Caffeine and Nicotine Caffeine and nicotine are not found naturally within the

Caffeine and nicotine caffeine and nicotine are not

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Caffeine and NicotineCaffeine and nicotine are not found naturally within the body. Both of these nonregulated drugs have an excitatory effecton membranes of neurons in general and have a stimulatory effect on the cardiac centers specifically, causing an increase inHR. Caffeine works by increasing the rates of depolarization at the SA node, whereas nicotine stimulates the activity of thesympathetic neurons that deliver impulses to the heart.822CHAPTER 19 | THE CARDIOVASCULAR SYSTEM: THE HEARTThis content is available for free at
Although it is the world’s most widely consumed psychoactive drug, caffeine is legal and not regulated. While precisequantities have not been established, “normal” consumption is not considered harmful to most people, although it may causedisruptions to sleep and acts as a diuretic. Its consumption by pregnant women is cautioned against, although no evidenceof negative effects has been confirmed. Tolerance and even physical and mental addiction to the drug result in individualswho routinely consume the substance.Nicotine, too, is a stimulant and produces addiction. While legal and nonregulated, concerns about nicotine’s safetyand documented links to respiratory and cardiac disease have resulted in warning labels on cigarette packages.Factors Decreasing Heart RateHR can be slowed when a person experiences altered sodium and potassium levels, hypoxia, acidosis, alkalosis, andhypothermia (seeTable 19.1). The relationship between electrolytes and HR is complex, but maintaining electrolyte balanceis critical to the normal wave of depolarization. Of the two ions, potassium has the greater clinical significance. Initially,both hyponatremia (low sodium levels) and hypernatremia (high sodium levels) may lead to tachycardia. Severely highhypernatremia may lead to fibrillation, which may cause CO to cease. Severe hyponatremia leads to both bradycardia andother arrhythmias. Hypokalemia (low potassium levels) also leads to arrhythmias, whereas hyperkalemia (high potassiumlevels) causes the heart to become weak and flaccid, and ultimately to fail.Heart muscle relies exclusively on aerobic metabolism for energy. Hypoxia (an insufficient supply of oxygen) leads todecreasing HRs, since metabolic reactions fueling heart contraction are restricted.Acidosis is a condition in which excess hydrogen ions are present, and the patient’s blood expresses a low pH value.Alkalosis is a condition in which there are too few hydrogen ions, and the patient’s blood has an elevated pH. Normal bloodpH falls in the range of 7.35–7.45, so a number lower than this range represents acidosis and a higher number representsalkalosis. Recall that enzymes are the regulators or catalysts of virtually all biochemical reactions; they are sensitive topH and will change shape slightly with values outside their normal range. These variations in pH and accompanyingslight physical changes to the active site on the enzyme decrease the rate of formation of the enzyme-substrate complex,

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