Anatomy of the Human Blood Vessels

Introduction to the Cardiovascular System and Blood Vessel Types

The cardiovascular system is a vital transport network comprising the heart, blood vessels, and blood. Blood vessels function as specialized tubes that transport blood away from the heart to various body tissues and return it back to the heart. This vascular network organizes itself hierarchically, where larger vessels branch into increasingly smaller divisions. The primary site of physiological interaction occurs in the capillaries, which are vessels with extremely thin walls that facilitate gas exchange, specifically Oxygen ($O_2$) and Carbon Dioxide ($CO_2$). Beyond gas exchange, these vessels manage nutrient delivery and the pickup of metabolic waste products between the blood and the tissues.

There are five primary types of blood vessels categorized by their function and structure. Arteries are responsible for carrying blood from the heart to the other tissues of the body; these vessels are characterized by having an elastic muscular wall. Arterioles represent the smallest divisions of the arterial system. Capillaries form a thin-walled network where the vital exchange of nutrients and wastes occurs between the plasma and the interstitial fluid. Structurally, capillaries consist of only a single layer of endothelial lining supported by a basement membrane to maximize exchange efficiency. Venules are the smallest veins that receive blood from the capillaries. Finally, Veins carry blood from the body tissues back toward the heart.

Classification of Blood Vessels by Diameter

Blood vessels are classified into two broad categories based on their diameter: Macrovasculature and Microvasculature. Macrovasculature includes vessels with a diameter greater than $0.1\,\text{mm}$. This category includes elastic arteries, which are the largest in the body, such as the aorta and the pulmonary trunk; these are essential for maintaining constant blood flow and blood pressure. It also includes muscular arteries, such as the radial and ulnar arteries, as well as muscular veins and large arterioles. Microvasculature refers to vessels with a diameter less than $0.1\,\text{mm}$. This group consists of arterioles, capillaries, and post-capillary venules.

General Microscopic Structure of Blood Vessels

The walls of arteries and veins (but notably not capillaries) are composed of three distinct layers or tunics. The innermost layer is the Tunica interna (also known as the intima), which is composed of endothelium formed by simple squamous epithelium. The middle layer is the Tunica media, consisting of smooth muscle and elastic tissue. This layer is responsible for controlling blood pressure through the physiological processes of vasoconstriction and vasodilatation. The outermost layer is the Tunica externa (also known as the adventitia), which is composed of connective tissue.

Distinctive Features and Hemodynamic Principles

There are several critical clinical and anatomical distinctions between arteries and veins. In arteries, the main or thickest layer is the Tunica media, whereas in veins, the Tunica adventitia is the dominant layer. Arteries contain an elastic lamina that is crucial for maintaining systemic blood pressure. Generally, the Tunica media is significantly thicker in arteries than in veins. While vessels throughout the body can adjust their diameter, vasoconstriction and vasodilatation occur primarily at the level of the arterioles; vasoconstriction specifically serves to rise the systemic blood pressure. Morphologically, the diameter of a vein is larger than that of its corresponding artery, but the thickness of the arterial wall exceeds that of the vein. A unique anatomical variance exists in the head and neck, where veins typically do not have valves, unlike veins in the rest of the body which possess them to prevent backflow. Furthermore, venous return is facilitated by the muscular pump and the respiratory pump.

Specialized Vascular Supplies: Vasa Vasorum and Vasa Nervosum

The walls of large blood vessels, particularly large arteries and veins, require their own blood supply due to their thickness. These small vessels are called the Vasa Vasorum. Their primary functions are to provide oxygen and nutrients to the outer layers of the large vessels and to remove metabolic waste products. Similarly, large nerves also require a dedicated blood supply; for example, the sciatic nerve is supplied by vessels known as the vasa nervosum.

Special Vascular Pathways: Shunts and Portal Systems

Certain areas of the body utilize specialized vascular pathways. An Arteriovenous Shunt is a bypass that allows blood to flow directly from an artery to a vein without passing through a capillary bed. These are primarily found in cold-sensitive areas such as the skin, hands, legs, ears, and nose, where they function to regulate body temperature.

Another specialized pathway is the Portal System, defined by a sequence of Capillary (\rightarrow) Vein (\rightarrow) Capillary. An essential example is the Hepatic-Portal System, where capillaries from the gastrointestinal (GI) system form the Hepatic portal vein, which then enters the liver to form another capillary bed. This configuration ensures that everything ingested passes through the liver first, facilitating the "First pass metabolism" of drugs. Another significant example is the Hypothalamo-hypophyseal portal system, which connects the hypothalamus to the patutery gland.

The Aorta and the Ascending Aortic Branches

The aorta is the largest artery in the human body, originating from the left ventricle. It is divided into the ascending aorta, the aortic arch, and the descending aorta (which is further subdivided into thoracic and abdominal segments). The ascending aorta gives rise to two primary branches: the Right coronary artery (RCA) and the Left coronary artery (LCA). These coronary arteries supply the heart muscle itself. The LCA further divides into the circumflex artery and the Anterior interventricular artery, also known as the LAD (Left Anterior Descending). The RCA divides into the marginal artery and the Posterior interventricular artery, also known as the PDA. In approximately $15\%$ of the population, the PDA branches from the LCA instead of the RCA. Notably, the LAD is the artery most commonly affected in coronary artery disease.

The Arch of the Aorta and Head/Neck Circulation

The arch of the aorta gives rise to three major branches from its superior aspect, ordered from right to left: the Brachiocephalic trunk, the Left common carotid artery, and the Left subclavian artery. The Left common carotid artery supplies the left half of the head and neck, while the Left subclavian artery supplies the left upper limb. The Brachiocephalic trunk, located on the right side, divides into the Right common carotid artery (supplying the right half of the head and neck) and the Right subclavian artery (supplying the right upper limb).

The Common Carotid Artery (CCA) provides the main supply to the head and neck. Each CCA divides into an Internal carotid artery (ICA) and an External carotid artery (ECA). This bifurcation occurs at the level of the superior border of the thyroid cartilage, corresponding to the vertebral level of $C3-C4$, where a pulse is easily felt. The ECA gives off $8$ branches in the head and neck and terminates behind the neck of the mandible (between the Temporomandibular joint, or TMJ, and the External auditory meatus, or EAM) by dividing into the Maxillary artery and the Superficial Temporal artery. The Maxillary artery is the largest branch of the ECA, providing $16$ branches and supplying the muscles of mastication via its muscular (middle) branch. The Superficial Temporal artery also offers an easily accessible pulse point. The ICA, conversely, has no branches in the neck; it enters the cranial cavity through the carotid canal to supply the brain.

Blood Supply of the Brain: The Circle of Willis

The Circle of Willis is a critical anastomotic circle supplying the brain. Its components include the Internal Carotid Arteries (ICA), which are the major source of anterior circulation, the Anterior Cerebral Arteries, the Anterior Communicating Artery, the Posterior Communicating Arteries, and the Posterior Cerebral Arteries. It is important to note that the Middle cerebral artery is NOT considered part of the Circle of Willis.

Arteries of the Upper Limb and Thoracic Region

The subclavian artery provides the blood supply to the upper limbs, passing below the clavicle before continuing as the axillary artery. The axillary artery then becomes the brachial artery. The brachial artery, which supplies the arm, gives off the profunda brachiia artery (the deep artery of the arm). At the cubital fossa in the elbow, the brachial artery terminates by dividing into the Radial artery and the Ulnar artery. The radial artery continues into the hand to form the deep palmar arch, while the ulnar artery forms the superficial palmar arch. These arches form an integrated vascular network to supply the hand and fingers.

The subclavian artery has several important branches: the Vertebral artery, which participates in brain supply by passing through the Transvers foramina of cervical vertebrae from $C6$ to $C1$ (notably skipping $C7$), and eventually joining its counterpart to form the basilar artery (which terminates as the two Posterior cerebral arteries); the Internal thoracic artery, which runs parallel to the sternum supplying the first through sixth intercostal spaces and dividing at the $6^{th}$ intercostal space into the musculophrenic and superior epigastric arteries; the Thyrocervical trunk, which supplies the thyroid and neck through the inferior thyroid, transverse cervical, and suprascapular arteries; and the Costocervical trunk, which supplies the ribs and neck via the deep cervical and highest (supreme) intercostal arteries (supplying the first and second intercostal spaces).

The Descending Aorta and Abdominal Vascularization

The thoracic aorta provides branches including the Posterior intercostal arteries (supplying all intercostal spaces except the $1^{st}$ and $2^{nd}$), the bronchial artery, the pericardial artery, and the esophageal artery.

The Abdominal Aorta terminates at the level of the $L4$ vertebra. Its branches can be organized by the "$3 \times 3 \times 3$" rule. There are three anterior unpaired branches: the Celiac trunk, the Superior mesenteric artery (SMA), and the Inferior mesenteric artery (IMA). There are three lateral paired branches: the Suprarenal artery, the Renal artery, and the Gonadal artery (testicular in males, ovarian in females). Finally, there are three terminal branches: the Left common iliac artery, the Right common iliac artery, and the Medial sacral artery.

Embryological Divisions and Blood Supply of the Gastrointestinal Tract

The gastrointestinal tract is divided embryologically into three segments, each with a specific arterial supply. The Foregut extends from the esophagus to the upper part of the dudenum (including the stomach) and is supplied by the Celiac trunk. The Midgut extends from the upper part of the duodenum to the proximal $2/3$ of the transverse colon (including the ascending colon) and is supplied by the Superior mesenteric artery. The Hindgut extends from the distal $1/3$ of the transverse colon to the upper part of the anal canal (including the descending colon and the rectum) and is supplied by the Inferior mesenteric artery.

Arteries of the Pelvis and Lower Limbs

The common iliac artery divides into the Internal and External iliac arteries. The External iliac artery continues into the leg as the Femoral artery once it passes below the inguinal ligament. The femoral artery supplies the lower limb and gives off the profunda femoris artery. It is situated superficially in the femoral triangle, covered only by skin and fascia, making it highly vulnerable to trauma. In the event of a femoral artery injury, the first step is compression over the inguinal ligament. The femoral artery continues as the popliteal artery behind the knee and terminates in the popliteal fossa into the Anterior tibial artery (which becomes the Dorsalis pedis artery in the foot, where a pulse can be felt) and the Posterior tibial artery (which gives off the Fibular artery).

General Principles of the Venous System and Major Venae Cavae

Veins generally follow the same paths as arteries and often share their names. They merge into larger vessels until they reach the heart. The two largest veins terminating in the right atrium are the Superior Vena Cava (SVC) and the Inferior Vena Cava (IVC). The SVC brings blood from the head, neck, and upper limbs; it is formed by the union of the Right and Left Brachiocephalic veins (the left being longer and larger than the right). Each brachiocephalic vein is formed by the union of the internal jugular vein and the subclavian vein. The IVC brings blood from the thorax, abdomen, and lower limbs and is formed by the union of the Right and Left common iliac veins.

Superficial and Deep Venous Drainage of the Limbs

Veins are categorized as deep or superficial. Deep veins follow the arteries and share their nomenclature. Superficial veins are located in the superficial subcutaneous fascia and eventually drain into the deep veins. In the upper limb, the primary superficial veins are the Cephalic vein (located laterally, draining into the Axillary vein) and the Basilic vein (located medially, draining into the Brachial vein). In the lower limb, the primary superficial veins are the Great (Long) saphenous vein (located medially, draining into the Femoral vein) and the Small (Short) saphenous vein (located laterally, draining into the Popliteal vein, which then drains into the Femoral vein).

The Azygos System of Veins

The Azygos system (from the Greek "unpaired") drains the posterior thoracoabdominal walls and exhibits significant anatomical variation. The Azygos vein itself is located on the right side of the vertebral column and drains directly into the Superior Vena Cava. The right side of the thorax drains directly into the Azygos vein. The left side of the thorax drains into the Azygos system via two vessels: the Hemiazygos vein (draining the left lower thorax) and the Accessory Hemiazygos vein (draining the left upper thorax).