Renal Physiology (Day 1) Flashcards
Functions of the Kidneys
Regulation of extracellular fluid volume and blood pressure
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%).
Overall path of renal circulation
see slide
Renal flow
flow of filtrate AND flow of blood through capillaries
Renal Blood Flow
22% of CO
–portal system: afferent arterioles–> glomerular bed –> efferent arterioles -> peritubular bed –> veins
more blood flow going to kidneys than they need to do their job (due to renal filtration)
Bowman’s space
were fluid enters the kidneys
Renal Filtration
The kidneys receive 22% of cardiac output (1.1 L/min); unlike other organs, renal blood flow far exceeds metabolic demand.
All blood flow travels through glomeruli; so mechanisms that regulate RBF are closely linked to control of GFR.
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%
The high GFR allows for precise control of fluid volume/composition and rapid removal of waste.
Formation of urine involves three basic processes:
- ultrafiltration of plasma by the glomerulus
- ->except proteins (stay in blood) - reabsorption of water and solutes from the ultrafiltrate
- secretion of selected solutes into the tubular fluid (active, requires energy).
- -Plasma filtered into tubular system
- -Needed substances reabsorbed back into plasma, while wastes stay in, eventually excreted in urine
Nephron Tubules & Associated Blood Vessels
1) Glomerular (Bowman’s) capsule surrounds the glomerulus. Together, they make up the renal corpuscle.
2) Filtrate produced in renal corpuscle passes into the proximal convoluted tubule.
3) Filtrate passes into the descending and ascending limbs of the loop of Henle.
4) Filtrate passes into the distal convoluted tubule.
5) Finally, fluid passes into the collecting duct.
6) Fluid is now urine and will drain into a 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
amount filtered - amount reabsorbed + amount secreted = amount of solute excreted
Pressure differences in nephron determine net FILTRATION vs ABSORPTION
ΔP in glomerulus (45) favors filtration.
Lower P in tubules (13), but still > interstitium (net ΔP = 7), yet don’t see filtration—see net
absorption
–> all numbers refer to hydrostatic pressure
–to understand why, need to review Starling’s Law.
Starling Forces
Pc = capillary hydrostatic pressure πi = interstitial oncotic pressure (= 0) πc = capillary plasma oncotic pressure (plasma proteins) Pi = interstitial hydrostatic pressure
Net Filtration Pressure = Pc – Pi – πc
NFP in glomerulus
net filtration from glomerular capillaries into bowman’s space
NFP in peritubular capillaries
Negative net filtration pressure –>
Net absorption from interstitial space
into peritubular capillaries (proteins stay in resulting in higher hydrostatic pressure)
Favored Processes in two capillary beds of the nephron
glomerular: high capillary pressure = filtration
peritubular: low capillary pressure = absorption
Glomerulus
- Capillaries of the glomerulus are fenestrated.
- Large pores allow water and solutes to leave but not blood cells and plasma proteins.
- Fluid entering the glomerular capsule is called filtrate
- Filters water & small solutes (ions, glucose, amino acids, etc), while restricting the passage of large molecules (proteins)
- has basement membrane
filtrate vs. plasma
filtrate has NO proteins (stays in capillaries)
Filtrate passes through:
- -Capillary fenestrae
- -Glomerular basement membrane
- -Visceral layer of glomerular capsule composed of endothelial cells (podocytes) with extensions called pedicles
Glomerular Filtration Rate
Volume of glomerular filtrate formed per unit time – 125 mL/min or 180 L/day
Influenced by two 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
- Permeability of interface between the capillary and Bowman’s capsule
Glomerular Filtration Fraction
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) x 100 = 19%
- higher GFF = filtering out more fluid = higher concentration/plasma oncotic pressure
- lower GFF= low renal function = less fluid filtered out = lower concentration
Factors Affecting GFR
Kidneys can regulate hydrostatic pressure in glomerular
& peritubular capillaries, thereby changing the rate of
glomerular filtration and/or tubular reabsorption
- Renal Blood Flow:
- Construction of afferent article
- Constriction of efferent arterioles
Glomerular Filtration: Renal Blood Flow
-increased flow rate increases Pgc and causes lower percentage increase in πgc
–> increase in GFR
Glomerular Filtration: Constriction of Afferent Arteriole
– constriction reduces in flow 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
Glomerular Filtration: Constriction of Efferent Arteriole
– Constriction causes decreased flow-through – fluid piles up which increases Pgc
– As fluid filters, get huge increase in πgc which counteracts the increase in Pgc
–> ↑ or ↓ GFR
Cause: Angiotensin II
GFR Regulation
-normally, GFR is fairly constant over a significant range of arterial blood pressures: AUTOREGULATORY RESPONSE
Myogenic response:
- Intrinsic ability of vascular smooth muscle to respond to pressure changes
- Similar to autoregulation in other systemic arterioles
Tubuloglomerular feedback
-Paracrine control
Hormones and autonomic neurons
- By changing resistance in arterioles
- By altering the filtration coefficient
Tubuloglomerular Feedback
- macula densa cells sense changes in flow rate and ion concentrations in tubular fluid
1. nephron loops back on itself so the ascending limp of loop of hole passes between the afferent and efferent arterioles
2. macula dense cells sense distal tubule flow and release paracrine that affect afferent arteriole diameter
change in GFR –> sensed by macula dense –> feedback –> dilation or constriction of afferent/efferent arterioles
