Basic Mechanisms of Urine Formation

Renal Clearance

Basic Mechanisms of Urine Formation

Filtration, secretion, reabsorption and excretion.

How do we determine these rates?

The master equation:

Rate of excretion = filtration + secretion – reabsorption

TERMS

Urinary output is a flow rate; how much urine are you producing per unit of time, ml/min

This refers to the amount of urine dripping into the urinary bladder, not the amount voided.

Rate of filtration is the flow rate of plasma fluid going into Bowman’s capsule. This plasma fluid is called “filtrate”. The filtrate is not the same as plasma since there are no plasma proteins in it (proteins don’t get through the filtration membrane, not just because of their size, but also because of their negative charge).

Glomerular Filtration Rate (GFR) is the amount of fluid going into Bowman’s capsule per unit of time. Normal GFR is 90 - 120 mL/min. Dissolved in that filtrate are electrolytes, glucose, and amino acids. We can also calculate how much of those molecules are entering Bowman’s capsule.

z Glomerular filtration rate (GFR) can be calculated by measuring any chemical that has a steady level in the blood, and is freely filtered but neither reabsorbed nor secreted by the kidneys.

z One such substance is creatinine.

z To determine GFR from creatinine, collect urine for 24-hours and draw blood before and after the 24-hour period.

z Then measure the amount of creatinine that was removed from the blood during that time. Then apply the results to a formula to determine GFR. You do not need to do this for this class. On test questions, I will tell you what the patient’s GFR is.

Filtered Load (or Tubular Load) is the amount of a specific solute dissolved in the fluid (electrolyte, glucose, or amino acid) that enters Bowman’s capsule. There is an equation for this, but you have to know two things:

§ Filtered Load (or Tubular Load) is the amount of a specific solute (electrolyte, glucose, or amino acid) dissolved in the fluid that enters Bowman’s capsule. There is an equation for this, but you have to know two things:

§ Patient’s GFR

§ Plasma concentration of that solute.

§ To determine this, draw blood, place in spectrophotometer.

Equation:

TL s = Ps x GFR

Filtrate vs. Solute

If I had a cup and I filled it with 125ml of water, and then added one packet of Crystal light, how much fluid (the filtrate) entered the cup? 125 ml. How much fluid went into the cup is the GFR. If you know how much solute (Crystal light) went into the cup, you can calculate the filtered load.

How to calculate Total Load

· If we want to know how much glucose was filtered into the nephron, we need to know that person’s blood plasma glucose levels, and we would need to calculate their GFR (normal is 90-120mL/min).

· Each individual solute will have its own filtered load calculation.

TL glucose = Pglucose x GFR

TL sodium = Psodium x GFR

TL chloride = Pchloride x GFR

Kidney Function and Aging

· With age, our filtration membranes deteriorate, and we also lose some of our glomeruli.

· With age, the permeability of our kidney membranes also declines, causing a decline in GFR.

· Thus, we have a decline of urine production, gradually over our lifetime.

· Some people age better than others with our skin, some age better with their GFR.

· Older people have to take a lower dosage of medicines that are excreted in the urine.

Solutes Suck!

· Sodium (Na+) is the most numerous of the solutes in our plasma.

· Remember, if solutes are being reabsorbed, water will come with it….solutes SUCK water!

· When salt is reabsorbed, water will be reabsorbed too. When salt is secreted, water is secreted too.

Ø Some solutes are being reabsorbed, pulling water with them, but other solutes are not being reabsorbed, so they stay in the kidney tubules and are trying to keep water with them.

Ø If there are more particles being reabsorbed, the water will mostly be reabsorbed also.

Ø What will happen to the concentration of the particles in the tubule that are not being reabsorbed, while water is leaving? Their concentration in the tubule will go up.

Ø Those things that are not reabsorbed will leave the body.

Tubular Filtrate Resembles Plasma

By far, filtered fluid is mainly NaCl and water.

It contains other salts and electrolytes, amino acids, small sugars, vitamins and other small molecules, such as wastes.

Na+ and Cl- are present in such large amts; they are over 99% reabsorbed along the length of the tubule. (Why? We need it to keep our blood pressure up)

Ø At a constant GFR, as plasma concentration of a freely filtered substance rises, the tubular load of the substance rises in direct proportion to plasma concentration

Solutes are reabsorbed into the blood stream

· The next photo is of a Proximal Convoluted Tubule (PCT), just beyond the glomerulus. The lumen of the tubule contains the filtrate which leaked out of the capillaries and into the glomerulus, entered Bowman’s capsule, and has now arrived in the PCT.

· There are cells (tubular cells) lining the lumen of the PCT. The cells have proteins that allow some solutes to diffuse into the cell, and right back out cell, and into the peritubular capillaries that surround the PCT. In that way, solutes re-enter the blood stream.

· If ADH levels are very high, most of the water will be reabsorbed in the PCT.

· Water reabsorption along the proximal convoluted tubule (PCT) occurs by osmosis resulting primarily from reabsorption of sodium

Proximal Convoluted Tubule

Ø In a state of acidosis, the PCT will secrete H+ ions.

Ø When the H+ ions are secreted, reabsorption of bicarbonate ions occurs at the same time.

Ø H+ ions are also secreted in the DCT, but bicarbonate cannot be reabsorbed there.

Pressures

· Because we drop off a lot of fluid (125 ml of filtrate) into Bowman’s capsule every minute, the water pressure in the peritubular capillary beds is low.

· Because proteins cannot get through, the osmotic force is very high here (proteins are trying to get in, but cannot).

· In the glomerulus, the forces favor fluid to leave the glomerulus and enter Bowman’s capsule.

· In the peritubular capillary bed, the forces favor the fluid to enter the capillary bed.

Solutes without transporters

· There are some solutes that cannot get through the tubular cell membranes because they have no protein transporters, so they become more concentrated in the tubular lumen.

· The concentration increases until they are forced to diffuse down their concentration gradient and then they can enter the tubular cells and be reabsorbed.

· One such solute is chloride (Cl-).

Active Transport

· Active transport, facilitative transport, and simple diffusion are all involved in renal clearance.

· It all starts in the membrane of the tubular cells, which have protein transporters that allow Na+, K+, glucose, and other substances to get through. This is called Active transport (it requires ATP, so it uses energy).

Glucose reabsorption

· Glucose and amino acids enter into the tubular cells, and then enter the peritubular capillaries. That is reabsorption.

· When any substance leaves the bloodstream and enters the lumen of the kidney tubules, it is secretion.

· By adjusting reabsorption and secretion, your body adjusts its acid-base balance; if too many H+ ions were kept, there is too much acid, and the H+ ions will start to be secreted more.

Transport Maximum

· When proteins are shuttling solutes, the rate of this shuttling has a maximum, called a transport max (Tm).

· If there are more solutes present than can be transported, the solutes will end up in your bladder. There will be a point at which you can saturate these transporters. For instance, when your blood glucose levels are elevated because you have a problem making insulin or responding to insulin, then you will have an increase in glucose load in Bowman’s capsule. This excess filtered load will cause a spill of glucose into the urine.

· After the PCT, there are no more glucose transporters to reabsorb it. 320 mg/ml is the max rate to filter glucose. If you filter 125 mg/ml and reabsorbed 125 mg/ml, how much did you excrete? None. If you filter 375 mg/ml but reabsorb 320 mg/ml (transport maximum), 55 mg/ml is excreted

Threshold

· Transport maximum is the total transport maximum throughout all of the nephrons in the kidney. They do not all have the exact number and type of transporters.

· One single nephron might get to maximum and a tiny amount of glucose will appear in the urine. As more nephrons reach their maximum, more glucose will appear in the urine.

· That appearance of glucose in the urine before you reach the overall Tm of the kidney is called threshold.

· Threshold is the plasma concentration at which a substance begins to appear in the urine

Calculate Glucose Excretion

§ Step 1: Calculate their filtered load: GFR x plasma glucose level 90ml/min x 2mg/ml

§ Step 2: Determine reabsorption of glucose (maximum is 150mg/min).

§ Step 3: Subtract 150 from 180, and that tells you their excretion.

§ Filtration – reabsorption = Excretion

§ Answer: 180 – 150 = 30 mg/min

ANSWER

Urea

Ø Urea is also reabsorbed in the PCT.

Ø This happens because its concentration in the tubule is high, so it diffuses down its concentration gradient, which means it will leave the tubule and enter the capillaries.

Ø This is an advantage because it is a particle, and it brings water with it.

Ø But urea is a waste product…how will we get rid of it?

Ø We will reabsorb it now and secrete it again further along in the nephron. By the time the reabsorbed urea travels in the vasa recta to the distal convoluted tubule, its concentration is higher in the bloodstream than it is in the tubule, so it diffuses back out of the capillaries and into the tubule to be excreted. The DCT is impermeable to water, so water does not follow it.

Salt concentration in the PCT

· As you go along the length of the PCT, the amino acids and glucose are reabsorbed, in addition to salt and water.

· The salt concentration in the filtrate stays the same as the blood plasma salt concentration.

· As we go through the rest of the nephron, the saltiness will diminish as it becomes more like urine and less like plasma.

Diuretics in the PCT

Ø cause diuresis by reducing net water reabsorption from the proximal convoluted tubule

Ø Some diuretics are “Potassium sparing” because they decrease potassium excretion

Ø Mannitol: potassium-sparing

Ø Lasix: not potassium-sparing

Diuresis in the PCT

Ø Diuresis means the person is excreting a lot of water. This is what happens in diabetes mellitus. It makes the person thirsty, so they drink a lot of water.

Ø However, that is not why they have diuresis.

Ø The reason for the diuresis is the large amount of glucose in the tubule draws water with it, since glucose is a particle.

Ø Thus, the large filtered load of glucose has an osmotic effect on the tubule

Summary of PCT functions

Reabsorbs 2/3 of the salt and water

Reabsorbs 100% of glucose and amino acids

Reabsorbs 65% of potassium

Reabsorbs 50% of urea

Regulates pH in the filtrate

Secretes creatinine (waste product after you eat protein) into the filtrate

Now we leave the PCT and enter the Loop of Henle

· Our kidneys are responsible for adjusting the urinary output.

· They are responsible for determining if you have a lot of urine which very dilute, or scant, concentrated urine.

· If you drink a lot of water, it will make your urine dilute.

· If you are dehydrated, your urine will be concentrated.

· Caffeine and alcohol are dehydrating, so they have the opposite effect of drinking water. They block antidiuretic hormone (ADH) too.

Before we move onto the Loop of Henle, be mindful of the following:

· How can you make a solution more concentrated? Take out water or add more solute.

· How can you make a solution less concentrated? Add water or take out more solute.

· Your kidneys play that game. To do that, we have to separate solutes and water. But how do we do that, since water follows particles? There are several parts of the nephron that are impermeable to water.

LOOP OF HENLE: Descending Limb

· Water is reabsorbed

· Na+ and urea are secreted

· What will your filtrate taste like here?

· We are removing water and adding salt.

Keep track of the water:

· 20% of water is reabsorbed in the descending limb, and 67% was reabsorbed in the PCT. That means a total of 87% of our filtered water has now been reabsorbed. The remaining 13% can be tapped into if your body needs it. If you are well hydrated, it will go into the toilet. If you are dehydrated, hormones will be needed farther along in the nephron in order to reabsorb more water.

· In the PCT, you have reabsorbed 50% of urea. Although it is a waste product, we use it to create an osmotic gradient to reabsorb glucose and other good solutes. Now, we want to get rid of the urea since we are done reabsorbing all the glucose.

· By the time the filtrate is at the tip of the LOH, all of the urea that had gone back into the blood will be secreted back into the lumen, plus some more. The descending limb contains 110% of the urea you initially filtered.