Overview

Definition

Veins are tubular collections of cells transporting deoxygenated blood and waste products fromcapillariesCapillariesCapillaries 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 in the periphery of the body back to the heart.

General characteristics

Characteristics of veins and the venous system include:

• Veins are more abundant thanarteriesArteriesArteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles.Arteries: Histology.
• Compared toarteriesArteriesArteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles.Arteries: Histology, veins have:
  • Large lumens 
  • Thin walls
  • Less smooth muscle andelasticElasticConnective Tissue: Histology tissue
• Blood passes through veins in order of increasing luminal diameter:
  • Starts with the smallest vessels (venules)
  • Ends in the largest vessel (the vena cava)
• Veins oftenaccompanyan artery:
  • Veins surround the artery in an irregular branching network.
  • Function as a countercurrentheatHeatInflammation exchange → allows cool blood returning from the periphery to be warmed before returning to the heart
• CapacitanceCapacitanceThe measure of a blood vessel’s ability to increase the volume of blood it holds without a large increase in blood pressure. The vascular capacitance is equal to the change in volume divided by the change in pressure.Venous Function vessels:
  • Collapsed when empty, but able to distend significantly (known ascompliance)
  • The venous system can hold up to 70%–80% of the blood volume at rest.

Pressure within the venouscirculationCirculationThe movement of the blood as it is pumped through the cardiovascular system.ABCDE Assessment

• VenouscirculationCirculationThe movement of the blood as it is pumped through the cardiovascular system.ABCDE Assessment is alow-pressure system:
  • Averages only 10 mm Hg (compared to 120 mm Hg in thearteriesArteriesArteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles.Arteries: Histology duringsystoleSystolePeriod of contraction of the heart, especially of the heart ventricles.Cardiac Cycle)
  • Minimal fluctuations betweensystoleSystolePeriod of contraction of the heart, especially of the heart ventricles.Cardiac Cycle anddiastoleDiastolePost-systolic relaxation of the heart, especially the heart ventricles.Cardiac Cycle
• Pressure is affected by gravity and proximity to the heart:
  • More proximal to the heart = ↓ pressure
  • Compared to standing, the recumbent position has ↓ pressure.
  • While standing:footFootThe foot is the terminal portion of the lower limb, whose primary function is to bear weight and facilitate locomotion. The foot comprises 26 bones, including the tarsal bones, metatarsal bones, and phalanges. The bones of the foot form longitudinal and transverse arches and are supported by various muscles, ligaments, and tendons.Foot: Anatomy pressure highest, head pressure the lowest
• Pressure istoo low to spontaneously push blood against gravity
• Moving blood up (against gravity) requires:
  • The pumping action of contractingskeletal musclesSkeletal musclesA subtype of striated muscle, attached by tendons to the skeleton. Skeletal muscles are innervated and their movement can be consciously controlled. They are also called voluntary muscles.Muscle Tissue: Histology
  • One-way venousvalves preventing retrogradeflowFlowBlood flows through the heart, arteries, capillaries, and veins in a closed, continuous circuit. Flow is the movement of volume per unit of time. Flow is affected by the pressure gradient and the resistance fluid encounters between 2 points. Vascular resistance is the opposition to flow, which is caused primarily by blood friction against vessel walls.Vascular Resistance, Flow, and Mean Arterial Pressure

Layers of the Vessel Wall

All veins have the same basic structure and are made up of 3 primary layers:tunica intimaTunica intimaThe innermost layer of an artery or vein, made up of one layer of endothelial cells and supported by an internal elastic lamina.Arteries: Histology,tunica mediaTunica mediaThe middle layer of blood vessel walls, composed principally of thin, cylindrical, smooth muscle cells and elastic tissue. It accounts for the bulk of the wall of most arteries. The smooth muscle cells are arranged in circular layers around the vessel, and the thickness of the coat varies with the size of the vessel.Arteries: Histology, andtunica adventitiaTunica adventitiaThe outermost covering of organs, blood vessels, and other such structures in the body.Arteries: Histology.

Tunica intimaTunica intimaThe innermost layer of an artery or vein, made up of one layer of endothelial cells and supported by an internal elastic lamina.Arteries: Histology

• Made of:
  • A single layer ofendothelial cells:
    • Simple, squamous epithelial cells
    • EndotheliumEndotheliumA layer of epithelium that lines the heart, blood vessels (vascular endothelium), lymph vessels (lymphatic endothelium), and the serous cavities of the body.Arteries: Histology in veins appears smooth.
  • Small amounts offibrousFibrousFibrocystic Changeconnective tissueConnective tissueConnective 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
  • Valves:
    • Formed from thickenedendotheliumEndotheliumA layer of epithelium that lines the heart, blood vessels (vascular endothelium), lymph vessels (lymphatic endothelium), and the serous cavities of the body.Arteries: Histology andconnective tissueConnective tissueConnective 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
    • Found in medium veins (most commonly in the limbs and veins inferior to the heart)
• Functions:
  • Acts as a selectively permeable barrier
  • Secretes vasoactive substances
  • Provides a smooth lining to the blood vessel (preventsaggregationAggregationThe 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 ofplateletsPlateletsPlatelets 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/orRBCsRBCsErythrocytes, 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)
  • Valves help prevent retrogradeblood flowBlood flowBlood 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, especially in the lower extremities.
• Vascularized by directdiffusionDiffusionThe 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
• IncapillariesCapillariesCapillaries 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: the only layer present in the vessel wall
• In veins:nointernal elastic membraneInternal elastic membraneArteries: Histology (present, however, in largerarteriesArteriesArteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles.Arteries: Histology)

Tunica mediaTunica mediaThe middle layer of blood vessel walls, composed principally of thin, cylindrical, smooth muscle cells and elastic tissue. It accounts for the bulk of the wall of most arteries. The smooth muscle cells are arranged in circular layers around the vessel, and the thickness of the coat varies with the size of the vessel.Arteries: Histology

• Made of: 
  • CollagenCollagenA 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
  • Smooth muscle (much less than inarteriesArteriesArteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles.Arteries: Histology)
  • ElasticElasticConnective Tissue: Histology tissue (sparse compared toarteriesArteriesArteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles.Arteries: Histology)
• Functions:provides strength and structure
• In veins:noexternal elastic membraneExternal elastic membraneArteries: Histology (present, however, in largerarteriesArteriesArteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles.Arteries: Histology)

Tunica adventitiaTunica adventitiaThe outermost covering of organs, blood vessels, and other such structures in the body.Arteries: Histology

• Also called thetunica externa
• Normally the thickest layer in veins
• Made of: 
  • The primary component iscollagenCollagenA 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, which merges withconnective tissueConnective tissueConnective 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 and surrounds neighboring vessels, nerves, and organs.
  • Longitudinal smooth muscle fibers (vena cava only)
• Functions:
  • Strengthens the vessel wall
  • Anchors the vessel
• Innervated by tiny nerves known asnervi vasorumNervi vasorumArteries: Histology
• Vascularized by tiny vessels known asvasa vasorumVasa vasorumNutrient blood vessels which supply the walls of large arteries or veins.Arteries: Histology

Types of Veins

Segmental differentiation distinguishes the 3 primary types of veins by overall size, function, and composition. Veins generally exist on a continuum with gradual changes in vessel morphology down the venous tree.

The 3 primary types of veins are:

1. Small veins and venules
2. Medium veins
3. Large veins

Venules and small veins

• Marks the beginning of the venouscirculationCirculationThe movement of the blood as it is pumped through the cardiovascular system.ABCDE Assessment (drainscapillariesCapillariesCapillaries 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)
• Venules:
  • Thinnest walls relative to their lumens
  • Range: 15–100 µm in diameter
  • Can be quite porous → exchange fluid with surrounding tissue (similar tocapillariesCapillariesCapillaries 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)  
  • Join together to form small veins
• Small veins:
  • Generally 100 µm–1 mm in diameter 
  • Can unite to form venous plexuses
  • Unnamed

Medium veins

• Diameter up to 1 cm
• Drain venous plexuses
• Accompany medium-sized (distributing)arteriesArteriesArteries are tubular collections of cells that transport oxygenated blood and nutrients from the heart to the tissues of the body. The blood passes through the arteries in order of decreasing luminal diameter, starting in the largest artery (the aorta) and ending in the small arterioles. Arteries are classified into 3 types: large elastic arteries, medium muscular arteries, and small arteries and arterioles.Arteries: Histology
• Embedded in skeletal muscle compartments → skeletal muscle acts as apumpPumpACES and RUSH: Resuscitation Ultrasound Protocols, moving blood towards the heart
• One-way venous valves prevent retrogradeblood flowBlood flowBlood 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 within veins → ensures blood keeps moving forward
• Includes most named vessels, for example:
  • Cephalic and basilic veins (upper limb)
  • Great and small saphenous veins (lower limb)

Large veins

• Examples:
  • Superior andinferior vena cavaInferior vena cavaThe venous trunk which receives blood from the lower extremities and from the pelvic and abdominal organs.Mediastinum and Great Vessels: Anatomy:
    • Largest veins in the body
    • Both lead to the right atrium of the heart
  • Portal veinPortal veinA short thick vein formed by union of the superior mesenteric vein and the splenic vein.Liver: Anatomy
• Vena cava have wide bundles oflongitudinal smooth muscle within theirtunica adventitiaTunica adventitiaThe outermost covering of organs, blood vessels, and other such structures in the body.Arteries: Histology → allowingelongationElongationPolymerase Chain Reaction (PCR) or shortening according to changes in posture

Venous sinuses

• Characterized by:
  • Especially thin walls
  • Large lumens
  • No smooth muscle
• Examples:
  • Coronary sinusCoronary SinusA short vein that collects about two thirds of the venous blood from the myocardium and drains into the right atrium. Coronary sinus, normally located between the left atrium and left ventricle on the posterior surface of the heart, can serve as an anatomical reference for cardiac procedures.Atrial Septal Defect (ASD) of the heart
  • Dural sinuses of thebrainBrainThe part of central nervous system that is contained within the skull (cranium). Arising from the neural tube, the embryonic brain is comprised of three major parts including prosencephalon (the forebrain); mesencephalon (the midbrain); and rhombencephalon (the hindbrain). The developed brain consists of cerebrum; cerebellum; and other structures in the brain stem.Nervous System: Anatomy, Structure, and Classification

Clinical Relevance

• Chronic venous insufficiencyChronic venous insufficiencyChronic venous disease is a spectrum of disorders characterized by venous dilation and/or abnormal vein function in the lower extremities resulting from venous hypertension. “Chronic venous insufficiency” refers to the more severe forms of chronic venous disease. Skin changes typically distinguish chronic venous insufficiency from milder forms of venous disease.Chronic Venous Insufficiency (CVI): spectrum of disorders characterized by venous dilation and/or abnormal vein function in the lower extremities, resulting from venoushypertensionHypertensionHypertension, 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. More severe cases present withskinSkinThe skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue.Skin: Structure and Functions changes, which may includeskinSkinThe skin, also referred to as the integumentary system, is the largest organ of the body. The skin is primarily composed of the epidermis (outer layer) and dermis (deep layer). The epidermis is primarily composed of keratinocytes that undergo rapid turnover, while the dermis contains dense layers of connective tissue.Skin: Structure and Functions pigmentation,stasis dermatitisStasis dermatitisChronic Venous Insufficiency,lipodermatosclerosisLipodermatosclerosisChronic Venous Insufficiency, and (eventually) the development of ulcers. Diagnosis is usually based on exam findings alone. The mainstay of management iscompressionCompressionBlunt Chest Trauma therapy, though a variety of surgical options also exist. Venous ulcers are common as the disease progresses.
• PhlebitisPhlebitisInflammation of a vein, often a vein in the leg. Phlebitis associated with a blood clot is called (thrombophlebitis).Glycopeptides and thrombophlebitis:PhlebitisPhlebitisInflammation of a vein, often a vein in the leg. Phlebitis associated with a blood clot is called (thrombophlebitis).Glycopeptides refers toinflammationInflammationInflammation 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 of a vein and is commonly due to the presence of a thrombus, which is known as thrombophlebitis. The conditions may occur in both deep and superficial veins. In certain situations, such as thepostpartum periodPostpartum periodIn females, the period that is shortly after giving birth (parturition).Postpartum Complications or an indwelling vascular catheter, the clot may become infected and lead toseptic thrombophlebitis. Management involvesanticoagulationAnticoagulationPulmonary Hypertension Drugs if a clot is present, and antibiotics if the clot is infected. Isolated superficialphlebitisPhlebitisInflammation of a vein, often a vein in the leg. Phlebitis associated with a blood clot is called (thrombophlebitis).Glycopeptides is typicallybenignBenignFibroadenoma and self-limited.
• Deep veinthrombosisThrombosisFormation and development of a thrombus or blood clot in the blood vessel.Epidemic Typhus (DVTDVTDeep vein thrombosis (DVT) usually occurs in the deep veins of the lower extremities. The affected veins include the femoral, popliteal, iliofemoral, and pelvic veins. Proximal DVT is more likely to cause a pulmonary embolism (PE) and is generally considered more serious.Deep Vein Thrombosis) andpulmonary embolismPulmonary EmbolismPulmonary embolism (PE) is a potentially fatal condition that occurs as a result of intraluminal obstruction of the main pulmonary artery or its branches. The causative factors include thrombi, air, amniotic fluid, and fat. In PE, gas exchange is impaired due to the decreased return of deoxygenated blood to the lungs.Pulmonary Embolism (PE):Deep veinthrombosisThrombosisFormation and development of a thrombus or blood clot in the blood vessel.Epidemic Typhus refers to a thrombus formed in a deep vein. ADVTDVTDeep vein thrombosis (DVT) usually occurs in the deep veins of the lower extremities. The affected veins include the femoral, popliteal, iliofemoral, and pelvic veins. Proximal DVT is more likely to cause a pulmonary embolism (PE) and is generally considered more serious.Deep Vein Thrombosis most commonly forms in a femoral, popliteal, iliofemoral, or pelvic vein.PatientsPatientsIndividuals participating in the health care system for the purpose of receiving therapeutic, diagnostic, or preventive procedures.Clinician–Patient Relationship present withpainPainAn unpleasant sensation induced by noxious stimuli which are detected by nerve endings of nociceptive neurons.Pain: Types and Pathways andswellingSwellingInflammation distal to the thrombus. A PE occurs if a portion breaks off, lodges in the pulmonary vasculature, and occludes the affected vessel. Pulmonary embolisms can be fatal. Ultrasound can visualize the thrombus andanticoagulationAnticoagulationPulmonary Hypertension Drugs is the primary mode of treatment. 
• EdemaEdemaEdema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity).Edema: Excess serous fluid accumulates in a body cavity or the interstitial space.Hydrostatic pressureHydrostatic pressureThe pressure due to the weight of fluid.Edema increases within a vein leading to increased pressure within thecapillariesCapillariesCapillaries 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. The increased pressure pushes fluid out of the vasculature and into theextracellular fluidExtracellular fluidThe fluid of the body that is outside of cells. It is the external environment for the cells.Body Fluid Compartments, resulting inedemaEdemaEdema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity).Edema. Symptoms vary depending on the location of theedemaEdemaEdema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity).Edema and the underlying etiology.EdemaEdemaEdema is a condition in which excess serous fluid accumulates in the body cavity or interstitial space of connective tissues. Edema is a symptom observed in several medical conditions. It can be categorized into 2 types, namely, peripheral (in the extremities) and internal (in an organ or body cavity).Edema is a symptom of a wide variety of illnesses.

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