Skills: The Blood System (DP IB Biology: SL)

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 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

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

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

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

• 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