renal physiology 1 Flashcards
what are the functions of the kidney
endocrine function, maintain balance of salt, water and pH, excrete waste products
what is the cardiac output, renal blood flow, urine flow
- Cardiac output = 5L/min
- Renal blood flow = 1L/min
- Urine flow = 1 ml/min
describe the renal blood supply
Renal artery ↓ Interlobar artery ↓ Arcuate artery ↓ Interlobular artery ↓ Afferent arteriole ↓ (Nephron) Glomerular Capillary Peritubular Capillary
where are the nephrons 2 capillary beds
glomerulus
peritubular area
what are the 2 sets of capillaries in the kidneys connected by
efferent arteriole
what is the renal corpuscle formed of
the glomerular tuft and the Bowman’s capsule
what do the peritubular capillaries join to form?
veins
what is the entire capillary covered in
podocytes
what percentage of the plasma flowing through the glomerulus filters into the bowman’s capsule
20%
what happens in the bowman’s space?
• The filtrate from the glomerulus collects in Bowman’s space before flowing into the proximal convoluted tubule
what part of the nephron is responsible for secretion and reabsorption
distal (tubule)
what are the 3 layers of the filtration barrier?
- Single-celled capillary endothelium
- Basement membrane (basal lamina)
- Single-celled epithelium lining of bowman’s capsule (podocytes with foot processes)
what molecules can pass through the filtration barrier ?
- Small molecules and ions up to 10kDa can pass freely e.g. glucose, uric acid, potassium, creatinine
- Larger molecules increasingly restricted
- Fixed negative charge in glomerular basement membrane repels negatively charged anions
what does crossing the filtration barrier depend upon?
• Pressure
o HYDROSTATIC PRESSURE constant along length of capillary
o OSMOTIC PRESSURE rises along length of capillary
• Size of the molecule
• Charge of the molecule
• Rate of blood flow
• Binding to plasma proteins e.g. calcium, hormones such as thyroxine
what do efferent arterioles do?
carry blood away from glomerulus then supply peritubular capillaries which supply proximal and distal convoluted tubules - also supply vasa recti which supplies the loop of Henle
• Both peritubular capillaries & vasa recti supply: water and solutes to be secreted into filtrate & blood to carry away water and solutes reabsorbed by the kidneys
what are the 2 types if nephron?
- 15% juxtamedullary – renal corpuscle lies in the part of the cortex closest to the cortical-medullary junction – reabsorption of water
- 85% cortical – renal corpuscles lie in the outer cortex and their loops of Henle do not penetrate deep
urinary protein
- Albumin has a molecular weight of around 66kDa but is negatively charged ∴ cannot easily pass into the tubule
- Filtered fluid is essentially protein-free
- Tamm Horsfall protein in urine produced by tubule
glomerular filtration rate (GFR) definition
the volume of fluid filtered from the glomeruli into Bowman’s space per unit time (minutes)
what is the GFR equation
GFR=Um x urine flow rate / Pm
what is the filtration coefficient a product of
the permeability of the filtration barrier and on the surface area available for filtration
what is GFR determined by
net filtration pressure, permeability of the corpuscular membranes
flow of the glomerular filtrate
- Glomerular capsule
- PCT
- Nephron loop
- DCT
- Collecting duct
- Papillary duct
- Minor calyx
- Major calyx
- Renal pelvis
- Ureter
- Urinary bladder
- Urethra
describe autoregulation
- Renal blood flow, capillary pressure and GFR maintained almost constant over systemic mean arterial pressure range 90-200 mmHg
- Occurs in denervated kidneys & in isolated perfused kidneys ∴ not dependent on nerve supply or on blood-borne substances
- Intrinsic property of vascular smooth muscle
- Pressure within afferent arteriole rises → stretches vessel wall →triggers contraction of smooth muscle → arteriolar constriction
- Prevents an increase in systemic arterial pressure from reaching the capillaries
- Reverse happens when systemic arterial pressure falls
Tubuloglomerular feedback
- GFR of individual nephron regulated by the rate at which filtered fluid reaches the distal tubule
- Cells of macula densa (distal tubule) detect NaCl arrival
- Macular densa cells release prostaglandins in response to reduced NaCl delivery
- This acts on granular cells, triggering renin release, activating the renin-angiotensin system
measurement of GFR
Calculated by measuring the excretion of a marker substance (M):
• Freely filtered (same concentration in blood and tubular fluid)
• Not secreted or absorbed in tubules
• Not metabolised
Amount excreted per minute = amount filtered per minute
measurement of GFR in clinical practice
- Normal GFR = 125ml/min
- Clinically, creatinine is used to estimate GFR
- Muscle metabolite; constant production
- Serum creatinine concentration will vary with muscle mass
- Freely filtered at the glomerulus
- Some additional secretion by the tubules
- Entire plasma volume is filtered around 60 times every 24 hours
filtration fraction equation
filtration fraction = GFR / renal plasma flow
what is renal clearance
The volume of plasma from which a substance is completely removed by the kidney per unit time (usually a minute)
clearance equation
urine conc x urine vol / plasma conc
examples of clearance values
- Urea 65ml/min – freely filtered then partially reabsorbed
- PAH (para-aminohippurate) 625ml/min – freely filtered and completely secreted
- Glucose 0ml/min – freely filtered and completely reabsorbed
what does it mean if a substance has a clearance lower than GFR (125mls/min)
then either it was reabsorbed from the tubular fluid or it is not freely filtered at the glomerulus (too large or bound to protein)
