HEART RATE AND ITS CONTROL
1. What is a generalization about the relationship between heart rate and size of animal?
2. What is the relationship between heart rate and physical conditioning, and factors associated with young versus mature animals?
3.
What are the effects of sympathetic stimulation and of parasympathetic stimulation on the heart?4. What is Starling’s law of the heart?
5. What is the response of the carotid and aortic sinus receptors to increased blood pressure?
6. Where are receptors for the Bainbridge reflex located, how.are they stimulated, and what is their effect?
Heart rate refers to the frequency of cardiac cycles and is usually measured by the number of beats per minute (bpm). Physiologic factors influencing the heart rate are excitement, muscular exercise, high environmental temperature, digestion, and sleep. Changes in heart rate are seen in a variety of pathologic conditions.
Metabolic Rate
In general, small animals have higher heart rates than larger animals. This is a consequence of the higher metabolic rate (and oxygen consumption) necessitated by their larger surface area per unit of body mass. The inverse relationship between heart rate and body size applies both within a species and among different species. For example, a small dog may have a resting heart rate of 120 bpm, whereas a large dog might show a resting heart rate of only 80 bpm or less. The resting heart rate of the mouse is about 600 bpm; rat, 400 bpm; guinea pig, 280 bpm; elephant,,30 bpm. Physical conditioning and the cardiac hypertrophy that occurs as a result of physical conditioning lower the resting heart rate in all animals. Young animals have a higher heart rate than mature animals, explained in part by their smaller size. Another factor is that tonic vagal inhibition is less developed in young animals. Resting heart rates for some domestic animal species and for the human are shown in Table 9-1.
| TABLE 9-1 HEART RATES IN ADULT, RESTING ANIMALS | |
| ANIMAL | HEART RATE (BEATS/MIN) |
| Horse | 32-44 |
| Horse (thoroughbred) | 38-48 |
| Dairy cow | 60-70 |
| Sheep and goat | 70-80 |
| Pig | 60-80 |
| Dog | 70-120 |
| Cat | 110-130 |
| Chicken | 200-400 |
| Human | 60-90 |
Autonomic Nervous System
Heart rates compared among species are usually obtained with the animals being at rest. A number of factors can influence heart rate, including activity, excitement, fever, heart disease, and altitude. The regulation of heart rate is a function of the autonomic nervous system. Sympathetic innervation to the heart occurs by way of efferent fibers from the stellate ganglia of the sympathetic trunk.
Parasympathetic innervation is supplied by fibers from the vagus nerves. Sympathetic stimulation increases all heart activities, and parasympathetic stimulation decreases all heart activities (see Chapter 4). Activities of the heart that are important in this regard are: (1) rate of contraction, (2) force of contraction, (3) rate of impulse conduction, and (4) amount of coronary blood flow.
Autoregulation
In addition to the nervous regulation of heart function and output, an autoregulation of cardiac output based on the amount of blood received also exists. In other words, the more the heart is filled during diastole, the greater is the volume of blood pumped out. This is known as Starling’s law of the heart.,The heart can do this because the greater volume of incoming blood stretches the heart muscle fibers, resulting in a greater force of contraction.
There are limits, however, to the amount of stretch by which the force of contraction increases. The stretch phenomenon is characteristic of all types of muscle.Reflexes
Several important reflexes within the cardiovascular system assist in its regulation. In the arch of the aorta and where the carotid artery branches to form the internal carotid (the aortic and carotid sinuses, respectively), there are many receptors that respond to stretching of these vessels. Their stretch is caused by increased blood pressure from within. The receptors fire with greater frequency when stretched. The impulses from the aortic arch are transmitted to the medulla by the vagus nerves and those from the carotids are transmitted to the medulla by the glossopharyngeal nerves. The responses of the greater number of impulses are directed toward lowering the blood pressure. This is done by greater stimulation of the cardioinhibitory center (which increases parasympathetic stimulation to the heart and decreases its activities) and by inhibition of the vasomotor center, thereby causing dilatation of the systemic blood vessels (lowers peripheral resistance). The effects of these responses (decreased heart rate and decreased peripheral resistance) lower the blood pressure (Figure Q-27). When blood pressure is low, the responses described for high blood pressure are reversed. Accordingly, a decrease in blood pressure causes fewer impulses to be transmitted from the aortic and carotid sinuses, whereby the cardioinhibitory center receives less stimulation and the vasomotor center receives less inhibition, producing an increase in blood pressure.
There are also receptors in the right atrium of the heart that are stimulated by stretch of that chamber, such as during exercise when greater amounts of blood are returned to the heart. The resulting reflex is known as the Bainbridge reflex. Stretch receptors transmit their impulses through the vagus nerves to the medulla of the brain. The effect of this reflex is to increase all activities of the heart to increase the circulatory effort necessary for the increased requirements.
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