href="#ulink_b23a0d2c-ff67-524b-8e79-8e077c3b6d6a">Table 3.1) consists of the heart (cardio) and the blood vessels (vascular). It is primarily a pumped system reliant on both the efficient coordinated contractions of the heart and a system of vessels which act as conduits through which the blood is circulated.
Heart structure and function
Position of heart within the thorax
The heart has a relatively central position within the thoracic cavity (Figure 3.1) with the bulk of its structure found behind the sternum (breastbone). On average, an adult human heart weighs around 250–350 g and is around 12–14 cm long and 9–11 cm wide. There is much variation in heart size, with larger people unsurprisingly having larger hearts compared to their smaller counterparts. It is commonly stated that an individual’s own heart is roughly the same size as their own clenched fist.
The heart is located in the mediastinum (region between the right and left lung) and is positioned obliquely between the second rib and the fifth intercostal space.
In individuals with a normal body mass index (BMI), the apex heartbeat can usually be detected in the left fifth intercostal space around 8 cm to the left of the central point of the sternum.
The pericardium
The heart is surrounded, protected and anchored in position by a compound membrane called the pericardium which consists of two major layers. The first of these is the fibrous pericardium, which forms an outer protective sheath and is composed of collagenous connective tissue. It anchors the heart in position within the thoracic cavity; anteriorly it is attached to the inner surface of the sternum and inferiorly to the diaphragm, the dome-shaped breathing muscle that separates the thoracic cavity from the abdominal cavity.
Figure 3.1 Overview of the cardiovascular system
Since the heart is continually beating, the fibrous pericardium is essential to stop excessive movement and drifting of the heart within the thorax. The serous pericardium is the inner portion of the pericardium and is itself composed of two distinct layers of tissue: the parietal pericardium, a fluid-producing membrane that is attached to the inner portion of the fibrous pericardium, and the visceral pericardium, which is actually the outer layer of the heart and is also known as the epicardium (see below).
Table 3.1
The two layers of the serous pericardium secrete a watery fluid termed pericardial fluid which fills the narrow pericardial space between the visceral and parietal pericardium. In health around 10–20 ml of this slippery pericardial fluid surrounds the heart, acting as a lubricant to prevent abrasion and damage to the beating heart and the surrounding tissues. This fluid also functions as an effective shock absorber, cushioning the heart against any knocks and bumps.
Pericarditis, pericardial effusions and cardiac tamponade
Viral and bacterial infections of the pericardium are not uncommon and these can trigger an inflammatory response within the pericardial membranes. This is termed pericarditis and is often accompanied by chest pain and a fever. A major issue with pericarditis is that infection can cause pericardial effusion where there is an increase in fluid volume within the pericardial space. This extra fluid typically consists of inflammatory exudate, pus or blood or a combination of these. Pericardial effusion can quickly develop into a life-threatening emergency as fluid builds up around the heart and begins to compress the ventricles, preventing the heart from functioning as an efficient pump.
This phenomenon is termed cardiac tamponade and is usually treated by inserting a needle into the pericardial sac (needle decompression) to drain off the excess fluid that is crushing the heart. Once decompressed, cardiac function is quickly restored since the ventricles can then fill and pump blood normally. Pericardial effusions that may lead to cardiac tamponade can also be caused by blunt trauma to the chest (e.g. following a car accident when the patient’s chest impacts the steering wheel).
Layers of the heart
The heart can be regarded as a simple muscular cone consisting of three distinct layers of tissue (Figure 3.2): the endocardium is the smooth inner layer of the heart that forms the lining of the chambers. The myocardium forms the thick, muscular, middle layer of the heart that makes up the vast bulk of the heart’s mass and is composed of specialised cardiac muscle fibres. The final outermost layer of the heart is called the epicardium; confusingly, this is the same layer of tissue that is also commonly referred to as the visceral pericardium. It is important that nurses are familiar with both terms as they are used interchangeably in clinical practice.
Figure 3.2 Layers of the pericardium and heart
Internal structure of the heart
Internally the heart consists of four distinct chambers (Figure 3.3).
Atria
The atria (sometimes known by the older term auricles) are the two superior (upper) chambers of the heart. These are thin-walled, elastic structures that function primarily as simple collecting chambers. The right atrium collects deoxygenated blood from the two great veins (the superior vena cava and the inferior vena cava). The left atrium receives highly oxygenated blood directly from the lungs via the pulmonary veins. Following collection the atria rapidly deliver blood to their corresponding ventricles. During foetal development the left and right atria are connected to each other via a small opening termed the foramen ovale.
When a baby is born and takes its first breath, changes in blood pressure cause this aperture to close, forming a thin interatrial septum which permanently separates the right- and left-hand sides of the heart. Sometimes this closure does not occur or is incomplete, resulting in a patent foramen ovale (PFO) which is often referred to as a ‘hole in the heart’. This is a common congenital birth defect (affecting around 25 per cent of the population) and frequently seen in babies with chromosomal abnormalities such as Down syndrome. Septal defects such as PFO allow abnormal mixing of oxygenated and deoxygenated blood which can lead to reduced blood oxygen saturation and also may increase the risk of stroke in later life. Such congenital defects, if severe, often require surgery to correct.
Ventricles
The ventricles are the two inferior (lower) chambers of the heart; these are much thicker than the atria, containing the bulk of the cardiac muscle mass of the myocardium. The ventricles function as the primary pumping chambers of the heart and are responsible for pumping around 7,200 litres of blood around the body per day. The left and right ventricles are separated by a thick muscular interventricular septum which effectively separates the heart into two distinct pumping mechanisms. Internally within the heart there are four valves which ensure that blood flows in the correct direction.
The two upper valves are semi-lunar valves called the pulmonary and aortic valves which close to prevent blood flowing back into the heart following contraction of the ventricles. The lower two valves are termed atrioventricular (AV) valves since they separate the atria from the ventricles. The AV valve on the right side consists of three flaps (cusps) of tissue and is therefore called the tricuspid, while the AV valve on the left consists of two cusps and is termed the bicuspid or mitral valve. The role of the AV valves is to close and prevent blood flowing back into the atria during ventricular contraction.
Figure 3.3 Internal structure of the heart showing direction