Chapter 6

• Circulatory system

  Its purpose is to transport substances around the body
• Components of blood

  • Plasma
  • Red blood cells
  • White blood cells
  • Platelets
• Plasma

  Yellowish liquid that transports blood cells, nutrients, hormones, and waste products
• Red blood cells

  • Contain haemoglobin to transport oxygen
  • Biconcave shape to increase surface area-to-volume ratio
  • No nucleus to make more space for haemoglobin
  • Flexible to move through narrow capillaries
• White blood cells

  • Phagocytes carry out phagocytosis to destroy pathogens
  • Lymphocytes produce antibodies to recognize and destroy pathogens
• Organ transplant or blood transfusion can cause tissue rejection due to the immune system recognizing the donated organ or blood as foreign
• Platelets are fragments of cytoplasm that

  • Produces the enzyme thrombokinase that catalyses the conversion of inactive prothrombin to active thrombin
  • Thrombin converts soluble fibrinogen to insoluble fibrin threads which entangles red blood cells to form a blood clot
• Blood clotting prevents excessive blood loss and entry of microorganisms
• Blood groups

  • A
  • B
  • AB
  • O
• Antigens

  Special proteins on the surface of red blood cells
• Antibodies

  Present in the blood and may react with antigens on the red blood cell of another person, causing agglutination which is the clumping of red blood cells
• Antigens on blood cells

  • A
  • B
  • A and B
  • None
• Antibodies in plasma

  • a
  • b
  • a and b
  • None
• Mixing incompatible blood groups can cause agglutination which is the clumping of red blood cells, obstructing blood flow
• Parts of the Circulatory System

  • Heart
  • The 3 main and 2 sub blood vessels
  • Arteries: carry blood away from heart
  • Arterioles: when arteries divide into smaller vessels
  • Blood capillaries: when arterioles further divide into very tiny blood vessels
  • Venules: when capillaries unite to form small vessels.
  • Veins: when venules unite to form larger vessels.
• Structure and Function

  In biology, structure is always related to function.
• Arteries

  • Transport blood away from the heart to the rest of the body.
  • Characteristics of blood carried in arteries: Oxygenated (except for pulmonary artery), Fast-flowing, High-pressure
• Adaptations of Arteries

  • Thick muscular and elastic walls to withstand the high blood pressure of blood flowing within.
  • The elasticity helps the artery wall to stretch and recoil to push blood along in spurts.
  • The contraction and relaxation of the arterial walls control the size of the artery lumen. This can help to divert blood to certain organs at certain times (e.g. dilation of arterioles of the skin to divert blood to skin capillaries during thermoregulation)
• Veins


  • Transport blood from the rest of the body back to the heart.
  • Characteristics of blood carried in veins: Deoxygenated (except pulmonary vein), Flows more slowly and smoothly, Lower blood pressure (hence, blood has a tendency to backflow).
  • The walls of veins are thinner than the walls of the arteries.
• Adaptations of Veins

  • The walls of veins less thick and muscular than the walls of the arteries since blood pressure in veins is lower (blood flows more smoothly and slowly).
  • Veins have internal valves to prevent backflow of blood.
• When we sit for too long without much movement

  Pressure can build up in the veins of the lower limbs. This causes poor blood circulation and blood to pool in the legs.
• Skeletal muscles and movement are important to ensure blood returns to the heart.
• Blood capillaries are where exchange of substances occur between blood and tissue cells in the body.
• Role of blood capillaries

  The exchange of substances occurs mainly by diffusion.
• Factors affecting diffusion

  • Temperature*
  • Surface area to volume ratio
  • Diffusion distance
  • Concentration gradient
• *temperature does not matter in the human body because it is constant
• Features of blood capillaries

  • One-cell thick walls: provide short diffusion distance
  • Branched in a large network: provide increased surface area to volume ratio, provide increased total cross-sectional area compared to arterioles, lowering the blood pressure and thereby slowing blood flow to give more time for exchange of substances
• To increase rate of exchange of substances, capillaries need:

  • Large surface area
  • Short diffusion distance
  • Steep concentration gradient
• Tissue fluid

  • colourless fluid that filled the spaces between cells in tissue
  • transport dissolved substances between blood capillaries and tissue cells
• Double Circulation


  • In a double circulation, the blood has to pass through the heart twice in one complete circuit.
  • Movement of blood in a double circulation: From the heart to the lungs and back to the heart, From the heart to the rest of the body back to the heart
• Names of the 10 Main Blood Vessels

  1. Upper vena cava

  2. Lower vena cava

  3. Pulmonary Artery

  4. Pulmonary Vein

  5. Aorta (coronary arteries emerge from the aorta to bring blood to the heart tissues)

  6. Hepatic artery

  7. Hepatic vein

  8. Hepatic portal vein

  9. Renal Artery

  10. Renal Vein
• Four chambers


  • Two atria (right atrium, left atrium)
  • Two ventricles (right ventricle, left ventricle)
• Walls of the heart chambers


  • Walls of the atria are thinner than that of ventricles
  • Walls of the right ventricle are thinner than that of the left ventricle
• Median septum

  Prevents the mixing of deoxygenated blood with oxygenated blood
• Hole in the median septum

  Mixing of oxygenated and deoxygenated blood, reducing the concentration gradient for oxygen and carbon dioxide, and reducing efficiency of gas exchange. This can cause problems like shortness of breath, fatigue and heart failure.
• Valves


  • tricuspid valves
  • bicuspid valves
  • aortic valves (semi-lunar valves in the aorta)
  • pulmonary valves (semi-lunar valves in the pulmonary artery)
• How Tricuspid and Bicuspid valve acts like a one-way door

  1. Blood moves from atrium to ventricle

  2. Blood is pumped from higher pressure to lower pressure area

  3. These valves close when ventricular pressure exceeds atrial pressure to prevent backflow
• Cardiac cycle


  1. Atrial systole (atrium contracts)

  2. Ventricular systole (ventricle contracts)

  3. Atrial diastole (atrium relaxes)

  4. Ventricular diastole (ventricle relaxes)
• One cardiac cycle consists of one ventricular systole and one ventricular diastole
• What happens during the cardiac cycle

  1. Atrial muscles contract to pump blood through the tricuspid and bicuspid valves into ventricles

  2. Ventricular muscles contract to pump blood through semilunar valves into the pulmonary artery and aorta

  3. The rise in blood pressure causes the tricuspid and bicuspid valves to close, creating a loud, ‘lub’ sound

  4. Meanwhile, atrial muscles relax to receive blood from the vena cava and pulmonary vein.

  5. Ventricular muscles relaxes, causing semilunar valves to close due to the fall in blood pressure, creating a softer, ‘dub’ sound