Topic 1

• Circulatory system


  The system that moves substances around the body (as well as enter and leave it) as the cells of all living organisms need a constant supply of reactants for metabolism
• Circulatory system in unicellular organisms

  • Large surface area to volume ratio
  • Substances can diffuse directly across the cell surface membrane
  • Short diffusion distances
  • Substances such as oxygen and glucose can be easily diffused to all parts of the cell very efficiently
  • Diffusion is usually fast enough to meet the requirements of the needs of single-celled organisms
• Circulatory system in multicellular organisms

  • Small surface area to volume ratio
  • Cells require energy (mostly via aerobic respiration, which require raw materials [oxygen and glucose] to be delivered to cells)
  • Too large to just rely on diffusion to move substances around to suffice their needs
  • Time taken for substances to diffuse to every cell in the body would be too long as the diffusion distance is too great
  • Larger organisms also have high energy requirements (to remain a constant temperature) and a higher metabolic rate
  • Require a mass transport system to move substances efficiently from specialised exchange organs to body cell and remove metabolic waste over long distances by mass flow
• Features of a mass transport system

  • A network to move through (e.g. blood vessels)
  • A medium for movement (a fluid)
  • Controlled direction – to move substance to/from where they are needed (e.g. the pressure gradient created by the heart moves the blood and valves control the direction)
  • Maintenance of speed (e.g. contraction of the heart maintains pressure, thus speed)
• Mass transport function


  • Moves substances quickly from one exchange site to another
  • Maintain diffusion gradients at exchange sites and between cells and surroundings
  • Ensure cell activity – bringing/removing reactants/waste products
• Open circulatory system


  • Blood/haemolymph circulates in open spaces, inside the body cavity and bathes the tissues and internal organs directly
  • Simple hearts pump the blood into the body cavity
  • Not directional and there is no control over concentration gradient/lack of pressure
  • Substances can easily diffuse between the blood and cells as there is a smaller diffusion distance
  • Diffusion is only fast enough in small animals as the rate of diffusion is inversely proportional to the distance
• Closed circulatory system


  • Animals that have a close circulatory system tend to be larger and more active
  • The blood is enclosed in vessels and highly branched
  • That the channels are narrower, meaning higher blood pressure
  • Blood travels at a more efficient speed
  • More control over blood flow and easier to be directed
  • Mechanism to maintain a one-way flow – ensured by the valves and high pressure of the heart
  • Mechanism for movement of medium inside the body – skeletal muscles
  • A means to control the movement to different parts of the organism to suit its needs at different times – smooth muscles
  • Takes up less space for other specialised organs
  • Always deoxygenated blood goes into the heart – more efficient
  • Blood leaves the heart and flows through the arteries, then the arterioles to capillaries (where substances are exchanges)
  • Blood returns to the heart by venules and then veins
• Single circulatory system


  • Consists of one loop that includes the lungs and the body
  • Heart pumps deoxygenated blood to the gills
  • Gaseous exchange occurs
  • The blood leaving the gills flows around the body and returns to the heart
  • Lower pressure to not damage the gills
  • The blood flows through the heart once for each complete circuit of the body
• Double circulatory system


  • Consists of two loops: one to the lungs and one to the body
  • Birds and mammals have double circulatory systems
  • Right ventricle pumps blood to lungs to get oxygen
  • Oxygenated blood returns to heart and pumped second time to the rest of the body
  • High pressure to reach various parts of the body
  • The blood flows through the heart twice for each complete circuit of the body
• Three chambered hearts


  Deoxygenated and oxygenated bloods is mixed and makes it inefficient
• Circulatory systems in humans

  • Humans have a double, closed circulatory system
  • The right side of the heart pumps deoxygenated blood to the lungs – pulmonary circulatory system
  • The left side of the heart pumps oxygenated blood around the body – systemic circulatory system
• Water


  • Composed of one oxygen atom and two atoms of hydrogen, which are joined together by shared electrons to form a covalent bond
  • There are weak intermolecular bonds between each molecule of water
  • Water is a polar substance because it has an uneven distribution of electrical charge
  • The hydrogen end of the molecule is slightly positive, and the oxygen end is slightly negative because the electrons are pulled towards the oxygen, so they are more concentrated at the oxygen end
  • This makes water a dipole – a partial negative charge on one side and a partial positive charge on the other
• Hydrogen bonding in water

  • Hydrogen bonds form between the positive and negatively charged regions of the water molecule
  • The slight negative oxygen atom attracts the slightly positive hydrogen atom in another molecule
  • The hydrogen bonds hold the water molecules together and need energy to break so it has high boiling point/liquid state at room temperature (whilst other small molecules are gases)
• Water as a solvent

  • Water is a universal solvent
  • The polar nature of water makes it a good transport medium and is also a medium for many metabolic reactions/biochemical reactions as most biological reactions take place in solution
  • Many substances (ionic and covalently bonded polar substances) can be dissolved in water due to it being a polar molecule
  • This allows metabolites to be transported around the body in a dissolved state
  • Water molecules can surround charged particles and the positive parts of water are attracted to the negatively charged particles and vice versa; the surrounded molecules break apart and get dissolved
  • Polar molecules become surrounded by water and go into solution, and they are known as being hydrophilic
  • Non-polar/hydrophobic substances are not soluble in water
  • Instead, lipids bind with proteins to create lipoproteins
• Cohesion and adhesion in water

  • Water's dipole nature makes water cohesive and a good solvent, making it good at transporting substances
  • Cohesion is the attraction between the same type of molecule
  • The dipolar nature of water molecules allows for hydrogen bonding between them, making water very cohesive
  • The cohesive nature of water makes it flow and good for transporting substances
  • Water can bond with other molecules via hydrogen bonding – adhesion
  • Water flows easily within the body
  • Water molecules pull other water molecules due to cohesion
  • Adheres to the sides of a vessel due to adhesion
• Thermal properties of water

  • Water has a high specific heat capacity, so a large amount of energy is needed to break the hydrogen bonds and raise the temperature
  • This means that water warms and cools down quickly
  • This is useful for organisms as it means that their internal temperature won't change drastically even when conditions outside vary
  • This means that the bodies of water that aquatic animals live in also don't change rapidly
• Water has a high surface tension due to hydrogen bonding
• Heart


  • A hollow, muscular organ located in the thorax
  • It's a double pump and is made of cardiac muscle
  • Diagrams are always shown as if we were facing the heart – that's why the left of the heart is on the right and vice versa
  • Protected in the chest cavity by the pericardium (tough and fibrous sac)
  • The heart has 4 chambers – the two atria at the top and the two ventricles at the bottom
  • The oxygenated blood is in the left side of the heart and the deoxygenated blood is on the right and it's separated by the septum (muscle and connective tissue) so they don't mix together
  • Humans have a double, closed circulatory system
• Journey of blood on the right side of the heart

  Vena cava (superior and inferior) -> right atrium -> (tricuspid valve ->) right ventricle -> (semilunar valve ->) pulmonary artery
• Journey of blood on the left side of the heart

  Pulmonary veins -> left atrium -> (bicuspid valve ->) left ventricle -> (semilunar valve ->) aorta
• Chambers of the heart – ventricles and atria


  • Ventricles have thicker walls (for strong contraction and higher pressure to push it further) compared to the atria because they must pump blood out of the heart (to the lungs or the rest of the body), whereas the atria just pump the blood into the ventricles
  • The wall of the left side of the heart is much thicker than the right as the left side of the heart is responsible for pumping oxygenated blood to the rest of the body and the right side only needs to pump it to the lungs (which is much closer in comparison)
  • The right atrium receives deoxygenated blood from the vena cava so then the blood is pumped into the right ventricle and into the pulmonary artery so that the blood can become oxygenated again
  • The left atrium receives oxygenated blood from the pulmonary vein and then the blood is pumped into the left ventricle and into the aorta, which carries the blood around the body
• Valves in the heart

  • The function of the valves is to prevent backflow
  • The valves open when the pressure of blood behind them is greater than the pressure in front of them (and vice versa for when they are close)
  • The atrioventricular separate the atria and the ventricles, and they stop the blood from going backwards into the atria when the ventricles contract
  • Tricuspid/mitral valve has three flaps – on the right of the heart
  • Bicuspid/aortic valve has two flaps – on the left of the heart
  • The atrioventricular valves have tendinous chords/valve tendons (that are connected to the wall of the ventricle and the valves) to prevent backflow and stop the valves from inverting
  • Semilunar valves separate the arteries from the ventricles, and they prevent backflow into the heart after the ventricles contract
  • The pulmonary valve is situated between the pulmonary artery and the right ventricle
  • The aortic valve is situated between the aorta and the left ventricle
  • The name 'semi-lunar' is from the crescent moon shape of the flaps that make up the valve
• Blood vessels in the heart

  • There are four main blood vessels that carry blood to and from the heart
  • Pulmonary vein carries oxygenated blood from the lungs to the left atrium
  • Aorta (artery) carries oxygenated blood from the left ventricle to the body
  • Vena cava (vein) carries deoxygenated blood from the body to the right ventricle
  • Pulmonary artery carries deoxygenated from the right atrium to lungs
  • The muscle of the heart is also supplied with blood by coronary arteries, which are wrapped around the heart to supply blood to cardiac muscles
• Blood vessels


  • Arteries
  • Arterioles
  • Veins
  • Venules
  • Capillaries
• Blood flow through blood vessels

  Blood flows through the lumen of blood vessels
• Arteries

  • Transport blood away from the heart
  • Usually at high pressure
  • The artery walls consist of three layers: the endothelium (a.k.a. the tunica intima) – the inner layer, the smooth muscle and elastic tissue (a.k.a. tunica media) – the middle layer, and the outer wall (a.k.a. tunica externa)
  • The endothelium is very smooth to minimise the resistance of the blood and is highly folded to allow the arterial walls to expand under high pressure
  • The smooth muscle and elastic tissue layer is very thick to withstand the high blood pressure so the vessels won't burst
  • The (smooth) muscles control where the blood flows through vasoconstriction/vasodilation
  • The outer wall contains collagen, which is connective tissue and a structural protein to protect blood vessels from damage by over-stretching
  • Arteries have a narrow lumen which helps to maintain a high blood pressure to carry the blood to the rest of the body
  • A pulse is present in arteries as they stretch to accommodate an increased volume of blood with each heartbeat
• Arterioles


  Arteries branch into narrower blood vessels called arterioles which transport blood into capillaries
• Veins


  • Veins return the blood back into the heart
  • They receive blood that has passed through capillary networks, so the blood pressure is very low
  • Veins contain the same layers as arteries but in different proportions - their muscle and elastic tissue layer is much thinner as the blood pressure is much lower
  • They have a much larger lumen to ensure that blood returns to the heart at an adequate speed
  • A large lumen reduces friction between the blood and the endothelium - rate of blood flow is slower but the volume of blood delivered per unit of time is equal to arteries due to the larger lumen
  • Veins have valves to prevent backflow as the blood has low pressure
  • A pulse is absent in the veins due to the increased distance from the heart
• Venules


  These vessels transport blood between the capillaries and the veins
• Capillaries


  • There are networks of capillaries in tissues – capillary beds – which increase the surface area for exchange
  • They have a narrow lumen so that red blood cells pass through in single file and forces the blood to travel slowly to ensure more time for diffusion to occur
  • The wall of the capillary have much thinner walls (one cell thick to minimise diffusion distance and increase the rate of diffusion)
• Veins


  • Contain the same layers as arteries but in different proportions
  • Muscle and elastic tissue layer is much thinner as the blood pressure is much lower
  • Have a much larger lumen to ensure that blood returns to the heart at an adequate speed
  • A large lumen reduces friction between the blood and the endothelium - rate of blood flow is slower but the volume of blood delivered per unit of time is equal to arteries due to the larger lumen
  • Have valves to prevent backflow as the blood has low pressure
  • Pulse is absent due to the increased distance from the heart
• Venules


  Vessels that transport blood between the capillaries and the veins
• Capillaries


  • There are networks of capillaries in tissues - capillary beds - which increase the surface area for exchange
  • Have a narrow lumen so that red blood cells pass through in single file and forces the blood to travel slowly to ensure more time for diffusion to occur
  • Have much thinner walls (one cell thick to minimise diffusion distance and increases the rate of diffusion) as they are where metabolic exchange occurs - substances are exchanged between the cells and the capillaries
  • Have a single layer of endothelial cells
  • The cells of the wall have gaps called pores which allow blood plasma to leak out and form tissue fluid
  • White blood cells can combat infection in affected tissues by squeezing through the pores in the capillary walls
• Cardiac cycle


  The sequence of contraction and relaxation of the atria and ventricles to allow blood to continuously circulate through the body
• Contraction of the heart muscle

  Causes a decrease in volume in the corresponding chamber of the heart, which increases again when the muscle relaxes
• When volume decreases


  Pressure increases and vice versa
• The volume of the atria and the ventricles


  Change as they contract and relax, thus changing pressure due to the changes in the chamber volume
• If there's higher pressure behind a valve


  It's forced open
• If pressure is higher in front

  The valve is forced shut
• Stages of the cardiac cycle


  1. Atrial systole

  2. Ventricular systole

  3. Diastole