heart volume Flashcards
(7 cards)
Contractility
is the ability of cardiac muscle to generate tension. Contractility is influenced by factors other than stretch, including the amount of calcium ions present, sympathetic nervous stimulation, and hormones such as epinephrine and thyroxine. As contractility increases, stroke volume increases.stroke volume (SV) is the amount of blood pumped by one ventricle per heartbeat.
Afterload
is the amount of pressure the ventricles must overcome to eject blood. Afterload is determined by the blood pressure in the pulmonary trunk and aorta pressing back on the semilunar valves. In order to pump blood, the ventricles must generate a force greater than the afterload. If afterload increases, it will be more difficult for the ventricles to pump blood, and stroke volume will decrease.
Factors Modifying Basic Heart Rate
In healthy people, stroke volume tends to be relatively constant. However, when blood volume drops suddenly or when the heart has been seriously weakened, stroke volume declines, and cardiac output is maintained by a faster heartbeat. Although heart contraction does not depend on the nervous system, its rate can be changed temporarily by the autonomic nerves. Indeed, the most important external influence on heart rate is the activity of the autonomic nervous system. Several chemicals, hormones, and ions also modify heart rate. Some of these factors are discussed next (see also Figure 11.9).
Neural (ANS) controls.
During times of physical or emotional stress, the nerves of the sympathetic division of the autonomic nervous system more strongly stimulate the SA and AV nodes and the cardiac muscle itself. As a result, the heart beats more rapidly. This is a familiar phenomenon to anyone who has ever been frightened or has had to run to catch a bus. As fast as the heart pumps under ordinary conditions, it really speeds up when special demands are placed on it. Because a faster blood flow increases the rate at which fresh blood reaches body cells, more oxygen and glucose are made available to them during periods of stress. When demand declines, the heart adjusts accordingly. Parasympathetic nerves, primarily vagus nerve fibers, slow and steady the heart, giving it more time to rest during noncrisis times.
Hormones and ions.
Various hormones and ions can have a dramatic effect on heart activity. Both epinephrine, which mimics sympathetic nerves and is released in response to sympathetic nerve stimulation, and thyroxine, a thyroid hormone, increase heart rate and contractility. Electrolyte imbalances pose a real threat to the heart. For example, recall that calcium ions are required for muscle contraction. A reduced level of ionic calcium in the blood depresses the heartbeat, whereas an excessive level of blood calcium ions causes such prolonged contractions that the heart may stop entirely. Either excess or lack of needed ions such as sodium and potassium also modifies heart activity. A deficit of potassium ions in the blood, for example, causes the heart to beat feebly, and abnormal heart rhythms appear.
Physical factors
A number of physical factors, including age, gender, exercise, and body temperature, influence heart rate. Resting heart rate is fastest in the fetus (140–160 beats per minute) and then gradually decreases throughout life. The average adult heart rate is faster in females (72–80 beats per minute) than in males (64–72 beats per minute). Heat increases heart rate by boosting the metabolic rate of heart cells. This explains the rapid, pounding heartbeat you feel when you have a high fever and accounts in part for the effect of exercise on heart rate (remember, working muscles generate heat). Cold has the opposite effect; it directly decreases heart rate. As noted previously, exercise acts through nervous system controls (sympathetic division) to increase heart rate (and also, through the action of the muscular pump, to increase stroke volume).
Homeostatic Imbalance 11.6
Here’s a concise flashcard-style summary of Congestive Heart Failure (CHF):
The pumping action of the healthy heart maintains a balance between cardiac output and venous return. But when the pumping efficiency of the heart is reduced so that circulation is inadequate to meet tissue needs, congestive heart failure (CHF) occurs. Congestive heart failure is usually a progressive condition that reflects weakening of the heart by coronary atherosclerosis (clogging of the coronary vessels with fatty buildup), hypertensive heart disease, or multiple myocardial infarctions (repaired with noncontracting scar tissue). In these patients, the heart pumps weakly and is nearly “worn out.” The weak contractions of a heart in CHF result in a lower stroke volume. For those patients, the drug digitalis is routinely prescribed. It enhances contractility and increases stroke volume of the heart, resulting in greater cardiac output.
Because the heart is a double pump, each side can fail independently of the other. If the left heart fails, pulmonary congestion occurs. The right side of the heart continues to propel blood to the lungs, but the left side is unable to eject the returning blood into the systemic circulation. As blood “backs up” in the lungs, they become swollen with blood, the pressure within them increases, and fluid leaks into the lung tissue, causing pulmonary edema. If untreated, the person “drowns” in these fluids.
If the right side of the heart fails, peripheral congestion occurs as blood backs up in the systemic circulation. Edema is most noticeable in the distal parts of the body: The feet, ankles, and fingers become swollen and puffy. Failure of one side of the heart puts a greater strain on the opposite side, and eventually the whole heart fails.
