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Biology · Blood, Blood Vessels and the Heart

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Biology · Edexcel Paper 2

Blood, Blood Vessels and the Heart

The Structure and Functions of Blood

Every part of the blood has a structure that suits its job: red blood cells carry oxygen, white blood cells defend, platelets clot and plasma transports.

  • Plasma The pale yellow liquid part of the blood, which carries dissolved substances, hormones, heat and blood cells.
  • Red blood cell A blood cell with no nucleus that is full of haemoglobin and carries oxygen (an erythrocyte).
  • Haemoglobin The red protein in red blood cells that contains iron and combines with oxygen.
A tube of blood before and after spinning in a centrifuge, with the plasma, white blood cells and platelets and the red blood cells in separate layers.
Plasma is a little over half of the blood, the red blood cells are most of the rest and the white cells and platelets are less than 1 percent.

Red blood cells and white blood cells

  • Red blood cells No nucleus; Packed with haemoglobin; A biconcave disc about 7 micrometres across
  • White blood cells A nucleus, which may have lobes; No haemoglobin; Round or irregular, and larger than red cells

How a blood clot forms

  • The vessel is cut Blood leaks out and the damaged wall is exposed.
  • Platelets collect Platelets stick to the damaged area and release chemicals.
  • A mesh forms The chemicals change the soluble protein fibrinogen into insoluble threads of fibrin.
  • The mesh traps cells Red blood cells and platelets are trapped in the mesh, which forms a clot.

Arteries, Veins and Capillaries

Arteries are strong, veins are wide with valves, and capillaries are thin and tiny, because the structure of each vessel suits its job.

  • Artery A blood vessel that carries blood away from the heart at high pressure.
  • Vein A blood vessel that carries blood towards the heart at low pressure.
  • Capillary A tiny blood vessel with a wall one cell thick, where substances are exchanged with the cells.
An artery with a thick muscular wall and narrow lumen, a capillary with a wall one cell thick, and a vein with a thin wall, a wide lumen and a valve.
Each vessel has a structure that suits its job: strong, thin or one-way.

Arteries and veins

  • Arteries Carry blood away from the heart; High pressure; Thick wall with muscle and elastic fibres
  • Veins Carry blood towards the heart; Low pressure; Thin wall with little muscle

The route of blood around the body

  • Artery The heart pumps blood at high pressure into the arteries.
  • Smaller arteries The arteries branch into smaller vessels that take the blood to each organ.
  • Capillaries The smallest vessels run between the cells.
  • Small veins The capillaries join up to form small veins, and the pressure is now low.
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Biology · Edexcel Paper 2

Blood, Blood Vessels and the Heart continued

The Heart and Double Circulation

The heart is two pumps in one, and a double circulation sends blood to the lungs and then back to the heart so that it can be pumped to…

  • Atrium An upper chamber of the heart that receives blood from the veins (plural atria).
  • Ventricle A lower chamber of the heart with a thick muscular wall that pumps blood into the arteries.
  • Septum The wall of muscle that separates the left and right sides of the heart.
The heart in section, showing the four chambers, the valves, the septum, the vena cava, the pulmonary artery, the aorta and the pulmonary vein, with a thin wall on the right ventricle and a thick wall on the left ventricle.
The left ventricle has the thickest wall because it pumps blood all the way round the body.

The right side and the left side of the heart

  • Right side Receives deoxygenated blood from the body through the vena cava; Pumps the blood to the lungs through the pulmonary artery; The right ventricle has a thinner wall
  • Left side Receives oxygenated blood from the lungs through the pulmonary vein; Pumps the blood to the body through the aorta; The left ventricle has a much thicker wall

The route of the blood

  • Vena cava Deoxygenated blood from the body flows in through the vena cava.
  • Right atrium to right ventricle The right atrium contracts, and the blood passes through the…
  • To the lungs The right ventricle contracts and forces the blood through the…
  • Gas exchange In the lungs the blood loses carbon dioxide and picks up oxygen.

Heart Rate, Stroke Volume and Cardiac Output

\text{cardiac output} = \text{stroke volume} × \text{heart rate}

  • Heart rate The number of times the heart beats in one minute, measured in beats per minute.
  • Stroke volume The volume of blood pumped out of a ventricle in one heartbeat.
  • Cardiac output The volume of blood pumped out of the heart in one minute, equal to stroke volume times heart rate.
A graph of heart rate against time showing a resting rate of 70 beats per minute, a rise to 160 during exercise and a slow fall during recovery.
The heart rate rises quickly when exercise starts, levels off, and falls slowly during recovery.

Investigating the effect of exercise on heart rate

  • Rest The volunteer sits quietly for five minutes.
  • Exercise The volunteer steps up and down on a step, or jogs on the spot.
  • Measure straight away Immediately find the pulse again, counting for 15 seconds and multiplying by 4.
  • Recovery Measure the pulse every minute until it returns to the resting value.