LITMIR.BIZ

      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:

      Use the following terms to complete your flow diagram:

      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.