4. Intro to glomerulus Flashcards
What are 4 general functions of the kidney?
- Regulation: controls concentrations of key substances in ECF
- Excretion: excretes waste products
- Endocrine
- Metabolism
Give examples of kidney endocrine function.
Renin, erythropoietin, prostaglandin
Give examples of kidney metabolism function.
Activate VitD, catabolism of insulin, PTH and calcitonin
How much water, Na, Cl, Bicarbonate, Glucose and Amino acids are reabsorbed?
- Over 99% of filtered water recovered
- Over 99% of filtered sodium and chloride ions recovered
- 100% of bicarbonate recovered
- 100% of glucose and amino acids recovered
- Just a few waste products not recovered
Define euvolemic
fluid balance
What is the glomerular filtration rate?
Rate at which kidneys filter plasma = 120mL/min/1.73m2
180L/day
How much urine is produced each day?
approx. 1.5L
Describe the electrolyte composition of intra cellular fluid
high K+, low Na+, many large organic anions
Describe the electrolyte composition of extracellular fluid
low K+, high Na+, main anion Cl- and HCO3-
what maintains the differences in intracellular and extracellular electrolyte composition and what happens if it is not controlled?
- Differences maintained by active transport
- Sodium pumps
- Failure to control extracellular electrolytes will affect transport and electrical functions
What is the difference between osmolarity and osmolality?
Osmolarity = number of osmoles of solute per litre Osmolality = solute particles per Kg of solvent
What is oncotic pressure?
Osmotic force due to proteins
What is normal renal plasma flow (RPF)?
800ml/min
What is filtration fraction and what is the normal value?
percentage of total plasma volume that filters into the tubule = GFR/RPF = 20% usually
what happens to 80% of blood that is not filtered at glomerulus?
exits via efferent arteriole (unfiltered)
what are the two types of nephrons?
- Cortical
* Juxtamedullary
What is the normal range for GFR?
90-120mL/min/1.73m2
What is the normal composition for glomerular filtrate
- Contains no blood cells or platelets
- Contains virtually no proteins
- Is composed of mostly organic solutes with a low molecular weight and inorganic ions
Compare the amount of glucose, Na+, urea and creatinine in blood and filtrate
Substance Plasma Ultra-filtrate
Glucose mg/100mL 100 100
Na+ mmol/L 140 140
Urea mg/dL 15 15
Creatinine mmol/L 60-120 60-120
Have the same concentration of each substance.
What are the different components of the filtration barrier?
Capillary endothelium, basement membrane, podocytes
What is a property of the glomerular capillaries endothelium?
They are fenestrated, to allow selective filtration of the plasma
What is the basement membrane composed of and what charge does it have?
It is acellular gelatinous, made up of glycoproteins which have negative charges
How is the basement membrane involved in selective filtration?
The negative charge makes it selective for positively charged substances.
What are podocytes?
Specialised type of epithelial cell which directly invest the glomerular capillaries.
What part of the podocytes make contact with the basement membrane and what is the opening between these called?
They have ‘foot processes’ (pedicels), between these are filtration slits which allow passage of substances into the bowman’s space
Molecules of what radius will pass through the filtration barrier?
1.48nm
In disease where negative charge of basement membrane is lost, what can pass through?
In many disease processes the negative charge on the filtration barrier is lost so that proteins are more readily filtered - a condition called proteinuria
What 3 forces determine net filtration pressure?
- Hydrostatic pressure in capillary (PGC)
- Hydrostatic pressure in Bowman’s capsule (PBC)
- Oncotic pressure difference between the capillary and tubular lumen (piGC)
How is Net Filtration calculated?
PGC – (PBS + PiBS)
Which part of the glomerular capillaries have higher oncotic pressure?
End closer to efferent arteriole, same amount of protein in a smaller volume (as plasma has been filtered) so higher protein conc
At what blood pressures is autoregulation able to maintain GFR within normal limits?
90-200mmHg
What 2 mechanisms are responsible for autoregulation of GFR and RBF?
- Myogenic mechanism
* Tubuloglomerular feedback
Which vessels does myogenic autoregulation predominantly occur in?
It is a property predominantly of the preglomerular resistance vessels
• Accurate
• Interlobular
• Afferent arteriole
What is myogenic autoregulation?
The ability of vascular smooth muscle to regulate its own state of contraction. When GFR increases vessels contract to reduce blood flow, vice versa
Describe the myogenic mechanism when an increase in renal arterial pressure is detected
Afferent arterioles vasoconstrict so less blood entering capillary bed and Prevents transmission of ↑ BP to
glomerular capillary and maintains normal glomerular
capillary pressure and RBF
efferent arterioles dilate so more blood leaving capillary bed so as to decrease GFR
Describe the myogenic mechanism when a decrease in renal arterial pressure is detected
Efferent arteriole vasoconstrict so less blood leaving so GFR increases –> Maintains GFR and RBF
Afferent arterioles dilate so more blood entering capillary bed
What are the 3 types of cells that contribute to the juxtaglomerular apparatus?
- the macula densa, a part of the distal convoluted tubule of the same nephron
- juxtaglomerular cells, (also known as granular cells) which secrete renin
- extraglomerular mesangial cells
At what region does the juxtaglomerular apparatus exist?
At the vascular pole of the glomerulus
In what part of the nephron does the macula densa exist?
Distal convoluted tubule (has larger nuclei than other cells of the DCT)
What do the macula densa detect an increase in?
Increase in NaCl.
What is the TG feedback if GFR increases?
• Macula densa cells of the DCT epithelium detect osmolality or the rate of movement of Na+ or Cl- movement into the cells.
• Increase in BP increases RBF which increases hydrostatic pressure increasing GFR
• The higher the flow of filtrate the higher the Na+ concentration in cells.
• An ATP signal is sent via the juxtaglomerular cells, triggered by an increase in NaCl concentration of distal tubular fluid.
• Further vasoconstriction of the smooth muscle of the
adjacent afferent arterioles and therefore ↓ RBF which in turn ↓GFR.
What is the TG feedback if GFR decreases?
- Decrease in BP decreases RBF which decreases hydrostatic pressure which decreases GFR
- The lower the flow of filtrate the lower the Na+ concentration in cells.
- Macula densa cells of the DCT epithelium detect low levels of Na or Cl-
- reduced release of ATP signal from macula densa cells so reduced vasoconstriction of afferent arteriole
- Release of prostaglandins from macula densa cells – attenuate systemic constriction of afferent arteriole
- Renin released by juxtaglomerular cells
What is the action of prostaglandins in low GFR?
When BP is low, systemic vasoconstriction occurs to direct blood flow to other organs, this is bad for renal function so prostaglandin released from macula densa cells acts to vasodilate afferent arteriole to reduce the effect of systemic vasoconstriction and increase GFR.
what 3 stimuli cause renin release?
- Sympathetic nerve stimulation
- ↓ stretch of afferent arteriole
- Signals generated by macula densa cells in response to ↓NaCl delivery
What is the action of rennin?
renin converts angiotensinogen to angiotensin 1 which is converted to angiotensin 2 in lungs.
angiotensin 2 causes constriction of efferent arterioles so that GFR increases
Why does increasing GFR help increase BP in severe low BP?
increased GFR means blood flowing to rest of body has higher oncotic pressure so reabsorbs more fluid and NaC| increasing volume of blood. angiotensin 2 also increases aldosterone which increases Na and water reabsorbtion
