Flashcards in Session 4 - Changes in Plasma Volume (Cells of the tubules) Deck (62)
What is in predominant in the ECF?
What ion is predominant in the ICF?
What occurs at the glomerulus?
• Filters of 180l/d (bulk filter)
What occurs at the PCT?
• Freely permeable membrane
• Reabsorbs electrolytes, glucose (100%), urea (50%) and amino acids (100%)
• Reabsorbs large amount of fluid (66%)
Reduces water content
What occurs at the thin descending tube of the loop of henle?
• High conc of sodium
• Concentrates filtrate due to loss of water
What happens at the thin ascending tube of the loop of henle?
• Pulls Cl- and Na+ out of filtrate without H20
• Causes filtrate to become dilute
Why is the DCT special?
• Selective reabsorption
What happens at the DCT?
• Reabsorbs water and concentrates urine via action of ADH
• Reabsorbs Na and water as a result of aldosterone action
• Secretes K+ as a result of aldosterone
What occurs in the collecting duct?
• Permeability affected by ADH
• Absorbs or secretes K+, Na+, H+ and ammonia according to body's needs
What are the excretory ranges of sodium?
• Low salt diet 0.5g/d
Why can't we directly excrete water?
• No water pump
• Must follow solute
Why is important to control the volume of the ECF?
• Includes the vascular system (blood pressure), the volume of which needs to be controlled within very tight limits
What is sodium balance?
The kidneys must match input of sodium with output
Why does ECF expansion occur
• If Na+ excretion is less than intake, it is retained in the body in the ECF
• This causes water to be drawn into the ECF from the nephron, causing increase in volume
• Blood volume and arterial pressure increase
• Oedema may follow
Why does ECF contraction occur?
• If Na+ excretion is greater than intake (patient is in negative balance) the Na+ content decreases
• Less water drawn out of nephron, so ECF volume decreases along with blood volume and arterial pressure
Does an increase in Na+ mean you get an increase in ECF osmolarlity?
• If conc of Na+ in the ECF increases, then so does the volume
• Increased volume gives increased CO, so increase Na+ excretion
State the % of Sodium filtered at each point of the nephron
• PCT - 67%
• Descending thin limb of Henle's loop - 0%
• Ascending thin and thick limb of Henle's loop - 25%
• Distal convoluted tubule - 5%
• Collecting duct system - 3%
State the % of water filtered at each point
• PCT - 65%
• Descending thin limb of Henle's loop - 10-15%
• Ascending thin and thick limb of Henle's loop - 0
• Distal convoluted tubule - 0
• Collecting duct system - 5 (>24% during dehydration)
How much Na+ is filtered in glomerulus?
How much Na+ is reabsorbed in the PCT?
What is glomerular tubular balance?
• Reabsorption of sodium is always around 67%
• Blunts Na+ excretion response
What do all transporters depend on?
The action of Na+/K+ATPase
What are the two regions of the PCT?
• Section 1 (early)
Section 2 + 3(late)
Give one transporters found in the basolateral membrane of the S1 section of PCT
Give five transporters found in apical membrane of S1 of PCT
• Co-Transported with glucose
• Na-H exchange
• Co-transport with AA/Carboxylic Acids
• Co-transport with phosphate (increase with [PTH])
What happens to Urea and Cl- in S1 of the PCT?
• Remain in filtrate, to counterbalance loss of glucose/AA/phosphate/HCO3
• Gives conc grad for Cl-
What is found in basolateral membrane of S2-S3?
What drives reabsorption of Cl- in S2-S3?
• Conc gradient
How is Cl- primarily reabsorbed in S2-S3?
• Na-H exchanger
• Paracelluar Cl- reabsorption
• Transcellular Cl- reabsorption
Why is PCT known as a bulk transporter of water?
• Highly water permeable
What is water-reabsorption in the PCT driven by?
• Solute reabsorption
• ENa (Na+ in)
What does the high water permeability of the PCT allow?
Reabsorption to be isoosmotic with plasma
What is the reabsorption of water in the PCT driven by? (3)
• Osmotic gradient established by sodium reabsorption
• Hydrostatic forces in interstitium
• Oncotic force in the peritubular capillaries - 20% of filtrate lost a glomerulus, but cells and proteins remain in the blood
Give three methods of autoregulation
• Myogenic action
• Tubulo-glomerular feedback
• Glomerulotubular balance
What is glomerulotubular balance?
• Glomerulotubular balance is the balance between glomerular filtration rate and the rate of reabsorption of solutes
• PCT can adjust the amount of sodium it reabsorbs (67%) in order to regulate any changes in glomerular filtration rate
How is more Na+ excreted if ECF volume increases?
• Increase in ECF volume causes
• Increase in cardiac output
Increase in GFR
How does Glumerulotubular balance work?
• Macula densa in JGA detect low osmolarity of Na+
• AG2 or prostaglandins release which act as a vasoconstrictor of afferent arterioles
• Reduces GFR
How are the descending limb and ascending limb of the loop of Henle different?
• Descending limb reabsorbs water but on NaCl
• Ascending limb reabsorbs NaCl but not water
What occurs in thin descending limb?
• Aquaporins secrete water from lumen to interstitium down conc grad provided by excretion of ions by thick ascending limb into interstitium
Give a structural features of the thin descending limb which facillitates the movement of water
• No tight junctions between cells, which allows paracellular reuptake
Give two transporters found in luminal side of thick ascending limb?
• ROMK (K+ out down conc grad)
Why is ROMK necessary on luminal aspect of thick ascending limb?
• To drive NaKCC2, which requires K+
What is NaKCC2 the target of, and what condition does this cause?
• Loop diuretics
• Increased loss of K+ in the urine causes hyperkalaemia
Give two transporters found in the ECF membrane of thick ascending limb
• Cl- transporters
• Na/K+ ATPase
Why is thick ascending limb particularly sensitive to hypoxia?
Uses more energy than anywhere else in nephron
Describe changes in concentration of filtrate from thin descending limb to the thick ascending limb
• Normal filtrate enters TDL
• Lots of water lost
• High conc
• Tal excretes large amount of ions
• Hypo-osmotic filtrate produced
Why is the thick ascending limb of the loop of henle known as the diluting segment?
• NaCl leaves filtrate without removal of water
• Tubule fluid leaving loop is hypo-osmotic compared to plasma
What is water reabsorption of early DCT based on?
• Active Na+ reabsorption
• Actively transported by NaCC transporter, driven by 3NaK+-ATPase
What is water permeability like in the Distal Convoluted Tubule?
• Fairly low
What major ion is reabsorbed in the early DCT?
Outline the two transporters on the luminal side of early DCT
• NaCC transporter (Na+ in as well as Cl-)
Give three transporters found on ECF side of early DCT
• NCX (Ca2+ into ECF, Na+ in)
• Na+/K+ ATPase
What part of DCT is sensitive to thiazide diurectics?
What drives reabsorption of Ca2+ in early DCT?
How does the fluid entering the DCT compare to the ECF?
What is water reabsorption in late DCT and collecting duct driven by?
Water permeability dependent on ADH
What are the two cell types found in late DCT and early collecting tubule?
• Principle cells (reabsorption of Na+ via Enac)
• Type B intercalated cells (active reabsorption of Chloride)
How is Na+ reabsorbed in collecting duct?
• Na+ pumped out into ECF by Na+/K+ATPase
• Drives eNaC (epithelial Na+ channel)
What are the two transporters found on luminal side of principle cells in late DCT and early collecting tubule?
• eNa (sodium in)
• ROMK (K+ out)
What proportion of collecting duct cells are principle?
What is the main feature of principle cells?
• Produce lumen charge
○ Electrical gradient for paracellular Cl- absorption
○ K+ secretion into lumen
• Variable uptake through aquaporin 2
○ Dependent on ADH