The right ventricle has:
- Right atria-ventricular opening guarded by the tricuspid valve.
- A rounded opening of the pulmonary artery, guarded by semi lunar valves. The semi lunar valves prevent backflow of the blood.
- The chordae tendinae are as in the left ventricle, but they are not as strong.
- The papillary muscles are conical in shape with their bases attached to the walls of the ventricle and their apices directed towards the ventricular cavity.
Cardiac Muscle
The contractile proteins of cardiac muscle are arranged into sarcomeres, fibrils and fibres as in voluntary striated muscle, but in cardiac muscle the fibres are arranged into branched short individual cells with elaborate junctions (intercalated discs) between them. These discs are highly folded interlocking areas of the cell membrane (sarcolemma) consisting of areas where rapid ion transport can occur from cell to cell thus allowing waves of depolarisation (contraction) and repolarisation (relaxation) to pass through the tissue. Between the cells are numerous capillaries of the coronary circulation.
Within the cells are many mitochondria enabling the tissue to have a very high metabolic rate for long periods when needed. The tissue is said to be myogenic which means that it will contract and relax rhythmically of its own accord throughout life. The nervous system is only involved in altering the frequency and the force of these contractions to meet body needs.
The sheets of the cardiac muscle making up the walls of the heart are firmly anchored to a rough ring of collagen fibres which makes up the skeleton of the heart. This is situated in the wall of the heart between the atria and the ventricles and runs around the full circumference of the heart in this region.
It consists of roughly 50% of the heart by mass.
Detailed composition of the heart
The heart comprises of three layers, the epicardium, myocardium, and endocardium surrounding the inner chambers and is enclosed in a fourth protective layer known as the pericardium, as can be seen from the diagram.
The pericardium
Within the mediastinum, the heart and origins of the great blood vessels are enclosed in a loose fitting sac termed the pericardium. This protective sac is composed of two layers separated by a space called the pericardial cavity. The outer layer is termed the parietal pericardium, and the inner layer is called the visceral pericardium.
The parietal pericardium consists of an outer layer of thick, fibrous connective tissue and an inner serous layer. The serous layer, consisting largely of mesothelium together with a small amount of connective tissue, forms a simple squamous epithelium and secretes a small amount of fluid. Normally the total volume of this fluid is only about 25 to 35 ml.
This fluid layer lubricates the surfaces to allow friction free movement of the heart within the pericardium during its muscular contractions.
The fibrous layer of the parietal pericardium is attached to the diaphragm and fuses with the outer wall of the great blood vessels entering and leaving the heart. Thus, the parietal pericardium forms a strong protective sac for the heart and serves also to anchor it within the mediastinum. So preventing it from thrashing around when it is contracting. It also protects it from external shocks and over-distension of the heart with blood in extreme conditions
The visceral pericardium is also known as the epicardium and as such comprises the outermost layer of the heart proper.
The epicardium (or visceral pericardium) forms the outer covering of the heart and has an external layer of flat mesothelial cells. These cells lie on a stroma of fibrocollagenous support tissue, which contains elastic fibres, as well as the large arteries supplying blood to the heart wall, and the larger venous tributaries carrying blood from the heart wall. The large arteries (coronary arteries) and veins are surrounded by adipose tissue, which expands the epicardium.
The coronary arteries originate from the first part of the aorta just above the aortic valve ring and pass over the surface of the heart in the epicardium, sending branches deep into the myocardium. This superficial location of the arteries is of great practical importance since it permits surgical bypass grafting of blocked arteries.
The endocardium
The internal lining of all four heart chambers is the endocardium, which is composed of three layers:
-a layer in direct contact with the myocardium
-a middle layer
-an innermost layer.
The outmost layer is composed of irregularly arranged collagen fibres that merge with collagen surrounding adjacent cardiac muscle fibres. This layer may contain some Purkinje fibres, which are part of the impulse conducting system as we shall see later.
The middle layer is the thickest endocardial layer and is composed of more regularly arranged collagen fibres containing variable numbers of elastic fibres, which are compact and arranged in parallel in the deepest part of the layer. Occasional myofibroblasts are present.
The inner layer is composed of flat endothelial cells, which are continuous with the endothelial cells lining the vessels entering and emerging from the heart.
The endocardium is variable in thickness, being thickest in the atria and thinnest in the ventricles, particularly the left ventricle. The increased thickness is due almost entirely to a thicker fibroelastic middle layer. Localised areas of endocardial thickening (jet lesions) are common, particularly in the atria, and result from turbulent blood flow within the chamber.
Cardiac cycle
Prior to mechanical events there are electrical impulses, which can be recorded in a cathode ray tube and so we would know what’s happening with time. This is called ECG (or EKG), and it gives a time based electrical record on cardiac cycle.
1. Ventricular systole
http://classes.kumc.edu/son/nurs420/ unit4/cardiaccycle.html
Ventricular systole (contraction), and the cardiac cycle, begin with ventricular depolarization, represented by the QRS complex on the ECG. The electrical impulse travels from the AV node through the Bundle of His, right and left bundle branches, and Purkinje fibres to supply the myocardial cells.
Having received the electrical impulse, the ventricular walls, septum, and papillary muscles stiffen to provide a stable outflow tract.
The ventricular pressure (blue waveform) is greater than atrial pressure (yellow waveform), therefore the AV valves closes causing the S1 heart sound, the tendinous tissue/ papillary muscles prevent blowback/ valve inversion.
The volume of the ventricle is constant because the semi lunar valves are closed, because the ventricular pressure is smaller than the aortic pressure.
The ventricular pressure (blue waveform) exceeds aortic pressure (red waveform), causing the semi lunar valves to open.
The ventricle contracts and most of its volume are ejected rapidly into the systemic circulation. (The resulting increase in aortic pressure is reflected in the red waveform). As the ventricle contracts, the atrioventricular valve bulges back into the atrium, causing a slight increase in atrial pressure (yellow waveform) called the c wave.
When aortic pressure (red waveform) exceeds ventricular pressure (blue waveform), the semi lunar valves close, causing the S2 heart sound, and relaxation begins ventricular diastole.
Note the slight increase in aortic pressure (red waveform) which occurs immediately following; this is caused by the backsplash of blood against a closed aortic valve and is called the dicrotic notch.
2. Ventricular diastole
The increase in atrial pressure resulting from atrial filling is represented by the v wave and causes atrial pressure (yellow waveform) to exceed ventricular pressure (blue waveform).
As a result, the atrioventricular valves open, and blood under pressure begins to enter the ventricles in a period of rapid ventricular filling.
Even before the atria enter systole, the ventricles are filled with blood to approximately 70% of their capacity. When the atria do finally contract, additional blood enters the ventricles and elevates the intraventricular pressure. As the ventricles contract, blood is forced backward, closing the AV valves and a sharp rise in ventricular pressure occurs.
Blood, now under less pressure, continues to enter the ventricles in a period of reduced ventricular filling called diastasis. Via ventricular suction.
As the atria contract, the remaining blood is ejected into the ventricles, accounting for about 20% of cardiac output. Atrial and ventricle pressure increase slightly due to the weak atrial contraction.
This increases the stretch of the ventricle, therefore a lot more power for the ventricle systole. Therefore creating a bigger systole, than otherwise possible.
This period of increased atrial pressure is represented by the a wave on the atrial (yellow) waveform. The end of atrial systole marks the beginning of ventricular systole, and the cardiac cycle begins again.
The volume at the end of ventricle diastole is around 260 cm3.
The blood forced intot eh aorta is around 130 cm3, and is called the stroke volume. (the volume of blood ejected in one stroke)
