Introduction to Body Fluids

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The fluids in your body are composed of water and dissolved substances, including electrolytes, which are crucial for body function. Your goals for learning are: To list the general functions and importance of water and electrolytes in the body. To identify the fluid compartments and the relative concentrations of electrolytes within those fluid spaces. What you need to know: The definition of osmosis which is the diffusion of water through a selectively permeable membrane from a higher water concentration (and lower solute concentration) to a lower water concentration (and higher solute concentration). Membrane transport mechanisms We ingest water and electrolytes through the gastrointestinal, or GI, tract. Fluids are absorbed into the plasma in the intestine. The fluids circulate within the plasma, bathing the cells in the body. The kidneys remove excess ions and water from the body through the urine. Although water is also lost at other sites. Water performs several important functions in the body. Water helps maintain body temperature. When water vaporizes off the skin, it takes large quantities of heat with it. This process cools the body temperature down. Because water has a high heat capacity, it can absorb and release large quantities of heat before significantly changing temperature. Our bodies are composed of 50 to 70 percent water and that large percentage of water holds heat in the body and helps prevent fluctuation in body temperature. Water acts as a protective cushion in amniotic fluid and cerebral-spinal fluid. Water acts as a lubricant in the serous fluids, joints, and gastrointestinal tract. [EATING SOUND] Water is the reactant for hydrolysis reactions that occur in the body. Starch is hydrolyzed in the mouth by the enzyme salivary amylase. With the help of this enzyme, water molecules are added to the bonds between the glucose unit in the starch hydrolyzing the starch to glucose. Note that water can also be formed during some chemical reactions in the body such as the reactions that produce metabolic water. Water acts as a solvent to dissolve molecules and ions in the body. For example, if you eat a salty pretzel, the water in your saliva will dissolve the salt. Water is a polar molecule. When water dissolves ions, the partial negative charge on the oxygen attracts positive ions such as sodium and the partial positive charge on the hydrogen attracts negative ions such as chloride. Except for the salts deposited in bone and teeth, most other ions in the body are dissolved because of water’s ability to act as a solvent. Water within cells is an important solvent. It dissolves many of the proteins and other solutes. We are now looking at a blood vessel. Because of water’s ability to dissolve ions and molecules within the body fluids, water functions as a medium for the delivery of nutrients and the removal of wastes from the cells through the plasma. The percentage of water in a person depends on the amount of fat tissue which is only about 20 percent water compared to lean body mass or muscle mass which is about 65 percent water. Let’s determine the approximate percentage of water in the bodies of each of these people. A healthy young man who is muscular and does not have a lot of fat in his body is about 60 percent water. Newborns have the highest percentage of water in their bodies at 73 percent. A healthy young woman naturally has more fat and less muscle than a man and is about 50 percent water. The more fat a person has in his or her body, the less water is present. Older people tend to have less lean body mass and therefore contain less water. Water along with its dissolved solutes occupies three main compartments within the body. Intracellular fluid is the fluid within cells, it is also known as cytosol. Extracellular fluid is the fluid found outside of cells. There are two major kinds of extracellular fluid. Interstitial fluid is the fluid surrounding the cells. Plasma is the fluid component of the blood. Let’s consider a 70-kilogram man. 60 percent of his weight or approximately 40 liters is fluid. Approximately 62% of the body’s fluid is intracellular. [TAP WATER SOUND] Approximately 30 percent of the body’s fluid is interstitial. Approximately 8 percent of the body’s fluid is plasma. You are looking at plasma, a typical body fluid. The term “body fluid” refers to the water in the body and all the dissolved substances which are also known as solutes. Since the water dissolves the solutes, it is the solvent. A typical body fluid contains electrolytes also known as ions. Proteins are considered to be colloids when dispersed in body fluids. Compared to simple ions, proteins are huge molecules. Because they bear a negative charge, we will consider them to be electrolytes. Non-electrolytes are uncharged molecules found in body fluids. Glucose is an example of a non-electrolyte. Blood cells do not dissolve in water. They are suspended particles and are not considered to be a part of the body fluid. Electrolytes are charged particles or ions that are dissolved in body fluids. The major positive ions, or cations, in body fluids, are; Sodium ion, Potassium ion, Calcium ion, and Magnesium ion. The major negative ions, or anions, in body fluids, are; chloride ion, bicarbonate ion, phosphate ions, sulfate ion, organic acids, and proteins. Each fluid compartment needs just the right types and levels of electrolytes for proper functioning of neurons, muscle cells, and other cells in the body. The electrolyte composition of extracellular fluids and intracellular fluids have significant differences. Filling in the pie graph will help illustrate these differences. In addition to sodium, extracellular fluid contains the positive ions; potassium, calcium, and magnesium. The intracellular fluid contains the positive ions; potassium, sodium, and magnesium. The extracellular fluid contains the following negative ions: chloride, protein, bicarbonate, phosphate, organic acid, and sulfate. The intracellular fluid contains the negative ions; phosphate, protein, bicarbonate, chloride, and sulfate. The composition of interstitial fluid is almost identical to that of plasma, except for one negative electrolyte, protein. There is very little protein in interstitial fluid. To summarize; the major positive ion of the extracellular fluid is sodium and its major negative ion is chloride. The major positive ion of the intracellular fluid is potassium and its major negative ions are protein and phosphates. Let’s count the number of positive and negative ions, or particles, in this sample of intracellular fluid and see if the number of positive ions equals the number of negative ions. There are 9 positive ions and 6 negative ions here, so the number of ions are not equal. Now let’s count the number of positive and negative charges in this sample of intracellular fluid. There are 11 positive charges and 11 negative charges in this intracellular fluid. The laws of chemistry dictate that within a fluid compartment, the total number of positive charges must be equal to the total number of negative charges. Even though there are six negative ions and nine positive ions here, the charges balance because the individual ions have different charges, there may not be the same number of positive and negative ions in the compartment, the charges will always balance. Now, let’s look at a few of the many important functions electrolytes perform in the body. Electrolytes serve as cofactors for enzymes. A very important reaction in the body occurs when carbon dioxide and water form carbonic acid. We can speed up this reaction with the enzyme carbonic anhydrase. Cofactors are non-protein substances that act along with enzymes to speed up reactions in the body. Calcium, Magnesium, and other cations such as Zinc can serve as cofactors for enzymes. Zinc is a cofactor for this enzyme. Let’s add it the enzyme. This enzymatic reaction shown here cannot occur without a zinc ion present. Many other enzymes in the body require positive metal ions as cofactors in order to function. Electrolytes in the form of sodium and potassium ions also contribute to membrane potential in all cells and are responsible for action potentials in neuron and muscle cells. [BODY ORGAN SOUND] Calcium ions are important electrolytes because they are involved in the secretion and action of hormones and neurotransmitters. The blue balls represent vesicles containing neurotransmitters. Calcium is also involved in the contraction of muscles including the heart. Some electrolytes such as bicarbonate phosphate and protein help maintain acid /base balance. By pumping sodium out of the cell, the sodium potassium pump uses ATP to keep the concentration of sodium low in the cells. During secondary active transport, some transport proteins will allow sodium to diffuse from areas of high to low concentration and drag with it a molecule or another ion such as glucose seen here from an area of lower to higher concentration. Glucose enters the proximal convoluted tubular cells of the kidney tubulars and the intestine by secondary active transport. Finally, electrolytes, including proteins, also play a major role in promoting the movement of water between fluid compartments through osmosis. Let’s take a closer look at what happens during osmosis. Osmosis is the movement of water across a membrane from the side that has more water and therefore less solute to the side that has less water and therefore more solute. When there are equal amounts of water particles on either side of the membrane, water moves freely back and forth across the membrane in both directions at the same rate. We say that the two solutions are isotonic, meaning that they have the same total concentration of non-penetrating solute particles. Let’s see what happens when more solute particles are added to the right side of the container. Now more solute is present on the right side and it is hypertonic compared to the left side. Because less solute is present on the left side, we say that solution is hypotonic compared to the right side. Note that if you increase the concentration of solute, you decrease the concentration of water. Water still moves freely across the membrane in both directions. However less water will move from right to left because the higher concentration of water on the left side creates a greater chance of a collision between a water molecule and a channel on the left side. As a result, more water will move from the left side to the right side of the container. Let’s see how solutions of different tonicity affect real cells. In a hypertonic solution, water moves out of cells and they shrink, or crenate. In a hypertonic solution, water moves into cells and they expand. We measure the ability of a solution to cause osmosis in terms of osmotic pressure which is expressed in millimeters of mercury, a unit of pressure. Osmotic pressure is defined as the external pressure applied to the top of the fluid to prevent osmosis from occurring. The greater the number of solute particles dissolved in solution, the higher the osmotic pressure. Here we see a patient ready to receive an intravenous solution. Normally patients are given an isotonic IV solution. In this patient, the hypertonic and hypotonic IVs are contraindicated. Let’s see what happens to this patient’s blood cells when each IV is given. If we put a red blood cell in a hypotonic solution, the cell will expand. If too much water enters the cell, it could eventually undergo hemolysis or break open. When a cell is placed in a more dilute solution, more water hits water channels on the outside of the membrane than on the inside causing more water to move to the into the cell. This is why hypotonic IV solutions are only used in specific clinical situations. If we put a red blood cell in a hypertonic solution, the cell will shrink, or crenate because more water hits water channels in the membrane on the inside of the cell than on the outside of the cell. There is a net flow of water out of the cell that causes the cell to shrink. This is why hypertonic IV solutions are only used in specific clinical situations. If we put a red blood cell in an isotonic solution, the cell volume will remain constant. Here’s a summary of what we’ve covered. Fluids are composed of water and all the substances, or solutes, dissolved in the water in the body. Most fluids within the body exist in three major compartments: intracellular, plasma, and interstitial. The interstitial compartment and the plasma are the extracellular compartments. Within a solution, positive and negative charges must balance regardless of the number of ions present. The concentrations of dissolved ions, or electrolytes, in the intracellular compartment are very different than the concentrations of electrolytes in the extracellular compartment. Both water and electrolytes have many important functions in the body. Osmosis is the movement of water across a membrane from the side that has more water to the side that has less water.