Fluid Compartments, Body Fluid Composition and Movements of Fluid between Compartments
Water Intake, Regulation of Water Intake, Water Output, and Regulation of Water Output
Sources of Hydrogen Ions, Strengths of Acids and Bases, and Regulation of Hydrogen Ion Concentration
Maintaining electrolyte and water balance in the body requires that equal amounts of these substances entering the body must leave the body. Recall that electrolytes are substances that release ions. Changing the water balance also changes the electrolyte balance in the body.
Fluids of the body are distributed into different compartments. Each compartment will have a fluid that varies in composition. Movements of fluids between these departments must be regulated.
The fluid compartments of the body include:
1) intracellular compartments - includes the fluids inside cells; about 63% of all the fluid in the body.
2) extracellular compartments - includes all fluids found outside of cells; includes fluids between cells called interstitial fluids; also includes the plasma of blood and fluids found in lymphatic vessels; also includes cerebrospinal fluid, fluids of the eye, synovial fluids, serous fluids, mucus, saliva and any other secretion of exocrine glands; the fluids in extracellular compartments make up about 37% of all body fluids.
Intracellular fluids contain high concentrations of potassium, phosphate, magnesium and sulfate ions. They contain relatively low concentrations of sodium, chloride, and bicarbonate ions. Intracellular fluids also have a relatively high concentration of proteins.
Extracellular fluids generally contain high concentrations of sodium, chloride and bicarbonate ions. They have relatively low concentrations of potassium, calcium, magnesium, phosphate and sulfate ions. Of the extracellular fluids, plasma of blood contains the highest concentration of proteins.
Two major factors influence the movement of fluids between compartments - 1) hydrostatic pressure (pressure created by a fluid) and 2) osmotic pressure (pressure created by solids that pull water towards them).
Hydrostatic pressure (blood pressure) forces fluid out of plasma and osmotic pressure returns fluids to blood.
Hydrostatic pressure drives fluid into lymphatic vessels.
Osmotic pressure generally regulates the movement of fluids in and out of cells.
Remember, wherever sodium ions go, water will follow!
Water balance is created when the amount of water you take in is the amount of water you put out either through urine, sweating or breathing.
Most of the water we take in is through drinking beverages. About 30% of the water we get is through moist foods and 10% of the water in our bodies is made by cells through chemical reactions. The water our cells generate is called water of metabolism.
The primary stimulus that encourages water intake is thirst. The hypothalamus is the structure in the body that triggers the feeling of thirst.
Water can be lost by the body through urine, feces, sweat, and breathing. Most water is lost through urine.
Water output in urine is primarily controlled by the hormone called ADH (antidiuretic hormone). Recall that when ADH is released, the tubules of the kidney become more permeable and more water is returned to the blood stream. Therefore less water ends up in urine!
Electrolyte balance occurs when the amount of electrolytes taken in equals the amount of electrolytes lost from the body.
Most electrolytes are taken in through eating food. The most important electrolytes are sodium, potassium, calcium, phosphate, sulfate, bicarbonate, magnesium, hydrogen, and chloride ions.
Most people take in enough electrolytes in their diets. However, if a severe electrolyte deficiency develops, a person may experience a salt craving.
Electrolytes are lost from the body through sweat, urine, and feces.
The kidneys are the major organs that help to maintain electrolyte balance in the body. Hormones also play major roles in maintaining electrolyte balance.
Recall that aldosterone is a hormone released by the adrenal glands. This hormone causes the body to retain sodium. When sodium is retained potassium is lost through urine.
Recall that PTH (parathyroid hormone) and calcitonin affect calcium concentrations in the blood. PTH is released when calcium levels in the blood fall below normal. PTH then stimulates calcium and phosphate ion concentrations to increase in the blood. Calcitonin is released when calcium levels rise about normal. Calcitonin causes calcium levels to decrease in blood.
Recall that acids are substances that release hydrogen ions. Bases are substances that combine with hydrogen ions.
Most hydrogen ions in the body are made by cells through chemical reactions but some hydrogen ions are taken in through foods.
Acids that release a lot of hydrogen ions are called strong acids. Those that release only a few hydrogen ions are called weak acids. For example, the acid in the stomach (HCl) is a strong acid. The an acid found in blood called carbonic acid is a weak acid.
Strong bases are bases that can combine with a lot of hydrogen ions. Bases that can combine with only a few hydrogen ions are called weak bases. For example, sodium hydroxide (NaOH) is a strong base while chloride ions and bicarbonate ions are weak bases.
The pH of body fluids, or the amount of hydrogen ions in body fluids, is regulated by acid-base buffer system, the respiratory center, and the nephrons of the kidneys.
Buffer systems prevent a change in pH. For example, blood normally has a pH of around 7.2. The blood therefore contains buffers to prevent the pH from changing dramatically.
Buffer systems get rid of free hydrogen ions when the pH of a fluid goes down (becomes more acidid). Buffer systems can also release free hydrogen ions when the pH of a solution goes up (becomes more basic).
The most important buffer systems in body fluids are 1) the bicarbonate buffer system, 2) the phosphate buffer system and 3) the protein buffer system.
Recall that the respiratory center is located in the brain stem. It controls hydrogen ion concentrations by regulating the rate and depth of breathing. If the concentration of hydrogen ions increases in the blood, the respiratory center will increase the rate and depth of breathing. If hydrogen ion concentrations fall below normal, the respiratory center will decrease the rate and depth of breathing. Recall that if carbon dioxide levels in the blood increase, the hydrogen ion concentration will increase as well. So breathe to get rid off all that carbon dioxide!
Nephrons of the kidney can regulate hydrogen ion concentration by excreting excess hydrogen ions into urine.
The acid-base buffer systems quickly regulate hydrogen ion concentrations. The kidneys and the respiratory center alter hydrogen ion concentrations more slowly.