recognise and draw red blood cells, monocytes, neutrophils and lymphocytes from microscope slides, photomicrographs and electron micrographs

Circulatory System – Cambridge International AS & A Level Biology (9700)

Learning Objectives

• Describe the structure and function of the closed double‑circulatory system in mammals.
• Identify the main blood vessels and explain the direction of blood flow.
• Explain the anatomy of the heart (chambers, valves, wall layers) and outline the cardiac cycle.
• Explain how gases, nutrients and waste products are transported in the blood.
• Recognise and accurately draw the four major white‑blood‑cell types and red blood cells from:

  • light‑microscope slides,
  • photomicrographs, and
  • electron‑microscope images.

• Understand the composition of plasma, formation of tissue fluid and the mechanisms of capillary exchange (Starling forces).
• Plan, carry out and evaluate a simple circulatory‑system investigation (Paper 3/5 practical skills).

1. Overview of the Mammalian Circulatory System

The mammalian circulatory system is a closed, double‑circuit (parallel) system:

• Systemic circuit – carries oxygen‑rich blood from the left heart to body tissues and returns deoxygenated blood to the right atrium.
• Pulmonary circuit – carries deoxygenated blood from the right heart to the lungs and returns oxygen‑rich blood to the left atrium.

1.1 Main Blood Vessels and Direction of Flow

2. Heart Anatomy and the Cardiac Cycle

2.1 Wall Layers (from inside to outside)

• Endocardium – smooth lining that reduces friction.
• Myocardium – thick contractile muscle; thickness varies (ventricles > atria).
• Pericardium (viscera + parietal layers) – protective sac containing a small amount of lubricating fluid.

2.2 Chambers and Valves

2.3 The Cardiac Cycle (one complete heartbeat)

1. Atrial systole – atria contract, pushing the remaining ~30 % of ventricular filling through the open AV valves.
2. Isovolumetric ventricular contraction – ventricles begin to contract; AV valves close (producing the first heart sound, “lub”). No blood leaves the ventricles yet.
3. Ventricular ejection – pressure exceeds arterial pressure, semilunar valves open; blood is expelled into the pulmonary artery (right) and aorta (left).
4. Isovolumetric ventricular relaxation – ventricles relax; semilunar valves close (second heart sound, “dub”). No change in volume.
5. Rapid ventricular filling – AV valves open; blood rushes from atria into ventricles.
6. Diastasis (slow filling) – flow slows; the heart is at rest until the next atrial systole.

3. Gas Transport in the Blood

• Oxygen (O₂)

  • ≈ 98 % bound to haemoglobin (Hb) in red blood cells (RBCs) as oxyhaemoglobin.
  • Each Hb molecule binds four O₂ molecules; binding is cooperative, giving a sigmoidal O₂‑dissociation curve.
  • Right‑shift factors (decrease affinity): ↑ CO₂, ↓ pH (Bohr effect), ↑ 2,3‑BPG, ↑ temperature.
  • Left‑shift factors (increase affinity): ↓ CO₂, ↑ pH, ↓ temperature.

• Carbon dioxide (CO₂)

  • ≈ 70 % transported as bicarbonate (HCO₃⁻) in plasma.
  • Reaction (catalysed by carbonic anhydrase in RBCs):
    

    CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻

  • Chloride shift – HCO₃⁻ leaves the RBC in exchange for Cl⁻ to maintain electroneutrality.
  • Remaining CO₂ is carried as carbamino‑Hb (bound to the globin chain) or dissolved in plasma.

4. Plasma, Tissue Fluid and Capillary Exchange

4.1 Composition of Plasma

4.2 Formation of Tissue Fluid

Plasma is filtered at the arterial end of capillaries by hydrostatic pressure, forming a filtrate that becomes interstitial (tissue) fluid. At the venous end, plasma proteins exert a colloid osmotic (Starling) pressure that draws water back into the capillary.

4.3 Mechanisms of Capillary Exchange (Starling Forces)

1. Diffusion – movement of gases (O₂, CO₂) and small solutes down concentration gradients.
2. Osmosis – water follows solute movement; contributes to fluid balance.
3. Bulk flow (filtration & reabsorption) – driven by the net difference between capillary hydrostatic pressure (Pₕ) and plasma colloid osmotic pressure (πₚ) versus interstitial hydrostatic pressure (Pᵢ) and interstitial colloid osmotic pressure (πᵢ).
   

   Net filtration pressure = (Pₕ – Pᵢ) – (πₚ – πᵢ)

4. Transcytosis – vesicular transport of macromolecules (e.g., proteins) across endothelial cells.

5. Blood‑Cell Identification (RBCs, Monocytes, Neutrophils, Lymphocytes)

5.1 Checklist for Identifying Blood Cells on Slides or Images

1. Measure overall cell diameter with a calibrated eyepiece micrometer.
2. Determine presence of a nucleus.

   • Absent → RBC.
   • Present → note shape (round, lobed, indented).

3. Observe granules (if any).

   • Fine pink granules uniformly distributed → neutrophil.
   • No visible granules → monocyte or lymphocyte (use nucleus shape to decide).

4. Assess cytoplasmic‑to‑nuclear (C:N) ratio.

   • High C:N (more cytoplasm) → neutrophil or monocyte.
   • Low C:N (large nucleus) → lymphocyte.

5. Compare size with an adjacent RBC for a relative reference.

6. Microscopy Techniques

1. Light Microscopy (LM) – bright‑field, oil‑immersion (≈ 1000×). Used for routine blood‑smear examination and photomicrography.
2. Photomicrography – digital capture of LM images; provides colour reference and a permanent record for comparison.
3. Transmission Electron Microscopy (TEM) – magnifications up to 100 000×; reveals ultrastructural details such as granule type, membrane folds, and organelles.

7. Practical Skills (Paper 3/5 – Investigation)

Typical practical tasks related to the circulatory system include:

• Preparing and staining a peripheral blood smear (Wright‑Giemsa stain).
• Measuring cell size with an eyepiece micrometer and calculating average diameters.
• Estimating the proportion of each white‑blood‑cell type (differential count).
• Investigating the effect of temperature or pH on the oxygen‑dissociation curve using a haemoglobin‑oxygen saturation assay.
• Analysing plasma protein concentration with a refractometer and relating it to colloid osmotic pressure.

7.1 Planning a Practical Investigation

1. State a clear hypothesis (e.g., “Increasing temperature shifts the O₂‑dissociation curve to the right”).
2. Identify independent, dependent and controlled variables.
3. Include safety and ethical considerations (e.g., handling human blood, disposal of sharps).
4. Write a detailed method that allows replication.
5. Present data in well‑labelled tables/graphs, calculate uncertainties, and evaluate sources of error.

8. Drawing Tips for Examinations

1. Start with a light pencil outline of the overall cell shape (disc, round, lobed).
2. Place the nucleus accurately:

   • Central, round – lymphocyte.
   • Indented, kidney‑shaped – monocyte.
   • 2‑5 lobes – neutrophil.

3. Indicate granules only for neutrophils (small dots evenly distributed).
4. Show the biconcave centre of an RBC as a pale area (central pallor).
5. Label each part clearly (e.g., “biconcave disc”, “multi‑lobed nucleus”, “granules”).
6. Include a scale bar or note “≈ 1 RBC = 7 µm” to demonstrate size awareness.

9. Summary

Mastering the circulatory system for Cambridge AS & A Level Biology requires an integrated understanding of:

• The parallel systemic and pulmonary circuits and the direction of blood flow through major vessels.
• The anatomy of the heart (chambers, valves, wall layers) and the sequence of events in the cardiac cycle.
• Mechanisms of gas transport, the role of haemoglobin, and the factors that modify the O₂‑dissociation curve.
• Plasma composition, tissue‑fluid formation, and capillary exchange explained through Starling forces.
• Recognition and accurate drawing of RBCs, monocytes, neutrophils and lymphocytes across LM, photomicrograph and TEM views.
• Practical investigation skills, including slide preparation, measurement, differential counting, data analysis and critical evaluation.

Regular practice with the comparison tables, checklists and drawing steps will build the confidence needed for both written and practical exam components.