5.1 Body Structure and Function: Anatomy for Shipboard First Aid

5.1 Body Structure and Function: Anatomy for Shipboard First Aid

Core Principle: Column 2 of Table A-VI/1-3 lists appreciation of body structure and functions as a mandatory knowledge item, immediately after assessing the casualty and threats to your own safety. The reason is practical: the ABC priority order, the decision to splint or not to splint, and the recognition of shock all depend on knowing what is underneath the skin and what it is supposed to be doing.

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1. The Skeletal System — the Frame and the Armour

The adult skeleton has 206 bones. It gives the body shape, protects the organs, provides the levers the muscles pull on, stores calcium, and manufactures blood cells in the marrow.

Joints are where two bones meet: hinge (elbow, knee), ball-and-socket (shoulder, hip), pivot (neck), and gliding (wrist, ankle). Ligaments tie bone to bone; tendons tie muscle to bone. A joint that is deformed, painful and immobile is treated as a fracture until proven otherwise.

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2. The Muscular System

• Skeletal (voluntary) muscle moves the body and generates heat — shivering is skeletal muscle producing heat on demand.
• Smooth (involuntary) muscle lines the gut, the airways and the blood vessels. Vasoconstriction in shock and in cold-water immersion is smooth muscle at work.
• Cardiac muscle is unique: it contracts rhythmically without external stimulation, driven by the heart's own electrical conduction system.

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3. The Respiratory System — Getting Oxygen In

   Nose / mouth -> pharynx -> larynx (voice box, epiglottis)
        -> trachea -> right and left bronchi -> bronchioles
        -> ALVEOLI  <-- gas exchange happens here, and only here

• The diaphragm and intercostal muscles do the mechanical work: contraction enlarges the chest, pressure falls, air flows in. Breathing out at rest is passive elastic recoil.
• Gas exchange occurs across the alveolar membrane into the surrounding capillaries.
• Inspired air is about 21% oxygen; exhaled air still contains about 16%. This is exactly why expired-air (mouth-to-mask) rescue breathing works.
• Normal adult tidal volume is roughly 500 mL, and normal adult respiratory rate is 12–20 breaths per minute.

Why the airway comes first: in an unconscious supine casualty the tongue loses muscle tone and falls back against the pharyngeal wall. It is the commonest cause of preventable airway obstruction, and it is corrected by a head-tilt chin-lift or a jaw thrust in seconds (Section 5.2).

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4. The Circulatory System — Moving Oxygen Around

• The heart is a four-chambered pump: right atrium and ventricle drive the pulmonary circuit to the lungs; left atrium and ventricle drive the systemic circuit to the body. The left ventricle is the thick-walled chamber that generates blood pressure.
• Arteries carry blood away from the heart under high pressure with thick, elastic, muscular walls — which is why arterial bleeding spurts in time with the pulse.
• Veins return blood at low pressure and contain one-way valves; venous bleeding flows steadily.
• Capillaries are one cell thick and are where oxygen, nutrients and waste actually cross into and out of the tissues.
• Blood volume in an average adult is roughly 5 litres, about 70 mL per kilogram of body mass. Losing about 1 litre (20%) produces clinical shock; losing 1.5–2 litres (30–40%) is immediately life-threatening.

Pulse Points Worth Knowing

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5. The Nervous System — the Reason for the Clock

• The central nervous system is the brain and spinal cord; the peripheral nervous system is everything branching off it.
• The autonomic division runs the things you do not think about: heart rate, breathing drive, vessel tone, digestion.
• The brain stores no oxygen. Consciousness is lost within about 10 seconds of circulatory arrest, and irreversible neuronal damage begins within 4 to 6 minutes. Every rule about minimising compression pauses in Section 5.3 follows from this single fact.
• The spinal cord runs inside the vertebral canal. Damage above the level of the fourth cervical vertebra can paralyse the diaphragm and stop breathing altogether — the anatomical reason for the jaw-thrust airway technique in suspected spinal trauma.

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6. The Skin and Thermoregulation

The skin is the largest organ and has three layers: epidermis (outer barrier), dermis (nerves, blood vessels, sweat glands, hair follicles) and the subcutaneous fat layer beneath. It is a waterproof barrier against infection, the primary organ of thermoregulation, and the site of the sensations you use in assessment.

Burn depth in Section 5.5 is simply a statement of how far down through these layers the injury has gone — which is why a full-thickness burn is painless in its centre: the dermal nerve endings have been destroyed.

Normal core body temperature is about 37°C (98.6°F). Below 35°C is hypothermia (Section 2.2); above 40°C with altered mental status is heat stroke (Section 5.5).

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7. Normal Adult Vital Signs — Your Reference Baseline

Record a full set of vitals, with the time, and repeat them. A single set tells you where the casualty is; a series tells you which way they are going, and that is what the telemedical service needs to hear.