Arteries: Histology

Overview

Definition

Arteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body.

General characteristics

All arteries have the following characteristics:

• Thicker walls with smaller lumens than veins Veins Veins are tubular collections of cells, which transport deoxygenated blood and waste from the capillary beds back to the heart. Veins are classified into 3 types: small veins/venules, medium veins, and large veins. Each type contains 3 primary layers: tunica intima, tunica media, and tunica adventitia. Veins: Histology 
• Appear round on histology
• Are resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing vessels:
  • Create most of the resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing in the circulation Circulation The movement of the blood as it is pumped through the cardiovascular system. ABCDE Assessment
  • High-pressure system
  • Can regulate vascular resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing and blood flow velocity Flow velocity Vascular Resistance, Flow, and Mean Arterial Pressure by contracting or relaxing the smooth muscle within their walls 
• Vessels for the transport of flowing blood: The blood passes through arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta Aorta The main trunk of the systemic arteries. Mediastinum and Great Vessels: Anatomy) and ending in the smallest (the arterioles) before entering the capillary beds Capillary beds Groups of 10–100 individual capillary vessels supplied by a single metarteriole. Capillaries: Histology.

Layers of the Vessel Wall

All arteries have the same basic structure and are made up of three primary layers: the tunica intima, tunica media, and tunica adventitia (also known as tunica externa).

Tunica intima

• Made up of:
  • A single layer of endothelial cells (simple, squamous epithelial cells)
  • Small amounts of fibrous Fibrous Fibrocystic Change connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology
• Functions:
  • Acts as a selectively permeable barrier
  • Secretes vasoactive substances
  • Provides a smooth lining to the blood vessel, preventing aggregation Aggregation The attachment of platelets to one another. This clumping together can be induced by a number of agents (e.g., thrombin; collagen) and is part of the mechanism leading to the formation of a thrombus. Coagulation Studies of platelets Platelets Platelets are small cell fragments involved in hemostasis. Thrombopoiesis takes place primarily in the bone marrow through a series of cell differentiation and is influenced by several cytokines. Platelets are formed after fragmentation of the megakaryocyte cytoplasm. Platelets: Histology and/or RBCs RBCs Erythrocytes, or red blood cells (RBCs), are the most abundant cells in the blood. While erythrocytes in the fetus are initially produced in the yolk sac then the liver, the bone marrow eventually becomes the main site of production. Erythrocytes: Histology when intact
• Endothelium appears wavy owing to constriction of the smooth muscle.
• Vascularized by direct diffusion Diffusion The tendency of a gas or solute to pass from a point of higher pressure or concentration to a point of lower pressure or concentration and to distribute itself throughout the available space. Diffusion, especially facilitated diffusion, is a major mechanism of biological transport. Peritoneal Dialysis and Hemodialysis from the lumen
• In capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries: Histology: This is the only layer present in the vessel wall.
• In larger vessels: There is an internal elastic Elastic Connective Tissue: Histology membrane separating the tunica intima from the tunica media.

Tunica media

• Made up of:
  • Smooth muscle
  • Elastic Elastic Connective Tissue: Histology fibers (e.g., elastin)
  • Collagen Collagen A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of skin; connective tissue; and the organic substance of bones (bone and bones) and teeth (tooth). Connective Tissue: Histology
• Functions:
  • Smooth muscle allows the vessels to adjust blood flow Blood flow Blood flow refers to the movement of a certain volume of blood through the vasculature over a given unit of time (e.g., mL per minute). Vascular Resistance, Flow, and Mean Arterial Pressure through vasoconstriction Vasoconstriction The physiological narrowing of blood vessels by contraction of the vascular smooth muscle. Vascular Resistance, Flow, and Mean Arterial Pressure and vasodilation Vasodilation The physiological widening of blood vessels by relaxing the underlying vascular smooth muscle. Pulmonary Hypertension Drugs.
  • Elastin in larger vessels allows for the vessel to recoil.
  • Collagen Collagen A polypeptide substance comprising about one third of the total protein in mammalian organisms. It is the main constituent of skin; connective tissue; and the organic substance of bones (bone and bones) and teeth (tooth). Connective Tissue: Histology provides strength and structure.
• Usually the thickest layer (can vary in size)
• In larger vessels: There is an external elastic Elastic Connective Tissue: Histology membrane separating the tunica media from the tunica adventitia.

Tunica adventitia

• Also called the tunica externa
• Made up of connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology:
  • Collagenous and elastic Elastic Connective Tissue: Histology fibers
  • Merges with connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology surrounding neighboring vessels, nerves, and organs
• Functions:
  • Strengthens the vessel wall
  • Anchors the vessel
• Innervated by tiny nerves known as nervi vasorum
• Vascularized by tiny vessels known as vasa vasorum

Types of Arteries

There are 3 primary types of arteries, based on their overall size, function, and composition (known as segmental differentiation). The arteries generally exist on a continuum, with gradual changes in vessel morphology moving down the arterial tree.

The 3 primary types of arteries are:

• Large elastic Elastic Connective Tissue: Histology arteries (also known as conducting arteries)
• Medium muscular arteries (also known as distributing arteries)
• Small arteries and arterioles

Large/ elastic Elastic Connective Tissue: Histology arteries

This group includes the largest arteries in the body; all large/ elastic Elastic Connective Tissue: Histology arteries are close to the heart. The aorta Aorta The main trunk of the systemic arteries. Mediastinum and Great Vessels: Anatomy is the largest artery in the body.

Large/ elastic Elastic Connective Tissue: Histology arteries include: 

• Aorta Aorta The main trunk of the systemic arteries. Mediastinum and Great Vessels: Anatomy
• Pulmonary trunk Pulmonary Trunk Truncus Arteriosus and arteries
• Brachiocephalic trunk
• Common carotid arteries Carotid Arteries Either of the two principal arteries on both sides of the neck that supply blood to the head and neck; each divides into two branches, the internal carotid artery and the external carotid artery. Carotid Arterial System: Anatomy
• Subclavian arteries
• Common iliac arteries

Characteristics:

• Large lumens
• Tunica intima includes a subendothelial Subendothelial Membranoproliferative Glomerulonephritis layer containing additional smooth muscle.
• Many elastic Elastic Connective Tissue: Histology fiber networks within the media
  • Elastic Elastic Connective Tissue: Histology fibers have fenestrations:
    • Small gaps, or “windows,” within the elastic Elastic Connective Tissue: Histology fibers
    • Allow the diffusion Diffusion The tendency of a gas or solute to pass from a point of higher pressure or concentration to a point of lower pressure or concentration and to distribute itself throughout the available space. Diffusion, especially facilitated diffusion, is a major mechanism of biological transport. Peritoneal Dialysis and Hemodialysis of nutrients throughout the wall
  • Elastic Elastic Connective Tissue: Histology fibers are the dominant tissue type within the aorta Aorta The main trunk of the systemic arteries. Mediastinum and Great Vessels: Anatomy tunica media.
  • Allow these vessels to recoil
• Recoil: Vessels can stretch and return to their original shape after receiving the stroke volume Stroke volume The amount of blood pumped out of the heart per beat, not to be confused with cardiac output (volume/time). It is calculated as the difference between the end-diastolic volume and the end-systolic volume. Cardiac Cycle of blood ejected by the left ventricle during systole Systole Period of contraction of the heart, especially of the heart ventricles. Cardiac Cycle.
• Effects of stretching and recoil on blood pressure:
  • Minimizes changes in blood pressure downstream during systole Systole Period of contraction of the heart, especially of the heart ventricles. Cardiac Cycle
  • Maintains blood pressure during diastole Diastole Post-systolic relaxation of the heart, especially the heart ventricles. Cardiac Cycle
• Windkessel function:
  • During systole Systole Period of contraction of the heart, especially of the heart ventricles. Cardiac Cycle, the vessel walls stretch, enabling blood that is ejected during systole Systole Period of contraction of the heart, especially of the heart ventricles. Cardiac Cycle to be momentarily “stored.”
  • During diastole Diastole Post-systolic relaxation of the heart, especially the heart ventricles. Cardiac Cycle, the vessel walls recoil, and this “retracting force” moves the stored blood forward to the periphery.

Medium muscular arteries

• Primary role: distribution of blood to the organs
• Also known as distributing arteries
• Tunica media has a predominance of smooth muscle:
  • Average 25–40 layers of smooth muscle (approximately ¾ of the wall thickness) 
  • Significant ability to vasoconstrict and vasodilate, resulting in significant ability to regulate blood flow Blood flow Blood flow refers to the movement of a certain volume of blood through the vasculature over a given unit of time (e.g., mL per minute). Vascular Resistance, Flow, and Mean Arterial Pressure
• These arteries are more distant from the heart than large arteries.
• Most “named” blood vessels, which are not large are medium muscular arteries (e.g., brachial, femoral, and splenic arteries).

Small arteries and arterioles

• These vessels are usually not named.
• Most of these vessels can be observed only under magnification.
• Also known as resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing vessels
• Relatively little elastic Elastic Connective Tissue: Histology tissue
• The tunica media is thick in proportion to its lumen relative to other arteries.
• Small arteries:
  • Have up to 25 layers of smooth muscle
  • Small arteries narrow toward the capillary network and become arterioles.
• Arterioles:
  • Last vessels prior to entering the capillary beds Capillary beds Groups of 10–100 individual capillary vessels supplied by a single metarteriole. Capillaries: Histology
  • Primary point at which the body controls relative blood flow Blood flow Blood flow refers to the movement of a certain volume of blood through the vasculature over a given unit of time (e.g., mL per minute). Vascular Resistance, Flow, and Mean Arterial Pressure to the capillary beds Capillary beds Groups of 10–100 individual capillary vessels supplied by a single metarteriole. Capillaries: Histology of various organs 
  • Only 1–3 layers of smooth muscle
• Metarterioles:
  • Short vessels linking arterioles and capillaries Capillaries Capillaries are the primary structures in the circulatory system that allow the exchange of gas, nutrients, and other materials between the blood and the extracellular fluid (ECF). Capillaries are the smallest of the blood vessels. Because a capillary diameter is so small, only 1 RBC may pass through at a time. Capillaries: Histology
  • Have precapillary sphincters Precapillary Sphincters Capillaries: Histology: individual smooth muscle cells encircling the entrance to a capillary, regulating blood flow Blood flow Blood flow refers to the movement of a certain volume of blood through the vasculature over a given unit of time (e.g., mL per minute). Vascular Resistance, Flow, and Mean Arterial Pressure to the capillary beds Capillary beds Groups of 10–100 individual capillary vessels supplied by a single metarteriole. Capillaries: Histology

Clinical Relevance

Vasculitides Vasculitides Vasculitides are a group of conditions characterized by vasculitis, ischemia, and damage to the organs supplied by the affected vessels. The affected arteries are of different sizes and locations and vary by the type of vasculitis. Vasculitides

Vasculitides Vasculitides Vasculitides are a group of conditions characterized by vasculitis, ischemia, and damage to the organs supplied by the affected vessels. The affected arteries are of different sizes and locations and vary by the type of vasculitis. Vasculitides are a group of autoimmune conditions characterized by immune-mediated blood vessel inflammation Inflammation Inflammation is a complex set of responses to infection and injury involving leukocytes as the principal cellular mediators in the body’s defense against pathogenic organisms. Inflammation is also seen as a response to tissue injury in the process of wound healing. The 5 cardinal signs of inflammation are pain, heat, redness, swelling, and loss of function. Inflammation and wall damage. Loss of vessel integrity can lead to bleeding, as well as to downstream ischemia Ischemia A hypoperfusion of the blood through an organ or tissue caused by a pathologic constriction or obstruction of its blood vessels, or an absence of blood circulation. Ischemic Cell Damage and necrosis Necrosis The death of cells in an organ or tissue due to disease, injury or failure of the blood supply. Ischemic Cell Damage. These processes can be primary or secondary, and they tend to affect only vessels of a specific type or in a specific location.

Diseases involving abnormalities of the arterial walls (structure and/or function)

• Hypertension Hypertension Hypertension, or high blood pressure, is a common disease that manifests as elevated systemic arterial pressures. Hypertension is most often asymptomatic and is found incidentally as part of a routine physical examination or during triage for an unrelated medical encounter. Hypertension: elevated blood pressure, defined as > 130 mm MM Multiple myeloma (MM) is a malignant condition of plasma cells (activated B lymphocytes) primarily seen in the elderly. Monoclonal proliferation of plasma cells results in cytokine-driven osteoclastic activity and excessive secretion of IgG antibodies. Multiple Myeloma Hg systolic or > 80 mm MM Multiple myeloma (MM) is a malignant condition of plasma cells (activated B lymphocytes) primarily seen in the elderly. Monoclonal proliferation of plasma cells results in cytokine-driven osteoclastic activity and excessive secretion of IgG antibodies. Multiple Myeloma Hg diastolic: Hypertension Hypertension Hypertension, or high blood pressure, is a common disease that manifests as elevated systemic arterial pressures. Hypertension is most often asymptomatic and is found incidentally as part of a routine physical examination or during triage for an unrelated medical encounter. Hypertension can result from either an increase in cardiac output Cardiac output The volume of blood passing through the heart per unit of time. It is usually expressed as liters (volume) per minute so as not to be confused with stroke volume (volume per beat). Cardiac Mechanics or from systemic vascular resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing. Vascular resistance Resistance Physiologically, the opposition to flow of air caused by the forces of friction. As a part of pulmonary function testing, it is the ratio of driving pressure to the rate of air flow. Ventilation: Mechanics of Breathing is largely determined by the relative constriction and/or dilation of the arterial walls. Hypertension Hypertension Hypertension, or high blood pressure, is a common disease that manifests as elevated systemic arterial pressures. Hypertension is most often asymptomatic and is found incidentally as part of a routine physical examination or during triage for an unrelated medical encounter. Hypertension is a risk factor for many secondary diseases, such as stroke, congestive heart failure Heart Failure A heterogeneous condition in which the heart is unable to pump out sufficient blood to meet the metabolic need of the body. Heart failure can be caused by structural defects, functional abnormalities (ventricular dysfunction), or a sudden overload beyond its capacity. Chronic heart failure is more common than acute heart failure which results from sudden insult to cardiac function, such as myocardial infarction. Total Anomalous Pulmonary Venous Return (TAPVR), myocardial infarction Myocardial infarction MI is ischemia and death of an area of myocardial tissue due to insufficient blood flow and oxygenation, usually from thrombus formation on a ruptured atherosclerotic plaque in the epicardial arteries. Clinical presentation is most commonly with chest pain, but women and patients with diabetes may have atypical symptoms. Myocardial Infarction, and chronic kidney disease Chronic Kidney Disease Chronic kidney disease (CKD) is kidney impairment that lasts for ≥ 3 months, implying that it is irreversible. Hypertension and diabetes are the most common causes; however, there are a multitude of other etiologies. In the early to moderate stages, CKD is usually asymptomatic and is primarily diagnosed by laboratory abnormalities. Chronic Kidney Disease. 
• Atherosclerosis Atherosclerosis Atherosclerosis is a common form of arterial disease in which lipid deposition forms a plaque in the blood vessel walls. Atherosclerosis is an incurable disease, for which there are clearly defined risk factors that often can be reduced through a change in lifestyle and behavior of the patient. Atherosclerosis: common form of arterial disease in which lipid deposition forms a plaque Plaque Primary Skin Lesions within large, medium, and small blood vessels throughout the body: Endothelial injury triggers deposition of lipids Lipids Lipids are a diverse group of hydrophobic organic molecules, which include fats, oils, sterols, and waxes. Fatty Acids and Lipids, release of inflammatory molecules, smooth muscle hyperplasia Hyperplasia An increase in the number of cells in a tissue or organ without tumor formation. It differs from hypertrophy, which is an increase in bulk without an increase in the number of cells. Cellular Adaptation, and the increased formation of connective tissue Connective tissue Connective tissues originate from embryonic mesenchyme and are present throughout the body except inside the brain and spinal cord. The main function of connective tissues is to provide structural support to organs. Connective tissues consist of cells and an extracellular matrix. Connective Tissue: Histology in the region, ultimately giving rise to the atherosclerotic plaques. These plaques lead to narrowing of the vessel lumen, loss of elasticity Elasticity Resistance and recovery from distortion of shape. Skeletal Muscle Contraction, and reduced or obstructed blood flow Blood flow Blood flow refers to the movement of a certain volume of blood through the vasculature over a given unit of time (e.g., mL per minute). Vascular Resistance, Flow, and Mean Arterial Pressure. Ruptured plaques can become embolic, leading to acute end-organ damage.
• Arteriosclerosis: condition involving hardening of the vessels and loss of their intrinsic elasticity Elasticity Resistance and recovery from distortion of shape. Skeletal Muscle Contraction: Arteriosclerosis is most commonly associated with hypertension Hypertension Hypertension, or high blood pressure, is a common disease that manifests as elevated systemic arterial pressures. Hypertension is most often asymptomatic and is found incidentally as part of a routine physical examination or during triage for an unrelated medical encounter. Hypertension and diabetes Diabetes Diabetes mellitus (DM) is a metabolic disease characterized by hyperglycemia and dysfunction of the regulation of glucose metabolism by insulin. Type 1 DM is diagnosed mostly in children and young adults as the result of autoimmune destruction of β cells in the pancreas and the resulting lack of insulin. Type 2 DM has a significant association with obesity and is characterized by insulin resistance. Diabetes Mellitus mellitus. 
• Aortic aneurysm Aortic aneurysm An abnormal balloon- or sac-like dilatation in the wall of aorta. Thoracic Aortic Aneurysms: abnormal dilation in the aorta Aorta The main trunk of the systemic arteries. Mediastinum and Great Vessels: Anatomy resulting from a thinning and/or weakening of the aortic wall, causing it to bulge out: Aortic aneurysms may occur in the thoracic cavity (thoracic aortic aneurysms that affect primarily the ascending aorta Ascending aorta Mediastinum and Great Vessels: Anatomy) or in the abdominal cavity (abdominal aortic aneurysms). Rupture of an aortic aneurysm Aortic aneurysm An abnormal balloon- or sac-like dilatation in the wall of aorta. Thoracic Aortic Aneurysms is a life-threatening emergency.