L12- Reabsorption, secretiona and urine formation

What % of plasma that passes through he glomerulus is filtered at how many litres does this equate to

20%→ 180L/day

What % of filtered plasma gets excretesd and what plume does this equate to

1%→ 1.5L/day

What % o filtrate gets reabsorbed

99%

What % of re absorption happens in thePCT

66%

Equation for working out the amount excreted

Amount filtered- amount reabsorbed + amount secreted

What lines the nephron

A single layer of epithelial cells

What cells are there in the PCT

Tall columnular epithelial cells weight lots of mitochondria and a microvilli border

What cells are there in the thin segment of teh LoH

Cells with no microvilli border that are very tightly bound together and with les mithcondria

Why do cells in the LoH have fewer mitochondria

They are more involved in passive reabsoprtion

Cells in teh DCT

Columnular epithelial cells with more mitochondria than in the LoH but with no microvilli border

Cells in teh CD

Columnular epithelial cells tightly connected

Reabsorption

Movement of solutes/fluid out of the filtrate and into capillaries via epithelial transport mechanisms

Where is the apical border

Between the epithelial cells and the filtrate

Where is the basolateral memrbaen

Between epithelial cells and extracellular fluid

What happens at both the apical and basolateral memebranes

Passive and active transport

Methods of reasborption (6)

• leak Chanel

• Para cellular transport

• Co transport

• Antiporters

• Membrane pump

• Memrbaen carreir

What do leak channels allow. Solids to do

Pass down the concentration gradient and move through the cell to the extracellular fluid

Paracellular trasnsport

movement between the epithelial cells via osmosis or electrical conencrtation gradient

Where does accretive transprt occur on the apical memrbaen

Through co transporters nad Antiporters

Where does active transprt happen on the basolateral membrane

through memebrane pumps and carriers

Why does filtrate from the extracellular fluid enter the peritubular capillary

• There is a ent filtration pressure that moves it into the peritubular capillary

• osmotic pressure is higher than the hydrostatic pressure int eh peritubular capillary

• There is no hydrostatic pressure in the extracellular fluid

What determines the route taken by solutes to be reabsorbed

Depends not heir electrochemical gradient and permeability of epithelial junctions

What functions is the PCT specialised for (2)

• reabsorption

• Secretion

How do microvilli on the apical sugrace provide a beneficial adaptation for the PCT

maximise surface ara available for reabsoprtion

What is there at the basolateral membrane that aids secretion

Has indterdigiatations which are folds int eh memrbane increasaeing SA and reducing the distance to the mitochondria reducing diffusion distance for active transports to occur

How is sodium reabsorbed at the PCT

• absorbed from filtrate in tubule lumen at the apical border through sodium leak channels and through Na/H Antiporters (H+ is lost from cell)

• 3Na+ reabsorbed into extracellular fluid at basolateral memrbane through Na/K+ atp pump

• costransported with essential solutes eg glucose, amino acids actively at the basolateral memrbaen

How is H2O and Cl- eabsporbed in PCT

Move through paracellular route

How is glucose reabsorbed at PCT

• cotransported into cell at apical border through Co transporter with Na+

• Glucose and sodium ATPase pump at basolateral memebrane removes it from cell to extracellular fluid

Tm

Transport maximum rate= rate at saturation of renal transporter/carreir

Renal threshold

Plasma concentration of substrate at transport maximum

Diabetes mellitus

Excessive glucose concentration saturates numebr of carriers and excess glucose appears in urine

What is the tubular fluid that leaves teh PCT isoosmotic with and what is this value

Plasma→ 300mOsm

How does osmolarity change through the nephron and how des this affect final urine conc (4)

• isoosmotic fluid leavening the PCT gets progressively more concentrated Ii teh descending limb of teh LoH

• Removeal of solute in ascending limb creates Hypo osmotic fluid

• Permeability to water and solutes in the DCT and CD is regulated by hormones

• Final urine osmolarity de SDs on Reabsorption in teh CD

Why does osmolarity ddecrease in the descending loop

Is permeability to water but not oolsutes so onyl water is reabsorbed leaving a concentrated solution in the LoH

Wher is NaCl transported to in the ascending limb of the LoH

The interstitium

How si H2O reabsoprtion increased (3)

• ADH makes teh CD more permeable to H2O

• H2O gets reabsorbed passively driven by the osmotic gradient int eh medullary interstitium

• Countercurrent system maintains the osmotic gradient in the medullary interstitium

Properties of the countercurrent exchange systems (3)

• 2 flows moving in opposite directions

• Vessels are anatomically very close together

• Passive transfer of molecules from one vessel to another eg CD and ascending vasa recta

What is the countercurrent multiplier system enhance by

The activtiy transport of solutes

How dos the countercurrent multiple I exchange affecte filtrate in the LoH (3)

• filtrate entering descending limb gets progressively more concentration as it loses wtaer

• Blood in the vasa recta removes the water leaving the LoH

• The ascending limb pumps out Na+, K+ and Cl- and the filtrate becomes hyposmotic

What is the normal plasma pH range

7.38- 7.42

What are 3 ways of regulating plasma pH

• use of buffers

• Respiratory adjustment (CO2)

• Renal adjustment

Examples of buffers used (4)

• cellular proteins

• Haemoglobin

• HPO4 2-

• HCO3-

How does renal adjustment work (2)

• directly by excreting or reabsorbing H+

• Indirectly by excreting for reabsorbing HCO3-

How does acidosis occur

Type A intercalated cells in the CD excrete H+ and reabsorbing HCO3-

How does alkalosis occur

Type B intercalated cells in the CD excrete HCO3- and reabsorb H+

What do intercalated cells have a high amount of

Carbonic a hydrate

We’re does acidosis occur

Int eh extracellular fluid

How is acidosis overcome

• high H+ in extracellular fluid reacts with HCO3- present forming CO2

• CO2 enters cell via basolateral memrbane

• CO2 reacts with H2O. In the intercalated cells forming H2CO3

• This is broken down by carbonic anhydrsase to H+ + HCO3-

• HCO3- is transported out of basolateral memrbaen and Cl- enters in its place to maintain charge balance

• H+ leaves via atp pump on apical memrbane as well as K+ entering in its place

What can the process of overcoming acidosis lead to

An excess of K+ ions in teh extracellular fluid

How is alkalosis overcome

• The low H+ in extracellular fluid reacts with HCO3- forming CO2

• entres the cells through basolateral memrbane where it then reacts it’s H2O

• From H2CO3 which gets broken down by carbonic a hydrate o HCO3- an H+

• H_ moves out of cell through basolateral memrbane via atp and ATP/K+ pump restoring extracellular fluids

• Hc3o- moves out of cell at apical memrbane and cl- entes

• K_ is removed through apical memrbaen and both k+ and hco3- are excreted