5.1 Respiratory & Circulatory System Structure and Function

5.1 Respiratory & Circulatory System Structure and Function

NFPT Blueprint Focus: The content outline asks candidates to identify components of the respiratory system and circulatory system under Domain 1, and to recognize the function of body systems and how they interact under Domain 2. Knowing how cardiac output changes during exercise is not enough — the exam also asks what the parts are called and what each one does.

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The Respiratory System: Components

Respiration has two meanings on this exam, and they are routinely confused. Pulmonary ventilation is the bulk movement of air in and out of the lungs. External respiration is gas exchange at the alveoli; internal respiration is gas exchange at the tissues; and cellular respiration is the mitochondrial ATP production covered later in this chapter.

The Conducting Zone (Anatomical Dead Space)

Air travels through structures that warm, humidify, and filter it but perform no gas exchange. This volume — roughly 150 mL in an average adult — is called anatomical dead space.

1. Nose and nasal cavity — filters particulates, warms and humidifies incoming air.
2. Pharynx — shared passage for air and food.
3. Larynx — the voice box; the epiglottis seals it during swallowing.
4. Trachea — the windpipe, held open by C-shaped cartilage rings.
5. Bronchi — the trachea bifurcates into right and left primary bronchi, one per lung.
6. Bronchioles — progressively smaller branches whose walls are smooth muscle. Sympathetic stimulation dilates them during exercise; bronchoconstriction is what makes an asthma attack an emergency.

The Respiratory Zone (Gas Exchange)

1. Alveoli — roughly 300 million thin-walled sacs providing a total exchange surface of about 70 square meters, wrapped in pulmonary capillaries. Oxygen diffuses from alveolus to blood and carbon dioxide diffuses from blood to alveolus, both down their partial-pressure gradients. No active transport is involved; diffusion alone does the work.

The Respiratory Muscles

• Diaphragm — the primary muscle of inspiration. It contracts and flattens, enlarging the thoracic cavity, dropping intrathoracic pressure below atmospheric pressure, and drawing air in. Quiet expiration is passive, driven by elastic recoil.
• External intercostals — assist inspiration by elevating the ribs.
• Accessory muscles — sternocleidomastoid, scalenes, and pectoralis minor recruit during forced or heavy-exercise breathing. Visible accessory recruitment in a resting client is a red flag worth a referral.
• Forced expiration muscles — internal intercostals and the abdominals, which matter for the trainer because abdominal contraction against a closed glottis is the Valsalva maneuver.

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The Circulatory System: Components

The Heart

The heart is a four-chambered double pump. The right side handles the pulmonary circuit (to the lungs); the left side handles the systemic circuit (to the body).

Exam trap: The pulmonary arteries are the only arteries carrying deoxygenated blood, and the pulmonary veins are the only veins carrying oxygenated blood. Arteries are defined by direction (away from the heart), not by oxygen content.

The Cardiac Conduction System

Sinoatrial (SA) node (the intrinsic pacemaker, ~60–100 impulses/min) → atrioventricular (AV) node (delays the impulse so atria empty before ventricles fire) → Bundle of His → right and left bundle branches → Purkinje fibers (rapid ventricular depolarization). Because the SA node is autorhythmic, the heart beats without neural input; the autonomic nervous system only modulates the rate it sets.

Blood Vessels

Veins depend on the skeletal muscle pump and the respiratory pump to move blood back to the heart against gravity. This is the physiological reason a cool-down matters: stopping a hard run abruptly removes the muscle pump while the arterioles in the legs are still dilated, venous return collapses, and the client can faint.

Blood

• Plasma (~55% of volume) — water, proteins, electrolytes, hormones, nutrients.
• Erythrocytes (red blood cells) — carry oxygen bound to hemoglobin; about 98% of oxygen travels this way.
• Leukocytes (white blood cells) — immune defense.
• Platelets (thrombocytes) — clotting.
• Hematocrit — the percentage of blood volume made of red cells, typically ~42% in women and ~45% in men.

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How the Two Systems Interact During Exercise

The exam's Domain 2 objective is interaction, so learn the sequence rather than isolated facts:

1. Working skeletal muscle consumes oxygen and produces carbon dioxide, lowering tissue oxygen partial pressure.
2. Chemoreceptors and central command increase ventilation (tidal volume first, then rate) and increase heart rate and contractility.
3. Cardiac output rises: $Q = \text{HR} \times \text{SV}$, from roughly 5 L/min at rest to 20–25 L/min in a trained adult.
4. Arteriolar smooth muscle dilates in working muscle and constricts in the viscera, redistributing up to 85% of cardiac output to active muscle.
5. At the capillary, a widened arteriovenous oxygen difference reflects greater tissue extraction.
6. Carbon dioxide returns in the venous blood, diffuses into the alveoli, and is exhaled.

Clinical anchor: An oxygen–hemoglobin dissociation curve shifts right during exercise because working muscle is warmer, more acidic, and higher in carbon dioxide and 2,3-DPG. A rightward shift means hemoglobin releases oxygen more readily — the body's built-in mechanism for delivering more oxygen exactly where the metabolic demand is highest.