Human Anatomy: Skull Structure, Cardiac Circulation, and Vascular Systems

Posted on Oct 25, 2025 in Biology

Anatomy of the Skull and Face Bones

Bones Forming the Cranium

The following bones form the skull:

  • Frontal
  • Parietal (2)
  • Temporal (2)
  • Occipital
  • Sphenoid
  • Ethmoid

Key Structures of the Skull

Right Side Structures:

  • Coronal Suture
  • Frontal Bone
  • Ethmoid Bone
  • Lacrimal Bone
  • Nasal Bone
  • Zygomatic Bone (Malar)
  • Maxilla
  • Mandible

Left Side Structures:

  • Parietal Bone
  • Sphenoid Bone
  • Occipital Bone
  • Temporal Bone
  • Mastoid Process
  • External Acoustic Meatus (Hearing Canal)
  • Styloid Process

Cardiac Valves and Blood Flow

Semilunar Valves: Structure and Passive Operation

The semilunar valves are located between the ventricle and the artery. They consist of membranous folds suspended in the artery wall. Their action is primarily passive:

  • Blood pressure forces blood past the folds and into the artery walls.
  • Increased intraventricular pressure stops the weight of the column of blood, causing the membrane to close the opening.

Atrioventricular Valve Mechanism (Intracardiac Valves)

Intracardiac valves (tricuspid and mitral) are opened (active and passive) by:

  • Increased atrial pressure.
  • Tension of the chordae tendineae attached to the papillary muscles.

They close (passively) when increased intraventricular pressure causes the valve edges to balloon into the atrium.

Systemic Blood Circulation Path

  1. Vena Cava
  2. Right Atrium (RA)
  3. RA → Tricuspid Valve → Right Ventricle (RV)
  4. RV → Semilunar Valve → Pulmonary Artery
  5. Pulmonary Artery → Lungs (Oxygenation of the blood and CO2 exchange; venous blood becomes arterial blood)
  6. Pulmonary Veins (4 veins) → Left Atrium (LA)
  7. LA → Mitral Valve → Left Ventricle (LV)
  8. LV → Semilunar Valve → Aorta Artery
  9. Aorta Artery → Capillaries (Tissue oxygenation; arterial blood becomes venous) → Vena Cava.

Blood Vessels and Circulatory Circuits

Function of Blood Vessels in the Bloodstream

  1. Arteries: Carry blood away from the heart to organs.
  2. Capillaries: Supply the tissues via capillary networks, connecting the arterial and venous systems.
  3. Veins: Return blood to the heart from the capillaries.

Structure of Different Blood Vessels

  1. Arteries: Composed of an outer layer of connective tissue (tunica externa), a muscle layer (tunica media), and an inner endothelial layer (tunica intima).
  2. Veins: Have a thinner muscle and elastic fiber layer (tunica media) and an inner endothelial layer (tunica intima).
  3. Capillaries: Are very fine tubes formed by a single layer of endothelial cells.

The Arterial System

The arterial system consists of two main parts:

  • Pulmonary Artery System: Arises from the RV and divides into two branches (right and left lung).
  • Aorta Artery System: Runs from the Left Ventricle (LV) to the capillaries. It is divided into the Aortic Arch, the Thoracic Aorta, and the Abdominal Aorta.

Circuits Related to the Arterial System

The arterial system relates to two major circuits:

  • The 1st part corresponds to the Lesser Circulation (Pulmonary Arterial System) because it originates in the RV and travels only to the lungs.
  • The 2nd part corresponds to the Greater Circulation (Aorta System) because it starts in the LV and reaches the capillary networks of the major tissues throughout the body.

Major Lymphatic System Collectors

  1. The Thoracic Duct: Receives lymph from the left side of the body and limbs. It originates in the Cisterna Chyli (Tank of Pecquet), ascends toward the chest, and joins the left subclavian vein.
  2. The Great Lymphatic Vein (Right Lymphatic Duct): Collects lymph from the upper right quadrant of the body (areas not drained by the thoracic duct).

The Spleen: Location and Functions

What is the Spleen?

The spleen is a dark red, oval organ located in the upper left abdomen.

Functional Parts of the Spleen

The spleen performs functions through its two main parts:

  1. White Pulp: Consists of lymphoid tissue and produces white blood cells.
  2. Red Pulp:
    • Phagocytic Cells: Destroys aged red blood cells.
    • Venous Sinuses: Acts as a reservoir of blood, capable of dilation.