Larger organisms require circulatory systems to transport materials and nutrients efficiently.

Structure of the Circulatory System

"Know the structure of the heart, arteries, veins and capillaries."
  • Thick layer of smooth muscle
  • Elastic tissue allows recoil
  • This allows blood to flow under high pressure
  • Narrow lumen
  • Wider lumen
  • Thin layer of smooth muscle
  • Blood is under lower pressure
  • Veins therefore have semi-lunar valves preventing the backflow of blood
  • Narrow lumen increases the surface area
  • Single layered wall provides a short diffusion path
  • Made up of endothelium only
The Heart
  • Left and right sides separated by the septum
  • Vena cava enters the right atrium
  • Atrioventricular valve separates the atria and the ventricles
  • Semi-lunar valves separate each ventricle from the artery
  • Tendinous cords ensure valves are not turned inside out
  • Made up of involuntary cardiac muscle
  • AV Valves - tricuspid & bicuspid
  • SL Valves - aortic & pulmonary

Double & Single Circulatory Systems

"Understand the advantages of a double circulatory system in mammals over the single circulatory systems in bony fish, including the facility for blood to be pumped to the body at higher pressure and the splitting of oxygenated and deoxygenated blood."
A double circulatory system has a number of advantages over a single circulatory system.

Single Circulatory System
  • Present in bony fish
  • Heart pumps deoxygenated blood to the gills
  • Blood takes up oxygen
  • Releases carbon dioxide
  • Then delivers oxygen to body cells
  • Returns to the heart
Despite this system being suitable for small animals such as fish, the system raises a number of problems:
  • Blood pressure falls significantly
  • System does not separate oxygenated and deoxygenated blood
  • Results in less efficient exchange
Double Circulatory System
  • Suitable for organisms with greater metabolic demands
  • Blood is pumped to the lungs, oxygenated and returns to the heart
  • Is then pumped to the rest of the body under high pressure
This system is much more suitable for larger organisms and results in the following advantages:
  • Separates oxygenated and deoxygenated blood
  • Maintains a higher hydrostatic pressure
  • Allows for more efficient exchange

The Cardiac Cycle

"Know the sequence of events of the cardiac cycle."
This is a relatively simple process which has three main stages:
  1. Atrial systole - the atria contract forcing blood into the ventricles through the atrioventricular valve.
  2. Ventricular systole - the ventricles contract, the AV valves close and the blood exits through the semi lunar valves.
  3. Diastole - both the atria and the ventricles relax, blood flows into the atria and the cycle repeats.

Myogenic Stimulation of the Heart

"Understand myogenic stimulation of the heart, including the roles of the sinoatrial node (SAN), atrioventricular node (AVN) and bundle of His."
Heart tissue is myogenic and this means that the heart can contract without an external stimulus. The process is shown below:
  1. The sinoatrial node (SAN initiates an impulse in the wall of the right atrium, this spreads across both atria causing atrial systole.
  2. The impulse the reaches the atrioventricular node (AVN) which delays the impulse allowing atrial systole to complete.
  3. The AVN then passes the impulse along the bundle of his, into the Purkyne fibres causing ventricular systole.
  4. These electrical changes can be referred to as a wave of depolarisation.

ECG Traces

"Be able to interpret data showing ECG traces and pressure changes during the cardiac cycle."
These graphs show the amount of electrical activity in the heart during myogenic stimulation and the cardiac cycle.
  1. The P-wave shows atrial systole caused by the SAN
  2. QRS complex shows ventricular systole
  3. T-wave shows systole as the ventricles repolarise


"Know the structure of blood as plasma and blood cells, to include erythrocytes and leucocytes (neutrophils, eosinophils, monocytes and lymphocytes)."
This is a major transport medium made up of a number of molecules:
  • Erythrocytes - contain lots of haemoglobin which carry oxygen. They have a biconcave shape, have no nucleus and have lots of haemoglobin.
  • Plasma - made up of 92% water and contain proteins, ions, nutrients, gases and hormones.
  • Leucocytes - main role is protection and defence. There are 4 different types:
    • Lymphocytes - release antibodies and can engulf pathogens
    • Eosinophills - contain enzymes which detoxify foreign proteins
    • Neutrophils - engulf foreign material
    • Monocytes - respond to inflammation

Functions of the Blood

"Know the function of blood as transport, defence, and formation of lymph and tissue fluid."
The blood has three main functions:
  • transport of nutrients and waste
  • form tissue fluid
  • defend against foreign bodies

Blood Clotting

"Understand the role of platelets and plasma proteins in the sequence of events leading to blood clotting, including: platelets form a plug and release clotting factors, including thromboplastin, prothrombin changes to its active form, thrombin and soluble fibrinogen forms insoluble fibrin to cover the wound."
This is a cascade of events which results in a blood clot. A blood clots is an insoluble network of proteins which prevents blood loss.
  1. Vessel is damaged
  2. Platelets release thromboplastin
  3. This turns prothrombin into thrombin
  4. Thrombin turns fibrinogen into fibrin
  5. This forms an insoluble mesh
  6. This traps blood cells
  7. A blood clot (thrombus) is formed


"Understand the stages that lead to atherosclerosis, its affect on health and the factors that increase the risk of its development."
Atherosclerosis is a serious health problem that can have many negative health implications. It is the results of a very long process in which fats and cholesterol build up inside of artery walls.
  1. Cholesterol penetrates the artery wall
  2. Cholesterol is oxidised
  3. Area becomes inflamed and macrophages move in
  4. LDL is ingested to form foam cells
  5. Collagen is then released forming a fibrous cap
  6. An atheroma is formed
  7. Calcification occurs, hardening the artery removing its ability to expand
This can cause CVD & CHD. A number of factors increase the risk of developing atherosclerosis:
  • Age
  • Genes
  • Smoking
  • Alcohol
  • Lack of exercise
  • Obesity
  • Amount of LDL & HDL cholesterol in the blood