Circulatory System

• Transport in Animals

  Animals require transport / circulatory systems to bring in nutrients and O₂ for energy and remove metabolic wastes
• Unicellular Organisms


  • Rely on diffusion of materials across their cell membrane
  • Possible since they have high surface area to volume ratio
• Protoctista


  • Amoeba (NOT animals)
• Cnidarians

  • Have sac-like body, with body wall only 2-cell layers thick
  • Endoderm lining enteron / gastrovascular cavity (digestion + distribution of substances)
  • Ectoderm lining outer surface of body
  • All cells in direct contact with external environment, allowing direct exchange of substances and waste products by diffusion
  • Substances can easily diffuse between endoderm and ectoderm cells due to short distance for diffusion
• Cnidarians


  • Hydra
• Platyhelminthes


  • Planarians & other flat worms have highly branched gastrovascular cavity with single opening through which exchange can occur
  • Dorsoventrally flattened shape and branching of cavity ensures all cells bathed in suitable medium, no cells too far from cavity for efficient diffusion and large surface area across which diffusion can occur
• As the size of organisms increased, surface area to volume ratio decreased, and diffusion was no longer efficient enough for transport of material
• Larger multicellular organisms developed specialised transport systems for long-distance bulk/mass flow of material due to increased quantity of material required
• Circulatory System


  Transports fluid in one direction, powered by a pump that forces the fluid through vessels that reach throughout the whole body
• Components of Circulatory System

  • Fluid: serves as medium of transport
  • Vascular System: channels/vessels that conduct fluid around body
  • Contractile Pump: keeps fluid circulating
• Circulatory systems evolved in conjunction with respiratory systems and often functions overlap
• Open Circulatory System

  • Haemolymph bathes organs and tissues directly
  • No distinction between blood and interstitial/tissue fluid
  • Consists of heart and open-ended blood vessels leading to series of open spaces (haemocoel)
  • Fluid/haemolymph not enclosed in vessels but directly fills haemocoel and bathes tissues where exchange can occur
  • Heart contracts and pumps blood into haemocoel
  • Blood/haemolymph under low pressure moves slowly around tissues while exchange of material occurs
  • When heart relaxes, suction gathers haemolymph back into open-ended vessels/valved openings called Ostia
  • Valves at Ostia help keep blood flow in one direction
  • Body movements help keep blood circulating in sinuses
• Organisms with Open Circulatory System

  • Mollusca
  • Arthropoda
• Insects have respiratory system independent of circulatory system, still fairly active despite limitations of open circulatory system
• Closed Circulatory System

  • Blood circulates within closed blood vessels
  • Clear distinction between blood and interstitial fluid
  • Heart pumps blood around body at high pressure
  • Blood components remain within continuous series of vessels and never come into direct contact with tissues
  • Exchange occurs between blood and tissue fluid through walls of blood vessels
• Organisms with Closed Circulatory System

  • Annelids
  • Most vertebrates (including humans)
• Single Circulation (Fish)


  • Blood flows in one circuit pathway
  • 2-chambered heart: 1 atrium collects blood from body, 1 ventricle pumps blood to gills where it gets oxygenated, then flows to tissues and rest of body before returning to heart
  • Blood pressure (hydrostatic pressure) decreases due to increased cross-sectional area of capillaries in gill lamellae, so blood flowing from gills through aorta to rest of body is at low pressure and speed
• Double Circulation (Amphibians, Reptiles)

  • Beginnings of double circulation with 3-chambered heart: 2 atria, 1 ventricle with no/incomplete septum
  • Mixing of oxygenated and deoxygenated blood occurs within heart, so systemic and pulmonary circuits are not completely separated (less efficient)
• Double Circulation (Birds, Mammals)

  • 4-chambered heart divided into 2 halves: Right atrium and ventricle receive deoxygenated blood from body and pump it to lungs in pulmonary circuit at relatively low pressure, Left atrium and ventricle receive oxygenated blood from lungs and pump it to rest of body in systemic circuit at relatively high pressure
  • 2 completely separate circuits (pulmonary and systemic) with no mixing of blood within heart
• Vascular System Arrangement

  • In humans, vascular system arranged in parallel, allowing unobstructed flow - if passage is obstructed/blocked to one organ, flow to other organs not compromised
  • Ensures blood pressure, oxygen and nutrient content reaching each organ is approximately equivalent
  • Exception is portal vessels which link two organs (e.g. gut and liver) allowing modification and detoxification of blood before entering general circulation
• Mammalian Heart

  • Situated on ventral side of body, behind sternum and between two lungs in thorax
  • 3 layers: Pericardium (tough sac surrounding heart), Myocardium (muscular wall of heart), Endocardium (innermost lining of heart cavities)
  • 4 chambers: 2 upper atria with thin walls, 2 lower ventricles with thick walls
  • Right side receives deoxygenated blood from systemic circuit, left side receives oxygenated blood from pulmonary circuit
  • Ventricles have thicker walls to pump blood at higher pressure to systemic and pulmonary circuits
• One-Way Valves in Heart

  • Maintain unidirectional flow of blood through heart
  • Atrioventricular valves (tricuspid, bicuspid/mitral) separate atria and ventricles
  • Chordae tendinae and papillary muscles prevent inversion of AV valve flaps during ventricular contraction
  • Semi-lunar valves (aortic, pulmonary) open when ventricle contracts, close to prevent backflow when blood enters aorta/pulmonary artery
• Blood Vessels Supplying the Heart

  • Coronary arteries supply oxygenated blood to heart wall
  • Coronary sinus carries deoxygenated blood from heart wall to right atrium
• Cardiac Cycle

  • Heart beats approximately 75 times per minute, each heartbeat lasts around 0.83 seconds
  • Sequence of atrial contraction (systole), ventricular contraction (systole), and relaxation (diastole)
  • Lub-Dub heart sounds: Lub = AV valves closing, Dub = Semi-lunar valves closing
  • Heartbeat begins at sino-atrial (SA) node, which stimulates contraction
  • AV node picks up signal from SA node with slight delay, then transmits it via Bundle of His and Purkinje fibres to stimulate ventricular contraction from base upwards
• Desmosomes


  Stabilise position of cells, maintain shape and reinforce to withstand tension and pressure
• Gap Junctions


  Channels between cells which allows ions to flow rapidly
• Cardiac muscle fibres are in electrical contact with each other such that action potential can spread rapidly leading to coordinated contraction
• Cardiac muscle fibres are electrically isolated (atrio-ventricular septum) between atria and ventricles
• Cardiac Cycle

  1. Atrial Diastole

  2. Atrial Systole

  3. Ventricular Systole

  4. Ventricular Diastole
• After action potential initiated by SA node and spreads through atria, it is picked up by atrioventricular (AV) node
• AV node

  Only route for action potential to reach ventricles from atria
• AV node


  • Group of modified fibres located at junction between atria and ventricles which pick up action potential with slight delay
• AV node


  Branches into Bundle of His (runs down interventricular septum) and into specialised Purkinje fibres which conduct impulse (without contracting) and stimulate contraction of ventricle from base upwards
• Purkinje fibres are highly branched in RV and LV to increase surface area of contact with ventricular muscle
• When pressure in RA is greater than pressure in RV

  Blood passively trickles into RV
• When pressure in LA is greater than pressure in LV

  Blood passively trickles into LV
• High P in RA causes Ca2+ to enter SA node, generating impulse which spreads to atria only

  Impulse causes contraction (systole) of atria
• Impulse generated by SA node spreads quickly to RA and LA via electrical synapses


  Impulse causes contraction (systole) of atria
• AV node picks up impulse from SA node and conducts it down Bundle of His and to Purkinje fibres


  Causes RV and LV to contract with slight delay compared to RA and LA
• Ventricular Systole

  1. RV contracts

  2. LV contracts

  3. Isovolumetric Contraction

  4. Rapid Ventricular Ejection