2.3 Cardiovascular and respiratory systems

The cardiovascular and respiratory systems work together to deliver oxygen and nutrients to our body's tissues. These systems adapt to exercise, improving our ability to perform physical activities and maintain overall health.

During exercise, our heart pumps harder and faster, while our lungs breathe deeper and quicker. Over time, regular training leads to lasting changes in our heart, blood vessels, and lungs, making them more efficient at supporting our body's needs.

Heart Structure and Function

Chambers and Blood Flow

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• The heart is a muscular organ located in the chest cavity that pumps blood throughout the body via the cardiovascular system
• The heart is divided into four chambers: the right and left atria (upper chambers) and the right and left ventricles (lower chambers)
  • The right atrium receives deoxygenated blood from the body and the right ventricle pumps it to the lungs for oxygenation
  • The left atrium receives oxygenated blood from the lungs and the left ventricle pumps it to the body
• The heart has four valves that ensure unidirectional blood flow: the tricuspid valve, pulmonary valve, mitral valve, and aortic valve
  • The tricuspid valve is located between the right atrium and right ventricle
  • The pulmonary valve is located between the right ventricle and pulmonary artery
  • The mitral valve is located between the left atrium and left ventricle
  • The aortic valve is located between the left ventricle and aorta

Cardiac Cycle and Electrical Activity

• The cardiac cycle consists of systole (contraction) and diastole (relaxation) phases, which are regulated by the sinoatrial node (natural pacemaker) and the autonomic nervous system
  • During systole, the ventricles contract and pump blood out of the heart
  • During diastole, the ventricles relax and fill with blood from the atria
• Cardiac output, the volume of blood pumped by the heart per minute, is determined by heart rate and stroke volume
  • Heart rate is the number of times the heart beats per minute
  • Stroke volume is the volume of blood pumped out of the left ventricle with each contraction
• The heart's electrical activity can be measured using an electrocardiogram (ECG), which displays the P wave, QRS complex, and T wave, representing atrial depolarization, ventricular depolarization, and ventricular repolarization, respectively
  • The P wave represents the spread of electrical activity through the atria
  • The QRS complex represents the spread of electrical activity through the ventricles
  • The T wave represents the recovery of the ventricles after contraction

Gas Exchange in the Lungs

Alveolar Gas Exchange

• Gas exchange occurs in the alveoli of the lungs, where oxygen diffuses from the air into the blood and carbon dioxide diffuses from the blood into the air
  • Alveoli are tiny air sacs surrounded by capillaries where gas exchange takes place
  • The partial pressure gradients of oxygen and carbon dioxide between the alveoli and the blood drive the diffusion of these gases across the alveolar-capillary membrane
• The process of breathing involves inspiration (inhalation) and expiration (exhalation), which are controlled by the respiratory center in the medulla oblongata and the autonomic nervous system
  • During inspiration, the diaphragm and external intercostal muscles contract, increasing the volume of the thoracic cavity and drawing air into the lungs
  • During expiration, the diaphragm and external intercostal muscles relax, decreasing the volume of the thoracic cavity and forcing air out of the lungs

Exercise and Ventilation

• During exercise, the respiratory system increases its rate and depth of breathing (ventilation) to meet the increased oxygen demand of the working muscles and to remove the increased carbon dioxide production
  • Ventilation is the volume of air moved in and out of the lungs per minute
  • Tidal volume is the volume of air inhaled or exhaled with each normal breath
  • Respiratory rate is the number of breaths taken per minute
• The ventilatory threshold is the point during incremental exercise at which ventilation increases disproportionately to the increase in oxygen consumption, indicating the onset of anaerobic metabolism
  • Anaerobic metabolism produces lactic acid, which dissociates into lactate and hydrogen ions, stimulating chemoreceptors and increasing ventilation
  • The ventilatory threshold is an important marker of endurance performance and can be used to prescribe training intensities

Circulatory System and Tissue Delivery

Blood Vessels and Blood Flow

• The circulatory system consists of the heart, blood vessels (arteries, capillaries, and veins), and blood, which work together to transport oxygen, nutrients, hormones, and waste products throughout the body
  • Arteries carry oxygenated blood away from the heart to the tissues, while veins carry deoxygenated blood from the tissues back to the heart
  • Capillaries are the site of exchange between the blood and the tissues, where oxygen and nutrients diffuse from the blood into the cells, and waste products diffuse from the cells into the blood
• The circulatory system is divided into the pulmonary circulation, which carries blood between the heart and the lungs, and the systemic circulation, which carries blood between the heart and the rest of the body
  • In the pulmonary circulation, deoxygenated blood is pumped from the right ventricle to the lungs, where it picks up oxygen and releases carbon dioxide, and then returns to the left atrium
  • In the systemic circulation, oxygenated blood is pumped from the left ventricle to the body's tissues, where it delivers oxygen and nutrients, and then returns to the right atrium

Blood Pressure and Oxygen Delivery

• Blood pressure, the force exerted by the blood against the walls of the blood vessels, is determined by cardiac output and peripheral resistance
  • Cardiac output is the volume of blood pumped by the heart per minute
  • Peripheral resistance is the resistance to blood flow in the blood vessels, primarily determined by the diameter of the arterioles
• The redistribution of blood flow during exercise is regulated by the autonomic nervous system, which increases blood flow to the working muscles and decreases blood flow to non-essential organs
  • The sympathetic nervous system causes vasoconstriction in non-essential organs and vasodilation in working muscles
  • The parasympathetic nervous system causes vasodilation in non-essential organs and has little effect on working muscles
• The oxygen-carrying capacity of the blood is determined by the concentration of hemoglobin, an iron-containing protein in red blood cells that binds to oxygen
  • Each hemoglobin molecule can bind up to four oxygen molecules
  • The oxygen-hemoglobin dissociation curve shows the relationship between the partial pressure of oxygen and the percentage of hemoglobin saturated with oxygen

Exercise Adaptations in Cardiovascular and Respiratory Systems

Acute Adaptations to Exercise

• Acute cardiovascular adaptations to exercise include increases in heart rate, stroke volume, and cardiac output to meet the increased oxygen and nutrient demands of the working muscles
  • Heart rate increases due to the withdrawal of parasympathetic tone and the activation of sympathetic tone
  • Stroke volume increases due to increased venous return and increased contractility of the heart muscle
  • Cardiac output increases as a result of the increases in heart rate and stroke volume
• Acute respiratory adaptations to exercise include increases in breathing rate and depth (ventilation) to maintain adequate gas exchange and blood oxygenation
  • Tidal volume increases due to the increased contraction of the inspiratory muscles
  • Respiratory rate increases due to the increased stimulation of the respiratory center by the motor cortex and chemoreceptors

Chronic Adaptations to Training

• Chronic cardiovascular adaptations to endurance training include increased left ventricular size and wall thickness (cardiac hypertrophy), increased capillary density in skeletal muscles, and increased blood volume and red blood cell count
  • Cardiac hypertrophy allows the heart to pump more blood with each contraction, increasing stroke volume and cardiac output
  • Increased capillary density improves the delivery of oxygen and nutrients to the working muscles
  • Increased blood volume and red blood cell count enhance the oxygen-carrying capacity of the blood
• Chronic respiratory adaptations to endurance training include increased lung volumes and capacities, improved respiratory muscle strength and endurance, and increased efficiency of gas exchange at the alveolar-capillary membrane
  • Vital capacity, the maximum volume of air that can be exhaled after a maximal inhalation, increases due to the increased strength and endurance of the respiratory muscles
  • Respiratory muscle training can improve the strength and endurance of the diaphragm and other inspiratory muscles
  • The increased efficiency of gas exchange allows for more oxygen to be extracted from the air and delivered to the blood
• High-altitude training can induce additional adaptations, such as increased production of erythropoietin (EPO), which stimulates red blood cell production and enhances oxygen-carrying capacity
  • EPO is produced by the kidneys in response to the decreased partial pressure of oxygen at high altitudes
  • The increased red blood cell count allows for more oxygen to be delivered to the tissues, enhancing endurance performance
• Resistance training can lead to acute increases in blood pressure and chronic adaptations such as increased left ventricular wall thickness and improved peripheral vascular function
  • The acute increase in blood pressure during resistance exercise is due to the increased cardiac output and peripheral resistance
  • The chronic increase in left ventricular wall thickness is an adaptation to the increased afterload placed on the heart during resistance exercise
  • Improved peripheral vascular function may be due to the increased shear stress on the blood vessels during resistance exercise, leading to improved endothelial function
• Detraining, or the cessation of regular exercise, can lead to a reversal of many of these cardiovascular and respiratory adaptations over time
  • Cardiac hypertrophy, increased capillary density, and increased blood volume may decrease within a few weeks of detraining
  • Respiratory adaptations such as increased lung volumes and capacities may also decrease with detraining, but at a slower rate than cardiovascular adaptations