Kidneys Flashcards
What are the 4 main functions of the kidneys?
Regulation of body fluid volume
Regulation of body fluid composition
Excretion of metabolic waste & toxins
Endocrine functions
What are the 4 key processes of the kidneys?
Filtration
Reabsorption
Secretion
All leading to urine formation and excretion
Outline the microstructure/components of the kidneys
Numerous uriniferous tubules and associated blood vessels:
nephron + collecting duct:
renal corpuscle + PCT + loop of Henle + DCT + collecting duct
What are the components of the renal corpuscle and what are the components of the filtration barrier?
What does filtration barrier limit passage of?
Renal corpuscle = glomerulus + bowman’s capsule
Fenestrated glomerular capillary endothelium
Negatively charged basement membrane
Podocytes with interdigitating foot processes
Limits passage of substances based on size, charge and shape
Outline the blood supply to the kidneys
Renal artery > Segmental arteries > Interlobar arteries > Arcuate arteries > Interlobular arteries which give off afferent arterioles to renal corpuscle
Blood to the kidneys passes through 2 capillary beds…
High pressure glomerular capillaries for filtration
Low pressure peritubular capillaries for reabsorption and secretion
Describe bowman’s capsule structure and function
Filters blood to form initial filtrate
Double walled cup surrounding glomerular capillaries
Outer parietal layer, simple squamous cells
Inner visceral layer, podocytes (modified simple squamous)
What happens in the PCT? What epithelium?
H20, Na+, Cl-, amino acids and glucose reabsorption
Secretion of drugs and waste molecules
Simple cuboidal with microvilli brush border
What’s the role of the loop of Henle? Outline the 3 parts
Generate hyperosmolar interstitium in medulla to concentrate urine
Thin descending limb - H20 permeable
Thin ascending limb - H20 impermeable
Thick ascending limb - H20 impermeable and active reabsorption of Na+
What happens in the DCT? Epithelium?
Variable H20 permeability dependent on ADH
Active reabsorption of Na+ and other solutes
Secretion of K+ and H+
Simple cuboidal cells
What part of the nephron is the JGA close to?
What are the 3 components of the JGA?
Distal tubule
Macula densa cells
Lacis cells
Granular cells in afferent arteriole
What’s the role of the collecting duct? Epithelium?
Final site for urine processing - regulates degree of urine concentration
H20 permeability controlled by ADH
Simple columnar cells
What are the different body fluid compartments? How much of body weight is roughly fluid?
In 70kg male 42L, 60% bodyweight
Different compartments separated by semi permeable membranes
Intracellular fluid - 28L (in cells)
Extracellular fluid - 14L
Extracellular fluid has 2 compartments: interstitial fluid surrounding cells and plasma which is non-cellular component of blood
What are the main cations and anions in ECF and ICF?
ECF: NA+ and Cl-
ICF: K+ and PO4-
Define the glomerular filtration rate and what is it determined by?
GFR = the volume of filtrate formed by all the nephrons in both kidneys per unit time
Determined by: glomerular capillary filtration coefficient (Kf) and net filtration pressure (NFP)
GFR = Kf x NFP
What does GFR Kf (filtration coefficient) represent?
How will this alter GFR in disease states?
The surface area available for filtration and the hydraulic conductivity (permeability) of the barrier
Changes in Kf aren’t major part of physiological regulation by may be affected in disease processes
Reduced number of nephrons will reduce surface area or reduce permeability and therefore decrease GFR
Define NFP (net filtration pressure) in the kidneys
The sum of pressures acting across the filtration barrier (starling forces)
Sum of hydrostatic pressures and sum of colloid osmotic pressures, typical NFP is 10mmHg
What’s the significance of hydrostatic and colloid osmotic (oncotic pressures) within the kidneys?
Hydrostatic pressure is that exerted upon the walls of the kidneys (glomerular capillaries) from the fluid within
Osmotic pressure is that exerted by proteins in glomerulus, exerting pull to stop fluid moving across
Higher hydrostatic pressure to drive net movement of fluid out of glomerular capillaries into bowman’s capsule
Most physiological regulation changes hydrostatic pressure (PG) as it depends on: arterial pressure, afferent and efferent arteriole resistance
What’s the link between afferent and efferent arteriole resistances in determining GFR?
Afferent arteriole dilation and efferent arteriole constriction increases GFR
Efferent arteriole dilation and afferent arteriole constriction reduces GFR
What vasoactive substances can regulate GFR and to what effect?
Dilating afferent arteriole and increasing hydrostatic pressure and therefore GFR:
Angiotensin II, prostaglandins and ANP
Reducing hydrostatic pressure by constricting the afferent arteriole and therefore reducing GFR:
NA, adenosine, endothelin
Briefly outline the two mechanism of autoregulation which prevent large changes in renal excretion of water and solutes across a range of systemic blood pressures
Myotonic autoregulation - ability of smooth muscle in afferent arterioles to respond to changes in vessel circumference by contracting/relaxing by Ca2+ channels activated by stretch
Tubuloglomerular feedback- uses JGA macula densa cells to respond to changes in NaCl concentration of own tubule lumen to alter resistance of afferent arteriole
What clinical signs will renal dysfunction of the following lead to: Regulation of body fluid volume? Regulation of body fluid composition? Excretion of metabolic waste and toxins? Endocrine functions?
Hypertension/oedema
Electrolyte disorders
Acid base disorders
Uraemia
Drug toxicity
Anaemia
Renal bone disease
What investigations can be done for kidney disorders?
Urine - look at what’s being filtered/excreted
Blood - urea, creatinine, eGFR/GFR, Na+, K+
Imaging - ultrasound, CT, MRI, contrast studies
Biopsy
What are some indicators of renal decline?
Proteinuria - damage to filtration barrier, indicated by urine dipsticks
Haematuria
eGFR
Serum creatinine/urea
GFR is accepted as the best overall index of kidney function - how is it measured and what are the caveats?
Linked to surface area of body, age, sex and declines with increasing age
Hard to measure directly
Measured using renal clearance (volume of plasma from which a substance is completely cleared by kidneys per unit time)
For any substance that is only filtered (so fully enters urine) clearance = GFR
Can use inulin or radioisotopes but that’s technically difficult
Outline how creatinine clearance is used to calculate GFR
Creatinine is produced by the body, so easier to use than inulin/radioisotopes
Creatinine is breakdown of creatin (skeletal muscle component) as is freely filtered at glomerulus and not reabsorbed but a small amount is secreted meaning creatinine clearance will overestimate GFR = eGFR
Requires 24 hours urine collection so issues with compliance, time, reliability
What can you test in blood serum to assess kidney function? What would it indicate?
Serum urea and serum creatinine
If something that’s normally filtered by the kidneys builds up in the blood it indicates reduced GFR and therefore reduced renal function
Large proportion of AKI is drug induced. Why are the kidneys highly susceptible to drug induced damage?
Vascularity
Large surface area for binding/transport
Reabsorption of H20 from kidneys concentrates some drugs in the nephron
Main route of excretion for most drugs
Should consult BNF for drug dose adjustments in renal impairment or what other circumstances? Uses eGFR
Elderly patients >75 years
Toxic drugs
Patients of extreme weights
How does urine pH influence speed of drug excretion?
Most drugs are either weak acids or weak bases
In alkaline urine acidic drugs are more readily ionised, in acidic urine alkaline drugs are more readily ionised
Ionised substances are more soluble in water, so easier to be excreted by the kidneys
What is a diuretic and how do they work?
A substance that promotes the formation and excretion of urine, mainly by promoting renal excretion of sodium (natriuresis)
Why is it sometimes a good idea to combine loop diuretics or thiazides, with potassium sparing diuretics?
Eg co-amilofruse or co-amilozide
Prevents large losses of K+ but must beware of K+ retention
Loop diuretics increase amount of NaCl delivered to distal nephron
What is an ICIQ-IU form and what does it tell you?
Structured incontinence questionnaire - how much urine you leak, when and how much it bothers the patient
Allows you to establish the degree of urinary frequency and at night which can be indicative of underlying problems eg diabetes
What’s the definition of osmosis? What are osmoles?
Movement of water across a semi-permeable membrane from an area of low solute concentration to a region of high solute concentration
Osmoles = number of osmotically active particles in a solution
What’re the main osmotically active electrolytes in ECF and ICF?
ECF Na+
ICF K+
Where is EPO produced and what’s its role?
By interstitial cells in cortex and outer medulla
It’s a growth factor that stimulates production of RBC precursors in bone marrow (proerythroblasts from haemoatopoetic stem cells)
Released in hypoxic situations
What type of anaemia will kidney disease result in and why?
Normocytic normochromic anaemia
Reduced kidney function will reduce production/release of EPO for RBC production
What’s the role of the kidneys in vitamin D activation? What’s the implication of this in kidney disease?
Kidney produces enzyme 1a-OHase which converts 25(OH)D -> 1,25(OH)2D (calcidiol->calcitriol) the active form for absorption
Patients with renal failure = develop renal bone disease as unable to produce the enzyme and unable to absorb sufficient Ca2+ from diet
Why is reabsorption favoured in the peritubular capillaries (after blood leaves efferent arteriole)?
Lower hydrostatic pressure + higher colloid osmotic pressure = reabsorption, rather than filtration is favoured