drv@mtx5w.UUCP (10/31/86)
Does anyone know the underlying mechanism that "causes" the heart to beat faster during exercise? Dennis R. Vogel AT&T Information Systems Middletown, NJ (201) 957-4951
ts@masscomp.UUCP (Tony Schene) (11/02/86)
In article <768@mtx5w.UUCP> drv@mtx5w.UUCP writes: > Does anyone know the underlying mechanism that > "causes" the heart to beat faster during exercise? Basically, it's a physiological reaction to lower pH levels, and lower oxygen partial pressures in the blood (the lower pH is caused by increased carbon dioxide). Sensory receptors in the carotid and aortic arteries respond to lower oxygen levels, and a cardio-accelerator center in the hindbrain responds directly to pH values. I don't know for sure how this information gets to the heart itself, but presumably it is carried by sympathetic nerves to the pacemaker region of the heart, causing it to speed the heartbeat. By the way, these same carotic and aortic bodies, in conjuction with other pH sensors in the brain, also cause increased breathing rates during exercise. -Tony Schene UUCP: {decvax, ihnp4, ...}!masscomp!ts
jgro@ur-tut.UUCP (Jeremy Grodberg) (11/03/86)
>> Does anyone know the underlying mechanism that >> "causes" the heart to beat faster during exercise? > I don't know for sure how this information >gets to the heart itself, but presumably it is carried by sympathetic nerves >to the pacemaker region of the heart, causing it to speed the heartbeat. While some of the information may be carried by nerves, I am pretty sure that chemical signals, such as adrenalin, play an important role in raising heat rate. Heart transplant patients do not have their new hearts nerves connected, and rely solely on chemical signals to change heartbeat in response to stress. --Jeremy Grodberg UUCP: ...!{allegra, seismo, decvax, bullwinkle}!rochester!ur-tut!jgro ARPA: ur-tut!jgro@rochester.arpa CS: ur-tut!jgro@rochester.arpa (as far as I know) USPS: Box 29073, Rochester, NY 14627-5073 (as far as they know) (There's got to be an easier way to sign a letter.)
larry@kitty.UUCP (Larry Lippman) (11/03/86)
In article <768@mtx5w.UUCP>, drv@mtx5w.UUCP writes: > Does anyone know the underlying mechanism that > "causes" the heart to beat faster during exercise? The heart is a pump; the volumetric pumping rate of the heart is referred to as "cardiac output", which is about 5.6 liters per minute for an average young adult male. Since the heart is a cyclic, volume-displacement pump, there are two parameters which determine cardiac output: (1) its cyclic rate (i.e., heart rate), and (2) its stroke volume. Of these two parameters, the heart rate is responsible for the greatest variation in cardiac output. Cardiac output is essentially proportional to the overall metabolism of the body. Exercise results in an obvious increase in body metabolism. Exercise can result in body tissues requiring as much as TWENTY times the normal amounts of oxygen and other blood-transported nutrients. In order to supply this increased demand, the cardiac output may be required to increase as much as SIX times, to over 30 L/min. In the particular case of exercise, the following are some of the mechanisms which increase cardiac output: 1. The autonomic nervous system increases heart rate and stroke volume (by increasing strength of ventricular contractions) just by THINKING about exercise. Really! 2. Heart rate and stroke volume are further increased by the sympathetic nervous system; this results from the motor cortex activity actually associated with exercise. 3. Increased metabolism resulting from muscular activity results in the formation of chemicals which exhibit a vasodilator effect, thereby reducing "peripheral resistance" (i.e., blood vessels expand to allow a greater blood flow). The resultant increase in blood flow results in greater venous return to the right atrium, which causes an increase in right atrial pressure. This increase in right atrial pressure increases the volume of the right atrium, which stretches a specialied portion of heart muscle called the "sino-atrial node", resulting in increased heart rate. There is yet a second mechanism which comes into play when the right atrium increases volume; it is called the "Bainbridge Reflex". Stretch receptors in the right atrium transmit aferrent signals to the medulla of the brain via the vagus nerves. Reception by the medulla results in transmission of eferrent signals by both the vagus and sympathetic nerves back to the heart to further increase heart rate and stroke volume (by stronger ventricular contractions). Of the above, the vasodilation effects of exercise reult in the greatest overall increase in heart rate through the mechanisms as described in (3) above. ==> Larry Lippman @ Recognition Research Corp., Clarence, New York ==> UUCP: {allegra|decvax|rocksanne|rocksvax|watmath}!sunybcs!kitty!larry ==> VOICE: 716/688-1231 {hplabs|ihnp4|seismo|utzoo}!/ ==> FAX: 716/741-9635 {G1,G2,G3} "Have you hugged your cat today?"
werner@aecom.UUCP (Craig Werner) (11/03/86)
> Does anyone know the underlying mechanism that > "causes" the heart to beat faster during exercise? > > Dennis R. Vogel Trust me - you really don't want to know. There are many reasons. The easiest to explain is the fluid dynamic component. That is, exercise causes more blood to flow into the heart, so more has to go out. And since Cardiac Output = Stroke Volume * Heart Rate, and that stroke volume can only increase so much, the heart rate must go up (i.e., the heart beats when it is full). There is a lot that can be said about total peripheral resistance and venous tone at this point. The body also senses of lowering of the blood pH (sensed in the medulla of the brain). This causes a reflex down the Vagus nerve as well as the sympathetic chain to stimulate (actually shorten the refractory period and speed up the Phase IV depolarization) at the SA node, which is the hearts natural pacemaker. It also speeds up conduction at the AV node, which allows beats to pass much quicker from atrium to ventricle The release of Norepinephrine also increases the contractility of the muscle, which doesn't really affect rate, but allows the heart to pump more efficiently (albeit while using more energy). -- Craig Werner (MD/PhD '91) !philabs!aecom!werner (1935-14E Eastchester Rd., Bronx NY 10461, 212-931-2517) "Never go to a doctor whose office plants have died."
mikeb@tekfdi.UUCP (Mike Boyce) (11/04/86)
In article <1154@masscomp.UUCP>, ts@masscomp.UUCP (Tony Schene) writes: > In article <768@mtx5w.UUCP> drv@mtx5w.UUCP writes: > > Does anyone know the underlying mechanism that > > "causes" the heart to beat faster during exercise? > > Basically, it's a physiological reaction to lower pH levels, and lower oxygen > partial pressures in the blood (the lower pH is caused by increased carbon > dioxide). Sensory receptors in the carotid and aortic arteries respond > to lower oxygen levels, and a cardio-accelerator center in the hindbrain > responds directly to pH values. I don't know for sure how this information My psych prof (behaviorist) has described this mechanism in terms of taste. Anyone who has experienced oxygen starvation knows the "taste" of it. He described the receptors as "taste buds". From there on I assume that the circuit is "hardwired". In my (arm chair) opinion, a good explanation of these types of circuits is a book by Norbert Wiener, "Cybernetics, Control Mechanisms in Animal and the Machine". Skip the intense math. Interesting reading. As a side issue: What causes the euphoria induced by aerobic exercise. Micky Mick Mick At your ...