Body Fluids and Circulation

💧 Lymph (Tissue Fluid)

When blood flows through tiny capillaries in your tissues, water and small nutrients leak out into spaces between your cells. This fluid is called tissue fluid or interstitial fluid. It helps exchange oxygen, nutrients, and waste between blood and cells. Your lymphatic system (a network of vessels) collects this fluid, now called lymph, and returns it to your bloodstream. Lymph:

  • Is colorless and contains infection-fighting lymphocytes 🛡️
  • Carries nutrients and hormones
  • Absorbs fats from your intestines via lacteals

❤️ Circulatory Pathways

Two main types exist:

  • Open system (insects/snails): Blood flows freely in body cavities.
  • Closed system (humans/worms): Blood stays inside vessels (more efficient!).

Vertebrate Hearts Comparison

  • Fish 🐟: 2 chambers (1 atrium + 1 ventricle) → Single circulation (blood passes through heart once per loop).
  • Frogs/Snakes 🐸: 3 chambers (2 atria + 1 ventricle) → Incomplete double circulation (oxygenated/deoxygenated blood mix).
  • Birds/Humans 🐦👨: 4 chambers (2 atria + 2 ventricles) → Double circulation (no blood mixing!).

👤 Human Circulatory System

Made of: Heart ❤️, blood vessels, and blood.

Heart Structure

  • Size of your fist, protected by pericardium (fluid-filled sac).
  • 4 Chambers:
    • Upper: Right & Left Atria (receive blood)
    • Lower: Right & Left Ventricles (pump blood out)
  • Valves (ensure one-way blood flow):
    • Tricuspid: Right atrium → Right ventricle
    • Bicuspid (Mitral): Left atrium → Left ventricle
    • Semilunar: Exit valves from ventricles

Heart’s “Electric System” ⚡

  • Pacemaker (SAN): In right atrium → Starts heartbeat (70-75/min).
  • AV Node (AVN): Relays signals to ventricles.
  • Purkinje fibres: Spread signal through ventricles.

This system makes the heart beat automatically! 🎯

🔄 Cardiac Cycle (0.8 seconds)

Steps in one heartbeat:

  1. Relaxation (Diastole): Blood enters atria → flows into ventricles.
  2. Atria Contract (Systole): Push extra blood into ventricles.
  3. Ventricles Contract (Systole):
    • AV valves close → “Lub” sound ❤️
    • Blood pumps into arteries (semilunar valves open)
  4. Ventricles Relax: Semilunar valves close → “Dub” sound.

Cardiac Output = Stroke Volume (70 mL/beat) × Heart Rate = ~5 L/min in adults! 🏃‍♂️

📈 Electrocardiogram (ECG)

Records your heart’s electrical activity:

  • P-wave: Atria contraction
  • QRS complex: Ventricles contract
  • T-wave: Ventricles resetting

Doctors use ECG to check for heart problems! 🩺

🔄 Double Circulation

Two loops working together:

  • Pulmonary Circuit:
    Right Ventricle → Lungs → Left Atrium (blood picks up O₂)
  • Systemic Circuit:
    Left Ventricle → Body → Right Atrium (delivers O₂ to organs)

Blood Vessel Layers: All arteries/veins have 3 walls:
1. Inner: Tunica intima
2. Middle: Tunica media (muscle/elastic fibers)
3. Outer: Tunica externa (connective tissue)

🌟 NEET Must-Knows

  1. Heart Chambers: 2 (fish), 3 (frogs), 4 (humans) → Double circulation in 4-chambered hearts.
  2. Cardiac Cycle: Diastole (filling) → Systole (pumping) → Heart sounds (“Lub-Dub”).
  3. ECG Waves: P (atrial depolarization), QRS (ventricular depolarization), T (ventricular repolarization).
  4. Blood Flow Paths: Pulmonary circuit (heart-lungs), Systemic circuit (heart-body).
  5. Pacemaker: SAN generates heartbeat (70-75/min).

Keep revising – you’ve got this! 💪