Kidney and Glomerular Filtration Flashcards
What are the functions of the kidney?
- excretion of metabolic waste products and foreign chemicals
- regulation of water and electrolyte balances
- regulation of RBC production
- synthesis of vitamin D3
What are the 2 most important parts of the kidney?
cortex and medulla
What is the functional unit of the kidney?
the nephron
What does each nephron consist of?
- glomerulus
- tubule
What is the glomerulus formed by?
the invagination of a tuft of capillaries into the dilated blind end of the nephron (Bowman’s capsule)
What is Bowman’s capsule?
the cup-like structure formed when blood vessels form and invaginate in the tubule at the bottom of the basement membrane
What does the afferent arteriole do?
enter into the glomerulus and bring in blood before breaking off into capillaries that gather to form an efferent arteriole
What are the 2 cellular levels that separate the blood from the glomerular filtrate?
- capillary endothelium
- specialised epithelium of the capsule
What is the capillary endothelium surrounded by?
the glomerular basement membrane (GBM) and specialised cells called podocytes
What do podocytes do?
interdigitate to form filtration slits along the capillary walls
Where are mesangial cells located?
between the GBM and endothelium
What does the glomerulus open into?
the proximal convoluted tubule
What are the 7 components of the renal tubule?
- Bowman’s capsule
- proximal tubule
- descending limb of loop of Henle
- thin ascending limb of loop of Henle
- thick ascending limb of loop of Henle
- distal tubule
- collecting tubule
What is the juxtaglomerular complex?
where the efferent and afferent arterioles come together with the distal tubule
What does the juxtaglomerular complex consist of?
- macula densa cells in the initial portion of the distal tubule
- juxtaglomerular cells in the walls of the arterioles
What is the macula densa?
a specialised group of epithelial cells in the distal tubules that come in close contact with the afferent and efferent arterioles
What are functions of the juxtaglomerular complex?
- regulation of BP
- glomerular filtration
- sodium reabsorption
- synthesis of renin
What type of nephron is more rare and what do they do?
juxtamedullary nephrons that concentrate and dilute urine
What percentage of CO is renal blood flow?
~22% (1100 ml/min)
What are the 2 capillary beds of the renal circulation?
- glomerular capillaries
- peritubular capillaries
What is the GFR?
value
180L/day
What are the 4 degrees of filtration and absorption of the renal system?
- filtration only e.g. inulin
- filtration and partial reabsorption e.g. electrolytes
- filtration and complete reabsorption e.g. amino acids, glucose
- filtration and secretion e.g. certain drugs, creatinine
What appears in high and low volumes in the urine respectively?
- high = end products of metabolism e.g. urea, creatinine
- low = electrolytes e.g. sodium, chloride, bicarbonate ions
What is completely reabsorbed into the blood?
glucose and amino acids
How does the body respond to excess sodium?
- increased rate of sodium filtration
- reduced rate of sodium reabsorption
What is GFR?
the amount of plasma ultrafiltrate formed each minute
What is FF?
the fraction of renal plasma flow filtered across the glomerulus
What is the formula for FF?
FF = GFR/renal plasma flow
What are the 3 determinants of GFR?
- size of the capillary bed (surface area)
- permeability of the capillaries
- hydrostatic and osmotic pressure gradients across the capillary wall
What does the glomerular ultrafiltration coefficient (Kf) describe?
the ease (or difficulty) with which water migrates across the filtration membrane
What is the formula for Kf?
Kf = glomerular capillary permeability x filtration surface area
What does contraction of mesangial cells cause?
a reduction in the area available for filtration
What regulates mesangial contraction?
angiotensin II
What is the glomerular capillary permeable to?
- molecules less than 4nm
- positively charged non-proteins
What is albumin?
a negatively charged plasma protein repelled by the glomerular capillary despite its small size
What is glomerulonephritis?
a condition where immune cells attack and damage the glomerulus; the negative charge is lost and albumin can enter the urine
Which forces favour filtration?
- glomerular capillary BP
- net filtration pressure
Which forces oppose filtration?
- plasma-colloid osmotic pressure
- Bowman’s capsule hydrostatic pressure
What must happen for substrates to filter into Bowman’s capsule?
the pressure in the capillaries must be higher than in the glomerulus
What is hydrostatic and colloid osmotic pressure generated by respectively?
- hydrostatic = pressure of fluids
- colloid osmotic = proteins attracting water
What reduces filtration surface area?
- ageing
- diabetes
- renal diseases
What do kidney stones cause?
the Bowman capsule pressure to increase and oppose the filtration forces
What are the effects of increased sympathetic activity on GFR?
- ↑ sympathetic activity e.g vasoconstrictor hormones
- afferent arteriolar constriction
- ↓ renal blood flow
- ↓ hydrostatic pressure
- ↓ GFR
What are the effects of increased angiotensin II on GFR?
efferent arteriole constriction which increases GFR
How is GFR maintained at low BP?
angiotensin II constricts the efferent arterioles
What is tubuloglomerular feedback?
a special feedback mechanism that links changes in the sodium chloride concentration at macula densa with the control of renal arteriolar resistance and autoregulation of GFR
How is blood flow increased to the kidney?
the afferent arteriole dilates to decrease resistance
What is plasma clearance?
the volume of plasma necessary to supply the amount of substance excreted in the urine per unit of time
What is the formula for clearance rate?
clearance rate = (concentration in urine x urine flow)/concentration in plasma
What is the formula for urine flow?
urine flow = urine volume/time
When is the rate of excretion in urine equal to GFR?
when a substance is freely filtered and is not reabsorbed or secreted by the renal tubules
How is GFR measured?
using the excretion of substances that are non-toxic or metabolised by the body e.g. inulin or creatinine
How can an approximation of changes in GFR be obtained?
by measuring the plasma creatinine concentration (PCr)
What is the relationship between PCr and GFR?
inversely proportional
When is a substance secreted into renal tubules?
when its clearance is greater than GFR
When is a substance reabsorbed in the renal tubules?
when its clearance is less than GFR
