What Affects the Heart Rate?
There are a number of different factors that affect the control and response of heart rate. But, what controls the beat of the heart?
Neural and Hormonal Affects
There are two different factors involved in heart rate management: intrinsic and extrinsic controls. Intrinsic regulation of heart rate is the result of the unique nature of cardiac tissue - it is self-regulating and maintains it's own rhythm without direction. Extrinsic controls are those that come from both hormonal responses as well as the commands from the nervous system: the central nervous system and the autonomic nervous system. Extrinsic regulation can cause the heart rate to change rapidly because of chemicals that circular in the blood or by direct action of nerves that go to the heart.
A good example of this is to measure heart rate changes when certain words or emotions are said or felt without a muscle contracting. Say the words, "we are going to have a surprise test today" and watch heart rate extrinsically increase. Put on a heart rate monitor and sitting completely still watch a movie and watch heart rate jump during a car chase or action thriller. There is no cardiovascular or cardio respiratory change as a result of this change in heart rate; it's simply the affect on the heart of chemicals and nerves responding to an external experience.
The cardiovascular control center for the body is located in the ventro-lateral medulla. Here heart rate slows if activated by the cardio inhibitory center in the medulla or speeds up if activated by the cardio accelerator.
From this site, the two channels of the autonomic nervous system originate the sympathetic and parasympathetic components. The sympathetic components increase heart rate by releasing the neural hormone catecholamines - epinephrine and norepinephrine. These hormones are cardio accelerators. Acceleration of the heart rate is called tachycardia.
The parasympathetic nervous system located in the brain stem and upper or sacral portion of the spinal cord slows heart rate. The parasympathetic components decrease heart rate. These neurons release the neurohormone acetylcholine, which inhibits heart rate. The slowing of heart rate is called bradycardia.
The combination of the neural and chemical components regulates heart rate and other heart functions. When you begin to exercise in heart zones 1-3, heart rate increases because parasympathetic (cardioinhibitory) stimulation stops. During more strenuous exercise, heart zones 3-5, the heart rate increase occurs by direct activation of the sympathetic cardioaccelerator nerves.
Exercise excites the relationship between the sympathetic accelerators and the parasympathetic depressor neurons. This change in the balance in their activity called tonic activity leads to more involvement of the vagus nerves. The vagus nerves carry about 80% of the parasympathetic fibers, those responsible for slowing heart rate. With increased vagal dominance, heart rate values change and slow. One of the training effects is the slowing of resting and ambient heart rates. This is the result of the effect of fitness on the tonic activity and the favoring of greater activity by the vagus nerves to slow heart rate. These adaptations following zone 1-3 or aerobic training occur to those who are sedentary and begin and exercise program as well as those who maintain one. This is one of the benefits of training, a significant resting bradycardia.
The central nervous system plays the greatest role in control over heart rate during exercise. When you start a movement pattern, the central nervous system sends impulses through the cardiovascular center in the medulla. There is a coordinated and quick response of both the heart and the blood vessels to change blood pressure, tissue perfusion to respond to the requirements.
A good example of the central command involvement is with anticipatory heart rate. Before an event begins, if the individual anticipates with excitement and enthusiasm the event, heart rate increases dramatically without any muscular involvement. Anticipatory heart rate or your heart rate immediately before exercising in one experiment averaged 148 bpm when the announcer started giving starting commands to a group of sprinters. In this experiment, heart rates increased 140% in anticipation of the starting of this 60-yard dash. In fact, the body that the body increases heart rate in anticipation is good because it provides for the rapid mobilization of it's bodily reserves by revving the body's engine. Research shows that the longer the event, the lower the anticipatory heart rate changes.
Internal Body Changes
Almost any substance taken into the body affects the equilibrium of the comanism. Heart rate is one of the quickest changes that occur as a direct reflection of this change that results in disequilibrium. For example, beta blockers (Inderal, Propranolol, Lopressor, etc.) cause bradycardia or the heart rate to drop. Similarly, the antiarrhythmic agents (Cardioquin, Procaine, Quinidine, etc.) given to patients to improve cardiac function also causes a decrease in heart rate. Pulmonary bronchodilator drugs such as the sympathomimetics (Isoproterenol, Ephedrine, Bronkosol, etc.) cause tachycardia or increase in heart rate values. Drugs that act as stimulants such as caffeine, nicotine, methamphetamines, cocaine, PCP cause tachycardia and drugs that are depressants, barbiturates, tranquilizers, alcohol and quaaludes cause bradycardia. Some drugs like inhalants can cause either a quickening or depressing of heart rate and respiration.
Other changes can cause changes in heart rate. Lack of sleep, irritability, rapid changes in blood chemistry such as blood sugar levels, reactions to different types of ingested foods can both lower and raise resting and exercising heart rates. Emotions play a large role in heart rate response. Anger, fear, and anxiety cause tachycardia while depression usually results in lowering of heart rate. Feelings of love, compassion, happiness usually result in braycardia. Emotional stress causes heart rate to stay elevated.
Heart rate is affected by external stresses on the body such as heat, humidity, cold, wind, and altitude and air quality. With each stress, the human heart is affected and different compensatory changes occur, one of those being adjustment in the beat of the heart. Triathletes racing at the Hawaiian Ironman face most of these conditions simultaneously while racing in one of the most strenuous events in the world. As a result, a heart rate monitor can help provide them with key information on how their body is responding to the conditions and the duration of this high intensity racing throughout the event.
The following chart shows the affects on heart rate of certain environmental stresses:
Type of Stress
Heart Rate Changes
|Thermal Stress||Heat gain*||Elevated||Changes in heart rate are the result of changes in the core body temperature. Dressing appropriately is the most important consideration to maintain the bodies core temperature. Dehydration causes heart rate to increase.|
|Heat loss||Lowered||Thermoregulation adjustments result in improved exercise capacity to heat exposure but minimally to cold stress. This generally takes about 10 days. Shivering can increase the heart rate significantly to increase core temperature. Considerable water can be lost from the respiratory tract during cold exposure when exercising which results in elevated heart rates (dehydration effect on heart rate).|
|Humidity||Dry Air||Elevated||The water content in the ambient air affects the amount of water lost through sweating. In dry air, sweating can be profuse and decrease in blood volume from dehydration substantial. Each 1 pound of body weight loss corresponds to 15 ounces (450 mL) of dehydration. Hot dry climates are easier than humid ones because evaporation of sweat, which cools the skin, can be achieved.|
|Highly moist air||Elevated||Exercising in high humidity challenges the thermoregulatory system because the large sweat loss contributes little to evaporative cooling. Sweat does not cool the skin; evaporation of sweat cools the skin. Heart rate response is to increase blood flow to the skin for sweating therefore increasing heart rate.|
|Wind||Wind Chill||Lowered and Elevated||Wind caused by physical movement or air movement magnifies heat loss as the warmer insulating air on the skin is continually replaced by cooler, ambient air. Wind causes heat to decrease and hence heart rate to stay lower. Wind chill factor is an index that shows the effect of wind velocities on bare skin for different temperatures.|
|Altitude||High||Lowered maximum and training heart rates||There is a progressive reduction in the amount of oxygen and its partial pressure as altitude increases. As a result, the heart beats faster to compensate for less oxygen per breath. Maximum heart rate drops with increases in altitude approximately one beat per 1,000 feet of gain. There is some relief from the acclimatization process, which result in improved tolerance to altitude hypoxia.|
* Same response to thermic effect of food
The genes that you inherited are responsible for much in our lives. They too affect heart rates. It appears that the effect of your genetic makeup accounts for about 50% of the value of your maximum heart rate. This means that if your parents both have a low maximum heart rate, the odds are favorable that you will as well.
Protection from the degenerative process of heart disease
The state of fatigue or rest of the individual also affects heart rate. If a student is physically tired from over-exercising there is a decline in physiological performance. Overtraining is a complex series of conditions which can include nutrient-fatigue, muscle-fatigue, and neuromuscular-fatigue. Heart rate is affected differently by different kinds of fatigue.
Heart rate is not affected by body composition. It's not affected by body type. It is affected by heart size with smaller hearts typically having higher resting and ambient heart rates.
The fitter you are the less often your heart contracts thus saving heartbeats. Getting fitter is like putting money into your saving account, it's putting heartbeats into your physiological saving account. Through the phenomenon of the training effect, ambient and resting heart rates drop, by as much as 20-30 bpm. When extended over a lifetime, this can equate to hundreds of millions of heartbeats.
The athlete's heart as the fit cardiac muscle is sometimes referred to is different than the sedentary individuals cardiac pump. There are structural and dimensional changes to the hearts of athletes, which reflect the specific training demands. The effects of getting a fit heart leads to cardiac hypertrophy, a muscle adaptation as a result of increased work capacity. That is, there is a moderate increase in heart size and anatomies regardless of age as the result of an aerobic and anaerobic training program. Here's a list of changes that happen to the fit, athletic heart muscles:
- Improved cardiac output
- Lower resting and ambient heart rates
- Increased stroke volume
- Enlarged ventricular chamber
- Thickening of the heart walls Improved coronary blood flow
- Improved mitochondria mass Increase number of respiratory enzymes in the myocardium
Mode of Exercise
Many factors affect maximum and training heart rates. The type of exercise is singularly one of the most significant. Maximum heart rate is mode specific. Anaerobic threshold heart rates are also mode specific. The greater the quantity of muscle mass that is used for the exercise, the higher the training heart rates attained. The highest heart rate numbers are those from sports which use both lower and upper muscle groups simultaneously such as cross country skiing. The lowest are those in which the body is in a horizontal position or in cool temperatures such as swimming.
Heart rate and changes in heart rate are affected by many factors. Each can result in heart rate variability depending multiple factors that might be simultaneously interacting. For example, a student might be on medication, fatigued, doing a different form of exercise, after not sleeping, just eating a big complex carbohydrate meal, suffering from stress, de-conditioned, living at altitude, in high relative humidity, and it's the day before their birthday and they just returned from a long trip. In combination, these factors can make a big day-to-day variation in heart rate.
That's even more of a reason to use a heart rate monitor. It's a management tool. The heart muscle takes all of these situations and conditions into account when it sets the frequency of the beat. Both the heart muscle and the heart monitor are powerful - use them both, together.