Articles on Medical Diseases and Conditions

Tag: heart beats

  • The Heart and Other Body Systems

    Your heart beats and your blood circulates with little or no conscious awareness on your part. Even though circulation is an involuntary function, it is a dynamic one. Your cardiovascular system is constantly adjusting to changes in the external environment or to demands you place on it. It adapts quickly, or directs other systems to adapt to chang- ing conditions in order to maintain a constant ?ow of blood to body tis- sues. Even the simple act of standing up requires increased blood ?ow to the legs, because the heart must work harder to counteract the effects of gravity. This means that either blood ?ow to other parts of the body must be decreased or the heart must pump blood faster or in greater volume to accommodate the activity.
    The two main systems that help regulate cardiac function are ?rst, the brain and the nervous system, and second, the kidneys.

    The Brain and the Nervous System

    Nervous system receptors throughout your body constantly gather information about factors such as stretching of the arterial walls or the amount of oxygen in the blood. This information is relayed to the brain by chemicals called neurotransmitters. In the brain stem, at the base of the brain, regulatory centers involved with automatic body functions including heart rate, blood pressure, and respiration receive the mes- sages and formulate a response. Neurotransmitters such as adrenaline carry messages back that direct a response in the target tissue, such as commands to constrict the blood vessels or increase the rate of respira- tion to deliver more oxygen to your lungs.

    The Kidneys

    The kidneys in?uence the volume of ?uids in the body, so they can change the volume of circulating blood. In this way, they signi?cantly affect blood pressure. They release enzymes that can raise blood pres- sure by constricting blood vessels, raising sodium levels, and increasing water retention. The kidneys can adapt to changing environmental conditions by, for instance, concentrating your urine if your body is dehydrated. If, on the other hand, you eat a lot of salty foods and start to retain water, your kidneys will produce less urine.

  • Your Heart’s Performance

    Both the rate at which your heart beats and the volume of blood your heart moves in a single beat determine how ef?ciently your heart pumps blood. Cardiologists calculate cardiac output to measure your heart’s
    ef?ciency. Cardiac output is, quite simply, the amount of blood your heart pumps through your circulatory system in one minute. It is calcu- lated by multiplying how much blood the left ventricle squeezes out in a single contraction (stroke volume) by the number of times the heart contracts in a minute (heart rate).
    Most typically, when your body needs more blood (for instance, when you are running up stairs) the heart increases its output by beat- ing faster. If your heart beats at a fast rate for very long, the muscle begins to tire and the resting phase of the heartbeat becomes too short for the chambers to ?ll adequately. If you are physically ?t, your heart muscle is stronger and can pump more blood with each contraction. That is, your stroke volume is higher, so your heart can deliver adequate blood to your body without tiring as quickly. A physically ?t person may actually have a low resting heart rate, because he or she has strength- ened the heart muscle so that it can pump more blood, delivering adequate oxygen to the body with fewer strokes. When a ?t person exercises, he or she may have the same heart rate as someone who is less ?t, but the ?t person is able to do more work, such as run longer with- out tiring.
    A healthy resting heart rate is usually between 50 and 75 beats per minute. When you exercise, your heart rate may increase to as much as
    165 beats or more. Age plays a role in determining your maximum heart rate; the maximum number of beats per minute can be very roughly predicted by the formula 220 minus your age. A number of other factors can cause your heart rate to increase, including stress, some medica- tions, caffeine, alcohol, and tobacco. When a healthy person sleeps, his or her heart rate may dip to as low as 40 beats per minute. As you age, your heart rate may decrease somewhat.
    Stroke volume in most people is about 3 ounces. That means that the ventricles pump out about half the blood they contain. A good athlete may be able to increase his or her stroke volume by 5 percent or more. A diminishing stroke volume is one of the ?rst signs of a fail- ing heart.
    A pregnant woman’s body demands more blood ?ow and oxygen for the developing placenta. Stroke volume increases early in preg- nancy, and later the heart rate increases to maintain a cardiac output 40 to 50 percent above normal. These changes reverse after the baby is delivered.