The Heart, Blood Vessels, and Lymphatic Network Explained

Posted on Jan 9, 2026 in Biology

The Cardiovascular System

The cardiovascular system is composed of the heart and blood vessels. The heart pumps blood throughout the body.

Blood flow involves several stages: Deoxygenated (used) blood returns to the heart and is ejected to the lungs for oxygenation. Oxygenated blood then returns to the heart and is ejected through the aorta to circulate throughout the body.

Related Physiological Functions

  • The kidneys chemically purify the blood.
  • The liver performs blood detoxification.
  • In the brain, the blood-brain barrier prevents certain substances from penetrating via osmotic phenomena.

Heart Structure and Circulation

The heart is a contractile organ consisting of striated muscle fibers that contract involuntarily. It is divided by partitions into two main halves: the right side (handling venous/deoxygenated blood) and the left side (handling oxygenated blood).

The transverse wall divides the heart into its four chambers:

  • Right Atrium (RA)
  • Left Atrium (LA)
  • Right Ventricle (RV)
  • Left Ventricle (LV)

Path of Blood Flow

  1. Deoxygenated blood from the body enters the Right Atrium (RA) via the superior and inferior vena cava.
  2. Blood passes from the RA to the Right Ventricle (RV).
  3. The RV ejects blood through the pulmonary arteries to the lungs for oxygenation.
  4. Oxygenated blood returns from the lungs to the Left Atrium (LA) via the pulmonary veins.
  5. Blood passes from the LA to the Left Ventricle (LV).
  6. The LV ejects oxygenated blood to the rest of the body through the aorta.

Chamber Musculature and Grooves

The atria have a small, very elastic muscle layer, while the ventricles have a much more developed muscular layer.

The Left Ventricle (LV) is significantly more developed than the Right Ventricle (RV) because the RV only ejects blood to the lungs (a shorter distance), whereas the LV must eject blood throughout the entire body, requiring greater contractile effort.

Outwardly, the four chambers are delimited by grooves:

  • Atrioventricular groove
  • Interatrial groove
  • Interventricular sulcus

Heart Valves

The heart contains valves that ensure unidirectional blood flow:

  • Tricuspid Valve: Located between the Right Atrium (RA) and Right Ventricle (RV). (Three cusps)
  • Mitral Valve (Bicuspid): Located between the Left Atrium (LA) and Left Ventricle (LV). (Two cusps)

At the bottom of the ventricles, the valve cusps are attached to papillary muscles via chordae tendineae (referred to here as “piers veils muscle fibers”). When the ventricle contracts, these structures prevent the valve cusps from prolapsing, ensuring proper valve closure.

Cardiac Regulation and Layers

Intrinsic Conduction System

The heart possesses an intrinsic system that regulates its rhythmic movement (the heartbeat). This system includes:

  1. Sinus Node (Sinoatrial or SA Node)
  2. Atrioventricular Nodule (AV Node)
  3. Bundle of His
  4. Purkinje Fibers

The SA Node is typically the primary pacemaker, controlling the rhythm and timing of ventricular contraction.

The heart also receives extrinsic innervation from the spinal cord (autonomic nervous system).

Layers of the Heart Wall

The layers of the heart wall are:

  • Endocardium: The inner lining.
  • Myocardium: The middle, muscular layer (where the involuntary contractions occur).
  • Pericardium: The outer serosal layer.

Coronary Circulation

The heart muscle requires its own specific nutrition system supplied by the coronary arteries. These arteries arise directly from the aorta.

The main coronary arteries supply the left and right sides of the heart, ensuring proper cardiac function.

The Pericardium

The heart muscle is enveloped by the pericardium, a serosal layer. The pericardium folds upon itself to form two layers—the parietal and visceral layers—creating a space called the pericardial cavity.

This cavity contains pericardial fluid, which is essential to allow the pericardial layers to slide smoothly against each other during heart movement.

(Note on Aortic Valve: The aorta also contains a valve, typically with three cusps, which prevents blood from flowing back into the heart once ejected. The coronary arteries originate just above this valve.)

Lymphatic System and Blood Vessels

Lymph

Lymph is interstitial fluid found in the tissues, formed by the extravasation of blood plasma. Lymph is collected by lymphatic vessels and transported to lymph nodes before eventually draining back into the veins.

Veins (Vena)

Veins typically consist of three layers:

  1. Tunica Intima: Smooth inner layer (endothelium) to prevent clotting.
  2. Tunica Media: Smooth muscle layer (often very elastic).
  3. Tunica Adventitia: Outer serous layer.

Arteries (Artery)

In arteries, the muscle layer (Tunica Media) is much more developed than in veins, allowing them to withstand high pressure. Arteries carry the pulse (or beat) generated by the heart’s contraction.