Academy of Diagnostic & Osteopathic Medicine
ARTERIES
Arteries: The Body’s Highways for Oxygen Arteries are blood vessels that carry oxygen-rich blood away from the heart to tissues and organs. They have thick, muscular walls to withstand high pressure as blood is pumped with force. The tunica media, or middle layer, allows arteries to expand and contract, regulating blood pressure and flow. Major arteries like the aorta distribute blood throughout the body, while smaller arterioles control local circulation. When arteries become stiff or blocked—from high blood pressure, cholesterol, or plaque buildup—it can lead to heart disease, strokes, or poor circulation. Keeping arteries flexible is essential for cardiovascular health.
CAROTID
The carotid arteries are located on either side of the neck, running from the thorax to the base of the skull. Each carotid artery divides into a common carotid artery, an internal carotid artery that supplies the brain, and an external carotid artery that supplies the face and scalp. These arteries deliver oxygen-rich blood to the brain, face, and neck, and venous blood returns via the jugular veins. Their function is critical: they provide oxygenated blood to vital areas like the brain. Common conditions include carotid artery stenosis, where plaque buildup narrows the artery and increases the risk of stroke, and aneurysms, which are localized dilations that can rupture."
AORTA
The aorta begins at the left ventricle of the heart, arches in the thoracic cavity, and then descends through the abdomen. It is divided into four sections: the ascending aorta, the aortic arch, the thoracic aorta, and the abdominal aorta. This major artery supplies oxygenated blood to the entire systemic circulation. While it doesn’t drain blood itself, venous return from the body flows through systemic veins back to the heart. The aorta’s function is simple but vital: it carries oxygenated blood from the heart to the rest of the body. Common conditions include aneurysms, which are weaknesses in the aortic wall that can lead to rupture, and atherosclerosis, which is plaque buildup that narrows the artery and reduces blood flow.
ARTERIOLES
Arterioles are the small but powerful blood vessels that serve as the transition point between arteries and capillaries. They control the flow of oxygen-rich blood into tissues, acting as the primary regulators of blood pressure and circulation. Structurally, arterioles have muscular walls made of smooth muscle, allowing them to constrict or dilate as needed. This process, called vasoconstriction and vasodilation, determines how much blood enters the capillary networks. For example, during exercise, arterioles widen to increase blood flow to active muscles, while in cold environments, they constrict to preserve body heat by reducing circulation to the skin. Arterioles also play a critical role in blood pressure regulation. By adjusting their diameter, they determine the resistance blood encounters as it moves through the circulatory system. High blood pressure (hypertension) often results from chronic constriction of arterioles, which increases resistance and forces the heart to work harder to pump blood. When arterioles are damaged or dysfunctional, they can contribute to serious health issues such as poor circulation, tissue ischemia, and conditions like peripheral artery disease. Because they serve as the final control point before blood enters the delicate capillary networks, their ability to regulate flow is essential for maintaining overall cardiovascular health and preventing organ damage.
CAPILLARIES
Capillaries: The Body’s Exchange Network Capillaries are the smallest and most essential blood vessels in the circulatory system, acting as the connection point between arteries and veins. While arteries carry oxygen-rich blood away from the heart and veins return oxygen-depleted blood back, capillaries serve as the exchange sites where vital substances are transferred between blood and tissues. These microscopic vessels have thin, permeable walls made of a single layer of endothelial cells. This structure allows oxygen, nutrients, hormones, and waste products to pass between the bloodstream and surrounding cells. In the lungs, capillaries exchange carbon dioxide for oxygen, while in tissues, they deliver glucose and other nutrients to fuel cellular activity. Capillaries are also dynamic. They expand and constrict based on the body's needs. For example, during exercise, capillaries in muscles widen to allow for increased oxygen and nutrient delivery, while in colder conditions, capillary flow may decrease in extremities to conserve heat. Disruptions in capillary function can lead to serious health issues. Weak or damaged capillaries can cause swelling (edema), poor circulation, or increased fragility, which is seen in conditions like diabetes or hypertension. Because capillaries reach nearly every cell in the body, they are a key factor in overall health and disease prevention.
MEDIA
The media, or tunica media, is the middle layer of blood vessels, made of smooth muscle and elastic fibers that control vessel diameter and blood pressure. This layer allows vessels to contract (vasoconstriction) to increase pressure or relax (vasodilation) to improve blood flow. It’s strongest in arteries, where it helps handle high-pressure blood flow from the heart. When the media is damaged—by hypertension, aging, or arterial disease—vessels can stiffen, leading to high blood pressure, aneurysms, or poor circulation. The media’s flexibility is essential for a healthy cardiovascular system.
VEINS
Veins are the body’s return highways, carrying deoxygenated blood back to the heart. Unlike arteries, veins have thinner walls and rely on one-way valves to keep blood flowing in the right direction, preventing backflow. These vessels are low-pressure systems, assisted by muscle contractions and breathing to push blood upward—especially from the legs—against gravity. The largest vein, the inferior vena cava, collects blood from the lower body, while the superior vena cava gathers it from the upper body. But when veins weaken or valves fail, conditions like varicose veins, deep vein thrombosis (DVT), or chronic venous insufficiency can develop. Veins are critical to circulation, and their health is essential for overall vascular function.
JUGULAR
The jugular veins run parallel to the carotid arteries in the neck. They are divided into internal jugular veins, which drain blood from the brain, and external jugular veins, which drain the face and neck. Since these are venous structures, they don’t receive arterial blood but instead return blood to the superior vena cava for oxygenation in the heart. Their function is to drain deoxygenated blood from the brain, face, and neck back into circulation. Common issues include jugular vein thrombosis, where a blood clot causes neck swelling and pain, and external compression, which can lead to jugular vein distension
IVC
The inferior vena cava runs along the right side of the vertebral column in the abdominal cavity. It’s formed by the union of the common iliac veins and empties deoxygenated blood into the right atrium of the heart. Since it’s a vein, it doesn’t receive arterial blood. Instead, it drains blood from the lower body and returns it to the heart. Its main function is to return deoxygenated blood from the lower body to the heart for oxygenation. Two common pathologies are IVC thrombosis, where a blood clot blocks the vein, and compression syndrome, where external pressure from tumors or pregnancy obstructs blood flow.
VENULES
Venules: The Transition Between Capillaries and Veins Venules are the small blood vessels that bridge the gap between capillaries and larger veins, playing a crucial role in returning blood back to the heart. After blood delivers oxygen and nutrients to tissues through capillaries, it collects in venules before moving into larger veins for transport back to the heart and lungs. Structurally, venules have thin walls composed of endothelial cells, smooth muscle, and connective tissue. This allows them to accommodate changes in blood volume and pressure. Unlike capillaries, venules start to regain some of the structural support seen in larger veins, but they remain flexible to help regulate blood flow. Venules also play an important role in immune responses. They are involved in the movement of white blood cells from the bloodstream into tissues when the body is fighting infection or inflammation. This process, called leukocyte migration, is essential for immune defense and wound healing. When venules become damaged or dysfunctional, it can lead to circulation issues such as venous insufficiency, where blood struggles to return efficiently to the heart. This can contribute to swelling, varicose veins, or conditions like chronic venous hypertension. As an essential part of the circulatory system, venules ensure that deoxygenated blood makes its way back through the body’s intricate network of veins, keeping circulation balanced and efficient.
INTIMA
The intima is the innermost layer of blood vessels, made of smooth endothelial cells that regulate blood flow and prevent clotting. It produces nitric oxide, helping vessels relax and control blood pressure. When damaged—by high blood pressure, smoking, or cholesterol buildup—plaques can form, leading to atherosclerosis, increasing the risk of heart disease and stroke. The intima is the first line of defense in vascular health, making its integrity crucial for circulation and preventing blockages.
ADVENTITIA
The adventitia, or tunica adventitia, is the outermost layer of blood vessels, providing structure, strength, and protection. Made of connective tissue, collagen, and small blood vessels, it anchors arteries and veins to surrounding tissues and supplies nutrients to the vessel walls. Unlike the media, which controls vessel movement, the adventitia prevents overexpansion, reducing the risk of rupture. When weakened—by chronic inflammation, high blood pressure, or vascular disease—it can contribute to aneurysms or vessel weakening. The adventitia is the foundation of blood vessel integrity, ensuring durability and stability in circulation.
HEART
The heart is located in the thoracic cavity, between the lungs, resting on the diaphragm, and enclosed within the pericardial sac. It has four chambers: the right and left atria, and the right and left ventricles. The heart also contains valves, like the tricuspid and mitral valves, which ensure blood flows in the correct direction. Its walls are composed of three layers: the epicardium, myocardium, and endocardium. The heart receives oxygenated blood through the right and left coronary arteries, while deoxygenated blood is drained through the coronary veins into the coronary sinus, which empties into the right atrium. The heart’s primary function is to pump oxygenated blood to the body via the systemic circulation and deoxygenated blood to the lungs via the pulmonary circulation. Two common heart conditions include myocardial infarction, or heart attack, which happens when a coronary artery is blocked and heart tissue dies, and arrhythmias, which are abnormal heart rhythms that can disrupt blood flow.
CORONARY ARTERIES
Coronary Arteries: The Heart’s Lifeline The coronary arteries are the heart’s dedicated blood supply, delivering oxygen and nutrients to keep the muscle pumping. There are two main branches: The left coronary artery (LCA) splits into the left anterior descending (LAD) artery—called the “widowmaker” because blockages here can be fatal—and the circumflex artery, which supplies the left atrium and ventricle. The right coronary artery (RCA) supplies the right side of the heart and part of the conduction system, including the SA and AV nodes, which control heart rhythm. When these arteries become blocked by plaque, it can lead to angina, heart attacks, or cardiac arrest. That’s why keeping them clear and flexible is essential for heart health.
HEART VALVES
Heart Valves: The Gates of Circulation The heart has four valves, acting as one-way doors to control blood flow: Tricuspid Valve (Right Atrium → Right Ventricle) Pulmonary Valve (Right Ventricle → Lungs) Mitral Valve (Left Atrium → Left Ventricle) Aortic Valve (Left Ventricle → Body) These valves open and close with every heartbeat, preventing blood from backflowing. But when they stiffen (stenosis), leak (regurgitation), or fail to close properly (prolapse), the heart struggles to pump efficiently, leading to fatigue, shortness of breath, and heart failure. Valvular disease can sometimes be heard as a murmur, a sign that blood flow is being disrupted.
LUNGS
The lungs are located in the thoracic cavity, on either side of the heart, and are protected by the rib cage. The right lung has three lobes, while the left lung has two. The lungs contain airways such as bronchi and bronchioles, and tiny air sacs called alveoli, where gas exchange occurs. Blood is supplied to the lungs by two systems: the pulmonary arteries carry deoxygenated blood to the lungs for oxygenation, and the bronchial arteries supply oxygen-rich blood to the lung tissue. Oxygenated blood returns to the heart through the pulmonary veins. The primary function of the lungs is to facilitate gas exchange, allowing oxygen to enter the bloodstream and carbon dioxide to be expelled. Common lung conditions include pneumonia, an infection that inflames the alveoli and often fills them with fluid, and chronic obstructive pulmonary disease, or COPD, a progressive condition that limits airflow and is often caused by smoking.
LARYNX
The larynx, or voice box, sits in the neck, just below the pharynx and above the trachea. It’s made of cartilage, including the thyroid cartilage, which forms the Adam’s apple, and the cricoid cartilage, which provides structure. Inside, it houses the vocal cords, responsible for sound production, and the epiglottis, which acts like a lid, closing off the airway during swallowing. Blood reaches the larynx through the superior and inferior laryngeal arteries, which branch from the thyroid artery, and drains through the laryngeal veins into the jugular vein. Functionally, the larynx does two things—it creates sound for speaking and singing, and it protects the airway, preventing food and liquid from entering the lungs. If the larynx is damaged, conditions like laryngitis can cause hoarseness, vocal cord paralysis can make breathing and speaking difficult, and laryngeal cancer can lead to airway obstruction.
VOCAL CORDS
The vocal cords, also called vocal folds, are two elastic bands inside the larynx that vibrate to produce sound. They stretch across the airway, with a small opening in between called the glottis. Muscles adjust their tension and length to control pitch, while airflow determines volume. The vocal cords receive blood from the laryngeal branches of the superior and inferior thyroid arteries, and venous drainage flows into the jugular vein. Their main function is voice production—as air passes through, the cords vibrate to create sound, which is then shaped by the tongue, lips, and mouth into speech. Common issues include vocal cord nodules and polyps, which develop from overuse, laryngitis, which inflames the tissue, and spasmodic dysphonia, a neurological condition affecting voice control.
DIAPHRAGM
The diaphragm is a dome-shaped muscle that separates the chest and abdominal cavities. It sits just below the lungs and is the primary muscle of respiration. The diaphragm is supplied by the phrenic arteries and drains blood through the phrenic veins into the inferior vena cava. Its function is simple but essential—when it contracts, it moves downward, creating a vacuum that pulls air into the lungs. When it relaxes, air is pushed out. It also plays a role in coughing, sneezing, and even stabilizing posture. If the diaphragm is weakened or paralyzed, conditions like diaphragmatic paralysis can make breathing difficult, hiatal hernias can occur when part of the stomach pushes through the diaphragm, and COPD can cause the diaphragm to become overworked and flattened.
PHARYNX
The pharynx, or throat, is a muscular passage that connects the nasal cavity and mouth to the larynx and esophagus. It plays a role in both breathing and swallowing, directing air to the lungs and food to the digestive system. Divided into three sections—nasopharynx, oropharynx, and laryngopharynx—it also contributes to speech resonance. Dysfunction in the pharynx can lead to breathing difficulties, swallowing disorders, or infections like pharyngitis.
TRACHEA
The trachea, or windpipe, is a hollow tube connecting the larynx to the bronchi. It runs down the front of the neck, just in front of the esophagus. Its walls are reinforced with C-shaped cartilage rings, keeping it open while allowing flexibility. Inside, it’s lined with cilia and mucus, which help trap and remove dust, bacteria, and debris before they reach the lungs. Blood is supplied by the inferior thyroid artery, and venous drainage flows into the brachiocephalic veins. The trachea’s primary job is to act as a passageway for air, ensuring oxygen reaches the lungs and carbon dioxide is expelled. If the trachea becomes inflamed, tracheitis can cause breathing difficulties. If it narrows due to tracheal stenosis, airflow is restricted. And if the cartilage weakens, tracheomalacia can lead to airway collapse.
BRONCHI
The bronchi are the two main airways that split off from the trachea, delivering air into the left and right lungs. Each bronchus further divides into smaller bronchi and then into bronchioles, forming a branching system inside the lungs. The bronchi receive blood from the bronchial arteries, which arise from the thoracic aorta, and drain through the bronchial veins into the azygos system. Their main function is to distribute air evenly throughout the lungs while filtering out debris with mucus and tiny hair-like structures called cilia. Common conditions include bronchitis, where inflammation causes mucus buildup and coughing, asthma, which leads to airway narrowing, and bronchiectasis, where the bronchi become permanently widened, making it harder to clear mucus.