WEEK 1: Assessment of Kidney Function

Question 1

Outline at least 5 kidney functions.

Answer 1

*Maintain water balance
*Maintain body fluid osmolarity
*Regulates the quantity & concentration of most ECF ions
*Maintain proper plasma volume
*Participates in acid-base balance
*Metabolic waste product elimination
*Exogenous compounds elimination
*Erythropoietin production
*Renin production
*Converts vitamin D into its active form

Question 2

Define osmolarity.

State the units for osmolarity.

Answer 2

Osmolarity refers to the concentration of osmotically active particles, or solutes, in a solution.

Osmolarity is expressed in osmoles per liter (osmol/L) or milliosmoles per liter (mOsm/L).

Question 3

State the formula for calculating osmolarity.

Answer 3

Osmolarity can be calculated if you know 2 things:
1. The molar concentration of the solution and
2. The number of osmotically active particles present in solution

In simpler terms, you sum up the concentrations of all individual solutes in the solution, each multiplied by the number of particles it produces when it dissociates in the solution.

Question 4

Calculate the osmolarity of a solution that contains the following solutes:

0.2 moles of NaCl (sodium chloride)
0.3 moles of glucose (C₆H₁₂O₆)
0.1 moles of CaCl₂ (calcium chloride), which dissociates into three ions when it dissolves.

Answer 4

1. Sodium Chloride (NaCl):
   Concentration: 0.2 moles/L
   Number of particles produced: NaCl dissociates into two ions (Na⁺ and Cl⁻).

Contribution to osmolarity:
0.2 × 2 = 0.4 mOsm/L

2. Glucose (C₆H₁₂O₆):
   Concentration: 0.3 moles/L
   Glucose does not dissociate, so the number of particles produced is 1.

Contribution to osmolarity:
0.3×1=0.3mOsm/L

3. Calcium Chloride (CaCl₂):
   Concentration: 0.1 moles/L
   Number of particles produced: CaCl₂ dissociates into three ions (Ca²⁺, 2Cl⁻).

Contribution to osmolarity:
0.1×3=0.3mOsm/L

Now, sum up the contributions from each solute:

Osmolarity=0.4+0.3+0.3=1.0mOsm/L

So, the osmolarity of the solution is 1.0 mOsm/L.

Question 5

State the osmolarities in the different parts of the nephron.

Answer 5

Proximal Convoluted Tubule (PCT):

Osmolarity: ~300 mOsm/kg

Descending Limb of the Loop of Henle:

Osmolarity: Increases from ~300 mOsm/kg at the cortex to ~1200 mOsm/kg at the medulla.

Ascending Limb of the Loop of Henle:

Osmolarity: Decreases from ~1200 mOsm/kg at the medulla to ~100 mOsm/kg at the cortex.

Distal Convoluted Tubule (DCT) and Connecting Tubule (CNT):

Osmolarity: Varies, but generally remains lower than that of the medulla.

Collecting Duct:

Osmolarity: Varies depending on the body’s needs, ranging from ~100 mOsm/kg to as high as ~1200-1400 mOsm/kg under the influence of antidiuretic hormone (ADH).

Question 6

What is tonicity?

Answer 6

It is the effects of various osmotic solutions on cells.

Question 7

A solution can be either; Hypotonic or Hypertonic or Isotonic.

Define the 3 terms.

Answer 7

An isotonic solution does not cause an osmotic flow of water into or out of a cell (no change in cell volume).

A hypotonic solution will cause osmotic flow of water into the cell, causing the cell to swell. If the osmotic gradient is large enough, the cell may eventually burst/lyse. (An extreme example of a hypotonic solution is water)

A hypertonic solution will result in osmotic flow of water from inside to outside the cell, causing the cell to shrink.

Question 8

State the formula for rate of excretion.

Answer 8

Filtration (GF) + Secretion (TS) = Reabsorption (TR) + Excretion
or

Rate of excretion = rate of filtration + rate of secretion - rate of reabsorption

Question 9

What is renal blood flow?

Answer 9

It is the volume of blood that comes into the kidney in a given period of time.

Question 10

What is Renal plasma flow?

What is the standard RPF?

Answer 10

It is the proportion of this RBF that is plasma.

The volume of plasma that passes through the kidneys per unit of time.

600 to 700 milliliters per minute (mL/min) per kidney.
*650mL/min

Question 11

What is glomerular filtration rate?

What is the standard GFR?

Answer 11

It is the volume of plasma filtered per minute.

125ml/min

Question 12

The ratio of filtered blood (GFR) to the renal plasma (RPF) can also be calculated.

This is referred to as____________and is expressed in percentage.

Answer 12

Filtration fraction.

Question 13

Calculate the filtration fraction.

Answer 13

Therefore,
Filtration fraction= GFR/RPF x 100
= 125mL/min/650mL/min X 100
= 19.2%

Question 14

State the net filtration pressure?

Answer 14

10mmHg

Question 15

What is the Filtration coefficient?

State the filtration coefficient.

Answer 15

We can express GFR in terms of net filtration pressure.
This is referred to as the filtration coefficient.

Therefore,
Filtration coefficient= 125mL/min / 10mmHg
= 12.5 mL.min-1.mmHg-1

Question 16

What is Plasma Clearance?

Units?

Answer 16

Plasma clearance= volume of plasma completely cleared of a particular substance per minute.

UNITS: mL/min

Question 17

State the 3 parameters that are determined using Plasma Clearance.

Answer 17

GFR
Renal plasma flow
Renal blood flow (indirectly)

Question 18

To measure plasma clearance of particular substance what are the 3 things we need to know?

Answer 18

Volume of urine excreted per unit time
Concentration of the substance in the urine
Concentration of the substance in the blood

Question 19

State the formula for plasma clearance.

Answer 19

Clearance (C)= (U * V)/ P
Where U= conc of substance in urine (mg/mL)
P= conc of the substance in plasma (mg/mL)
V= volume of urine flow (mL/min)

Remember C1 * V1 = C2 * V2

Question 20

To determine GFR, we need a substance that is completely filtered, BUT not reabsorbed or secreted.

Name the substance used.

Answer 20

Inulin is completely filtered by the body; it is neither reabsorbed nor secreted.

Question 21

State the Inulin clearance.

Name the parameter which has the same value as Inulin clearance.

Answer 21

Inulin clearance= GFR=125mL/min

Question 22

What really is the meaning of inulin clearance?

Answer 22

It means that 125mL of inulin-free plasma is returned to the circulation every minute!! (in 1 minute the kidneys have removed (cleared) all the inulin present in 125mL of plasma)

Question 23

__________ clearance is now used to determine GFR (Partially secreted in small amounts!!).

Answer 23

Creatinine clearance is now used to determine GFR (Partially secreted in small amounts!!).

Question 24

Name the substance clinically to measure renal plasma flow (= 650 ml/min).

State its properties.

Answer 24

Para-amino hippuric acid (PAH) = exogenous organic anion

*Complete filtration
*Complete secretion
*Not reabsorbed
PAH is
1) freely filtered at glomerulus,
2) secreted into the tubule (not reabsorbed) & 3) completely cleared from the plasma

i.e. all the PAH in the plasma that escapes filtration is secreted from the peritubular capillaries.

Question 25

RBF is not measured directly but derived from the renal plasma flow (RPF) & percentage of plasma volume in blood.

Our knowledge of blood composition and characteristics is applied here!

Show how it is calculated.

Answer 25

We already have renal plasma flow = 650 mL/min

% plasma volume is related to packed cell volume or hematocrit (= 45%), therefore the volume occupied by plasma is 55%.

Renal blood flow= 650/ .55 =1182 mL/min

Question 26

State the clearance values for the following.
1. Glucose
2. Inulin
3. Creatinine
4. Para amino hippuric acid

Answer 26

Cglucose = 0

Cinulin = 125 ml/min
(However, does require constant infusion to ensure a constant [inulin]p  not very convenient)

Ccreatinine  GFR = ~125 ml/min
(Creatinine is a muscle metabolite produced at a near constant rate. It is freely filtered and not reabsorbed but is slightly secreted,  gives a close approximation of GFR and is easier to measure than inulin clearance)

CPAH = RPF = 650 ml/min

Question 27

Why does glucose have a clearance of 0?

Answer 27

Normally, 100% of glucose in the filtrate is reabsorbed in the proximal tubule.

Question 28

State 3 properties of clearance markers.

Answer 28

Ideally any substance used as a clearance marker should have the following properties:

(1) Non-toxic
(2) Inert (i.e. not metabolized) 
(3) Easy to measure

Question 29

State the propertied of GFR marker.

Name a substance that is a GFR marker.

Answer 29

A GFR marker should be filtered freely; NOT secreted or reabsorbed.

Inulin

Question 30

State the propertied of RPF marker.

Name a substance that is a RPF marker.

Answer 30

A RPF marker should be filtered and completely secreted.

Para-amino-hippuric-acid

Question 31

Define renal threshold.

Answer 31

The term “renal threshold” refers to the concentration of a substance in the blood at which the kidneys begin to excrete the substance into the urine. Different substances have different renal thresholds, and these thresholds play a crucial role in renal physiology.

One common example is the renal threshold for glucose. Under normal circumstances, the kidneys reabsorb almost all of the glucose filtered through the glomerulus back into the bloodstream. However, when the blood glucose concentration exceeds a certain threshold, the renal tubules cannot reabsorb all the glucose, and the excess is excreted in the urine. This threshold is typically around 180 mg/dL (milligrams per deciliter) for glucose. If blood glucose levels exceed this threshold, glucosuria (glucose in the urine) may occur, indicating a loss of glucose through the kidneys.