The Cardiovascular System paart 2 -lecture 2
Heart Rate
The easiest way to measure heart rate is to-
palpate the pulse in a superficial artery and
count beats per minute
In newborn infants, heart rate is 120 bpm or -
.greater
In young adult males is 64 to 72 bpm -
It rises again in the elderly -
Tachycardia is a persistent, resting adult heart rate above
100 bpm.
It can be caused by stress, anxiety, drugs, heart
It can be caused by stress, anxiety, drugs, heart
disease, or fever.
Bradycardia is a persistent, resting adult heart rate below
60 bpm.
It is common during sleep and in endurance
It is common during sleep and in endurance
trained athletes.
Endurance training enlarges the heart and increases its
stroke volume. Thus, it can maintain the same cardiac
output with fewer beats.
Hypothermia (low body temperature) also slows the heart
rate and may be deliberately induced in preparation for
cardiac surgery.
Factors that raise the heart rate are called positive
chronotropic agents, and factors that lower it are
negative chronotropic agents.
Chronotropic Effects of the Autonomic Nervous
System
Nervous system modulates rhythm and force.
The cardiac center of the medulla oblongata consists of
two neuronal pools, a cardioacceleratory center and
cardioinhibitory center.
The cardioacceleratory center sends signals by way of
sympathetic cardiac accelerator nerves to the SA node,
AV node, and myocardium.
These nerves secrete norepinephrine, which binds to
β-adrenergic receptors in the heart and increases its rate.
The cardioinhibitory center sends signals by way of
parasympathetic fibers in the vagus nerves to the SA and
AV nodes.
The vagus nerves secrete acetylcholine, which binds to
muscarinic receptors and opens K+ channels in the nodal
cells.
As K+ leaves the cells, the cells become hyperpolarized and
fire less frequently, so the heart slows down.
The cardiac center also receives input from receptors in
:the muscles, joints, arteries, and brainstem
• Proprioceptors in the muscles and joints quickly inform
the cardiac center of changes in physical activity. Thus, the
heart can increase its output even before the metabolic
demands of the muscles rise.
• Baroreceptors (pressoreceptors) are pressure sensors in
the aorta and internal carotid arteries. They send a continual
stream of signals to the cardiac center).
• Chemoreceptors sensitive to blood pH, carbon dioxide,
and oxygen are found in the aortic arch, carotid arteries,
and medulla oblongata.
They are more important in respiratory control than in
cardiovascular control but do influence the heart rate.
.(hypercapnia (CO2 excess
.+acidosis (pH= 7.35). CO2 + H2O HCO3 _ + H
Hypercapnia and acidosis stimulate the cardiac center to
increase the heart rate and restoring homeostasis.
. hypoxemia (oxygen deficiency): slow down the heart
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