Dissection of a Sheep's Heart
Human Anatomy and Physiology
Why is it important for medical practitioners to dissect sheep's hearts?
Dissection is important when studying anatomy, and practicing with a sheep's heart is a suitable choice since it shares many similarities with a human heart. It also allows researchers to look into issues that can cause problems and diseases related to the heart, therefore helping with a correct diagnosis (Welsh NGfL, 2008).
Dissection is also a useful tool as it helps with the advancement of science, as well as being more readily available and a better ethical alternative in comparison to human hearts.
Why did you do this experiment?
The main purpose behind dissecting a lamb's heart was to understand the different functions that each part of the heart performs from the atria at the top of the heart to the ventricles at the bottom, as well as why each section is so important to a heart functioning properly and correctly.
The investigation allowed the groups to use certain equipment (see Figure 1) depending on what needed to be accomplished; this would be advantageous for future practical sessions later in the course.
The dissection also provided an active, firsthand learning experience that helped to illustrate what has been previously heard and read (Offner, 1993).
The aim of this experiment was to investigate the structure and functions of a lamb's heart and to identify the similarities and differences between it and a human heart.
Materials and Methods:
* Lamb Heart
* Dissecting Tray
* Dissecting Scissors
* Lab Coat
* Camera (optional)
For this experiment, the group was split into pairs or groups of three to share a sheep's heart. Once all the safety procedures were outlined, the researchers had to identify the right and left side of the heart; this was done by looking for blood vessels that divided the heart in half, or by squeezing each side, knowing that the left side would be stronger and more muscular because it has the important job of regulating blood flow around the majority of the body.
The task required the groups to examine the darker tissue, also known as the auricles, before using these to identify the superior vena cava that directs blood to the right atrium at the top half of the heart. A probe was then used to locate the inferior vena cava that brings blood to the right atrium from the lower tissues. After finding the left auricle, it was possible to see the pulmonary vein, which brings blood from the lungs to the left atrium.
The final part of the external anatomy observation was to locate the largest blood vessel, the aorta, which transports oxygenated blood from the left ventricle to the rest of the body and the pulmonary artery, which takes blood from the right ventricle and transports it to the lungs.
As part of the internal anatomy dissection, it was necessary to cut through the right hand side of the heart using either the dissecting scissors or the scalpel to reveal the tricuspid valve that is situated between the right atrium and right ventricle. After making this incision, the papillary muscles could be seen, and by using the probe in the pulmonary artery, small membranous pockets called the pulmonary semilunar valve also had to be identified.
The probe was finally inserted into the aorta to carefully examine inside, looking for the aortic semilunar valve, which has the responsibility of preventing the backflow of blood into the left ventricle (Carola, 1992, pp. 583).
Similarities and Differences:
To understand the reasoning behind using a sheep's heart rather than a human heart, it is important to consider the similarities and differences between them both.
The main similarity between the two hearts is that they both share the same function and purpose, which is to keep the body alive by continually circulating blood around the body.
Miller (1998) stated that since both humans and sheep are mammals, the heart is likely to share similar characteristics, for example, having a four chambered heart. The circulatory systems of both hearts are also very alike, and although the valves are situated in a similar position, this was difficult to verify since the top half of the hearts was removed beforehand.
In contrast, research has indicated that there usually is a slight difference in the position of the heart in a sheep, and that a human heart is normally around 1.5 to 2 times bigger than the average sheep's heart.
The Functions of the Heart:
The heart is a hollow organ that contains four chambers: right atrium, left atrium, right ventricle and left ventricle. The atria contain the blood that has returned from the body and the lungs, whereas the ventricles are responsible for sending the stored blood to the lungs and the rest of the body. The auricles are attached to both the right atrium and left atrium, and they are near to the opening of the superior / inferior vena cava and the pulmonary vein respectively; all of these control the amount of blood that enters the atria from the lungs and the rest of the body. The walls of the ventricles are thicker than the atria walls because a higher pressure is needed to maintain the constant supply of blood.
The aorta is the largest blood vessel in the heart and is responsible for transporting the oxygenated blood from the left ventricle to the entire body. After leaving the heart, the aorta branches off, in order to transfer oxygen and nutrients around the body in the bloodstream. Situated behind the aorta is another large blood vessel known as the pulmonary artery, which takes the blood from the right ventricle and transport it to the lungs.
The tricuspid valve is located between the right atrium and ventricle and is connected to tiny papillary muscles as well as the chordae tendinae. The tricuspid valve plays an important role as it limits the amount of blood that enters the ventricle via the atrium, but also stops blood from moving backwards into the atrium when the ventricle contracts. The pulmonary semilunar valve and aortic semilunar valve share similar roles, with each one being restricted to one side of the heart, the pulmonary to the right and the aortic to the left.
Since this was the first time dissecting a sheep's heart, there was always a chance of problems occurring due to a lack of confidence and inexperience. Some of the steps within the observation could not be completed because the top half of the heart was removed prior to dissection, so it was not possible to examine parts such as the valves that are linked to the atria in the sheep's heart.
Another issue involved incorrectly cutting the heart, particularly when dividing the right ventricle and left ventricle, and this made it difficult later on, especially when lit was essential to locate some of the smaller parts within the heart. The number of groups doing the dissection at the same time was also a hindrance as it was not always possible to receive help immediately.
Overall, the dissection was successful, and it helped reiterate what was previously learnt in the lectures in a 3-dimensional format. Taking pictures on a camera simplified the task of annotating the diagrams in the results section too.
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