The cardiovascular and respiratory systems

Why your heart matters more than your biceps

Longevity correlates more strongly with cardiovascular fitness than any other physical metric. Top-quintile VO2 max reduces all-cause mortality by ~80%. As a trainer, neglecting cardio for clients is malpractice.

The oxygen pathway

1. Air enters via nose/mouth 2. Trachea → bronchi → bronchioles → alveoli (tiny air sacs in the lungs) 3. Alveoli → capillaries — oxygen diffuses into the blood, CO2 diffuses out 4. Pulmonary veins → left atrium → left ventricle → aorta → arteries → arterioles → capillaries 5. Capillaries → muscle cells — oxygen diffuses into the cell, used in mitochondria for ATP 6. Reverse path for CO2 back to lungs and out

Key cardiovascular metrics

Heart rate (HR) — beats per minute. Resting 60-100 normal; elite endurance athletes can be 30-40. Stroke volume (SV) — blood ejected per beat. Increases dramatically with training (a trained heart pushes more blood per beat → can beat slower for the same output). Cardiac output (CO) = HR × SV. Liters per minute. Doubles to triples during max exercise. Blood pressure — systolic / diastolic. <120/80 ideal. >140/90 = hypertension stage 2. VO2 max — maximum oxygen the body can use per kilogram per minute. The gold standard fitness metric. Improvable by ~15-20% with consistent training; about 50% is genetic.

Respiratory metrics

Tidal volume — air per normal breath (~500 ml at rest) Vital capacity — max air you can exchange in one breath (~4-5 L) Minute ventilation — total air moved per minute

During exercise, both HR and tidal volume rise. Untrained people hit minute ventilation max long before muscle limits → "out of breath" stops them.

Adaptations to training

Aerobic training (Zone 2 specifically):

Stroke volume up
Resting HR down
Mitochondrial density and size up (more places to make ATP)
Capillary density in trained muscles up
VO2 max up

High-intensity training:

Max HR slightly up
Lactate threshold up
Anaerobic capacity up

What this means for programming

Every client, regardless of goal, should do at least 150 minutes/week of moderate cardio (ACSM standard). Even strength-focused athletes benefit — better cardio = better recovery between sets, more volume tolerance.