BME Measurements and Analysis Laboratory

125:315 BME MEASUREMENTS AND ANALYSIS LABORATORY FALL 2004

10. LABORATOR Y #10: 11. RENAL PHYSIOLOGY I. Objectives

The objectives of this laboratory are to learn how:

1. The kidney filters unwanted chemicals from the blood 2. The kidney regulates fluid volume and salt concentration 3. The kidney regulates acid/base balance.

II. Introduction

The kidneys serve multiple functions essential to well-being. To understand the function of the kidneys, some background knowledge will be necessary. In this section, we briefly summarize how material diffuses, how differences in concentrations lead to differences in effective pressures, and finally how “counter-current” exchange occurs.

II.1. Diffusion

Suppose you inject a small quantity of dye into a jar of water, as illustrated below. At the left we show an aliquot of dye in clear water, and to the right we show how the dye spreads over time until it uniformly covers the available volume.

If C is the concentration of dye at time t and position x, then we would describe this behavior in one spatial dimension, x, according to the differential equation:

C 2 C  D , [1] t x 2

where D is a constant that describes how rapidly the dye spreads. This behavior is what we expect to see, and if this were what went on in our bodies, toxins would sooner or later build up and spread uniformly throughout our bodies. In fact, the kidneys reverse this natural course of events so as to concentrate toxins in the urine so that they can be removed from the body.

II.2. Diffusive ‘pressure’

Equation [1] tells us that concentrations tend to spread out – so if we dissolve red dye in water on one side of a membrane and have undyed water on the other side, then the dye will tend to spread across the membrane to equalize concentrations. This tendency to equalize pressures implies that the dye will tend to flow across the membrane, and in this sense diffusion is equivalent to a pressure.

As an example of this effective pressure, consider the problem of a semipermeable membrane separating two sides of a ‘U’ – tube, as shown below. If the left half of the tube contains dye molecules that are too large to fit through holes in the central membrane shown, then fewer water molecules will strike the membrane per unit time from the left than from the right – and so water will tend to flow from right to left through the membrane. This flow will produce a height difference, h (termed a ‘head’), as shown, and the flow will stop when the pressure due to the head, h, balances the flow.

The pressure generated by a solute of molar concentration c is given by van’t Hoff’s equation:

P  cRT, [2]

where R is the universal gas constant and T is the temperature in Kelvins. So if we dissolve more dye particles on the left of the figure above, c will grow, and the pressure difference across the membrane (and hence h) will increase proportionally.

II.3. Counter current exchange

From the last 2 subsections, two things are apparent. First, materials such as waste products (e.g. urea) or out of balance solutes (e.g. high salt concentrations from eating salted potato chips) would tend if left alone to spread throughout the body. Second, if salt or other solutes were concentrated on the other side of a semipermeable membrane anywhere in the body, it would tend – again if left alone – to absorb water from elsewhere. Evidently the body needs a mechanism to concentrate solutes such as salt and to pump water away from these concentrations. The kidney, among other things, does both, using an ingenious trick: the counter-current exchanger. In the kidney itself, this appears in the “Loop of Henle”; elsewhere the same trick is used to conserve heat (e.g. in the arctic fox, which is continually on contact with ice) and to reclaim water (e.g. in desert animals such as the kangaroo rat, which lives without ever drinking liquid water). Whether it is salt, or heat, or water being concentrated, the counter-current exchanger works as follows.

In the figure below, we show a schematic of the Loop of Henle. As indicated, the Loop has an outward flow (shown as down) and an inward flow (up). The color coding indicates an increase in concentration, say of salt. Because of the flow indicated, an initially concentrated region suggested by the dark shading will travel around the loop. This will produce the pattern of concentration sketched in (b). This concentration pattern will cause a dark, high concentration region on the right to be juxtaposed against a lighter, low concentration, region to its left. In the Loop of Henle, as well as elsewhere, the outward and inward tubes are in contact, so a flow of salt (indicated by the arrow in (c)) will result across the membrane separating these tubes. Consequently, the dark, higher salt concentration, region indicated near the bottom of the loop cannot escape the loop under influence of the flow: once it begins being drawn out of the tube, it sets up a gradient across the loop that recirculates the high salt concentration material.

Thus a high salt concentration is maintained (in truth with the help of active pumping further up the Loop that is beyond the scope of this discussion) by the counter-current geometry shown. So what? In the kidney, this high salt region is put to use by passing the collecting duct, on its way to the bladder, near this region. So collected urine passes this high salt concentration region, and in so doing water is extracted from the urine, and in this way, water is conserved and urine is concentrated.

This is only one of the kidney’s tricks, and its mechanism is suitably simplified in the discussion above, but hopefully this will give you an appreciation for one of the many clever ways that your body maintains balances necessary for life.

III. Purpose

To understand body fluid distribution, how different types of fluid infusions change the fluid distributions in the body, to learn about the role of kidneys in filtration and to understand acid-base physiology.

IV. Materials and Methods

IV.1. Apparatus

Hardware  PC running Windows

Software

 SimBioSys Software

IV.2. Procedure

1. Open the SimBioSys software 2. Double-click on the section called ‘Fluid Balance’. You will see three sub-sections which are: (17) Fluid Compartments, (18) Kidneys and Filtration, (19) Acid-Base Physiology 3. Read through the sub-section (17): Fluid Compartments

 Double click on the ‘Exercises’ in that sub-section to see the ‘Fluid Volumes’ exercise.  Before starting any simulation, record the baseline (normal) values (volumes) of body fluid compartments (Total body water, Blood Volume, ISF and ICF volume).  The instructions in the exercise are modified. The exercise is modified as follows: o You will infuse different types of fluids, to see how the fluid compartment values change with each type of fluid used for replacement. o From the toolbox in the bottom left corner, click on ‘Drug and Fluid Infusor’. You will infuse the following fluids: . 250 ml of Normal Saline . 250 ml of Hypertonic Saline (5% Nacl) . 250 ml of FFP (Fresh Frozen Plasma) o You have to infuse each fluid one by one: When you are done with one type of infusion, make sure you RECORD the new body fluid compartment volumes. To start a new infusion, RESET simulation. o Create a table just like the one shown in SimBioSys that will help you to analyze your data and make sure you include your data and the answers to the questions in part V.2 in your report.

4. Read through the sub-section (18): Kidneys and Filtration.

 Double click on the ‘Exercises’ in that sub-section to see the ‘Glomerular Filtration’ exercise.  Do the exercise as instructed (How GFR changes when afferent renal fraction is varied). Record values; use the values to answer the related question in part V.2.

5. Read through the sub-section (18): Acid-Base Physiology.

 Double click on the ‘Exercises’ in that sub-section to see the related exercises.  Do the following exercises as instructed: CO2 Effects and Relationship of H and pH to SID (Strong Ion Difference).  Record values; use the values to answer the related question in part V.2. V. Lab Report Guidelines

V.1. Report Format

Introduction: Motivate lab in your own words (personal assessment of topic importance)

Methods: Description of the lab procedure. Description of the various simulations.

Results: Data (Objective summary of results; results of your simulations). Focus on the organization of the data. Include figures and tables when needed.

Analysis: Meaning of the results; make sure you refer to your results and data when making analysis or conclusions. What conclusions can or cannot be made and why? Make sure you include answers to each question in V.2, below.

Summary & Conclusions: Include only overall trends or findings from Analysis Section. There need be no more than 3 or 4 main points.

References: (quote all sources used, including web sources, no plagiarism)

V.2. Analysis Guide Questions

1. Describe body fluid compartments, use a block diagram showing each compartment and include normal values (volumes) for each compartment. 2. Explain briefly how fluid transport is dependent on hydrostatic pressure, osmotic pressure and oncotic pressure. 3. Analyze the effect of different types of fluid infusions. Is the distribution of each fluid infusion the same in body fluid compartments? Explain, why the effect is the same or different for each fluid infused (Normal saline, hypertonic saline, water, FFP). Fresh Frozen Plasma (FFP) is a colloid, explain why colloids are plasma volume expanders? 4. Explain what happens to the glomerular filtration rate when you change the afferent resistance fraction?

5. Explain what happens to the [H+] vs. PCO2 curve when you change the SID? 6. Explain what happens to the [H+] vs. SID curve when you change the carbon dioxide tension

(PCO2)?