Exercise Physiology is Fun! Understand Your Cardiovascular System

Your heart beats, your lungs breath, and muscles work for you - but do you understand how all this happens? Get a quick science review and master technique!

Science is fun!

Teach better classes with a good understanding of the science behind cardiovascular exercise.

Learn to appreciate exercise science, especially the physiology behind the cardiovascular/respiratory systems.   I hope you enjoy this Science Review as much as I do!

Systems Review

Cardiovascular system

The cardiovascular system consists of the heart and blood vessels.  A vital component of this system is the delivery of blood to the working cells (and muscles).

  • Cardio = heart
  • Vascular = blood vessels

Cardiac output is a product of heart rate and stroke volume; both important components of this system.

  • Heart rate = how fast the heart beats each minute (bpm)
  • Stroke volume = how much blood comes through with each heart beat
  • Cardiac output = the volume of blood pumped by the heart per minute (ml blood/min)
Cardiorespiratory system

While the cardiorespiratory system is the heart and lungs.

  • Cardio = heart
  • Respiratory = lungs (breathing)

VO2max is the product of respiration rate and tidal volume; important factor for determining maximum amount of effort a person can perform.

  • Respiration rate = number of breaths per minute
  • Tidal volume = amount of air inhaled & exhaled per minute
  • VO2max = maximum amount of oxygen that can be consumed for a given period of time
Cardiorespiratory endurance

Cardiorespiratory endurance is defined as the capacity of the heart and lungs to deliver blood and oxygen to the working muscles (both at rest and during exercise).  A person’s ability to perform aerobic exercise depends largely on the interaction of the cardiovascular and the respiratory systems.  These systems provide oxygen to be transported in the blood to the active cells for muscular contraction to occur (through ATP production), while also removing metabolic waste.

What happens when you’re working out << Cardio Exercise

During bouts of cardio activity, the cardiovascular and respiratory systems respond in ways that allow the body to perform at its best.  This includes the following increases as a responses to exercise:

  • Heart rate (HR) = beats of the heart per minute (bpm)
  • Breathing rate (aka. Respiration rate) = number of breaths taken per minute
  • Blood flow (aka. Stroke volume)= amount of blood pumped out with each heart beat
  • Oxygen flow (aka. Oxygen consumption) = amount of oxygen used by the body for activity
During exercise
  1. The heart rate (HR) and stroke volume (SV) increase, causing more blood to be pumped out to the working muscles with each heartbeat.
  2. Breathing rate increases and more oxygen is needed to help move the working muscles.
  3. The harder a person works, the more blood and oxygen the body is going to need. It will reach max levels for each, known as:
    • Max heart rate (MHR): max number of heart beats per minute
    • VO2max: maximum amount of oxygen that can be consumed for a given period of time
Goal of training
  1. Decrease the amount of oxygen needed for harder activities, allowing the body to do more challenging exercises with less effort.
  2. Decrease the amount of work the heart is doing for a given minute. With a decreased RHR and a similar stroke volume, the heart is getting more blood to the body with less effort.

Quick Science Review << Exercise Physiology

The body needs energy in order to complete any activity in a safe and effective manner.  The body has a limited supply of stored ATP (only enough for 1-2 seconds).  Therefore, it recruits help from other sources while you properly refuel and recovery after any movement.

ATP (adenosine tri-phosphate) is the body’s usable source of energy.  It is created through a process known as glycolysis and the krebs cycle. These processes help to breakdown essential compounds necessary to combine with and produce ATP.

ATP requires the breakdown of glucose to achieve max benefits.  The process includes:

  • Breakdown of carbohydrates to glucose
  • Glucose stored in muscles as glycogen
  • Glycolysis begins as the metabolic process of breaking down glucose to create ATP
Energy systems

Cardiovascular exercises can typically be broken into two primary categories based on the energy systems required to complete the bout of work.  These include:

  • Aerobic: with oxygen (air)
  • Anaerobic: without oxygen (air)
Aerobic vs. Anaerobic
Energy system
Type of activity
Rate of atp production
Muscle fiber utilization
Phosphagen system Very high to max 1-10 seconds Power & speed Fast Type iib/x
Anaerobic glycoloysis High 10 seconds – 3 minutes Speed & strength Moderate Type iia
Aerobic system Low to moderate 3+ minutes Endurance Slow Type i
Aerobic exercise in action

Aerobic training includes activities in which a person uses oxygen and a combination of fat/carbohydrate stores to complete the exercise.  These are low to moderate intensity bouts, greater than 3-minutes, which slowly produce ATP stores in order to maintain endurance activity.

The primary source of fuel during these activities is a combination of fat and carbohydrates which are found within the body.  These are used slowly to maintain and conserve as much energy as possible.  With training, an individual is better able to utilize fat stores for energy versus carbohydrate stores, allowing for lower levels of fat in the body.  This requires lots of endurance exercise and enough fat stores in the body to gather the energy from.

  • Running
  • Cycling
  • Swimming
  • Moderate intensity strength training

All of these are examples of a time when a person elevates their heart rate to a moderate level (60-80% of max) and maintains it for a given period of time.  This is commonly referred to as steady state training as the body can maintain the given effort for long periods of time.  As seen in the below graph of a typical steady state training session.  Learn more about EPOC.


Steady State EPOC

The body predominantly uses slow-twitch muscle fibers (type 1) to complete periods of steady state training, because they:

  • Contract slowly
  • Produce little force
  • Are highly resistant to fatigue
Anaerobic exercise in action

Anaerobic exercise are short bursts of energy that increase the heart rate to a very uncomfortable and breathless zone.  Bouts of work last from 10-seconds up to 3-minutes.  The longer the bout of work, the less anaerobic it becomes (lowering the heart rate).

There are two energy systems that produce fuel during anaerobic activity, these include:

  • Phosphagen system: super high to max intensity levels (90-100%), 1-10 seconds of speed and power moves
  • Anaerobic glycolysis: high intensity (80-90%), 2-3 minutes of speed and strength moves

Both of these systems produce ATP stores without the presence of oxygen, which is why they can only be sustained for a short period of time.

  • 100m sprint
  • Shot-put
  • Speed skater
  • Power lifting

These examples show short bursts of effort of 80-100% max heart rate.  This is commonly referred to as interval training as the body is only able to maintain that level of performance for short periods of time before rest is required (image below).  Learn more about EPOC


Interval Training EPOC








Fast-twitch muscle fibers are the main source of work and production in these activities because they:

  • Contract quickly
  • Have moderate to high force production
  • Fatigue easily

Quick Science Review << Interval Training

Interval training is simply a type of cardiovascular training that incorporates bouts of high-intensity work followed by lower intensity bouts of work, or rest, that is repeated for a specific number of repetitions.

Exercise scientists have demonstrated that interval training can:

  • Boost the performance of competitive athletes
  • Improve the health of recreational exercisers
  • Provide the benefits of continuous endurance training with fewer workouts

The rest periods are just as important as the work periods as they allow the body to get rid of lactic acid from the blood.  This better prepares the body to push max limits in the next interval cycle.

Rest-to-work ratios

As seen in research, properly spaced work-to-rest intervals allow more work to be accomplished at higher exercise intensities with the same or less fatigue than during continuous training at the same relative intensity.  Showing that more training can be accomplished at higher intensities with interval training.

One of the most challenging parts of interval training is determining the appropriate ratio’s of work-to-rest as research has shown benefits in a variety of different time intervals.  Therefore, understanding the time intervals and recovery periods for each energy system is key for maximizing intervals.

While definitive research still needs to be done to determine the best intervals, the below chart outlines general suggestions for each energy system.

% effort
Energy system
Work duration
Recovery duration
Work-to-rest period ratios
10-seconds 30-60 seconds 1:3 – 1:6 6-10
Anaerobic glycolysis
1-minute 2-minutes 1:2 4-8
3-minutes 3-minutes 1:1 6-12


  • American College of Sports Medicine. (2010). ACSM’s Guidelines for Exercise Testing and Prescription. Baltimore, MD: Lippincott Williams & Wilkins.
  • Baechle, T. R. (2008). Essentials of Strength Training and Conditioning. Champaign, IL: Human Kinetics.
  • Iserman, M., & Walker, K. (2014). NETA: The Fitness Professionals Manual. Minneapolis, MN: National Exercise Trainers Association.
  • Laursen, P. B., & Jenkins, D. G. (2002). The Scientific Basis for High-Intensity Interval Training. Sports Medicine, 53-73.
  • Powers, S. K., & Howley, E. T. (2004). Exercise Physiology: Theory and Application to FItness and Performance. New York, NY: McGraw-Hill Companies, Inc.
  • Swain, D. P., & Leutholtz, B. C. (2007). Exercise Prescription: A casestudy approach to the ACSM Guidelines, 2nd Edition. Champaign, IL: Human Kinetics.

Exercise Physiology: Cardiovascular System for Fit Pros

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