Modifications of both tubule and arteriole walls where they come into contact with each other o Modified tubular epithelium  macula densa o Modified arteriolar wall  specialized smooth muscle cells called granular cells ( aka JG cells) - Granular cells secrete enzyme renin o Involved in salt and water balance - Macula densa cells release paracrine factors that acts on smooth muscle of afferent arteriole Tubuloglomerular Feedback and Autoregulation of GFR 1. Increased GFR 2. Increased flow through tubules 3. Increased flow of NaCl past macula densa 4. Paracrine signal from macula densa acts on afferent arteriole 5. Afferent arteriole constricts  decrease renal blood flow, decreased Ph, decreased GFR

Endocrine and Autonomic Control of GFR Autonomic effects on GFR: - Via sympathetic innervation of afferent and efferent arterioles - Moderate symp activity has little effect on GFR o Extreme conditions (bleeding, severe dehydration)  Sharp drop in BP  symp vasoconstriction  decrease in GFR Endocrine effects: - Angiotensin II – potent vasoconstrictor - Prostaglandins – vasodilators o Both can also affect filtration coefficient through actions on podocytes (alters size of slits) and mesangial cells (shape of glomerular capillaries) 29 Effects of Sympathetic and Parasympathetic Discharge Effector Organ Parasympathetic Sympathetic pupil constricts dilates heart  rate  rate and force lungs bronchioles constrict bronchioles dilate digestive tract  motility & secretion  motility & secretion bladder release urine retain urine arterioles, veins - depends on receptor adrenal medulla - epinephrine secretion kidney -  renin secretion adipose tissue - fat break-down selected from Fig. 11-6 4B Renal Function Reabsorption - 180L/day filtered & 1.5L/day excreted - filtration removes foreign/toxic substances & endogenous material o high rate of filtrations clears thing faster

o default of filtered  excreted makes it easy to clean solutes without specific transport mechanisms Principles Governing Tubular Reabsorption - filtrate in capsule has same solute concentration as ICF - to reclaim solute, tubular cells must create gradients (chemical/electrical) by active transport - Pump Na out of tubule into interstitium o Anions follow o Water follows - Loss of water from tubule o Remaining solute in tubular fluid at high conc (K, Ca, urea) o These solutes move into interstitium (only if epithelial cells are permeable to it) 1. Na is reabsorbed by active transport 2. Electrochemical gradient drives anion reabsorption 3. Water moves by osmosis, following solute reabsorption. Conc of other solutes increase as fluid volume in lumen decreases 4. Permeable solutes reabsorbed by diffusion through membrane transporters of by paracellular pathway Different Ways of Crossing Epithelial Layer - Para-cellular pathway o Through cell-cell junctions - epithelial (trans-cellular) transport o crossing apical and basolateral membranes of epithelial cells o mechanism depends on driving force  down gradient – ‘leak’ channels or facilitated diffusion 