Term
| The Cardiac Cycle: Stages |
|
Definition
Stage 1. Filling
Stage 2. Atrial systole
Stage 3. Onset of ventricular systole
Stage 4. Ejection
Stage 5. Isovolumic relaxation
Stage 6. Onset of filling, Stage 1 |
|
|
Term
|
Definition
The atria and ventricle are relaxed. The mitral and tricuspid valves (the A-V valves) are wide open, the aortic and palmonary semilunar valves are closed. This state is called diastole.
The pressure in the venae cavae and atria is slightly higher than in the ventricles.About 70% of the ventricular filling occurs during this phase. |
|
|
Term
|
Definition
| THe atria contract, topping up the amount of blood in the ventricles by another 30%, filling the completely. The atria the relax, a stage known as atrial diastole. |
|
|
Term
Stage 3 Onset of ventricular systole.
isovolumic ventricular contraction |
|
Definition
This is the first phase of ventricular systole. When the ventricles contract, pressure rises in their and exceeds the pressure in the atria. This causes the A-V valves to close, which occurs passively in response to the pressure differential. As the ventricles contract, the papillary muscles also contract, pulling on and tensing the chordae tendineae. This prevents the A-V valves from turning inside out into the atria.
The ventricles continue to contract with all four valves closed, raising the pressure still more. At this stage, blood is flowing neither into, nor out of the ventricles, and the volume of blood in them does not change. |
|
|
Term
Stage 4. Ejection
ventricular diastole |
|
Definition
| Pressure in the ventricles reaches the point where it exceeds the pressure in the pulmonary arteries and aorta. The semilunar valves open passively, and blood is ejected from the ventricles into the pulmonary and systemic circuits, at first rapidly, then at a reduced rate. As blood is ejected, the pressure in the ventricles starts to fall, an effect that is accentuated when the ventricles cease to contract and enter the phase known as ventricular diastole. |
|
|
Term
|
Definition
| Pressure in the ventricles drops below the pressures in the pulmonary arteries and the aorta. The result is that the semilunar valves close passively. The pressure in the ventricles continues to fall. At this stage, all four valves are closed and blood is flowing neither into, nor out of the ventricles, and the volume of blood in them does not change. |
|
|
Term
Stage 6. Onset of filling, Stage 1
diastasis |
|
Definition
| Once the pressure in the ventricles falls bellow the pressure in the atria, the A-V valves open, allowing blood to rush into them and on into the relaxed ventricles. filling with blood is at first rapid, the its rate is reduced. This phase of reduced filling is sometimes called diastasis. |
|
|
Term
|
Definition
| The autonomic nervous can modify myocardial contractility. Myocardial contractility defines the force of contraction of the ventricles for a given end diastolic volume. In contrast, Starling’s Law of the Heart refers to the increase in force of contraction as the muscle fibers are stretched. That is, as the end-diastolic volume increases. This is an intrinsic property of cardiac muscle, and is independent of autonomic input. |
|
|
Term
|
Definition
| The autonomic nervous nervous system can also modify heart rate. In normal adults at rest, the average heart rate is 70 beats per minute, and may be as low as 50 or fewer beats per minute in athletes trained in endurance events. During sleep, the rate is usually 50-60 beats. During exercise or stress conditions, the rate may be as high as 190 beats per minute. This acceleration of heart rate results from the action of the autonomic nervous system on the pace maker cells in the sinoatrial node. |
|
|
Term
| The heart receives input... |
|
Definition
| by both the sympathetic and parasympathetic branches of the autonomic nervous systems. |
|
|
Term
|
Definition
Sympathetic stimulation acts on two regions of the heart.
-Sinus node
-Cardiac musle |
|
|
Term
|
Definition
| cases the heart rate to increase, - sympathetic branch |
|
|
Term
|
Definition
| Cardiac muscle of the ventricles (less importantly of the atria), causing it to increase its contractile force. An increase in contractile force is spoken of as an increase in myocardial contractility. An increase of myocardial contractility will usually increase the stroke volume, because the end-systolic volume is diminished by the more forceful systole of the ventricles. Since cardiac output is dependent on the heart rate multiplied by the stroke volume, sympathetic stimulation will increase cardiac output. |
|
|
Term
|
Definition
| Normally at rest, the parasympathetic inflow to the heart dominates over the sympathetic. This holds the heart rate in check. If the parasympathetic system is blocked (or the vagus nerves cut), there is marked increase in heart rate. |
|
|
Term
Central Control of the heart
vasomotor, cardiac center
hypothalamus
reticular formation
limbic system |
|
Definition
| Central control fo the heart is primarily from a region of the medulla called the vasomotor, cardiac center. Out put from this center controls both sympathetic outflow to the heart and the flow of nerve impulses down the vagus nerve. In turm, the vasomotor center is under higher control from many areas of the brain including the hypothalamus, reticular formation of the brain stem, and limbic system. |
|
|
Term
| Sympathetic action of the heart |
|
Definition
1) Acceralates heart rate (SA node).
2) Increses force of contraction of the ventricular myocardium.
3) Norepinephrine is released at sympathitic nerve terminals and acts on B-adrenergic receptors in the pacemaker cells of the sinoatrial node and on cardiac muscle cells of the ventricles.
4)Action of norepinephrine (and epinephrine) on the heart is blocked by B-blockers |
|
|
Term
| Parasympathetic actions on the heart |
|
Definition
1) Slows down heart rate
2) Dos not alter force of contraction of ventricles because it doesn’t innervate them.
3) Acetylcholine is released at parasympathetic nerve terminals and acts on nicotinic receptors on intraneural neurons which also release acetylcholine, which binds to muscarinic receptors in the pacemaker cells of SA node
4) Muscarinic receptors are blocked by atropine |
|
|
Term
| Heart contraction vs Skeletal muscle |
|
Definition
| a skeletal muscle such as the biceps brachii does not contract unless it receives a nervous command. In contrast, the heart’s contraction and relaxation is entirely spontaneous. |
|
|
Term
|
Definition
| In the intact animal, contraction and relaxation are modulated by the autonomic nervous system. The sympathetic nervous system increases heart rate. This is chronotropic effect. |
|
|
