Lecture 11. Dr Ebaa 4.
We will carry on from the “Thick ascending limb of Henle” and the types of transporters found there. Get up,
and get your slides! (Topic 3+4). It is very important to keep looking back at the diagrams. This will take you a
long way! Recall what ‘filtered’, ‘reabsorbed’, and ‘secreted’ mean.
3- Thick ascending limb of Henle
A) Transcellular Transport (50%)
1) Sodium Chloride Potassium channel
 Very important transporter
 It moves 1Na+, 2Cl-, and 1K+ in same direction towards reabsorption
 This happens with the help of Na+K+ATPase on the basal membrane.
 Drugs that block this channel: Furosemide or Lasix (diuretic)
o The thick ascending loop of Henle accounts for 25% of the reabsorption of Na+‎
o So blocking this channel is a very efficient way for reducing the reabsorption of solute (and
hence water)
o Prevents reabsorption of Na+, Cl-, and K+ and thus water. This causes increased loss of water.
Note that K+ is lost. On the long term this can cause hypokalemia.
o Given in emergencies when patient has very high blood pressure (to lower it).
o It has a very fast activity but needs a combination of other drugs to compensate this K+ loss.
2) Na+ H+ Exchange channel:
 Secretes H+ in exchange for Na+ with the help of the gradient caused by Na+K+ATPase.
 Both CO2 and H2O are found in the intercellular space of the epithelial cells.
o



carbonic anhydrase
H2O+CO2—
 H2CO3  H++HCO3-
H+ is secreted by the sodium hydrogen exchange channels
HCO3- (bicarbonate) is reabsorbed into the blood.
This is the normal situation because our body constantly produces acids so we need to remove the
H+ (secrete it into urine) and reabsorb the bicarbonate.
B) Paracellular Transport:


Positively charged ions can move this way due to the repulsion due to the high concentration of positive
charges on the membrane
The positive ions in the tubular fluid will be repelled and so will take the paracellular route and become
reabsorbed
So 50% of the reabsorption that takes place in the thick ascending limb of loop of Henle is active
transcellular and 50% is paracellular passively
Permeability to water in thick ascending: completely impermeable to water. Only solutes are reabsorbed.
1
Lecture 11. Dr Ebaa 4.
3- Distal tubule and collecting duct
The distal convoluted tubule is divided into early and late segment, each is unique. The late distal shares
characteristics with the collecting duct and so will be explained together.
EARLY DISTAL CONVOLUTED TUBULE
 Na+Cl- transporter:
o It’s a symporter, cotransporter ie it moves Na+ and Cl- in the same direction (direction of
reabsorption)
o This happens with the help of the gradient caused by Na+K+ATPase
o Permeability to water: it’s completely impermeable. Only solutes are transported.
o Drugs that block this channel: Thiazides (diuretic)
 Cause diuretic effect
 Indirectly cause hypokalemia
 Inhibiting this channel results in an increase in level of Na+ in the tubular fluid. In
other segments of the tubule (late distal), there will be an exchange between Na+
and K+ (increase in reabsorption of Na+ and secretion of K+)  loss of K+
hypokalemia
 Permeability to water: completely impermeable.
LATE DISTAL AND COLLECTING DUCT
 There are two types of cells:
1) Principal cells: site of action of Aldosterone hormone
 Epithelial Na+ K+ channel (ENac)
o Reabsorbs Na+ and secretes of K+
o Aldosterone works on it by increasing its activity (increases reabsorption of Na+)
o Mechanism of action of Aldosterone: increases the synthesis and insertion of this channel, hence
increasing Na+ reabsorption and secretion of K+.
o Aldosterone, which works to increase the blood pressure, is stimulated by:
 Low blood pressure: increases it by increasing Na+ reabsorption.
 High level of K+ in blood
o Drugs that block this channel: Amiloride.
 Notice that blocking this channel will not cause hypokalemia. On the contrary, it will
prevent Na+ reabsorption and K+ secretion (as opposed to Furosemide). This increases
the concentration of K+ in the body. Drugs that work to block this channel are called K+
sparing cells.

Na+K+ATPase: (look at diagram!)
2
Lecture 11. Dr Ebaa 4.
2) Intercalated cells:
 Specific for acid base balance
 Have carbonic anhydrase enzyme
 Have H+ transporter (secretes H+ mainly)
 There are also cells that are responsible for alkalosis:
o Reabsorb H+ (H+ to blood not urine)
o Are only active if alkalosis happens (lots of base).
 In normal cases, when there’s a lot of acid in the body, this pump is found on the apical surface
secreting H+ to tubular fluid and reabsorbing bicarbonate into the blood.

Permeability of late distal and collecting duct to water: variable
o Depends on presence ADH.
 Present: both permeable
 Absent: impermeable
o ADH inserts channels in these cells called aquaporins, which are the reason for water
permeability. When aquaporins are inside the cell, it is not permeable to water. When they
go to the surface, the plasma membrane becomes permeable to water.
o ADH is responsible for this insertion of aquaporins on cell membrane
o When body needs water in the urine, ADH is produced and the late distal and collecting duct
becomes permeable to water and reabsorption of water by osmosis occurs.
o Fluid in this late stage (late distal and collecting ducts) is very dilute because a lot of solute
reabsorption happens and no movement of water in earlier segment of the loop of Henle. As
a result, switching on aquaporins causes water to move by osmosis from tubular fluid to
blood (reabsorption).
o Action of ADH: prevents diuresis as it prevents lots of water in urine. It is also called
vasopressin because it directly increases blood pressure in body by vasoconstriction.
HORMONAL REGULATION FOR REABSORPTION
Some hormones have a direct effect on transporters:
Increase blood pressure:
1) Angiotensin II: when the blood pressure is low, AngII increases the reabsorption of Na+ (works on
transporters and Na+K+ATPase), an increase in water reabsorption follows.
2) Aldosterone: works on ENac. It stimulates Na+ reabsorption and K+ secretion, resulting in an increase
of water reabsorption mainly in distal and collecting duct.
3) Epinephrine and norepinephrine: due to sympathetic stimulation of the SNV or adrenal medulla.
-Reduce GFR: less loss of water and electrolytes increases the efficiency of reabsorption. This
results in more electrolytes and water in plasma.
-Increase reabsorption of Na+, so net result is an increase in volume and blood pressure.
4) ADH: works on aquaporins in late distal and collecting ducts causing the plasma membrane to become
water permeable. Water diffuses by osmosis from tubule to blood, making the urine more concentrated.
The volume of the extracellular fluid increases and this results in an increase in blood pressure.
3
Lecture 11. Dr Ebaa 4.
Decrease blood pressure:
1) Atrial Natriuretic Peptide (ANP) and Brain Natriuretic Peptide (BNP): exactly the same but ANP is
secreted from the atrium of the heart and BNP in from the brain. They are secreted as a result of the
increase in blood volume (blood pressure causes stretching and so they are secreted). ANP and BNP are
antihypertensives, that is, they decrease blood pressure. In filtration, they increased GFR as to get rid of
water and solutes. Here in reabsorption, they decrease the reabsorption of Na+ and thus fluids (act as
diuretics).
<only this example is required>
The next two slides, which are the last two slides in “Topic 3” are very important for the exam. Study them well!
TOPIC 4: HOW TO ASSES RENAL FUNCTION
It’s important to know if the function is normal, especially as pharmacists for example when looking at the dose
of dug for patients who already suffer from kidney diseases. How can we asses kidney function?
1) Blood plasma tests: look at level of waste products. If high, the kidney isn’t getting rid of them in an
appropriate way at a good rate. For example BUN (Blood Urea Nitrogen) test and Creatinine test.
2) Urine analysis:
 By observing the physical characteristics of the urine (eg. pH)
 Solute and electrolyte concentration
 Urine flow rate
MEASURING THE GFR BY CLEARANCE



One of the important ways to asses kidney function is by measuring the GFR. It should have a constant
value of 125ml/min (volume of plasma filtered per minute). GFR can be an indication as to whether the
membrane and vessels nourishing the nephron are intact. To calculate the GFR, it is not possible to bring two
kidneys and see the amount of fluid filtered per minute in Bowmans capsule. Instead, the “clearance rate” for
a certain substance is calculated.
The clearance rate is the volume of plasma that became free of this substance in one minute.
Clearance, unlike GFR is not constant, it varies depending on what the substance is (How does the kidney deal
with it? Efficient filtration or reabsorption of this substance? Does the kidney secrete it?)


Concentration of a substance will not become 0 in the plasma within a minute. It could take several minutes
or hours for the plasma concentration of this substance to become 0, that is IF it’s going to be zero. If the
body produces this substance, it will not become zero.
4
Lecture 11. Dr Ebaa 4.

The search for a substance with a clearance rate (using the equation) close to GFR began…
 Substance from our body: CREATININE
o Creatinine’s clearance rate was found to be 140ml/min, only a little more than GFR, which is
125ml/min. It is considered the most suitable substance for the estimation of GFR.
o Creatinine is a waste product that results from muscular activity. It is produced when
Creatinine phosphate in the muscles is broken down.
o Creatinine is constantly produced and constantly eliminated.
o Elimination = Production as this is a waste product which we do not want to store in our body.
There is a certain level it should not exceed.
o Why is Creatinine’s clearance so close to GFR? Because it has excellent filtration and very poor
reabsorption (it’s a waste product). If the reabsorption of a substance is high, clearance will be
a lot less (conc. in urine/conc. in plasma will be less).
o Why is Creatinine’s clearance a bit higher than GFR? Because some active secretion occurs.





Substance from outside our body: INULIN
o GFR equals clearance exactly
o It’s freely filtered, no reabsorption or secretion takes place (it’s IDEAL!)
o To accurately calculate GFR: Patient is admitted to hospital and given Inulin (has little toxicity).
Urine is collected and the plasma concentration of Inulin is measured after 24 hours. GFR is
then calculated. (using clearance equation previously mentioned)
o This is not practical, as it poses some danger on patient. Therefore, Creatinine is most often
used.
What is supposed to happen is uniform urine collection, measurement of concentration of Creatinine in urine,
and Creatinine in plasma  calculate urinary flow rate  apply the equation to get an estimation of GFR.
This also is not practical because the patients, at home, usually either over collect or under collect urine, so
the urinary flow rate is not accurate and thus GFR and even Creatinine concentration won’t be correct.
Due to the problem mentioned above, graphs were made to show relationship between GFR and plasma
Creatinine taking into consideration different factors like weight and gender. There are different graphs
depending on age, as one gets older, kidney function decreases. By knowing the plasma Creatinine, the
estimated GFR is found from the graph.
IN THE EXAM, values for Inulin or Creatinine will be given. You will have to apply the equation to get GFR.
Don’t forget, the whole point of finding clearance of substance with value close to GFR, is to estimate GFR
and thus assess kidney function.
When we said Creatinine indicates GFR we assumed that the production of Creatinine is constant and that
everyone has the same production of Creatinine. We only looked at how the kidneys dealt with Creatinine.
 In reality, Creatinine can be affected by
o Muscular activity: bed ridden people don’t have same Creatinine production as active people.
So when we get a blood test and find that the Creatinine production is low, that does not
mean that the kidney is working perfectly, they might have low production.
o Age
o Some antibiotics: can give a higher or lower value and this is makes our estimation incorrect
5
Lecture 11. Dr Ebaa 4.
PHYSICAL CHARACTERISTICS OF URINE
o
o
o
o
o
Cloudy: that might indicate an infection in the urine.
Odour: ketone bodies have odour in diabetic patients, it’s a fruit smell odour.
Colour: Some diet supplements affect the colour. Also, the more concentrated the urine, the darker its
colour.
pH: urine has a high range of pH, it could be highly acidic or highly alkaline. This is because urine is a
way to adjust body pH, so it should be able to stand if there was alkalosis or acidosis (excess base or
acid). For example, if there was an excess in the acids in the body, the urine will be very acidic (as to
reduce the body’s acidity back to normal) and likewise for bases.
Specific gravity: an indication for the concentration of the solutes and must also have a specific range.
Normally in urine, there should be no proteins or albumin or glucose.
MICTURITION REFLEX








It is the elimination of urine outside the body, when the urinary bladder empties its contents.
It’s mediated by micturition centers in the spinal cord but takes input from higher centers in brain.
Main mediator: parasympathetic stimulus
Relaxed status of urinary bladder: when the volume in the bladder hasn’t exceeded a certain volume
(about 200ml)
Before reaching a certain volume (this volume may vary depending on many factors such as age),
there is tonic discharge on the internal and external urinary sphincters to keep them contracted.
These sphincters are basically muscles; the internal sphincter is made of smooth muscle whereas
external has skeletal muscle.
There are stretch receptors on the walls of the urinary bladder, which become stimulated, when the
urinary bladder fills and reaches a certain volume. Signals are sent to the micturition centers in the
spinal cord. Sensory neurons extend from the urinary bladder to the micturition center in spinal cord
where there are they synapse with two interneurons. 2 actions occur, both of which are
parasympathetic:
1. Tonic (excitatory) discharge on walls of urinary bladder to contract it
2. Inhibitory discharge on internal and external sphincters: this causes them to relax
In the final stage of urination: voluntary control of the skeletal external sphincter. Input from the
consciousness of the person when it is suitable to start micturition reflex. So under the person’s
choice, urination starts.
Urinary incontinence: this is the lack of voluntary control over micturition. It can happen due to:
o Young age: need training
o Old age: irritation due to stones, anxiety, stress
This is the full sheet. Good luck and study well!
Sarah Bloukh
6