For homeostasis to remain balanced throughout the body millions of respiring cells need to discard carbon dioxide and waste products and also replenish with oxygen and nutrients. For this transaction to occur a complex transportation network called the cardiovascular system initiates. The cardiovascular system consists of the heart, arteries and veins.
The heart is a double pumping organ which is the driving force of the cardiovascular system, although only weighing approximately three hundred grams the heart is powerful enough to beat over seventy times a minute pumping blood around the body. The heart is located on the left hand side of the diaphragm lying within the mediastinum in the thoracic cavity. Resembling a pyramid on an oblique angle the heart is hollow and composed of three layers, myocardium, endocardium and pericardium. Myocardium formulates the majority of the heart; this is composed of specialised cardiac muscle occurring only in the heart. Endocardium is a smooth delicate membrane, which lines the interior surface of the heart chambers and valves, and the pericardium; which is a connective tissue, this acts as a protective barrier, the fibrous pericardium fuses with arteries which pass through it to form attachments which help to anchor the heart to its surrounding structures The interior of the heart is divided into two sides the right and left, nearly mirror image of each other a few differences can be recorded (see conclusion).
As figure 1 shows there is a complex network of arteries and veins which branch into the heart. It is through these arteries and veins that blood is transported throughout the body. The arteries carry oxygenated blood from the heart to tissues and cells throughout the body whereas the veins carry deoxygenated blood received to the heart.
As the heart is attached only by soft tissue it can change position in the diaphragm while it contracts and relaxes (diastole and systole) as in figure 2 and 3.
Figure 2Figure 3
For a better understanding of the structure and workings of the heart a heart dissection was performed, below are the conclusions gathered from the experiment.
o Distinguish between the dorsal and ventral sides of the heart. (The ventral side is more rounded than the dorsal side, and the thick walled arteries arise from this side).
The right and left atria (auricles) and right and left ventricles,
Pulmonary artery and aorta arising from right and left ventricles,
Anterior and posterior vena cava opening into right atrium,
Coronary vessels in the heart wall,
Figure 4 shows the ventral side of the heart clearly showing the pulmonary artery and aorta, the left and right atria and ventricles and the coronary vessels of the heart wall.
o Clamp posterior vena cava, then run water through anterior vena cava, from the water runs through the pulmonary artery, this is the route of the pulmonary circulation which receives deoxygenated blood from the systemic circulation and transported to the lungs to be oxygenated.
Now run water through the pulmonary vein, which vessel does the water emerge?
The water runs through the aorta, this is the route of the systemic circulation, the systemic circulation takes oxygenated blood away from the heart to oxygenise respiring cells throughout the body.
Figure 5 shows the systemic circulation in red and the pulmonary circulation in blue.
o Expose the left ventricle by a longitudinal cut through ventral wall of the ventricle, note your findings. Through the cut in the left ventricle wall the following observations were recorded. (see figure 3)
The left ventricular walls were visible, consisting of thick myocardium (cardiac muscle); the reason for the thickness of the left ventricle is because the left ventricle is responsible for the pumping of blood at high hydrostatic pressure throughout the systemic arterial system.
The bicuspid valve, this valve is used to prevent backflow of blood from the left ventricle to the right atrium.
Chordae tendinae, used to anchor the flaps of the bicuspid valve to the papillary muscle to prevent the valve turning inside out due to pressure.
Papillary muscle, this is part of the myocardium of the ventricle and contains irregular shaped columns called trabeculane carnae.
o Turn the heart upside down and run water into the ventricle through the slit you have cut, note your findings.
The water ran through the aorta, as the left ventricle pumps blood into the aorta to be transferred via the systemic circulation.
Figure 6 shows the interior of the heart.
o now turn the heart the right way up, run water through into the cut end of the aorta, note your findings
The water appeared through the trabeculare carnae (irregular shaped columns in the papillary muscle) as a shower.
o Cut open the left atrium and aorta by continuing your ventricular cut upwards, note your findings. Through the extended cut in the left ventricle the following were visible.
Left atrium (auricle) this is the chamber which lies superior to the left ventricle, smaller than the right atrium (auricle) this houses the pulmonary veins which bring oxygenated blood from the lungs.
The aortic valve (semi-lunar) this prevents backflow of blood from the aorta into the left ventricle.
The pulmonary vein which returns oxygenated blood from the lungs to be pumped around the body in the systemic circulation.
o Expose the interior of the right ventricle by a longitudinal slit through ventral wall, note your findings. The cut in the right ventricle exposed: (see figure 3)
The right ventricle wall was visible consisting of thinner myocardium (cardiac muscle) than the left ventricle this is due to less hydrostatic pressure required to push blood into the pulmonary artery, this is known as the pulmonary circulation.
Tricuspid valve this is to prevent backflow of blood into the right atrium from the right ventricle.
Chordinae tendinae used to anchor the flaps of the tricuspid valve to the papillary muscle this is to prevent the valve been turned inside out by pressure.
Papillary muscle part of the myocardium and contain fewer trabeculare carnae than the left ventricle.
The right ventricle has a larger funnel shaped area of smooth wall known as the conus arteriosus or infunibulum.
o Slit open the right atrium and pulmonary artery by continuing your ventricular slit upwards, note your findings. Through the extended slit in the right ventricle clearly visible was:
The right atrium, this is larger than the left atrium the two great veins (superior and inferior) vena cava deposit deoxygenated blood from around the body into the right atria.
The pulmonary valve, this prevents backflow of blood into the right ventricle.
Coronary sinus which returns blood from the cardiac veins to the heart.
o Note the opening of the coronary vein on the left hand side of the atrium; it is possible you may see a small oval depression this is the fossa ovalis, what do you suppose this is?
The fossa ovalis is situated in the interatial septum (dividing wall of atria). During the stage of foetal development the blood flow is different from a newborn. The blood passes from the right atrium directly into the left atrium to be pumped around the body, this is made possible by a connecting tube called the foramen ovale, when a newborn baby inhales its first breath of air the pressure closes the foramen ovale and leaves behind the fossa ovalis, on some occasions a gap may be left this is referred to as a hole in the heart.
Figure 7 shows the depression of the fossa ovalis situated in the right atrium
o Do you expect the foetal heart to differ from the adult heart? Why?
Yes the foetal heart differs from the adult heart.
The foetus although fully formed at twelve weeks is reliant on its mother until birth; the remaining twenty eight weeks are spent with maturation of the foetuss tissues and organs. The foetuss heart forms in the embryonic stage, beginning to beat at around week eight of gestation; although the heart is fully functional at this stage the lungs which play an essential part in the oxygenisation of blood in the cardiovascular system are not functional until birth. As the blood still needs to reoxygenise respiring cells a temporary substitute is the placenta, often referred to as the foetal lung it is responsible for filtering and supplying the foetus with oxygen and nutrients received from the maternal blood. For this process to take place the route the blood takes through the body needs to be diverted away from the lungs, as described above the foramen ovale passes blood through the interracial septum from the right atria to the left atria, this enables blood to bypass the right ventricle which intern stops the blood being pumped up the pulmonary artery. There is also a bypass system which connects the pulmonary trunk to the aorta; this called the ductus arteriosous which again enables blood to bypass the lungs. The ductus arteriosous and the foramen ovale close at birth with the first breath of the infant, this results in the two circulations of the heart (systemic and pulmonary) working alongside each other to bring homeostasis to the body.