Circulatory System Overview

Circulatory System and Circulation

• The circulatory system is a vital system in the body, facilitating the exchange of substances between tissues, the body, and external environments. It transports materials between bodily organs.

Importance

• The circulatory system is essential for the exchange of substances between all tissues in the body and the external environment. It transports various substances between body organs.

Functions

• Provides essential chemical substances to cells for metabolic processes.
• Transports waste products to organs for excretion.
• Plays a key role in preventing infections and defending against disease-causing microorganisms.
• Maintains normal body temperature and homeostasis of tissue fluids (electrolyte and acid-base balance).

Components: Heart and Blood Vessels

• Heart: A strong, muscular organ that contracts periodically (pulsates) to pump blood to various parts of the body.

  • Atria primarily receive blood.
  • Ventricles pump blood out.
  • The heart has 3 layers: endocardium, myocardium, and epicardium. The myocardium is the thickest layer and is primarily muscle tissue; the endocardium is the inner layer which is thin; the epicardium is the outer layer containing cells and connective tissue.
  • The heart has a fibrous ring which separates the atria from the ventricles, and the atria from each other are separated by an interatrial septum. The ventricles share a common wall called the interventricular septum. Atria and ventricles are connected by fibrous A-V valves (tricuspid and mitral).

• Blood Vessels:

  • Arteries: Strongest blood vessels able to expand and contract; their walls have elastic and muscle fibers facilitating blood transport.
  • Arterioles: Smaller arteries with constricting, regulated by the nervous system.
  • Capillaries: Smallest vessels where nutrient and waste molecule exchange occurs across thin walls.
  • Venules: Collect blood from capillaries.
  • Veins: Collect blood from venules and return to the heart; valves help prevent back flow of blood.

Cardiac Cycle

• Definition: The sequence of one systole (contraction) followed by one diastole (relaxation).
• Duration: Approximately 1/8 of a second.
• Systole: Rhythmic contraction of the auricles and ventricles.
• Diastole: Relaxation phase of the heart chambers.
• Contractions and relaxations happen in a specific order, starting with the auricles, then the ventricles (following by diastole of both).

Heartbeat

• The contraction (systole) and relaxation (diastole) of the heart constitute a heartbeat.
• Stroke volume: Amount of blood pumped out by each ventricle per heartbeat (approximately 70 mL in humans).
• Heart rate: Number of heartbeats per minute (approximately 70 in humans).
• Cardiac output: Total blood volume pumped by the heart per minute (heart rate x stroke volume).

Circulation

• Pulmonary circulation: Deoxygenated blood flows from the right atrium to the right ventricle, then to the pulmonary trunk, to two pulmonary arteries, to arterioles and capillaries in the lungs. After passing through lung capillaries around the alveoli, reoxygenated blood returns to the left atrium through pulmonary venules and veins.
• Systemic circulation: Oxygenated blood flows from the left atrium to the left ventricle, then the aorta, then branches of the aorta to coronary arteries (for the heart) and other body arteries. Blood flows through the body, then returns through veins to heart.
• Portal system: A unique system where blood flows through two sets of capillaries: capillaries in digestive organs, then venules to the hepatic portal vein (major vein), then breaks into capillaries in the liver, then finally exits through the hepatic vein into the vena cava.