Calisthenics Instructor Certification Chapter 9: The Circulatory System

Chapter 9: The Circulatory System

The circulatory system ensures the transport of oxygen and nutrients to body tissues and the removal of carbon dioxide and waste products. It consists of the heart, blood vessels, and the blood flowing through them, as well as the lymphatic system, which carries lymph. The heart is the central organ of this system.

The Heart

The heart is a hollow, muscular organ shaped like an inverted cone, about the size of its owner’s fist. It lies in the lower part of the anterior mediastinum, enclosed by the pericardium (a fibrous sac), and tilts obliquely so that about two-thirds of its mass lies to the left of the midline.

It has four chambers:

  • Two atria (right and left), smaller with thinner walls.
  • Two ventricles (right and left), larger with thicker walls.

The heart functions as a muscular pump, made up of specialized cardiac muscle fibers (myocardium) that contract synchronously. Its activity is coordinated by intrinsic pacemakers:

  • The sinoatrial (SA) node, located in the right atrium wall.
  • The atrioventricular (AV) node, located at the junction of the atrial and ventricular septa.

Right Atrium

  • Larger chamber with thinner walls.
  • Receives venous blood from:
    • The superior vena cava (head, neck, arms, and thorax).
    • The inferior vena cava (lower body).
  • Passes blood through the atrioventricular orifice into the right ventricle.
  • The opening is guarded by the tricuspid valve.

Right Ventricle

  • Has an apex, base, and three walls.
  • Receives venous blood from the right atrium through the tricuspid valve.
  • Pumps blood through the pulmonary valve into the pulmonary artery, which carries deoxygenated blood to the lungs.

Left Atrium

  • Cube-shaped chamber forming most of the posterior surface of the heart.
  • Receives oxygenated blood from the four pulmonary veins (two from each lung).
  • Pumps blood through the mitral valve into the left ventricle.

Left Ventricle

  • Cone-shaped, longer, and with the thickest wall of all four chambers.
  • Forms the apex of the heart.
  • Pumps oxygenated blood through the aortic valve into the ascending aorta.
  • The left ventricle generates higher pressures because it supplies the entire systemic circulation.

Great Vessels

  • Aorta: The body’s largest artery, originating from the left ventricle. Divided into:
    • Ascending aorta (gives rise to the coronary arteries).
    • Aortic arch (branches: brachiocephalic artery, left common carotid, left subclavian).
    • Descending aorta (thoracic and abdominal).
  • Pulmonary artery: Originates from the right ventricle, splits into right and left pulmonary arteries, carrying venous blood to the lungs.
  • Superior vena cava: Collects venous blood from the head, neck, upper limbs, and thorax, draining into the right atrium.
  • Inferior vena cava: Collects venous blood from the abdomen, pelvis, and lower limbs, draining into the right atrium.

Coronary Circulation

The heart receives its own blood supply from the right and left coronary arteries, both arising from the ascending aorta.

  • Right Coronary Artery (RCA):
    • Runs along the right coronary sulcus.
    • Divides into the posterior descending artery (supplies ventricles and septum) and smaller branches.
  • Left Coronary Artery (LCA):
    • Divides quickly into:
      • Anterior descending artery (LAD): runs down the anterior interventricular sulcus, supplying both ventricles and septum.
      • Circumflex artery: runs along the left coronary sulcus, supplying the left atrium and ventricle.

The coordinated supply of these vessels ensures the continuous contraction-relaxation cycle of the myocardium.

 

Blood Vessels and Circulation

Types of blood vessels

  • Arteries: carry blood away from the heart toward the periphery.
  • Capillaries: the site of gas and nutrient exchange between blood and tissues.
  • Veins: return blood back to the heart.

Circulatory loops

  • Pulmonary (small) circulation: Right ventricle → Pulmonary artery → Lungs (gas exchange: CO₂ released, O₂ absorbed) → Pulmonary veins → Left atrium.
    Note: The pulmonary artery uniquely carries deoxygenated blood, while pulmonary veins carry oxygenated blood.
  • Systemic (large) circulation: Left ventricle → Aorta → Capillaries throughout the body → Superior and Inferior vena cava → Right atrium.

The cardiovascular system ensures that oxygen and nutrients reach tissues while metabolic byproducts are removed. The heart works as a dual pump: one side sends oxygen-rich blood into systemic circulation, the other sends oxygen-poor blood into the pulmonary circuit.

Blood Pressure

Definitions

  • Systolic pressure: the peak pressure in arteries during ventricular contraction (≈120 mmHg at rest, can rise to 200 mmHg during exercise).
  • Diastolic pressure: the lowest pressure just before the next contraction (≈80 mmHg at rest).
  • Pulse pressure: the difference between systolic and diastolic values (e.g., 125 – 75 = 50 mmHg).

Mechanism

  • Only about one-third of the stroke volume exits arteries during systole; the remainder distends arterial walls, which then recoil in diastole, maintaining continuous flow.
  • Flow is determined by the pressure difference between regions and opposed by resistance.

At rest

  • Typical values: systolic ≈120 mmHg, diastolic ≈80 mmHg.
  • Elastic recoil of arteries sustains blood flow between beats.

During exercise

  • Cardiac output: rises from ~5 L/min to as much as 35 L/min.
  • Blood flow: vasodilation in active muscles, vasoconstriction in kidneys and gut.
  • Mean arterial pressure: usually rises only slightly.
  • Pulse pressure: rises significantly due to greater stroke volume and faster ejection.
  • Mechanism: increase is driven primarily by heart rate, with modest contribution from stroke volume and enhanced venous return.

Disorders

  • Hypertension: chronic elevation ≥140/90 mmHg. Usually linked to increased total peripheral resistance (narrowed arterioles). Risk factors include obesity and sedentary lifestyle.
  • Hypotension: abnormally low blood pressure. May result from blood loss or severe dehydration (fluid and sodium loss through sweating). Main danger is reduced perfusion of the brain and heart.

Blood and Plasma

Composition

  • Plasma (~55% of total blood volume): 91.5% water, 7% proteins (albumin, globulins, fibrinogen), 1.5% other solutes (nutrients, hormones, gases, electrolytes, vitamins, nitrogenous wastes).
  • Formed elements (~45%): red blood cells, white blood cells, platelets.
  • Overall: ~22% solid components, ~78% water.
  • Functions: transport of gases (O₂, CO₂, N₂), nutrients, hormones, lipids, proteins, and waste.

Blood sampling

  • Venous samples: most common (e.g., from veins near the elbow).
  • Arterial samples: used for specialized tests (e.g., blood gases). Typically taken from the radial artery at the wrist. More uncomfortable due to higher nerve density in arterial walls. After arterial puncture, firm pressure must be applied for ~5 minutes to prevent bleeding.

 

Types of Exercise and Blood Pressure

Aerobic Exercise

During aerobic exercise, blood vessels in active muscles dilate, increasing blood flow throughout much of the body. The alternating contraction and relaxation of the muscles acts like a pump, propelling blood back to the heart.

  • Systolic pressure: rises sharply in the first minutes of moderate aerobic activity, typically reaching 140–160 mmHg.
  • Diastolic pressure: remains relatively unchanged.
  • Mechanism: the increase in cardiac output is greater than the reduction in total peripheral resistance, leading to a rise in mean arterial pressure. Pulse pressure also increases because both stroke volume and ejection velocity rise.

Resistance Exercise

In resistance training, blood pressure increases dramatically. The sustained muscle contraction compresses peripheral arteries, creating high resistance to blood flow.

  • This extra workload on the heart can be risky for individuals with hypertension or cardiovascular disease.
  • Blood flow to the active muscles is reduced once contractions exceed 10–15% of maximal strength, because the mechanical compression of vessels overrides the local vasodilation.
  • As a result, isometric contractions can only be sustained briefly before fatigue, while also driving mean arterial pressure significantly upward.

Upper vs. Lower Body Exercise

For the same percentage of VO₂max, arm exercise produces higher arterial pressure compared to leg exercise.

  • This is due to the smaller muscle mass and vascular network of the arms, which create greater resistance to flow.
  • Additionally, blood must be pumped upward to supply the arms, requiring higher systolic pressure and imposing greater cardiovascular strain.

 

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