Renal system 1 & 2 Flashcards
name the key features of the uraniry system
Kidneys
Renal Pelvis (into which urine is drained)
Ureters (carry urine to urinary bladder)
Bladder (stores urine)
Urethra (tube between bladder and external environment)
main function of kidney?
filtration
reabsorption
homeostasis
how much weight is the kidneys in the body?
1%
Yet receive about 25% of Cardiac Output
¼ of all ‘work’ performed by heart is to perfuse kidneys
smallest Functional unit = is the nephron
Approx 2.5 million
Each approx 5 cm in length about 50 µm in diameter
Approx 125 km of tubing (not including blood vessels)
Processes 180 L plasma/day
Only about 1.5 drops per nephron
Of which ~ 1% ends up as urine
About 600g of sodium reabsorbed per day
What are the Substances filtered and reabsorbed
- WATER Glomerular filtrate = 180-200 l/day approx
Urinary volume = 1.5 l/day approx (ie most of water filtered is reabsorbed)
- SODIUM CHLORIDE
Nearly all Na+ and Cl- is reabsorbed to maintain internal O.P. - GLUCOSE
Reabsorbed as long as plasma glucose < 200 mg/100ml (10 mM) - HYDROGEN IONS –
Normal diet generates H+ ions and kidney eliminates them. - UREA H2N - CO - NH2
End product of protein metabolism
2/3 of that filtered is passed out in urine
Remainder is reabsorbed - TOXIC SUBSTANCES
Body metabolites Drugs + Drug metabolites
Some of these may be actively secreted (removed)
High coinc of what does urine include?
Urea
Ureic acie
Creatinine
K+
Other substances that are toxic
rest reabsorbed
blood supply travels to the kidney and substance are filtered ____
out
artery divides =
interlobular arteries - where they come into contact with the nephron (incl. swealling = where blood supply interacts with nephron)
blood means afferent to efferent
afferent - towards the glomerulus
efferent - from something (out efferent arterial)
what two components does the nephron include?
vascular (Blood)
Tubular (filtered fluid)
Some of the nephron is in renal cortex
outer region
granular
Some of the nephron is in renal medulla
inner region,
made up of triangles (renal pyramids)
The Nephron: Vascular component
Interlobular artery > afferent arteriole (aa)
splits > the glomerulus (g)
a ball of capillaries
responsible for filtration
rejoin > efferent arteriole (ea)
(in other beds, a venule)
splits > peritubular capillaries
a second capillary bed!
supply renal tissue with blood
receives compounds reabsorbed by tubule
source of compounds secreted by tubule
The Nephron: Tubular component
composed of?
epithelial cells
The Nephron: Tubular component
Bowman’s capsule (BC)
encloses glomerulus
in cortex
where filtration occurs
Proximal Convoluted Tubule (PCT)
in cortex
responsible for most reabsorption/secretion
Loop of Henle (LH)
cortex/medulla
responsible for osmotic gradient in medulla
Distal Convoluted Tubule (DCT)
in cortex
‘fine-tuning’ of solute/water reabsorption
Collecting Tubules/Ducts (CD)
in cortex/medulla
‘fine-tuning’ of urine concentration
Vascular and Tubular nephrons
peritubular capillaries
Bowman’s capsule and
Juxta glomerular apparatus
JG cells in afferent arteriole
MD in DCT
JG cells and MD form JG apparatus
regulates blood pressure and the filtration rate of the glomerulus
podocytes
found in epithelium of BC and surround capillaries (filtration)
MD - densely staining cells of DCT
detect low Na+ in DCT and can alter flow through glomerulus and water volume (fine tuning)
JG cells (afferent arteriole) –
responsible for releasing hormones (Renin) that control how much water is reabsorbed (fine tuning)
Can also control filtration rate
Roles of Kidneys
Excretion of waste products (eg urea) / foreign compounds (eg drugs)
Long term control of blood pressure
Via regulation of H2O and electrolytes
Homeostatic regulation of pH/
Long term acid/base balance (H+ and bicarbonate HCO3)
Production of hormones
Control of RBC number via erythropoietin
Production of/conversion to active form of Vit D
Important to regulate H2O because of…
‘volume’ effects ↔ osmotic/hydrostatic forces
Circa 40 L H2O in ‘average’ body
fluid compartments mean
water can move between different compartments
what are the fluid compartment barriers
- cell membrane
Very thin
Little barrier to H2O movement
Barrier to solute movement - Capillary wall (Interstitial fluid ↔ plasma)
Thin
Variable barrier to H2O movement
Little barrier to solute movement (except protein)
fluid moevments (ml/min) = Dynamic equilibrium
Renal adjustment of plasma composition affects other compartments
Renal adjustment of plasma volume affects blood pressure
Link between H2O & solutes
Water regn through solute regn
‘Most important’ solute is Na+
Moved via Na-pump
Molecular mechanism
Energy-consuming (ie ATP)
‘Up’ concentration gradient
There is no equivalent molecular H2O pump
H2O only ever moves via osmotic & hydrostatic forces
ie pumping of heart or osmotic effects of Na-pump
What are the Implications for Renal Function?
1) Need to maintain solute/H2O balance in body
2) Filtration into nephrons
3) Reabsorption back into peritubular capillaries
renal function = 1) Need to maintain solute/H2O balance in body
Intake is ‘unpredictable’
Therefore need both hypertonic and hypotonic urine ( hypertonic sol - greater conc of solutes than another etc)
renal function = 2) Filtration into nephrons
125 ml/min (180 L/day, 60x plasma vol)
Protein-free plasma
Topologically on surface of body
renal function = 3) Reabsorption back into peritubular capillaries
Major task is to reabsorb most of 180 L/day
-(Secretion is important, but fairly minor in comparison)
Circa 99% fluid reabsorbed
-H2O only ever moves via osmotic & hydrostatic forces
-Renal relies on osmotic forces
Basic Renal Processes
a) filtration
b) secretion
c) reabsorption
Excreted = Filtered + Secreted - Reabsorbed
AA: Afferent arteriole
GC: Glomerular capillaries
EA: Efferent arteriole
PC: Peritubular capillaries
BC: Bowman’s capsule
T: Kidney Tubule
How is Urine Formed?
FILTRATION
Passive ultrafiltration at Bowmans Capsule
ACTIVE TRANSPORT (resorption and secretion)
Energy driven retrieval of valuable substances along nephron
OSMOSIS (resorption)
Mainly in Loop PCT+ Collecting duct
Recap: Renal Processes
write it out remember
Filtration (BC)
Mass movement of water and solutes from plasma to the renal tubule
Depends on Glomerular capillary pressure
Reabsorption
Movement of water and solutes from the tubule back into the plasma
Secretion
Secretion of additional substances into the tubular fluid
Excretion: Components of urine
amount excreted = amount filtered - amount reabsorbed + amount secreted
What is the structure of renal corpuscle?
combination of glomerulus and Bowman’s capsule
What happens in renal corpuscle?
Filtration takes place in the renal corpuscle
Epithelium around glomerular capillaries -modified into podocytes
Filtration
Movement a balance between osmotic/hydrostatic forces
Filtered substances pass through endothelial pores and filtration slits.
What are the barriers that are involved in filtration?
Glomerular capillary basement membrane
Basal lamina/membrane
Bowmans capsule epithelia (podocytes)
Filtering membrane:
1) Endothelial layer what are they?
flat cells - fenestrated cytoplasm
cytoplasmic pores (0.9u)
Filtering membrane:
2) Basement Membrane
Muco-polysaccharide 800A thick (only intact membrane in filter surface)
podocytes
highly specialized cells of the kidney glomerulus that wrap around capillaries and that neighbor cells of the Bowman’s capsule
split pores 70A
Pedicels
Tubule
(vol of fluid/ unit time) Filtration??
GFR high 125 ml/min (vol of fluid /unit time )
1/5 plasma filtered into BC (4/5 plasma, proteins, cells PTC)
Lumen of efferent arteriole smaller than afferent
Capillary Pressure Causes Filtration > net pressure and flow into ______
tubule
which components retained in plasma (NOT filtered)?
Cells: Fenestrations in capillary wall
Proteins: Basement lamina membrane ‘-’ charged, proteins also ‘-’charged –like charges repel
Size (> 7nm 40KDa excluded AND charge)
Hematuria
Red blood cells in urine
not Normal
Sign of damage (to barrier)
Maybe from outside or inside kidney
Hematuria - outside
Kidney stones, tumours (renal pelvis, ureter, urinary bladder, prostate, urethra)
UTI (inflammation of urinary bladder, urethra, prostate)
May also get WBC in urine
Hematuria - inside
Inflammation of glomeruli (eg glomeruli nephritis) – affects filtration
Infarct – necrosis of kidney
Proteinuria
Protein in urine
Very little protein found in the urine of healthy people
Tubular Re-absorption
180/day liters fluid filtered into the tubules
Only about 1.5/2 liters excreted
>99% of fluid entering the tubules must be reabsorbed into the blood
Most takes place in the LH (fluid)/
proximal tubule (solutes)
Some in the distal segments of nephrons (fine tuning)
Re-absorption may be Active or Passive
Water regn through solute (Na) regn
PCT: Reabsorption of Na+
- Movement of Na
- Facilliated diffusion (concentration gradient)
Active transport of Na into interstitial space
Na/k pump (k can diffuse out of cell) - Diffuse into PTCap
[role of ATP - active transport]
Reabsorption in the PCT
Reabsorption of most solutes is linked to the diffusion of Na +
Carrier molecules for other molecules that co-transported
Each carrier molecules binds specifically to that substances to be transported and to Na+
Move with Na + into tubule cell
As solutes are transported out of the lumen, through proximal convoluted tubule cells, into the interstitial fluid, water follows by osmosis
volume has been reduced by approximately 65%.
Reabsorption: Can Reach Saturation
Glucose co-transported with Na+
Uses specific transporters
Finite number of these
Tubular load glucose normally < transport maximum
All glucose reabsorbed – not excreted
But if excess load then not all glucose reabsorbed and some excreted
What is Secretion?
How is it efficient?
Transfer of molecules from extracellular fluid into the nephron
Metabolites produced in the body / substances brought into the body / or xenobiotics ( eg drugs)
Make excretion is even more efficient
Depends mostly on membrane transport systems
What is the Loop of Henle?
makes the centre of the kidney v concentrates
(bottom is the most concentrated = the hook [bachdro])
Loop of henle - breakdown
Cortical (80%) and Juxtamedullary (20%) Nephrons
Juxtamedullary nephrons: hypertonic medulla
Vasa recta: counter current
Long peritubular capillaries that dip into the medulla
Blood flow in the vasa recta moves in the opposite direction from filtrate flow in the loops of Henle
[hypertonic environment in medulla]
What does the loop of henle absorb?
Further resorption of NaCl and H2O by
COUNTERCURRENT MULTIPLIER
Descending Loop
Permeable to water
Ascending Loop
Impermeable to water
Actively transports Na+ into interstitial space followed by Cl-
Osmotic gradients in medulla
Descending limb: Filtrate becomes more concentrated as it loses water
Ascending limb: pumps out ions, filtrate
What is the ascending loop responsible for?
Multiply the concentrations of Na+ deep in the medulla
(hypertonic, high concentration of solutes)
What does the vasa recta do?
The blood in the vasa recta removes water leaving the loop of Henle
Fine-tuning’ urine osmolarity (DCT and CD)
Hormonal mechanisms
Renin-angiotensis-aldosterone (RAAS) mechanisms
Antidiuretic hormone (ADH) mechanism
Atrial naturetic peptide (ANP) mechanisms
RAAS and ANP: more sensitive to changes in BP
ADH: more sensitive to blood concentration
Fine-tuning’ under hormonal control = Low BP
Angiotensin (RAAS)
Macula Densa cells senses low Na in filtrate in dct
JG cells also detect reduced stretch in afferent arteriole
- Renin release from kidney (from JGC)
Converts angiotensinogen (found in plasma) into angiotensin I
ACE (pulmonary capillaries) converts angiotensin I into angiotensin II
Angiotensin II: vasoconstrictor
- Increases TPR
Stimulates release of aldosterone (and thirst)
Fine-tuning’ under hormonal control
Low BP
Aldosterone =
Steroid hormone / Released from adrenal cortex
Travels to DCT and CD
Bind to receptors
Stimulates Na+ reabsorption from CD into capillaries
Cl- co-transported
Water follows
Increases Blood volume
Increases BP
Fine-tuning’ under hormonal control: Low BP or increased solutes in blood
Normally dct and collecting ducts impermeable to water
What is ADH released by?
Released from posterior pituitary
Release sensitive to…
Osmoreceptors (hypothalamus) –
sense when body fluids become concentrated
Volume stretch receptors (right atrium) (decrease) which causes ________ osmolarity
HIGH or low blood pressure cause vasopressin (ADH) release
What happens if ADH is released?
Increases permeability of dct and collecting ducts to water
Increases reabsorption in collecting ducts
- Small volume concentrated urine
Atrial Natriuretic Peptide (ANP) Opposite effect: Na+ excretion increased
ANP Released from right atrium when blood volume increases
Leads to increased loss/excretion of Na+
Decreases Na+ reabsorption
- Na + remains in tubules
Water moves towards Na +
Increases Urine volume while reducing blood volume and BP
- Can inhibit ADH
Proteinuria
CHARGE, SIZE
Albumin slightly < 7nm (urine minute amounts)
Some protein hormones smaller but actively reabsorbed
V little protein found in the urine of healthy people
Haemoglobin smaller than albumin
V little passes from blood to filtrate
