Heart

• Composed of two pumps:
  • Right side: Pumps deoxygenated blood to the lungs for oxygenation.
  • Left side: Pumps oxygenated blood throughout the body.

The heart has two ventricles, the right ventricle and the left ventricle, which play crucial roles in the circulatory system by pumping blood to different parts of the body. The heart also functions as a muscular pump that generates cardiac output (CO), defined as heart rate (HR) × stroke volume (SV).

Stroke volume (SV) is the amount of blood ejected from the ventricle with each beat, and is influenced by factors such as preload, afterload, and contractility. Ejection fraction (EF) represents the percentage of blood ejected from the ventricle relative to its total filling volume, serving as an important measure of cardiac efficiency.

1. Right ventricle:
   • Pumps deoxygenated blood from the right atrium to the lungs through the pulmonary arteries.
   • Plays a key role in the pulmonary circulation, where blood is oxygenated in the lungs.
   • Generates lower pressure compared to the left ventricle as it only pumps blood to the lungs.
2. Left ventricle:
   • Pumps oxygenated blood from the left atrium to the rest of the body through the aorta.
   • Plays a key role in systemic circulation, ensuring all tissues receive oxygen and nutrients.
   • Has a thicker muscular wall than the right ventricle due to the high pressure required to pump blood throughout the entire body.

Valves of the heart

• Tricuspid and mitral valves: Prevent backflow into the atria during contraction.
• Pulmonary and aortic valves: Prevent backflow into the ventricles.

Electrical conduction of the heart

The heart’s rhythmic contractions are controlled by an intrinsic electrical system, as shown in the image below.

1. Sinoatrial (SA) node: The natural pacemaker of the heart.
   • Discharges at 60–80 beats per minute.
2. Atrioventricular (AV) node: Delays the electrical signal, allowing the atria to contract before the ventricles.
   • Discharges at 40–60 beats per minute.
3. Purkinje fibers: Rapidly conduct the signal to the ventricles, ensuring synchronized contraction.

Electrocardiogram (ECG)

• A graphical representation of the heart’s electrical activity.
• Key components:
  • P-wave: Atrial depolarization.
  • QRS complex: Ventricular depolarization.
  • T-wave: Ventricular repolarization.
  • PR interval: Represents conduction time through the AV node.
• ST segment: Elevation or depression can indicate underlying pathology, such as myocardial ischemia or infarction.

Blood vessels

The circulatory system consists of arteries, capillaries, and veins, each serving specific functions.

Arteries

• Carry oxygenated blood away from the heart.
• Walls are muscular and elastic to handle high pressure.

Capillaries

• Facilitate exchange of gases, nutrients, and waste between blood and tissues.
• Walls are thin and permeable.

Veins

• Return deoxygenated blood to the heart.
• Equipped with valves to prevent backflow.
• Operate under low pressure, relying on pressure gradients, the muscle pump, and valves to aid venous return.

Blood

Blood performs critical functions, including:

• Transporting oxygen and nutrients.
• Removing carbon dioxide and waste.
• Regulating body temperature and pH levels.

Hemoglobin

• The oxygen-carrying protein in red blood cells.
• Plays a key role in buffering blood acidity.
• Its ability to bind and release oxygen is described by the oxygen–hemoglobin dissociation curve, which shows the relationship between oxygen partial pressure (PO₂) and hemoglobin saturation. A rightward shift (caused by factors such as increased temperature, CO₂, or acidity) promotes oxygen release to working muscles, while a leftward shift favors oxygen binding in the lungs.

The respiratory system

The respiratory system facilitates gas exchange, ensuring the body receives oxygen and removes carbon dioxide.

The respiratory system facilitates the exchange of oxygen and carbon dioxide. Air is transported through the trachea, bronchi, and bronchioles before reaching the alveoli, where gas exchange occurs. The system relies on pressure gradients to draw air into the lungs (inspiration) and expel it (expiration).

Key pressures include:

• Pleural pressure: The pressure in the narrow space between the lung pleura and chest wall, which is normally negative during respiration.
• Alveolar pressure: The pressure inside the alveoli, which must decrease below atmospheric pressure during inspiration to allow airflow.

Gas exchange occurs via diffusion at the alveoli, driven by partial pressure differences of oxygen (PO₂) and carbon dioxide (PCO₂). Hemoglobin in red blood cells plays a critical role in oxygen transport and buffering blood acidity.

During exercise, respiratory muscles like the diaphragm and intercostals work harder to meet oxygen demands and remove carbon dioxide. Regular training improves respiratory efficiency and strengthens breathing muscles. The Valsalva maneuver can help stabilize the trunk during heavy lifting but also carries risks, such as increased blood pressure, making it an important consideration in resistance training.

Structure

• Upper airway: Nasal cavity, pharynx, larynx.
• Lower airway: Trachea, bronchi, bronchioles, alveoli.

Exchange of gases

• Occurs in the alveoli, where oxygen diffuses into the blood, and carbon dioxide diffuses out.

Inspiration and expiration

• Inspiration: Diaphragm contracts, creating negative pressure that draws air into the lungs.
• Expiration: Diaphragm relaxes, forcing air out of the lungs.

Acute responses to exercise: Increased tidal volume, respiratory rate, and minute ventilation to meet the heightened oxygen demand.

Chronic adaptations: Enhanced capillarization of lung tissue, improved efficiency of oxygen diffusion, and stronger respiratory muscles, leading to greater overall ventilatory capacity.