Pulmonary and systemic circuits of blood
The heart can conveniently be thought of as two separate pumps which circulate blood in series (Figure 3.4). The right-hand side of the heart pumps deoxygenated blood to the lungs via the pulmonary circuit and the left-hand side of the heart pumps oxygenated blood from the lungs to all other organ systems via the systemic circuit. The pulmonary arteries and pulmonary veins are unusual blood vessels; unlike most arteries (which usually carry oxygenated blood) the pulmonary arteries carry deoxygenated blood to the lungs, and unlike most veins (which usually carry deoxygenated blood) the pulmonary veins carry oxygenated blood from the lungs.
Nurses are expected to know the basic circulation of blood through the heart including the names of the major blood vessels, chambers of the heart and heart valves through which the blood will pass. Activity 3.1 will help you to learn the names and the correct sequence of blood flow.
Now that you understand the circulation of blood through the pulmonary and systemic circuits, we will explore the nature of the heartbeat.
Figure 3.4 Circulation of blood and major blood vessels of the pulmonary and systemic circuits
Activity 3.1 Research and revision
Beginning at the vena cavae and ending at the aortic arch, draw a flow diagram using arrows to illustrate the passage of blood as it passes sequentially through the pulmonary and systemic circuits. The first three steps are:
Vena Cavae → Right Atrium → Tricuspid Valve → ..............................→ Aortic Arch
Use the following terms to complete your flow diagram:
Left Atrium, Lungs, Bicuspid Valve, Pulmonary Valve, Right Ventricle, Aortic Valve, Pulmonary Veins, Left Ventricle, Pulmonary Artery.
You may find it useful to design your own mnemonic to use as a memory aid.
There are some possible answers to all activities at the end of the chapter, unless otherwise indicated.
The beating heart
Following vigorous exercise or when we are resting and quiet, most people become aware of the sensations and sounds of their beating heart. In young healthy adults, typical resting heart rates range from 60 to 80 beats per minute. In general the resting heart rate relates to the individual’s current level of physical fitness; indeed, some athletes may have resting heart rates in the 40s while those who are physically unfit tend to have higher rates.
There are many ways of measuring a patient’s heart rate including: taking their pulse, listening to the heart using a stethoscope or other device such as a Doppler probe or viewing the visual display on an electrocardiogram (ECG) machine.
Bradycardia and tachycardia
When recording a patient’s resting heart rate sometimes it may be either slower or faster than expected. The term bradycardia is used to describe a resting heart rate of 60 bpm or lower. In most cases bradycardia is not indicative of disease; indeed, the most common cause of a slow heart rate is a high level of physical fitness. Bradycardia is also associated with the use of certain medications, particularly beta blockers (β blockers) which are frequently used to treat high blood pressure by slowing the heart. However, there are some medically significant causes of bradycardia such as hypothermia (see Chapter 2) or damage to the electrical conductive tissues of the heart.
Tachycardia is the opposite of bradycardia and is defined as a resting heart rate of 100 bpm or above. Tachycardia can have many causes ranging from increased release of adrenaline when a person is frightened, to more serious causes such as a major infection or severe haemorrhage.
It is essential to recognise that the terms bradycardia and tachycardia are only applied to a patient’s resting heart rate; so a heart rate above 100 bpm during exercise would not be referred to as tachycardia.
Systole and diastole
Contraction of the heart’s chambers is referred to as systole while relaxation is referred to as diastole. For the heart to function as an efficient pump, it needs to contract and relax in a precisely timed sequence. This sequence ensures the upper chambers contract first (atrial systole), allowing the ventricles to fill, which then contract (ventricular systole) to eject blood from the heart.
The cardiac cycle
Each heartbeat can be divided into series of five distinct phases termed the cardiac cycle (Figure 3.5). Each phase is perfectly timed and coordinated by the cardiac conductive system to ensure that the chambers of the heart contract and relax at the correct time, allowing the heart to function as an efficient pump.
Figure 3.5 Phases of the cardiac cycle
Source: OpenStax (2013) Anatomy and Physiology. Rice University. Available at: https://opentextbc.ca/anatomyandphysiology/chapter/19-3-cardiac-cycle
The first phase is called the passive ventricular filling stage, where for a short time both the atria and the ventricles of the heart are in diastole and the atrioventricular valves (bicuspid and tricuspid) are open. This allows approximately 70 per cent of atrial blood volume to flow passively from the atria into the dilated ventricles under the influence of gravity and the elastic recoil of the atria.
This is followed by phase two, atrial systole, where the atria contract, forcing the remaining 30 per cent of atrial blood volume into the dilated ventricles.
In phase three, isovolumetric contraction, the ventricles are rapidly undergoing systole, forcing closure of the atrioventricular valves. This phase is very fast, occurring in around 0.05 seconds. The term isovolumetric means at constant volume, reflecting the fact that the volume of the left and right ventricle is identical.
Phase four is known as ventricular ejection; as the pressure in the ventricles increases, the aortic and pulmonary valves are forced open and blood is ejected into the pulmonary and systemic circuits.
The final fifth phase is isovolumetric relaxation; here the ventricles undergo diastole and blood begins to flow back against the aortic and pulmonary valves, snapping them shut. Pressure continues to fall within the ventricles until it is below that of the atria. The elastic recoil of the atria and effects of gravity push blood onto the atrioventricular valves which open, returning the heart back to phase one, passive filling.
Cardiac muscle
Unlike skeletal muscle which consists of parallel running fibres, the cardiac muscle, which forms the myocardium, has a branched structure consisting of individual cells joined together by tight junctions termed intercalated discs. These allow efficient and rapid movement of electrical signals through the myocardium, allowing the component fibres to contract in synchrony to ensure the myocardium contracts as a single unit. Cardiac muscle has a unique feature called intrinsic rhythm which is an inbuilt ability to contract at a regular rate, allowing the heart to beat at a relatively regular rhythm even if its natural pacemaker is damaged.
The cardiac conductive system of the heart
As we have seen above, each heartbeat involves precise and accurately timed contraction and relaxation of the heart’s chambers during the cardiac cycle. The five phases of the cardiac cycle are coordinated and timed to split-second accuracy via the cardiac conductive system. This consists of a series