Heart Rate – Cardiac Output (Nursing)

00:01 So now that we've talked about the regulation of stroke volume, let's look at the regulation of heart rate.

00:08 So if for example, the stroke volume decreases as a result of a decreased blood volume or weakened heart, the body responds and maintains our cardiac output by increasing the heart rate and the heart's contractility.

00:27 Positive chronotropic agents or factors can increase the heart rate and negative chronotropic factors decrease the heart rate.

00:39 So heart rate can be regulated by our autonomic nervous system, certain chemicals and also some other factors.

00:48 So first, looking at the autonomic nervous systems regulation of heart rate, we find that the sympathetic nervous system can be activated by our emotional or physical stressors.

01:02 This causes the release of norepinephrine which then binds to beta-adrenergic receptors on the heart.

01:12 The binding of these receptors causes the pacemaker cells to fire more rapidly and this increases our heart rate.

01:22 As the heart rate is increased, the end diastolic volume is decreased because there is a decrease in the amount of time that the heart has to refill.

01:33 As well, norepinephrine can also lead to an increase in the contractility of our muscle cells.

01:41 This increase in contractility is going to lead to a decrease in the end systolic volume and this is because the increased volumes of ejected blood, so we are ejecting more blood and so there's less left in the ventricle.

01:59 Because both the end diastolic volume and the end systolic volume decrease, the stroke volume will remain unchanged because they are balancing each other out.

02:13 When it comes to the parasympathetic nervous system which is going to oppose sympathetic effects, acetylcholine will lead to the hyperpolarization of our pacemaker cells by causing the opening of potassium channels.

02:30 This in turn will slow the heart rate but has a very little effect on contractility.

02:39 At this point, the heart at rest is going to exhibit vagal tones.

02:45 So during rest, the parasympathetic system or parasympathetic nervous system is going to be the dominant influence on our heart rate.

02:56 The vagal tone will lead to a decrease in the heart rate of about 25 beats per minute at rest.

03:04 If you were to cut the vagal nerve or if you had damage to this nerve, this could lead to heart rates going up to about 100 beats per minute since you are lacking this parasympathetic activity.

03:20 So when the sympathetic nervous system is activated, the parasympathetic nervous system is inhibited and vice versa so they kinda work off of each other.

03:32 The atrial reflex also referred to as the Bainbridge reflex is a sympathetic reflex that is going to be initiated by increased venous return thus it increases atrial filling.

03:47 It is caused by the atrial walls being stretched because of this increased volume which is going to simulate that SA node, which is then going to increase the heart rate.

04:01 It's also gonna stimulate atrial stretch receptors that are going to activate sympathetic reflexes.

04:11 So now let's look at how the heart rate can be regulated by way of chemicals.

04:18 So there are two main chemicals that are gonna regulate our heart rate.

04:21 We have our hormones such as epinephrine from the adrenal medulla which is going to increase heart rate and contractility, and then we have thyroxine or thyroid hormone which is going to increase heart rate and also enhance the effects of norepinephrine and epinephrine from the adrenal medulla.

04:43 Other chemicals include ions.

04:46 You have your intracellular and extracellular ion concentrations of things like calcium and potassium that must be maintained for a normal heart function.

04:57 If there is any imbalance in your ions, this can be very dangerous to the heart because it's going to affect the ability of polarization and depolarization to occur and for that contraction to occur.

05:14 Other factors are gonna be things like age.

05:18 So as you're growing or as you're developing in utero, the fetus has the fastest heart rate and as you get older starting from birth and beyond, your heart rate is going to decline with age.

05:35 Also, females generally have a faster heart rate than that of males, and also with exercise you're gonna get an increase in your heart rate and you will find however, that if you are trained athlete even though you're exercising, your heart rate does not go up the way an untrained athlete or just a normal person who doesn't work out all the time, the way their heart rate would increase.

06:05 And finally, heart rate can also be influenced by body temperature.

06:10 There is a direct relationship between heart rate and body temperature so as your body heats up, the heart rate also goes up.