state the functions of tissue fluid and describe the formation of tissue fluid in a capillary network

The Circulatory System – Cambridge IGCSE/A‑Level Biology

Learning Objective

State the functions of tissue fluid and describe how tissue fluid is formed in a capillary network.

1. Overview of the Human Circulatory System

• Double circulation: the heart pumps blood through two separate circuits – the systemic circuit and the pulmonary circuit – before the blood returns to the heart.
• Systemic circuit:

  • Oxygen‑rich blood leaves the left ventricle via the aorta.
  • It is distributed to all body tissues.
  • De‑oxygenated blood returns to the right atrium through the superior and inferior venae cavae.

• Pulmonary circuit:

  • Oxygen‑poor blood leaves the right ventricle through the pulmonary artery.
  • It passes through the lung capillaries where gas exchange occurs.
  • Oxygen‑rich blood returns to the left atrium via the pulmonary veins.

• Major vessel types: arteries → arterioles → capillaries → venules → veins.

2. Structure & Function of Blood Vessels

3. Blood‑Pressure Regulation

• Arteriolar tone – the main determinant of total peripheral resistance; controlled by sympathetic nerves and circulating hormones.
• Baroreceptor reflex – stretch receptors in the carotid sinus and aortic arch send signals to the medulla; an increase in arterial pressure triggers parasympathetic activation (↓ heart rate) and sympathetic inhibition (vasodilation).
• Renin–angiotensin–aldosterone system (RAAS) – low renal perfusion → renin release → angiotensin II formation → vasoconstriction and aldosterone‑mediated sodium/water retention, raising blood volume and pressure.
• Antidiuretic hormone (ADH) – released from the posterior pituitary when plasma osmolality rises; increases water re‑absorption in the kidneys, expanding blood volume.

4. Practical Tip – Recognising Vessels and Blood Cells in Slides

• Arteries: thick, elastic walls; lumen relatively small; internal elastic lamina visible.
• Veins: thin walls, larger lumen, valves (triangular “flaps”).
• Capillaries: barely visible; appear as a fine branching network.
• Red blood cells (RBCs): biconcave discs, ~7 µm diameter, no nucleus.
• Leukocytes (required for the syllabus):

  • Neutrophils – multilobed nucleus, fine granules; primary phagocytes.
  • Lymphocytes – large, round nucleus, scant cytoplasm; adaptive immunity.
  • Monocytes – large nucleus, abundant cytoplasm; become macrophages in tissues.
  • Eosinophils – bilobed nucleus, large orange‑red granules; combat parasites and modulate allergic responses.

5. Blood Composition & the Role of Water

• Plasma (≈55 % of blood volume) – ~90 % water; the solvent for:

  • Ions (Na⁺, K⁺, Ca²⁺, Cl⁻, HCO₃⁻)
  • Hormones (insulin, adrenaline, ADH, etc.)
  • Metabolites (glucose, urea, amino acids)
  • Plasma proteins:

    • Albumin – maintains colloid (oncotic) pressure.
    • Globulins – transport of lipids, metal ions; immune functions.
    • Fibrinogen – clot formation.

• Water’s high specific heat and solvent properties allow:

  • Efficient transport of heat (temperature regulation).
  • Dissolution and distribution of metabolic substances throughout the body.

6. Functions of Tissue Fluid (Interstitial Fluid)

• Provides a medium for the exchange of O₂, CO₂, nutrients, and metabolic wastes between capillary blood and body cells.
• Maintains a stable extracellular environment (homeostasis) essential for cellular metabolism.
• Acts as a transport pathway for hormones, enzymes and immune cells to their sites of action.
• Serves as a reservoir for plasma proteins; excess fluid that cannot re‑enter the capillary is collected by lymphatic capillaries and returned to the circulation.

7. Formation of Tissue Fluid in a Capillary Network

Fluid movement across the capillary wall is governed by Starling’s forces – the balance between hydrostatic and oncotic pressures.

Step‑by‑step formation of tissue fluid

1. Blood enters the capillary at the arterial end; high P_c forces plasma (water + small solutes) out of the lumen.
2. Filtration creates interstitial (tissue) fluid that bathes the cells.
3. As blood progresses, resistance in the arteriolar‑capillary network lowers P_c.
4. Reabsorption occurs at the venous end where π_c exceeds the reduced P_c, pulling most of the filtered fluid back into the capillary.
5. Any fluid that remains in the interstitial space is collected by lymphatic capillaries, becomes lymph, and is returned to the venous circulation via the thoracic duct.

8. Clinical Link – Edema

Disturbance of Starling forces leads to excess interstitial fluid (edema). Common causes examined in Cambridge papers:

• ↑P_c – e.g., heart failure or venous obstruction.
• ↓π_c – e.g., hypo‑albuminaemia (liver disease, nephrotic syndrome).
• ↑π_i – e.g., inflammation, increased capillary permeability.
• ↓ lymphatic drainage – e.g., filariasis.

9. Summary Flowchart (text description for diagram)

Draw a single capillary tube:

1. Left (arterial) end – arrow outward labelled “Filtration (P_c > π_c)”.
2. Middle – label “Equilibrium (NFP ≈ 0)”.
3. Right (venous) end – arrow inward labelled “Reabsorption (π_c > P_c)”.
4. Outside the capillary, show an arrow from the interstitial space to a lymphatic capillary labelled “Excess fluid → lymph → venous circulation”.

Key Points to Remember for the Exam

• Hydrostatic pressure pushes fluid out of capillaries; oncotic pressure (mainly albumin) pulls fluid in.
• Typical pressures: P_c 35 mmHg (arterial) → 15 mmHg (venous); π_c ≈ 25 mmHg; P_i ≈ 0–2 mmHg; π_i ≈ 1 mmHg.
• Net filtration occurs at the arterial end, net reabsorption at the venous end; the balance creates tissue fluid.
• Functions of tissue fluid tie directly to the three main roles of the circulatory system: transport, regulation and protection.
• Edema results when filtration exceeds reabsorption or lymphatic return is impaired.
• Know the four leukocyte types (neutrophil, lymphocyte, monocyte, eosinophil) and one distinguishing feature of each.
• Remember that veins act as capacitance vessels, storing a large proportion of the blood volume.