The Structure of the Heart
Heart structure
- The heart has two sides:
- The left side pumps oxygenated blood around the body in the systemic circulation
- The right side pumps deoxygenated blood to the lungs in the pulmonary circulation
- The left and right sides of the heart are separated by a wall of muscular tissue called the septum.
- The septum is very important for ensuring blood doesn’t mix between the left and right sides of the heart
- The heart is divided into four chambers
- The two top chambers are atria (singular atrium) - they receive blood from the veins and pump it through to the ventricles
- The bottom two chambers are ventricles - they receive blood from the atria and pump it out into the arteries
- Valves are important for keeping blood flowing forward in the right direction and stopping it flowing backwards. They are also important for maintaining the correct pressure in the chambers of the heart
- The atria and ventricles are separated by the atrioventricular valves
- The right atrium and ventricle are separated by the tricuspid valve
- The left atrium and ventricle are separated by the bicuspid valve
- The ventricles and the arteries that leave the heart are separated by semi-lunar valves
- The right ventricle and the pulmonary artery are separated by the pulmonary valve
- The left ventricle and aorta are separated by the aortic valve
- The atria and ventricles are separated by the atrioventricular valves
- There are two blood vessels bringing blood into the heart; the vena cava and pulmonary vein
- There are two blood vessels taking blood out of the heart; the pulmonary artery and aorta
The human heart has four chambers and is separated into two halves by the septum
The pathway of blood through the heart
- Deoxygenated blood coming from the body flows through the vena cava and into the right atrium
- The atrium contracts and the blood is forced through the atrioventricular (tricuspid) valve into the right ventricle
- The ventricle contracts and the blood is pushed through the semilunar valve into the pulmonary artery
- The blood travels to the lungs and moves through the capillaries past the alveoli where gas exchange takes place
- Low pressure blood flow on this side of the heart prevents damage to the capillaries in the lungs
- Oxygenated blood returns via the pulmonary vein to the left atrium
- The atrium contracts and forces the blood through the atrioventricular (bicuspid) valve into the left ventricle
- The ventricle contracts and the blood is forced through the semilunar valve and out through the aorta
- Thicker muscle walls of the left ventricle produce a high enough pressure for the blood to travel around the whole body
Coronary arteries
- The heart is a muscle and so requires its own blood supply to enable its cells to carry out aerobic respiration
- The heart receives blood through arteries on its surface called coronary arteries
- It’s important that these arteries remain clear of blockages called plaques, as this could lead to angina or a heart attack (myocardial infarction)
The coronary arteries cover the outside of the heart, supplying it with oxygenated blood
Dissection of a mammalian heart
- Dissections are a vital part of scientific research
- They allow for the internal structures of organs to be examined so that theories can be made about how they function
Apparatus
- Scissors
- Scalpel
- Tweezers / Forceps
- Dissection board
- Paper towels
- Biological specimen
- Pins
- Gloves
- Goggles
Method
- A lab coat, gloves and eye protection should be worn
- This is done to avoid contamination with biological material which could cause an allergic reaction or contain harmful microorganisms
- Place the specimen on the dissecting board
- Use the tools to access the desired structure
- When using the scalpel cut away from your body and keep your fingers far from the blade to reduce the chance of cutting yourself
- Scissors can be used for cutting large sections of tissue (cuts do not need to be precise)
- Scalpel enables finer, more precise cutting and needs to be sharp to ensure this
- Use pins to move the other sections of the specimen aside to leave the desired structure exposed
Identifying structures during the dissection
- Observe the outside of the heart to identify the coronary arteries supplying the cardiac muscle with a oxygenated blood and nutrients
- The coronary arteries branch off the aorta near to the semilunar valves
- The coronary arteries are often surrounded by white, fatty tissue
- Position the heart and study it from the top, you should be able to identify
- Arteries leaving the heart - these can be identified by the thicker walls
- Poke a glass rod through the aorta to feel the thicker walls of the left ventricle and through the pulmonary artery to feel the thinner walls of the right ventricle
- Veins entering the heart - these can be identified by their thinner walls
- Poking a glass rod through the pulmonary vein will lead to the left atrium and poking a glass rod through the superior or inferior vena cava will lead to the right atrium.
- Arteries leaving the heart - these can be identified by the thicker walls
- Lay the heart down on its flatter side, this is the dorsal side.
- Dorsal refers to the back of an organism
- The ventral side is now closest to you with the pulmonary artery facing outwards and in front of the aorta
- Ventral refers to the front of an organism
- Make an incision from the Aorta, underneath the pulmonary artery and around the apex of the heart to cut the heart into halves
- This should open up the heart to show the ventricles so that you can compare the thickness of the walls
- The atrioventricular valves should also be visible as a white flap of connective tissue attached by tendinous cords which prevent inversion of the valve
- The tendinous cords are stringy in appearance and are sometimes referred to as the heartstrings
- The atria are wrinkled in appearance and can be tricky to see compared with the ventricles.
- They sit just above the atrioventricular valves and are the entry point for the veins (the pulmonary vein and the vena cava)
- They are significantly smaller than the ventricles
- It is possible that the atria and blood vessels may be sliced off some specimens
- Observe the septum of the heart which separates the left and right-hand sides. The septum contains the conductive fibres which stimulate contraction, however, these are not visible to the eye
Following a careful method during a heart dissection will allow you to identify the different structures
Examiner Tip
Remember:
- Arteries carry blood away from the heart
- Veins carry blood into the heart
When explaining the route through the heart we usually describe it as one continuous pathway with only one atrium or ventricle being discussed at a time, but remember that in reality, both atria contract at the same time and both ventricles contract at the same time
Also, the heart is labelled as if it was in the chest so the left side of a diagram is actually the right hand side and vice versa