The Circulatory System: A Vital Transport Network

Posted on May 29, 2024 in Biology

The Function of Circulation

  • Multicellular organisms are organized into tissues and organs.
  • Organ Systems require Nutrients and Oxygen to function.

How are these Vital Nutrients and Gases moved throughout the body?

Via a Transport System which is necessary to:

  1. Transport gases, nutrient molecules, and wastes.
  2. Regulate internal temperature and transport chemical substances around the body.
  3. Protect against blood loss from injury and against diseases and toxic substances.

Major Components of the Circulatory System

The Circulatory System has Three Main Components:

  • Heart: Muscular organ that pumps blood through the body.
  • Blood Vessels: System of hollow tubes through which blood moves.
  • Blood: Fluid that transports nutrients, O2, CO2, and many other materials.

The Human Heart

  • Location: Found slightly to the left of the middle of the chest.
  • Size: The size of your fist, weight 250-350 grams.
  • Composition: The heart is made of cardiac muscle AKA Myocardium.
  • The muscle cells contract and relax rhythmically and involuntarily without becoming fatigued.
  • Sample: How is Cardiac Muscle Different from Skeletal Muscle?
  • Oxygen-rich blood is kept separate from oxygen-poor blood.
  • The Heart has Four Chambers: two top Atria and two bottom Ventricles.
  • Often referred to as the Double Pump!
  • The left and right sides are separated by the Intraventricular Septum.

Don’t Break My Heart!

  • Protection: The heart is surrounded by a tough protective sac called the Pericardium.
  • The Epicardium: Is the Outer Layer of the Heart.
  • Myocardium: AKA the Cardiac Muscle lies beneath the Epicardium.
  • Endocardium: The layer of tissue that lines the inside of the heart.

Why the gap? Pericardial Cavity.

The Four Chambers of the Heart: The Flow of Blood

  • The right atrium receives blood from the body’s Veins via the Vena Cavae.
  • Blood is then moved into the right ventricle where it is pumped to the lungs (where it picks up oxygen) through the Pulmonary Arteries.
  • The oxygenated blood returns from the lungs via the Pulmonary Veins, enters the left atrium, and travels to the left ventricle.
  • The heart pumps it through the Aorta to flow to the rest of the body.
  • Valves control the flow of blood through the heart:
  • Atrioventricular Valves: Tricuspid (on the right) and bicuspid (mitral) (on the left) let blood from the atria into the ventricles.
  • Semilunar Valves: Pulmonary (on the right) and aortic (on the left) let blood flow through the ventricles into the large arteries.

Excitation of The Heart: Cardiac Myocardium Cells Are Excitable!

  • Human life depends on the consistent beating of the heart and its ability to adapt to various circumstances.
  • Exercise. Sleep.
  • Fight or Flight Response.

What Tells The Heart How & When To Beat?

  • Within the heart, the sinoatrial (SA) node stimulates the muscle cells to contract and relax rhythmically.
  • The SA node is in the right atrium.
  • Without Nervous Intervention, SA node allows for 70-80 bpm.
  • It generates an electrical signal that spreads over both the atria via internodal pathways so they contract top down, simultaneously.
  • The signal then reaches the Atrioventricular (AV) node, which transmits the signal through a bundle of fibres known as the bundles of His.
  • The signal travels to the right and left Bundle Branches.
  • These relay the signal to Purkinje fibres; which are then sent to the myocardium that forms the ventricles.
  • This initiates the almost simultaneous contraction of the ventricles.

The Electrocardiogram (ECG)

  • Most Heart Function is analyzed by measuring the electrical pulses generated by the Nerve Signals that stimulate contractions.
  • These create millivolt charges that can be detected with electrodes on the skin.
  • An ECG allows doctors to determine if the heart is generating signals of Normal Strength, Frequency, and Duration.
  • P Wave: 1st Wave represents depolarization AKA the spreading of the electrical signal through the Atria (- to +).
  • By the end of the P Wave, the signal has reached the AV Node.
  • After the atria depolarization, they repolarize (+ back to – reset readying themselves for another contraction). Not visible on ECG.
  • QRS Wave: Represents Depolarization of the Ventricle.
  • T Wave: Represents Repolarization of the Ventricle.

Coronary Circulation

  • Blood is supplied to the heart through two main Arteries.
  • The Right and Left Coronary Arteries.
  • Arteries branch off of the Aorta and divided numerous times to supply every region of the Myocardium with O2 rich blood.
  • With every subsequent division of the artery comes a decrease in diameter.
  • Arteries, Arterioles, Capillaries (gas exchange occurs here), Venules, Veins, & Coronary Sinus. All Coronary Veins eventually form the Coronary Sinus, which drains into the right atrium. This completes the path of Coronary Circulation.