chapter 12: the cardiovascular system: the heart 12-4 (blood volume…
chapter 12: the cardiovascular system: the heart 12-4
the term heart dynamics, or cardiodynamics, refers to the movements and forces generated during cardiac contractions.
the amount ejected by a ventricle during a single beat is the stroke volume (SV)
the stroke volume can vary from beat to beat, so physicians are often more interested in the cardiac output (CO), or the amount of blood pumped by each ventricle in ventricle in 1 minute
cardiac output provides an indication of the blood flow through the peripheral tissues; without adequate blood flow, homeostasis can't be maintained
CO (mL/min) = HR ( beats/min) X SV (mL/beat)
cardiac output is highly variable, however, because a normal heart can increase both its rate of contraction and its stroke volume
when necessary, stroke volume can almost double, and the heart rate can be increased by 250 percent.
cardiac output is precisely regulated so that peripheral tissues receive an adequate blood supply under a variety of conditions.
the major factors that regulate cardiac output often affect both the heart rate and stroke volume simultaneously
these primary factors include blood volume reflexes, autonomic innervation, and hormones
secondary factors include the concentration of ions in the extracellular fluid and body temperaturef
blood volume reflexes
cardiac muscle contraction is an active process, but is entirely passive
the force needed to return cardiac muscle to its precontracted length is provided by the blood pouring into the heart, aided by elasticity of the cardiac skeleton
as a result, there's a direct relationship between the amount of blood entering the heart and the amount of blood ejected during the next contraction
the atrial reflex involves adjustments in heart rate in response to an increase in the venous return, the flow of venous blood into the heart
the amount of blood pumped out of a ventricle each heartbeat depends not only on venous return but also on the filling time- the duration of ventricular diastole, when blood flow can flow into the ventricles.
the pacemaker cells of the SA node establish the basic heart rate, but this rate can be modified by the autonomic nervous system (ANS)
postganglionic sympathetic fibers extend from neuron cell bodies located in the cervical and upper thoracic ganglia
the vagus nerves (N X) carry parasympathetic preganglionic fibers to small ganglia near the heart.
both ANS divisions innervate the SA node and AV nodes as well as the atrial and ventricular cardiac muscle cells
epinephrine and norepherine secreted by the suprarenal medullae act to increase both heart rate and force of contraction
thyroid hormones and glucagon also act to increase force of contraction
autonomic effects on the heart rate
autonomic effects the heart rate primarily reflect the responses to the SA node to the acetylcholine (ACh) and norepinephrine(NE)
ACh released by force of cardiac contractions
both autonomic divisions are normally active at a steady background level, releasing ACh and NE both at the nodes and into the myocardium
cutting the vagus nerves increases the heart rate, and sympathetic blocking agents slow heart rate.
through dual innervation and adjustments in autonomic tone, and the ANS can make very delicate adjustments in cardiovascular functions
the coordination of autonomic activity
the cardiac centers of the medulla oblongata contain the autonomic headquarters for cardiac control
the cardioacceleratory center controls sympathetic motor neurons that increase heart rate; the nearby cardioinhibitory center controls the parasympathetic motor neurons that slow heart rate
the cardiac centers respond to changes in blood pressure and in the arterial concentrations of dissolved oxygen and carbon dioxide