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
• 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.
• 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

• Occlusion?can be defined as
  • The narrowing of the arteries due to a blockage
• The arteries can be blocked by the process of?atherosclerosis
• Atherosclerosis?results in a build-up of layers of fatty material known as plaque inside?arteries
• The main cause of atheroma development is the presence of low-density?lipoprotein?(LDL) which forms from saturated fats and cholesterol
• LDL builds up in regions of the arteries and?phagocytes?move to these areas, engulfing the LDL by?endocytosis
• The enlarged phagocyte cells are then covered by?smooth muscle cells?which cause a bulging of the?endothelium?in the artery
• Deposition of?calcium ions?can worsen the situation by hardening the endothelium
• This?narrows the lumen?of the artery, reducing the space for blood flow

Consequences of occlusion of the arteries

• When an atheroma builds up enough to cause impeded blood flow, tissues do not receive the required level of?oxygen?and?nutrients
  • This can?inhibit cell functions
• Occlusion?of the?coronary arteries?in particular can lead to significant health issues such as?coronary heart disease
  • The flow of blood through the coronary arteries is reduced, resulting in a lack of oxygen and nutrients for the heart muscle
  • Partial blockage?of the coronary arteries creates a restricted blood flow to the cardiac muscle cells and results in severe chest pains called?angina?as the heart muscle beats faster to try to increase blood supply
  • Complete blockage?means cells in the area of the heart not receiving blood will not be able to respire aerobically; these cells will be unable to contract, leading to a heart attack?
  • Atheromas can sometimes rupture,?leading to the development of a?blood clot
    • Blood clots can worsen existing blockages, or break off and travel into smaller blood vessels
      • Blood clots that travel to the coronary arteries can cause a heart attack
      • Blood clots that travel to the brain can lead to a stroke

Buildup of plaque in the coronary arteries narrows the lumen

The effect of a narrowed lumen in a coronary artery is reduced blood flow to the heart

Risk factors for atherosclerosis

• There are several?risk factors?which will increase the chances of coronary heart disease:
  • Consumption of?trans-fats
    • These damage the endothelium of the artery initiating formation of an atheroma
  • Constant high blood sugar
    • This is usually the result of consumption of foods high in carbohydrate
  • High blood pressure
    • This increases the force of the blood against the artery walls and consequently leads to damage of the vessels
    • Blood pressure can increase due to smoking or stress
  • High blood concentrations of LDL
    • Speeds up the build up of fatty plaques in the arteries, leading to blockages
  • Smoking
    • Chemicals in smoke cause an increase in plaque build up and an?increase in blood pressure
    • Carbon monoxide also reduces the oxygen carrying capacity of the red blood cells
  • Infection by certain microbes
    • Chlamydia pneumoniae?can infect the arterial wall and trigger?inflammation?which promotes atherosclerosis
    • Microbes of the small intestine produce the?chemical?trimethylamine N-oxide which promotes atherosclerosis

• The body contains several different types of blood vessel
• The walls of each type of blood vessel have a structure that relates to the function of the vessel
  • Arteries?transport blood?away from the heart (usually at high pressure). They are structured as follows:
    • Three thick layers in their walls
    • A high proportion of muscle and elastic fibres
    • Narrow lumen
    • No valves
  • Capillaries?transport blood from arteries to veins, and are located between the cells in the tissues. They are structured as follows:
    • Walls are only one cell thick (endothelial layer only)
    • No muscle or elastic fibres
    • No valves
  • Veins?transport blood?into the heart (usually at low pressure). They are structured as follows:
    • Thin walls with three layers
    • Low proportion of muscle and elastic fibres compared to arteries
    • Large lumen
    • Valves (usually) present

The three main categories of blood vessel can be identified by comparing their structures