Many factors affect HR and SV, and together, they contribute to cardiac function. HR is largely determined and regulated by autonomic stimulation and hormones. There are several feedback loops that contribute to maintaining homeostasis dependent upon activity levels, such as the atrial reflex, which is determined by venous return.
SV is regulated by autonomic innervation and hormones, but also by filling time and venous return. Venous return is determined by activity of the skeletal muscles, blood volume, and changes in peripheral circulation. Venous return determines preload and the atrial reflex. Filling time directly related to HR also determines preload. Preload then impacts both EDV and ESV. Autonomic innervation and hormones largely regulate contractility. Contractility impacts EDV as does afterload. CO is the product of HR multiplied by SV. SV is the difference between EDV and ESV.
afterload
force the ventricles must develop to effectively pump blood against the resistance in the vessels
autonomic tone
contractile state during resting cardiac activity produced by mild sympathetic and parasympathetic stimulation
atrial reflex
(also, called Bainbridge reflex) autonomic reflex that responds to stretch receptors in the atria that send impulses to the cardioaccelerator area to increase HR when venous flow into the atria increases
Bainbridge reflex
(also, called atrial reflex) autonomic reflex that responds to stretch receptors in the atria that send impulses to the cardioaccelerator area to increase HR when venous flow into the atria increases
baroreceptor reflex
autonomic reflex in which the cardiac centers monitor signals from the baroreceptor stretch receptors and regulate heart function based on blood flow
cardiac output (CO)
amount of blood pumped by each ventricle during one minute; equals HR multiplied by SV
cardiac plexus
paired complex network of nerve fibers near the base of the heart that receive sympathetic and parasympathetic stimulations to regulate HR
cardiac reflexes
series of autonomic reflexes that enable the cardiovascular centers to regulate heart function based upon sensory information from a variety of visceral sensors
cardiac reserve
difference between maximum and resting CO
ejection fraction
portion of the blood that is pumped or ejected from the heart with each contraction; mathematically represented by SV divided by EDV
filling time
duration of ventricular diastole during which filling occurs
Frank-Starling mechanism
relationship between ventricular stretch and contraction in which the force of heart contraction is directly proportional to the initial length of the muscle fiber
heart rate (HR)
number of times the heart contracts (beats) per minute
negative inotropic factors
factors that negatively impact or lower heart contractility
positive inotropic factors
factors that positively impact or increase heart contractility
stroke volume (SV)
amount of blood pumped by each ventricle per contraction; also, the difference between EDV and ESV
target heart rate
range in which both the heart and lungs receive the maximum benefit from an aerobic workout
1. The force the heart must overcome to pump blood is known as ________.
A) preload
B) afterload
C) cardiac output
D) stroke volume
B
2. The cardiovascular centers are located in which area of the brain?
A) medulla oblongata
B) pons
C) mesencephalon (midbrain)
D) cerebrum
A
3. In a healthy young adult, what happens to cardiac output when heart rate increases above 160 bpm?
A) It increases.
B) It decreases.
C) It remains constant.
D) There is no way to predict.
B
4. What happens to preload when there is venous constriction in the veins?
A) It increases.
B) It decreases.
C) It remains constant.
D) There is no way to predict.
A
5. Which of the following is a positive inotrope?
A) Na+
B) K+
C) Ca2+
D) both Na+ and K+
C
1. Why does increasing EDV increase contractility?
Increasing EDV increases the sarcomeres’ lengths within the cardiac muscle cells, allowing more cross bridge formation between the myosin and actin and providing for a more powerful contraction. This relationship is described in the Frank-Starling mechanism.
2. Why is afterload important to cardiac function?
Afterload represents the resistance within the arteries to the flow of blood ejected from the ventricles. If uncompensated, if afterload increases, flow will decrease. In order for the heart to maintain adequate flow to overcome increasing afterload, it must pump more forcefully. This is one of the negative consequences of high blood pressure or hypertension.