Back to Basics in Physiology: Fluids in the Renal and Cardiovascular Systems

By Juan Pablo Arroyo and Adam J. Schweickert

This original six chapter book will briefly review and integrate the basic concepts behind water distribution and movement in the body. This fills a knowledge gap that most medical and undergraduate physiology students acquire when these topics are studied separately. As of now, there is no textbook that fully integrates renal, cardiovascular and water physiology in a clear understandable manner. The book is intended primarily for medical students and undergraduate physiology students. Chapters include: 1) Water and its Distribution; 2) Water Dynamics; 3) Fluid Handling by the Heart and Blood Vessels; 4) Fluid Handling by the Kidneys; 5) Water and Oxygen Delivery; 6) Integration in the Response to Hemorrhage, Volume Depletion, and Water Redistribution.

• An easy-to-read, step by step explanation of how water is distributed, how it moves, how this aides in oxygen delivery and how this is regulated in the human body
• Presents a complex and detailed topic in an original way that will allow students to understand more complex textbooks and explanations

• Language: English
• Publisher: Academic Press
• Release date: Mar 20, 2013
• ISBN: 9780124071995

Juan Pablo Arroyo

After receiving his medical degree with honors from the University of La Salle in Mexico City, Dr. Arroyo pursued a PhD in Biomedical Science at the National Autonomous University in Mexico City (UNAM), focusing on renal ion transport physiology. During this time he was appointed Adjunct Professor of Physiology at the University of La Salle and the Panamerican University School of Medicine. He then completed a two year Post-Doctoral fellowship in Renal Genetics and Hypertension at the Genetics Department of the Yale University School of Medicine. Dr. Arroyo is now a Tinsley R. Harrison Society Scholar at Vanderbilt University, where he is undergoing his clinical training in Internal Medicine and Nephrology. Dr. Arroyo has been a guest speaker at several international renal physiology conferences, and is interested in clinical medicine, basic science research and active teaching in the classroom.

Book preview

1

How Fluid Is Distributed in the Body

Cells, Water, Salt, and Solutions

1.1 Why Is Water Amazing?

We often tend to not give much thought to water. We drink it, we know we need it, we know the bulk of the earth is covered by it, but that’s about it. When we talk about water in the body and its importance in chemistry, we think of it as sort of a boring liquid that just dissolves stuff. We seldom think about the key properties that make water so essential.

1. It’s everywhere—Water covers about 70% of the earth’s surface, it makes up 60% of the typical human body.

2. It’s polar—Water, as we know, is made up of two hydrogen atoms and an oxygen atom covalently bonded. However, oxygen is more strongly electronegative than hydrogen. So, the mutual partnership of electrons is uneven: more of the electrons are on oxygen’s side. This leads to a more negative charge on the oxygen side of the molecule and a more positive charge on the hydrogen side. This makes water a wonderful molecule for positive–negative interactions which help dissolve compounds (think electrolytes!).

3. It’s stable—Water is stable with respect to heat change. Hydrogen bonding is largely responsible for water’s very large heat capacity. Putting heat into a system where all the molecules are attracted to each other means it takes more energy to pull them apart. Conversely, it takes the removal of more energy for water to cool. This explains many naturally occurring phenomena! For example, it explains why coastal climates are more temperate, why sweating is an effective means of cooling (it absorbs more heat before evaporating from your skin), and why you only need to consume a relatively small amount of calories to maintain your body temperature.

4. It’s reactive (it plays well with others)—Water knows how to both give and take. It is known as the universal solvent. This is due mainly to its size and polarity as discussed above. This means that it tends to readily break the electrostatic attraction (positive–negative, magnet-type interactions) between stingy solutes. So, for example, when you put NaCl in water, it ionizes as it becomes Na+ and Cl-. So water’s chemistry is necessary for many of the body’s proteins to even function!

5. It contains hydrogen!—As mentioned earlier, water is small, polar, and highly concentrated within the body. Because ionization or hydration is involved in almost every biochemical process known, water is able to provide a nearly limitless supply of much-needed hydrogen ions!

1.2 What Happens When Water Makes Friends? Basic Properties of Solutions

A solution can be simply defined as liquid with other molecules mixed in. The mixture should be stable and uniform. The liquid, for example, water, is known as the solvent. The molecules mixed in, for example, table salt, are the solute. In general, the solvent is the thing there is more of in solution, and the molecules that are mixed in take on the properties of the solvent (e.g., a tablespoon of solid salt added to liquid water become liquid saltwater and looks just like pure water). When we talk about solutions in this book, we’ll be talking about liquid mixtures, but understand that you can have gas and solid solutions as well, for example, air. Many of the properties of solutions are beyond the scope of this text, but we will detail what properties are clinically relevant and attempt to integrate them throughout the book.

1.3 Quantifying Solutions

It should be noted that there are several properties of solutions that are entirely dependent on the amount of solute rather than what kind of solute. We can agree that a spoonful of salt added to a lake of water will be a different solution than a spoonful of salt added to a shot glass of water. But how do we quantify this difference? There are several ways, but the most useful way to quantify amount would be to know each solution’s concentration. Well, chemists use a few different units to describe concentration. Two of the most pervasive are molality and molarity. The root of the word comes from an arbitrary quantity known as a mole. So, you should remember from basic chemistry that chemists love reactions. They also love mixing stuff. Let’s say we want to play chemist and test this by making two different solutions, one with solute X and one with solute Y. We want to make the concentrations of each the same. The problem is that an individual X molecule weighs more than a Y molecule. But we can’t count the individual molecules, and we can’t just take 12 g of X powder and 12 g of Y powder and dump them into two equal glasses of water. If X molecule weighs twice as much as Y, then we’ll have half the concentration of X molecules compared to Y! This led to the development of the mole. Thus, 1 mole of X=1 mole of Y regardless of their individual weights.

Molarity=moles solute/L solution

Molality=moles solute/kg solvent

(Note that the arbitrary number was based on the number of atoms within a pure 12 g of normal carbon. This estimates to ~6.022×10−23 atoms/12 g ¹²-Carbon=1 mol).

1.4 Forces Affecting Static Solutions

Ok, so that’s solutions in a small nutshell. Now we’re going to look at three very important concepts involving behaviors of solutions: Diffusion, Gradients, and Osmosis.

Diffusion means to scatter, pour out. You should recall from your basic science classes that molecules—and thus matter—are made up of thermal energy. This energy is partly made up of self-propelled motion (i.e., kinetic energy). Well, back in 1827, a guy named Robert Brown, who was actually a Scottish botanist, was putting some pollen in a glass of water and watching it move around for hours (imagine his dinner party conversations!). He observed that this movement of pollen in water was random (later known as Brownian motion). People like Albert Einstein postulated that this applied to even smaller particles, including atoms. Turns out they were right. Particles will move constantly in random directions. Now imagine high concentration of these particles in solution. They will naturally collide and bounce off each other, right? You can imagine these particles will end up farther away from each other the more they collide, until they are dispersed evenly throughout the solution.

Now, let’s imagine what would happen if you were to put a high-concentration solution in contact with a low-concentration solution. As the high-concentration solution comes in contact with the low concentration, you would initially have a difference in the concentrations between solutions, this is known as a gradient. A gradient is basically a difference (concentration, pressure, temperature, etc.) between two points. In this example, the difference between the two solutions would be considered a concentration or diffusion gradient.

Key

Flow down a gradient is the movement from high concentration to low concentration!

This is easier to conceptualize and more applicable to the human body when you consider a large container separated by a semipermeable membrane (Figure 1.1A). Imagine the high-concentration solution on the left (A) and the low-concentration solution on the right (B). Which way do you think the particles (X) randomly wander toward? That’s right! They’d randomly drift around and collide, and over a short amount of time, the particle collisions would drive them down the gradient to the area of lower concentration (from 1→2) (Note that this would only apply to the particles capable of traversing the semipermeable membrane. Keep this concept in mind as we go