Urinary physiology - reabsorption and secretion

Question 1

Why is the peritubular capillary pressure low

Answer 1

Because the hydrostatic pressure of the blood has to overcome the frictional resistance of the efferent arteriole which leads to the peritubular capillaries

Question 2

Why is the oncotic pressure in the peritubular capillaries and efferent arteriole, higher

Answer 2

20% of the plasma is filtered into bowman’s capsule in the glomerulus and then the remaining blood which is in the efferent arteriole and peritubular capillaries has a higher concentration of plasma proteins

Question 3

What is a result of the high oncotic pressure and low pressure in the peritubular capillaries

Answer 3

Reabsorption is favoured

Question 4

What are glucose, amino acids, organic acids, sulphate and phosphate ions reabsorbed by

Answer 4

Carrier mediated transport systems

Question 5

What is the maximum transport capacity of carrier mediated transport systems and what happens when they are full

Answer 5

If the transport systems are full and fully saturated, the rest of the substrate cannot be reabsorbed so they are excreted in the urine

Question 6

What is the renal threshold

Answer 6

The plasma threshold at which saturation occurs

Question 7

What is the renal plasma glucose threshold

Answer 7

in men 10mmoles/L - All plasma glucose is filtered but up to 10mmoles/L will be reabsorbed with the rest being secreted

Question 8

What does it mean for glucose to be freely filtered

Answer 8

All plasma glucose is filtered

Question 9

What is the appearance of glucose in the urine of diabetics called and what causes it

Answer 9

Glycosuria and failure in insulin control

Question 10

Why is Tm set higher then the normal concentration of certain substrates

Answer 10

E.g for glucose and amino acids so that all the useful nutrients are reabsorbed normally

Question 11

What substances is the Tm saturation set at the plasma concentration

Answer 11

Phosphate and sulphate ion so if more then the plasma concentration is present, it gets excreted

Question 12

What is the normal sodium plasma concentration of sodium

Answer 12

142 mmoles/l

Question 13

Where does most of the reabsorption of sodium occur

Answer 13

Proximal tubule - it is not absorbed by a Tm mechanism but by active transplant

Question 14

Is sodium freely filtered (all plasma sodium is filtered)

Answer 14

yes

Question 15

Describe how sodium is reabsorbed at the proximal tubule

Answer 15

Sodium enters the proximal tubule cell through membrane proteins by passive transport (going down its chemical gradient

The Na/K ATPase pump then pumps the sodium into the interstitial fluid which keeps the concentration in the proximal tubule cell low so more sodium can move from the lumen into the cell and then again out into the interstitial fluid for reabsorption

Question 16

Why is sodium able to go through the proximal tubule when it is not permeable at cell membranes

Answer 16

The brush bored or the proximal tubule has a higher permeability for sodium ions due to the large surface are, high number of microvilli and vast supply of sodium ion channels which facilitate the passive transport of sodium into the cell

Question 17

How does sodium active transport mediate chlorine leaving the proximal tubule cell

Answer 17

The active transport of sodium out the cells allows negative ions such as chlorine to diffuse out of the cell due to the electrochemical gradient that is created

Question 18

How does the substrate in the tubule cell become more concentrated

Answer 18

Water follows chlorine out the cell which concentrates the substrates in the proximal tubule cell

Question 19

What substances is the tubular membrane impermeable to

Answer 19

Inulin and mannitol

Question 20

What is the effect of something that decreases active transport such as decreased blood flow

Answer 20

Disruption of the renal function

Question 21

Explain the sodium glucose symport at the tubule membrane

Answer 21

Sodium diffusing into the cell pulls glucose into the cell against it’s concentration gradient using the sodium- dependant glucose transporter

Glucose then diffuses out the tubule cell using GLUT protein

Sodium is pumped out by the NA/K ATPase pump

When there is high sodium concentration in the tubule - glucose transport is facilitated

When there is low sodium concentration in the tubule - glucose transport is inhibited

Question 22

What is tubular secretion

Answer 22

Secretory mechanisms transport substances from the peritubular capillaries into the tubule lumen for excretion

Question 23

Why is secretion beneficial

Answer 23

It is good for protein bound substances that cannot get in through the glomerulus and it allows harmful substances to be excreted more rapidly

Question 24

Why can carrier mechanisms carry more then their intended substrate

Answer 24

They are not very specific

Question 25

What is normal ECF potassium concentration

Answer 25

4mmoles/l

Question 26

What is the value of hyperkalaemia and what effect does it have

Answer 26

K concentration of >5.5mmoles/l - decreases the resting membrane potential of excitable cells so which leads to ventricular fibrillation

Question 27

What is the value of hypokalaemia and what is the effect

Answer 27

K concentration <3.5mmoles/l - causes increased resting membrane potential so it makes the resting potential more negative and makes the cardiac cells more excitable - this hyperpolarises muscle and cardiac cells which causes arrhythmias and eventually death

Question 28

What is hyperpolarisation

Answer 28

Makes the cell more negative which leads to easier send off of action potentials

Question 29

Where is potassium filtered and reabsorbed

Answer 29

Potassium is filtered at the glomerulus and reabsorbed primarily at the proximal tubule

Question 30

What influences increased potassium excretion

Answer 30

If there is more ingestion of potassium, there is more secretion into the renal tubule for excretion

Question 31

Why is potassium homeostasis so important

Answer 31

The effects it has on the muscles of the heart

Question 32

What regulates potassium secretion and how does it work

Answer 32

Aldosterone from the adrenal cortex if there is an increase in ECF potassium concentration which bathes the aldosterone secreting cells to stimulate aldosterone release to the renal circulation - this stimulates increased potassium secretion at the renal tubule

Question 33

Describe proton secretion

Answer 33

Protons are secreted directly from the tubule cells (not from the peritubular capillaries) into the lumen