsular space) over entire glomerulus GFR – regulation – renal autoregulation NFP a strong force effecting GFR: Higher blood pressure – higher PGC – higher NFP – higher GFR. Lower blood pressure – lower PGC – lower NFP – lower GFR. Renal autoregulation: pressures between 80 – 180 mmHg Intrinsic mechanisms within kidneys to try and keep GFR within a certain tolerance most of the time despite what blood pressure is doing (homeostasis of GFR). 2 renal autoregulation processes – myogenic mechanism and tubuloglomerular feedback. Above normal GFR detected Renal autoregulation generates response Upper GFR tolerance Normal GFR Lower GFR tolerance Below normal GFR Renal autoregulation generates detected response Time GFR – myogenic mechanism regulation Responds in seconds. Stimulus: blood pressure change effecting and stretch of blood vessel (arteriole). Response: alter smooth muscle surrounding arteriole contraction / alters arteriole radius / alters / alters NFP / alters GFR. Vasoconstrict afferent arteriole and/or vasodilate efferent arteriole – decreases / decreases NFP – decreases GFR. Vasoconstrict efferent arteriole and/or vasodilate afferent arteriole – increases / increases NFP –increases GFR. GFR – myogenic mechanism regulation – example ↓ blood pressure Kidney: ↓ / decreased arteriole stretch detected If uncorrected: If corrected: ↓ / ↓ NFP / ↓ GFR Smooth Muscle surrounding arteriole: vasodilates afferent arteriole - and/or vasoconstricts efferent arteriole ↑ / ↑ NFP / ↑ GFR Negative feedback prevents the uncorrected change from occurring. GFR is maintained at the same level despite blood pressure change. GFR – Tubuloglomerular feedback mechanism regulation A little slower to respond as more steps than myogenic mechanism. Stimulus: blood pressure change effecting and salt concentration delivery at macula densa cells of juxtaglomerular apparatus (JGA) Response: JGA alters release of nitric oxide (NO) / alters afferent arteriole radius (more or less vasodilation) / alters / alters NFP / alters GFR. GFR – Tubuloglomerular feedback mechanism regulation – example ↑ blood pressure Kidney: ↑ If uncorrected: If corrected: ↑ / ↑ NFP / ↑ GFR Macula densa cells of JGA: Increased salt concentration delivery - If corrected: Afferent arteriole: less If corrected: vasodilation JGA: secretes less NO ↓ / ↓ NFP / ↓ GFR Negative feedback prevents the uncorrected change from occurring. GFR – regulation Over typical blood pressure range (80 – 180 mmHg) GFR is largely maintained by intrinsic renal autoregulation. Under some circumstances recruit extrinsic mechanisms outside kidneys to alter GFR. Involve neural and/or hormonal changes. Changes must be stronger than renal autoregulation can correct for. GFR – neural regulation Venous, atrial, and arterial blood pressure changes monitored by baroreceptors in these locations. If baroreceptors detect significant changes alter signal to vasomotor centre and get altered sympathetic signal level to kidney. Alters vasoconstriction of afferent arteriole leading to changes in GFR. GFR – neural regulation – example Dehydration / sweat losses ↓ Plasma Vasomotor Center volume Local ↓ Venous baroreceptors ↑ sympathetic pressure detect firing ↓ Venous return Kidney: ↑ afferent arteriole ↓ Atrial vasoconstriction pressure ↓ EDV / ↓ SV / ↓ ↓ / ↓ NFP / ↓ GFR ↓ Arterial Additional arterial response – altered arterial pressure pressure can also change GFR without involving altering renal sympathetic firing.