Physiology of Body Fluids

Objectives

Upon completion of this chapter, the student should be able to answer the following questions :

How do body fluid compartments differ with respect to their volumes and their ionic compositions?
What are the driving forces responsible for movement of water across cell membranes and the capillary wall?
How do the volumes of the intracellular and extracellular fluid compartments change under various pathophysiologic conditions?

In addition, the student should be able to define and understand the following properties of physiologically important solutions and fluids :

Molarity and equivalence
Osmotic pressure
Osmolarity and osmolality
Oncotic pressure
Tonicity

Key Terms

Steady-state balance

Positive balance

Negative balance

Molarity

Equivalence

Osmosis

Osmotic pressure

van’t Hoff’s law

Osmolarity

Osmolality

Tonicity (isotonic, hypotonic, and hypertonic)

Isosmotic

Permeable

Impermeable

Effective osmole

Ineffective osmole

Reflection coefficient

Osmotic coefficient

Oncotic pressure

Specific gravity

Total body water

Intracellular fluid (ICF)

Extracellular fluid (ECF)

Interstitial fluid

Plasma

Third space

Ascites

Capillary wall

Starling forces

Capillary filtration coefficient (K _f )

Aquaporin (AQP)

One of the major functions of the kidneys is to maintain the volume and composition of the body’s fluids constant despite wide variation in the daily intake of water and solutes. In this chapter, the concept of steady-state balance is introduced. Also, the volume and composition of the body’s fluids are discussed to provide a background for the study of the kidneys as regulatory organs. Some of the basic principles, terminology, and concepts related to the properties of solutes in solution also are reviewed.

Concept of Steady-State Balance

The human body is an “open system,” meaning that substances are added to the body each day, and similarly substances are lost from the body each day. The amounts added to or lost from the body can vary widely depending on the environment, access to food and water, disease processes, and even cultural norms. In such an open system, the volume and composition of the body fluids is maintained through the process of steady-state balance .

The concept of steady-state balance can be illustrated by considering a river on which a dam is built to create a manmade lake. Each day, water enters the lake from the various streams and rivers that feed it. In addition, water is added by underground springs, rain, and snow. At the same time water is lost through the spillways of the dam and by the process of evaporation. For the level of the lake to remain constant (i.e., steady-state balance), the rate at which water is added, regardless of source, must be exactly matched by the amount of water lost, again by whichever route. Because the addition of water is not easily controlled, nor can the loss by evaporation be controlled, the only way to maintain the level of the lake constant is to regulate the amount that is lost through the spillways.

To understand steady-state balance as it applies to the human body, the following key concepts are important.

1.
There must be a set-point from which deviations can be monitored (e.g., the level of the lake in the previous example or setting the temperature in a room by adjusting the thermostat).
2.
The sensors that monitor deviations from the set-point must generate effector signals that can lead to changes in either input or output, or both, to maintain the desired set-point (e.g., electrical signals to adjust the spillway in the dam analogy or electrical signals sent to either the furnace or air conditioner to maintain the proper room temperature).
3.
Effector organs must respond in an appropriate way to the effector signals generated by the set-point monitor (e.g., the spillway gates must operate and the furnace or air conditioner must turn on and off as appropriate).
4.
The sensitivity of the system (i.e., how much of a deviation away from the set-point is tolerated) depends on several factors, including the nature of the sensor (i.e., how much of a deviation from the set-point is needed for the sensor to detect the deviation), the time necessary for generation of the effector signals, and how rapidly the effector mechanisms respond to the effector signals.

It is important to recognize that deviations from steady-state balance do occur. When input is greater than output, a state of positive balance exists. When input is less than output, a state of negative balance exists. Although transient periods of imbalance can be tolerated, prolonged states of extreme positive or negative balance are generally incompatible with life.

Physicochemical Properties of Electrolyte Solutions

Molarity and Equivalence

The amount of a substance dissolved in a solution (i.e., its concentration) is expressed in terms of either molarity or equivalence . Molarity is the amount of a substance relative to its molecular weight. For example, glucose has a molecular weight of 180 g/mol. If 1 L of water contains 1 g of glucose, the molarity of this glucose solution would be determined as:

\frac{1 g/L}{180 g/ mol} = 0.0056 mol/L or 5 .6 mmol/L

$\frac{1 g/L}{180 g/ mol} = 0.0056 mol/L or 5 .6 mmol/L$

For uncharged molecules, such as glucose and urea, concentrations in the body fluids are usually expressed in terms of molarity. ^a Because many of the substances of biologic interest are present at very low concentrations, units are more commonly expressed in the millimolar range (mmol/L).

a The units used to express the concentrations of substances in various body fluids differ among laboratories. The system of international units (SI) is used in most countries and in most scientific and medical journals in the United States. Despite this convention, traditional units are still widely used. For urea and glucose, the traditional units of concentration are milligrams per deciliter (mg/dL, or 100 mL), whereas the SI units are millimole per liter (mmol/L). Similarly, electrolyte concentrations are traditionally expressed as milliequivalent per liter (mEq/L), whereas the SI units are mmol/L (see Appendix B ).

The concentration of solutes, which normally dissociate into more than one particle when dissolved in solution (e.g., sodium chloride [NaCl]), is usually expressed in terms of equivalence. Equivalence refers to the stoichiometry of the interaction between cation and anion and is determined by the valence of these ions. For example, consider a 1 L solution containing 9 g of NaCl (molecular weight = 58.4 g/mol). The molarity of this solution is 154 mmol/L. Because NaCl dissociates into Na ⁺ and Cl ⁻ ions, and assuming complete dissociation, this solution contains 154 mmol/L of Na ⁺ and 154 mmol/L of Cl ⁻ . Because the valence of these ions is 1, these concentrations also can be expressed as milliequivalents of the ion per liter (i.e., 154 mEq/L for Na ⁺ and Cl ⁻ , respectively).

For univalent ions such as Na ⁺ and Cl ⁻ , concentrations expressed in terms of molarity and equivalence are identical. However, this is not true for ions having valences greater than 1. Accordingly, the concentration of Ca ⁺⁺ (molecular weight = 40.1 g/mol and valence = 2) in a 1 L solution containing 0.1 g of this ion could be expressed as:

\begin{matrix} \frac{0.1 g/L}{40.1 g/mol} = 2.5 mmol/L \\ 2 .5 mmol/L \times 2 Eq/mol = 5 mEq/L \end{matrix}

$\begin{matrix} \frac{0.1 g/L}{40.1 g/mol} = 2.5 mmol/L \\ 2 .5 mmol/L \times 2 Eq/mol = 5 mEq/L \end{matrix}$

Although some exceptions exist, it is customary to express concentrations of ions in milliequivalents per liter.

Osmosis and Osmotic Pressure

The movement of water across cell membranes occurs by the process of osmosis . The driving force for this movement is the osmotic pressure difference across the cell membrane. Fig. 1.1 illustrates the concept of osmosis and the measurement of the osmotic pressure of a solution.

Fig. 1.1, Schematic representation of osmotic water movement and the generation of an osmotic pressure. Compartment A and compartment B are separated by a semipermeable membrane (i.e., the membrane is highly permeable to water but impermeable to solute). Compartment A contains a solute, whereas compartment B contains only distilled water. Over time, water moves by osmosis from compartment B to compartment A. (Note: This water movement is driven by the concentration gradient for water. Because of the presence of solute particles in compartment A, the concentration of water in compartment A is less than that in compartment B. Consequently, water moves across the semipermeable membrane from compartment B to compartment A down its gradient). This causes the level of fluid in compartment A to rise and the level in compartment B to fall. At equilibrium, the hydrostatic pressure exerted by the column of water (h) stops the movement of water from compartment B to A. Thus, at equilibrium, the hydrostatic pressure generated by the height (h) of the column of water in compartment A is equal and opposite to the osmotic pressure generated by the solute particles in compartment A.

Osmotic pressure is determined solely by the number of solute particles in the solution. It is not dependent on factors such as the size of the solute particles, their mass, or their chemical nature (e.g., valence). Osmotic pressure (π), measured in atmospheres (atm), is calculated by van’t Hoff’s law as:

π = nCRT

$π = nCRT$

where n is the number of dissociable particles per molecule, C is total solute concentration, R is gas constant, and T is temperature in degrees Kelvin (°K).

For a molecule that does not dissociate in water, such as glucose or urea, a solution containing 1 mmol/L of these solutes at 37°C can exert an osmotic pressure of 2.54 × 10− ² atm as calculated by Eq. (1.3) using the following values: n is 1, C is 0.001 mol/L, R is 0.082 atm L/mol, and T is 310°K.

Because 1 atm equals 760 mm Hg at sea level, π for this solution also can be expressed as 19.3 mm Hg. Alternatively, osmotic pressure is expressed in terms of osmolarity (see the following discussion). Thus a solution containing 1 mmol/L of solute particles exerts an osmotic pressure of 1 milliosmole per liter (1 mOsm/L).

For substances that dissociate in a solution, n of Eq. (1.3) has a value other than 1. For example, a 150 mmol/L solution of NaCl has an osmolarity of 300 mOsm/L because each molecule of NaCl dissociates into a Na ⁺ and a Cl ⁻ ion (i.e., n = 2). If dissociation of a substance into its component ions is not complete, n is not an integer. Accordingly, osmolarity for any solution can be calculated as:

\begin{matrix} Osmolarity = Concentration \times Number of dissociable \\ particles \\ mOsm / L = mmol / L \times number of particles / mol \end{matrix}

$\begin{matrix} Osmolarity = Concentration \times Number of dissociable \\ particles \\ mOsm / L = mmol / L \times number of particles / mol \end{matrix}$

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