Ch. 17 Renal System (Day 1) Flashcards
Functions of the Kidneys (6)
- Regulation of extracellular fluid volume and BP
- Regulation of osmolarity
- Maintenance of ion balance
- Homeostatic regulation of pH
- Excretion of waste
- Production of hormones
Renal Structure
Gross anatomy: three clearly demarcated anatomical areas
Cortex constitutes the major portion of the kidney and receives a disproportionately higher percentage (90%) of the blood flow
Medulla (6-10%)
Papilla (1-2%)
Where does the nephron start? Where does it go?
Starts in cortex –> down to medulla –> back up to cortex –> repeats…
Overall path of renal circulation
Afferent arteriole –> Glomerulus (capillaries) –> Efferent article –> peritubular capillaries or vasa recta
Afferent vs. Efferent Arteriole
Afferent: going to glomerulus
Efferent: exiting glomerulus (to veins)
–Efferent = “E” for exit glomerulus
Renal Blood flow
22% of CO
Portal System: afferent articles –> glomerular bed –> efferent articles –> peritubular bed –> veins
Are tubular flow and blood flow the same, or are they 2 different things?
2 different things!!!!
Renal Filtration
Kidneys receive 22% of CO; unlike other organs, renal blood flow far exceeds metabolic demand
All blood flow is filtered through glomeruli; mechanisms that regulate renal blood flow (RBF) are closely linked to control of glomerular filtration rate (GFR)
(*Don’t worry about these absolute numbers, just get a general idea)
- Renal blood flow: 1100 mL/min
- Renal plasma flow: 605 mL/min (hematocrit: 45%)
- Glomerular filtration rate: 125 mL/min
- Filtration fraction: GFR/RPF (125/605 = 21%)
Higher GFR allows for precise control of fluid volume/composition and rapid removal of waste
Overall Renal Function
Formation of urine involves 3 basic processes:
- ultrafiltration of plasma by glomerulus
- reabsorption of water and solutes from ultra filtrate
- secretion of selected solutes into tubular fluid (active, requires energy)
Plasma filtered into tubular system
Need substances reabsorbed back into plasma, while wastes stay in, eventually excreted in urine
What cannot pass through the filter?
Proteins
Nephron Tubules and Associated Blood Vessels
- Glomerular (Bowman’s) capsule surrounds glomerulus. Together, they make up renal corpuscle
- Filtrate produced in renal corpuscle passes into the proximal convoluted tubule (PCT)
- Filtrate passes into descending and ascending limbs of Loop of Henle
- Filtrate passes into distal convoluted tubule (DCT)
- Finally, fluid passes into Collecting Duct
- Fluid is now urine and will drain into minor calyx
Glomerulus
Capillary bed; filtration
Proximal Tubule
Reabsorption (60%)
Secretion
Sodium and water similar
Loop of Henle
Descending limb is water permeable
Ascending limb is water impermeable
Distal Tubule/Collecting Duct
Water permeable in presence of ADH (vasopressin)
Equation of Secretion (*MEMORIZE)
Amount filtered (F) - Amount reabsorbed (R) + Amount secreted (S) = Amount of solute excreted (E)
F - R + S = E
Pressure Differences in Nephron
Determine net filtration vs absorption
Increase change in pressure in glomerulus favors filtration. Lower pressure in tubules, but still greater than that in interstitial, yet don’t see filtration - see net absorption
Gradient differences are important
Starling Forces
Pc = capillary hydrostatic pressure (pi)i = interstitial oncotic pressure (=0) (pi)c = capillary plasma oncotic pressure (plasma proteins) Pi = interstitial hydrostatic pressure
Net Filtration Pressure (NFP)
NFP = (Pc - Pi) - (pi(c) - pi(i)
So… NFP = Pc - Pi - pi(c)
NFP in Glomerulus
NFP = Pgc - Pbc - (pi)gc
gc = glomerular capillaries bc = Bowman's capsule
Net filtration from glomerular capillaries into Bowman’s capsule
–yields positive charge/number
NFP in Peritubular Capillaries
NFP = Ppc - Pi - (pi)pc
pc = peritubular capillaries i = interstitial fluid
Net absorption from interstitial space into particular capillaries
–yields negative charge/number
Glomerular vs Peritubular capillary pressure and processes
Glomerular:
- high capillary pressure
- process: filtration
Peritubular:
- low capillary pressure
- process: absorption
Glomerulus
Capillaries of glomerulus are fenestrated
–large pores allow water and solutes to leave but not blood cells and plasma proteins
Fluid entering the glomerular capsule called filtrate
Which has protein: filtrate or plasma?
Plasma has protein (stays in capillaries)
Filtrate has NO proteins
–if protein found in urine, it’s a problem
Glomerulus Filtration
Filters water and small solutes (ions, glucose, AAs, etc) while restricting passage of large molecules (proteins)
–proteins negatively charged, and so is basement membrane, so they can’t pass through b/c too large and b/c of charge repulsion
Basement membrane: primary barrier to large molecules, being both size and charge selective
Glomerular filtrate is similar to plasma in its composition, except no significant amount of protein
Filtrate passes through?
- Capillary fenestrae
- Glomerular basement membrane
- Visceral layer of glomerular capsule composed of cells (podocytes) w/ extensions called pedicles
Glomerular Filtration Rate (GFR)
Volume of glomerular filtrate formed per unit time - 125 mL/min or 180 L/day
Influenced by 2 factors:
- Net filtration pressure
- -> capillary hydrostatic pressure - capillary plasma oncotic pressure - hydrostatic pressure in Bowman’s capsule - Filtration coefficient
- -> surface area of glomerular capillaries available for filtration (most important aspect of filtration coefficient)
- -> permeability of interface between capillary and Bowman’s capsule
Glomerular Filtration Fraction (GFF)
Fraction of renal plasma flow that becomes glomerular filtrate
Normal plasma flow through both kidneys = 650 mL/min
Normal GFR = 125 mL/min
GFF = (125/650) * 100 = 19%
If filtration fraction goes up, filtering out more fluid (protein concentration also increases), higher oncotic pressure/concentration
If filtration fraction decreases, filtering out less fluid (protein concentration also decreases), lower oncotic pressure/concentration
Factors Affecting GFR
Kidneys can regulate hydrostatic pressure in glomerular and peritubular capillaries, thereby changing the rate of glomerular filtration and/or tubular reabsorption
Factor:
- -renal blood flow; effect: as increase RBF, increase GFR
- -constriction of afferent arterioles; effect: decrease GFR
- -constriction of efferent arterioles; effect: increase or decrease GFR
Effect of increasing renal blood flow on glomerular filtration
Increased flow rate increase Pgc and causes percentage increase in pi(gc)
Increase in GFR
Effect of constriction of AFFERENT arteriole on glomerular filtration
Constriction reduces inflow to glomerular capillaries leading to decreased flow and decreased Pgc
–decrease in GFR
Possible causes:
- Activation of renal sympathetic nerves, e.g. fight or flight, brain ischemia, severe hemorrhage
- Vasoconstrictor substances: norepinephrine, epinephrine, endothelin
Effect of constriction of EFFERENT arteriole on glomerular filtration
Constriction causes decreased flow through - fluid piles up which increases Pgc
As fluid filteres, get huge increase in (pi)gc which counteracts the increase in Pgc
–increase or decrease GFR
Cause: Angiotensin II
GFR Regulation
Normally, GFR is fairly constant over a significant range of arteriole blood pressures: auto regulatory response
Myogenic response:
- -intrinsic ability of vascular smooth muscle to respond to pressure changes
- -similar to auto regulation in other systemic arterioles
Tubuloglomerular feedback:
–paracrine control
Hormones and autonomic neurons:
- -by changing resistance in arterioles
- -by filtering filtration coefficient
Tubuloglomerular feedback
Macula densa cells sense change in flow rate and ion concentration in tubular fluid
Sympathetic Nerve Effects
- In a fight/flight reaction –> vasoconstriction of afferent arterioles
- Helps divert blood to heart and muscles
- Urine formation decreases to compensate for drop in BP
