McKenna's Pharmacology for Nursing, 2e

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C H A P T E R 7  Introduction to cell physiology

materials with no ionic charge move most freely through the channels. Substances with a negative charge move more freely than substances with a positive charge. Sub- stances that move into and out of a cell by diffusion include sodium, potassium, calcium, carbonate, oxygen, bicarbonate and water. When a cell is very active and is using energy and oxygen, the concentration of oxygen within the cell decreases. The concentration of oxygen outside the cell remains relatively high, so oxygen moves across the cell membrane (down the concentration gradient) to supply needed oxygen to the inside of the cell. Cells use this process to maintain homeostasis during many activities that occur during their life. Osmosis Osmosis , a special form of diffusion, is the movement of water across a semipermeable membrane from an area that is low in dissolved solutes to one that is high in dis- solved solutes. The water is attempting to equalise the dilution of the solutes. This diffusion of water across a cell membrane from an area of high concentration (of water) to an area of low concentration creates pressure on the cell membrane called osmotic pressure. The greater the concentration of solutes in the solution to which the water is flowing, the higher is the osmotic pressure. A fluid that contains the same concentration of solutes as human plasma is called an isotonic solution. A fluid that contains a higher concentration of solutes than human plasma is a hypertonic solution; it draws water from cells. A fluid that contains a lower concen- tration of solutes than human plasma is hypotonic ; it loses water to cells. If a human red blood cell, which has a cytoplasm that is isotonic with human plasma, is placed into a hypertonic solution, it shrinks and shrivels because the water inside the cell diffuses out of the cell into the solution. If the same cell is placed into a hypo- tonic solution, the cell swells and bursts because water moves from the solution into the cell (see Figure 7.5). Facilitated diffusion Sometimes a substance cannot move freely on its own into or out of a cell. Such a substance may attach to another molecule, called a carrier, to be diffused. This form of diffusion, known as facilitated diffu- sion , does not require energy, just the presence of the carrier. Carriers may be hormones, enzymes or proteins. Because the carrier required for facilitated diffusion is usually present in a finite amount, this type of diffusion is limited. Active transport Sometimes a cell requires a substance in greater con- centration than is found in the environment around it or needs to maintain its cytoplasm in a situation that would normally allow chemicals to leave the cell. When

Hypertonic solution A red blood cell placed in hypertonic solution will shrink and shrivel up as water moves out of the cell

Isotonic solution A red blood cell placed in isotonic solution is stable and will retain its shape

Hypotonic solution A red blood cell placed in hypotonic solution will swell and burst as water moves into the cell

FIGURE 7.5  Red blood cell, showing the cell’s response to hypertonic, isotonic and hypotonic solutions.

this happens, the cell must move substances against the concentration gradient using active transport, which requires energy. When a cell is deprived of oxygen because of a blood supply problem or insufficient oxy- genation of the blood, systems of active transport begin to malfunction, placing the cell’s integrity in jeopardy. One of the best-known systems of active transport is the sodium–potassium pump. Cells use active transport to maintain a cytoplasm with a higher level of potassium and a lower level of sodium than the extracellular fluid contains. This allows the cell to maintain an electrical charge on the cell membrane, which gives many cells the electrical properties of excitation (the ability to generate a movement of electrons) and conduction (the ability to send this stimulus to other areas of the membrane). Some drugs use energy to move into cells by active transport. Drugs are frequently bonded with a carrier when they are moved into the cell. Cells in the kidney use active transport to excrete drugs from the body, as well as to maintain electrolyte and acid–base balances.