Body Fluid Compartments Self-Study Guide PDF

Introduction to body fluid compartments: self-study guide Learning outcome: Describe the major body fluid compartments and explain the thirst mechanism. Introduction: One day you are going to be prescribing these… to all these different people… Those bags of fluids pictured above contain water,...

Introduction to body fluid compartments: self-study guide Learning outcome: Describe the major body fluid compartments and explain the thirst mechanism. Introduction: One day you are going to be prescribing these… to all these different people… Those bags of fluids pictured above contain water, electrolytes, and some of them contain glucose, also referred to as dextrose – the dextro-rotatory form (the most common optical isomer) of glucose. Some questions to think about: What is the purpose of the electrolytes? What about the glucose? Why are they in different concentrations? How will you know which one to prescribe and in what situation? Some clinical scenarios where you might have to prescribe IV fluids 26-year-old Liam is in ED with a ruptured spleen after a motor-cycle accident 3-month-old Piper has had diarrhoea and vomiting for 2 days, she is pale, quiet and lethargic 82-year-old Marion is nil by mouth, awaiting surgery for a bowel obstruction To answer these clinically relevant questions first you need to understand how water and electrolytes are distributed in the body and the many important physiological roles that they play. Depending upon our age and gender human beings are between 45-75% water. Healthy younger adult females are approximately 50-55% water and healthy younger adult males are around 60% water. Total body water (TBW) is highest infancy and declines with age. This means that the majority of us are mostly made of water. Some tissues contain more water than others, your brain is 85% water, skeletal muscle 75% whereas bone is 25% water and adipose tissue 10-20% water. The difference in TBW between genders reflect that females have relatively less skeletal muscle and slightly more adipose tissue than males. Newborn infants have low body fat and low bone mass so they can be up to 75% water. For the purposes of simplicity for the remainder of this document we’ll think of TBW as being 50-60% of total body mass. Compartments: The 50-60% of your body that is water resides in two main compartments in the body, the intracellular fluid (ICF) and the extra-cellular fluid (ECF). ICF is technically speaking divided into trillions of compartments – the cells. The barrier between the ICF and the ECF is the cell membrane. ECF constitutes all the fluid that is not contained within a cell membrane, it can be simply divided into the plasma, the interstitial fluid, and the transcellular fluid. Plasma is the volume of fluid contained within blood vessels – when we do blood tests of electrolyte levels it is the plasma concentrations that we are measuring. Plasma is about 20% of ECF. Interstitial fluid is the fluid in the tissues – it constitutes the external environment in which all the cells of the body sit. Interstitial fluid is about 75-80% of ECF. Transcellular fluid is the ECF fluid in locations such as the pleural cavity, the CSF, joints, the vitreous of the eye and G.I. tract secretions. It constitutes approx. 5-7% of ECF and normally plays a lubrication and less commonly a transport role. Transcellular fluid has the same solute composition as interstitial fluid so is considered part of that compartment. Composition of compartments: The water in both the ICF and ECF compartments contains solutes with the commonest and most physiologically important being: Electrolytes (chemical compounds that dissociate into ions in water – these charged particles can conduct an electrical current) Glucose Dissolved gases (oxygen, carbon dioxide) Proteins Lipids Products of metabolism (i.e. urea, creatinine) The arterial blood gas result below demonstrates the solutes in blood (part of the ECF), displaying the concentrations of common electrolytes, glucose, a metabolite (lactate) and the values of dissolved gases (in mmHg) Osmolarity and Osmolality Osmolarity is the total solute concentration of all particles in a litre of solution, it is expressed in milliosmoles per litre (mOsm/L). Osmolality is the total number of solute particles in a kg of water (mOsm/kg). When thinking about human physiology the values are essentially the same. Normal Osmolarity is 300mOmol/L in all compartments. Osmolality and osmolarity are used inter- changeably in this document. A sudden change in the osmolarity of a compartment will result in the movement of water down it’s osmotic gradient from one compartment to the other. This can be potentially dangerous when thinking about body compartments because it will mean that one physiological compartment may swell due to the sudden influx of water while the other shrinks. Na+ and Cl- are the major determinants of ECF osmolarity. If there was a decrease in ECF [Na+] that would result in a shift of water down its osmotic gradient into the ICF compartment resulting in swelling of cells. When CNS neurons swell within the rigid confines of the skull it can result in a decrease in CNS blood supply (the blood vessels are compressed by the swelling brain tissue) which causes depression of CNS function. If ongoing it will result in herniation of parts of the brain through the foramen magnum which can be fatal due to compression of vital centres in the brainstem. Tonicity Tonicity refers to the effect the osmolarity of a solution has on cells. Strictly speaking it is the ability of a solution to change the shape (or alter the cell membrane tension) of a cell by altering the cell’s internal water volume. The tonicity of a solution depends upon: 1. The solute concentration of the solution 2. The solute permeability of the cell membrane Isotonic solutions have the same solute concentration as the ICF and therefore have the same osmolarity as the ICF. Cells exposed to isotonic solutions, such as the common IV fluid 0.9% saline (also called normal saline) retain their normal shape – water does not pass out of or into the cell. Hypertonic solutions have a higher solute concentration than the ICF, they cause the cell to shrink as water moves down it’s osmotic gradient out of the cell. Hypotonic solutions have a lower solute concentration than the ICF, meaning water will enter the cell causing it to swell. Some IV fluids such as 5% dextrose are functionally hypotonic because the glucose is taken up into cells rapidly after the fluid is infused, leaving just water with no solutes behind. Resting membrane potential The ratio of electrolytes, particularly K+ in ECF to ICF (on one side of the cell membrane to the other) needs to remain roughly constant to maintain resting membrane potential (RMP). Resting membrane potential is -50 to -90mV – meaning that the cytoplasmic side of the cell membrane is negative with respect to the outside. The slow leak of K + out of the cell (down its concentration gradient) whilst leaving behind negatively charged protein anions is responsible for most of the RMP. The cell membrane is somewhat permeable to K + and impermeable to proteins. Maintenance of RMP is vital to preventing unwanted activity, overactivity, or inactivity of neural, muscle and cardiac cells. In the worst-case scenario, a significant imbalance of K + can result in disruption of co-ordinated activity of cardiac myocytes (cardiac muscle cells) resulting in cardiac arrest of the patient. This is why maintenance of the correct ECF electrolyte values is vitally important – this is something that you will have influence upon when you are an intern – measuring electrolytes and prescribing fluids with appropriate electrolyte compositions. Composition continued: The electrolyte composition of the intracellular and extracellular compartments is very different: Interstitial and plasma composition is very similar – the main exception being the much higher protein concentration in plasma – capillary walls being relatively impermeable to larger molecular weight molecules such as proteins. Na+ and K+ swap roles between ECF and ICF – in the ECF Na+ is the major cation, whilst in the ICF that role is taken by K+. Similarly, Cl- and HPO4- swap roles as the major anions (although proteins play a more significant anionic role in the ICF than they do in the ECF). Composition of ICF vs ECF ICF ECF Na 10 140 K 155 3.8 Cl 3 102 HCO3 10 28 Ca