Blood vessel

The blood vessels are the part of the circulatory system, and microcirculation, that transports blood throughout the human body.[1] These vessels are designed to transport nutrients and oxygen to the tissues of the body. They also take waste and carbon dioxide and carry them away from the tissues and back to the heart. Blood vessels are needed to sustain life as all of the body’s tissues rely on their functionality.[2]There are three major types of blood vessels: the arteries, which carry the blood away from the heart; the capillaries, which enable the actual exchange of water and chemicals between the blood and the tissues; and the veins, which carry blood from the capillaries back toward the heart. The word vascular, meaning relating to the blood vessels, is derived from the Latin vas, meaning vessel. Some structures -- such as cartilage, the epithelium, and the lens and cornea of the eye -- do not contain blood vessels and are labeled avascular.

Blood vessel
Circulatory System en
Simple diagram of the human circulatory system
Details
SystemCirculatory system
Identifiers
Latinvas sanguineum
MeSHD001808
TAA12.0.00.001
FMA63183
Anatomical terminology

Structure

The arteries and veins have three layers. The middle layer is thicker in the arteries than it is in the veins:

  • The inner layer, tunica intima, is the thinnest layer. It is a single layer of flat cells (simple squamous epithelium) glued by a polysaccharide intercellular matrix, surrounded by a thin layer of subendothelial connective tissue interlaced with a number of circularly arranged elastic bands called the internal elastic lamina. A thin membrane of elastic fibers in the tunica intima run parallel to the vessel.
  • The middle layer tunica media is the thickest layer in arteries. It consists of circularly arranged elastic fiber, connective tissue, polysaccharide substances, the second and third layer are separated by another thick elastic band called external elastic lamina. The tunica media may (especially in arteries) be rich in vascular smooth muscle, which controls the caliber of the vessel. Veins don't have the external elastic lamina, but only an internal one. The tunica media is thicker in the arteries rather than the veins.
  • The outer layer is the tunica adventitia and the thickest layer in veins. It is entirely made of connective tissue. It also contains nerves that supply the vessel as well as nutrient capillaries (vasa vasorum) in the larger blood vessels.

Capillaries consist of little more than a layer of endothelium and occasional connective tissue.

When blood vessels connect to form a region of diffuse vascular supply it is called an anastomosis. Anastomoses provide critical alternative routes for blood to flow in case of blockages.

There is a layer of muscle surrounding the arteries and the veins which help contract and expand the vessels. This creates enough pressure for blood to be pumped around the body. Blood vessels are part of the circulatory system, together with the heart and the blood.

The biggest difference in the structure of arteries and veins is the presence of valves. Backflow of blood is prevented in arteries by the heart. However in veins, one-direction valves are used to prevent backflow as a result of a decrease in blood pressure as the blood passes through the circulatory system.[3]

Types

Microvessel
Blood vessel with an erythrocyte (red blood cell, E) within its lumen, endothelial cells forming its tunica intima (inner layer), and pericytes forming its tunica adventitia (outer layer)

There are various kinds of blood vessels:

They are roughly grouped as "arterial" and "venous", determined by whether the blood in it is flowing away from (arterial) or toward (venous) the heart. The term "arterial blood" is nevertheless used to indicate blood high in oxygen, although the pulmonary artery carries "venous blood" and blood flowing in the pulmonary vein is rich in oxygen. This is because they are carrying the blood to and from the lungs, respectively, to be oxygenated.

Diagram of blood vessel structures

Function

Blood vessels function to transport blood. In general, arteries and arterioles transport oxygenated blood from the lungs to the body and its organs, and veins and venules transport deoxygenated blood from the body to the lungs. Blood vessels also circulate blood throughout the circulatory system Oxygen (bound to hemoglobin in red blood cells) is the most critical nutrient carried by the blood. In all arteries apart from the pulmonary artery, hemoglobin is highly saturated (95–100%) with oxygen. In all veins apart from the pulmonary vein, the saturation of hemoglobin is about 75%. (The values are reversed in the pulmonary circulation.) In addition to carrying oxygen, blood also carries hormones, waste products and nutrients for cells of the body.

Blood vessels do not actively engage in the transport of blood (they have no appreciable peristalsis). Blood is propelled through arteries and arterioles through pressure generated by the heartbeat. Blood vessels also transport red blood cells which contain the oxygen necessary for daily activities. The amount of red blood cells present in your vessels has an effect on your health. Hematocrit tests can be performed to calculate the proportion of red blood cells in your blood. Higher proportions result in conditions such as dehydration or heart disease while lower proportions could lead to anemia and long-term blood loss.[4]


Blood vessels also transport red blood cells which contain the oxygen necessary for daily activities. The amount of red blood cells present in your vessels has an effect on your health. Hematocrit tests can be performed to calculate the proportion of red blood cells in your blood. Higher proportions result in conditions such as dehydration or heart disease while lower proportions could lead to anemia and long-term blood loss.[4]

Permeability of the endothelium is pivotal in the release of nutrients to the tissue. It is also increased in inflammation in response to histamine, prostaglandins and interleukins, which leads to most of the symptoms of inflammation (swelling, redness, warmth and pain).

Vessel size

Vasoconstriction - Constricted blood vessel
Constricted blood vessel.

Arteries—and veins to a degree—can regulate their inner diameter by contraction of the muscular layer. This changes the blood flow to downstream organs, and is determined by the autonomic nervous system. Vasodilation and vasoconstriction are also used antagonistically as methods of thermoregulation.

The size of blood vessels is different for each of them. It ranges from a diameter of about 25 millimeters for the aorta to only 8 micrometers in the capillaries. This comes out to about a 3000-fold range.[5] Vasoconstriction is the constriction of blood vessels (narrowing, becoming smaller in cross-sectional area) by contracting the vascular smooth muscle in the vessel walls. It is regulated by vasoconstrictors (agents that cause vasoconstriction). These include paracrine factors (e.g. prostaglandins), a number of hormones (e.g. vasopressin and angiotensin) and neurotransmitters (e.g. epinephrine) from the nervous system.

Vasodilation is a similar process mediated by antagonistically acting mediators. The most prominent vasodilator is nitric oxide (termed endothelium-derived relaxing factor for this reason).

Blood flow

The circulatory system uses the channel of blood vessels to deliver blood to all parts of the body. This is a result of the left and right side of the heart working together to allow blood to flow continuously to the lungs and other parts of the body. Oxygen poor blood enters the right side of the heart through two large veins. Oxygen rich blood from the lungs enters through the pulmonary veins on the left side of the heart into the aorta and then reaches the rest of the body. The capillaries are responsible for allowing the blood to receive oxygen through tiny air sacs in the lungs. This is also the site where carbon dioxide exits the blood. This all occurs in the lungs where blood is oxygenated.[6]

The blood pressure in blood vessels is traditionally expressed in millimetres of mercury (1 mmHg = 133 Pa). In the arterial system, this is usually around 120 mmHg systolic (high pressure wave due to contraction of the heart) and 80 mmHg diastolic (low pressure wave). In contrast, pressures in the venous system are constant and rarely exceed 10 mmHg.

Vascular resistance occurs where the vessels away from the heart oppose the flow of blood. Resistance is an accumulation of three different factors: blood viscosity, blood vessel length, and vessel radius.[7]

Blood viscosity is the thickness of the blood and its resistance to flow as a result of the different components of the blood. Blood is 92% water by weight and the rest of blood is composed of protein, nutrients, electrolytes, wastes, and dissolved gases. Depending on the health of an individual, the blood viscosity can vary (i.e. anemia causing relatively lower concentrations of protein, high blood pressure an increase in dissolved salts or lipids, etc.).[7]

Vessel length is the total length of the vessel measured as the distance away from the heart. As the total length of the vessel increases, the total resistance as a result of friction will increase.[7]

Vessel radius also affects the total resistance as a result of contact with the vessel wall. As the radius of the wall gets smaller, the proportion of the blood making contact with the wall will increase. The greater amount of contact with the wall will increase the total resistance against the blood flow.[8]

Disease

Blood vessels play a huge role in virtually every medical condition. Cancer, for example, cannot progress unless the tumor causes angiogenesis (formation of new blood vessels) to supply the malignant cells' metabolic demand. Atherosclerosis, the formation of lipid lumps (atheromas) in the blood vessel wall, is the most common cardiovascular disease, the main cause of death in the Western world.

Blood vessel permeability is increased in inflammation. Damage, due to trauma or spontaneously, may lead to hemorrhage due to mechanical damage to the vessel endothelium. In contrast, occlusion of the blood vessel by atherosclerotic plaque, by an embolised blood clot or a foreign body leads to downstream ischemia (insufficient blood supply) and possibly necrosis. Vessel occlusion tends to be a positive feedback system; an occluded vessel creates eddies in the normally laminar flow or plug flow blood currents. These eddies create abnormal fluid velocity gradients which push blood elements such as cholesterol or chylomicron bodies to the endothelium. These deposit onto the arterial walls which are already partially occluded and build upon the blockage.[9]

The most common disease of the blood vessels is hypertension or high blood pressure. This is caused by an increase in the pressure of the blood flowing through the vessels. Hypertension can lead to more serious conditions such as heart failure and stroke. To prevent these diseases, the most common treatment option is medication as opposed to surgery. Aspirin helps prevent blood clots and can also help limit inflammation.[10]

Vasculitis is inflammation of the vessel wall, due to autoimmune disease or infection.

References

  1. ^ "Blood Vessels – Heart and Blood Vessel Disorders – Merck Manuals Consumer Version". Merck Manuals Consumer Version. Retrieved 2016-12-22.
  2. ^ "Heart & Blood Vessels: Blood Flow". Cleveland Clinic.
  3. ^ "Blood Vessel Structure and Function - Boundless Anatomy and Physiology". courses.lumenlearning.com.
  4. ^ a b "Hematocrit test - Mayo Clinic". www.mayoclinic.org.
  5. ^ "Blood Vessels - Encyclopedia.com". www.encyclopedia.com.
  6. ^ "How the Heart Works". WebMD.
  7. ^ a b c Anatomy Physiology: The Unity of Form and Function, Saladin, McGraw Hill, 2012
  8. ^ "Factors that Affect Blood Pressure" (PDF). Retrieved 21 Oct 2018.
  9. ^ Multiphase Flow and Fluidization, Gidaspow et al., Academic Press, 1992
  10. ^ "Blood Vessel Diseases - Mercy Health System". www.mercyhealth.org.
Aneurysm

An aneurysm is an outward bulging, likened to a bubble or balloon, caused by a localized, abnormal, weak spot on a blood vessel wall. Aneurysms are a result of a weakened blood vessel wall, and may be a result of a hereditary condition or an acquired disease. Aneurysms can also be a nidus (starting point) for clot formation (thrombosis) and embolization. The word is from Greek: ἀνεύρυσμα, aneurysma, "dilation", from ἀνευρύνειν, aneurynein, "to dilate". As an aneurysm increases in size, the risk of rupture increases, leading to uncontrolled bleeding. Although they may occur in any blood vessel, particularly lethal examples include aneurysms of the Circle of Willis in the brain, aortic aneurysms affecting the thoracic aorta, and abdominal aortic aneurysms. Aneurysms can arise in the heart itself following a heart attack, including both ventricular and atrial septal aneurysms.

Angiology

Angiology (from Greek ἀγγεῖον, angeīon, "vessel"; and -λογία, -logia) is the medical specialty which studies the diseases of the circulatory system and of the lymphatic system, i.e., arteries, veins and lymphatic vessels, and its diseases.

In the UK, this field is more often termed angiology, and in the United States the term vascular medicine is more frequent. The field of vascular medicine (angiology) is the field that deals with preventing, diagnosing and treating vascular and blood vessel related diseases.

Baroreceptor

Baroreceptors (or archaically, pressoreceptors) are sensors located in the blood vessels of all vertebrate animals. They sense the blood pressure and relay the information to the brain, so that a proper blood pressure can be maintained.

Baroreceptors are a type of mechanoreceptor sensory neuron that are excited by a stretch of the blood vessel. Thus, increases in the pressure of blood vessel triggers increased action potential generation rates and provides information to the central nervous system. This sensory information is used primarily in autonomic reflexes that in turn influence the heart cardiac output and vascular smooth muscle to influence total peripheral resistance. Baroreceptors act immediately as part of a negative feedback system called the baroreflex, as soon as there is a change from the usual mean arterial blood pressure, returning the pressure toward a normal level. These reflexes help regulate short-term blood pressure. The solitary nucleus in the medulla oblongata of the brain recognizes changes in the firing rate of action potentials from the baroreceptors, and influences cardiac output and systemic vascular resistance.

Baroreceptors can be divided into two categories based on the type of blood vessel in which they are located: high-pressure arterial baroreceptors and low-pressure baroreceptors (also known as cardiopulmonary or volume receptors).

Basilar skull fracture

A basilar skull fracture is a break of a bone in the base of the skull. Symptoms may include bruising behind the ears, bruising around the eyes, or blood behind the ear drum. A cerebrospinal fluid (CSF) leak occurs in about 20% of cases and can result in fluid leaking from the nose or ear. Meningitis is a complication in about 14% of cases. Other complications include cranial nerve or blood vessel injury.They typically require a significant degree of trauma to occur. The break is of at least one of the following bones: temporal bone, occipital bone, sphenoid bone, frontal bone, or ethmoid bone. They are divided into anterior fossa, middle fossa, and posterior fossa fractures. Facial fractures often also occur. Diagnosis is typically by CT scan.Treatment is generally based on the injury to structures inside the head. Surgery may be done for a CSF leak that does not stop or an injury to a blood vessel or nerve. Preventative antibiotics are of unclear use. It occurs in about 12% of people with a severe head injury.

Capillary

A capillary is a small blood vessel from 5 to 10 micrometres (µm) in diameter, and having a wall one endothelial cell thick. They are the smallest blood vessels in the body: they convey blood between the arterioles and venules. These microvessels are the site of exchange of many substances with the interstitial fluid surrounding them. Substances which exit include water (proximal portion), oxygen, and glucose; substances which enter include water (distal portion), carbon dioxide, uric acid, lactic acid, urea and creatinine. Lymph capillaries connect with larger lymph vessels to drain lymphatic fluid collected in the microcirculation.

During early embryonic development new capillaries are formed through vasculogenesis, the process of blood vessel formation that occurs through a de novo production of endothelial cells which then form vascular tubes. The term angiogenesis denotes the formation of new capillaries from pre-existing blood vessels and already present endothelium which divides.

Cerebral circulation

Cerebral circulation is the movement of blood through the network of cerebral arteries and veins supplying the brain. The rate of the cerebral blood flow in the adult is typically 750 milliliters per minute, representing 15% of the cardiac output. The arteries deliver oxygenated blood, glucose and other nutrients to the brain, and the veins carry deoxygenated blood back to the heart, removing carbon dioxide, lactic acid, and other metabolic products. Since the brain is very vulnerable to compromises in its blood supply, the cerebral circulatory system has many safeguards including autoregulation of the blood vessels and the failure of these safeguards can result in a stroke. The amount of blood that the cerebral circulation carries is known as cerebral blood flow. The presence of gravitational fields or accelerations also determine variations in the movement and distribution of blood in the brain, such as when suspended upside-down.The following description is based on idealized human cerebral circulation. The pattern of circulation and its nomenclature vary between organisms.

Collagen, type XXI, alpha 1

Collagen alpha-1(XXI) chain is a protein that in humans is encoded by the COL21A1 gene. The protein is an extracellular matrix component of blood vessel walls, secreted by smooth-muscle cells. The protein may contribute to the extracellular matrix assembly of the vascular network during blood vessel formation.

Endothelium

Endothelium refers to cells that line the interior surface of blood vessels and lymphatic vessels, forming an interface between circulating blood or lymph in the lumen and the rest of the vessel wall. It is a thin layer of simple, or single-layered, squamous cells called endothelial cells. Endothelial cells in direct contact with blood are called vascular endothelial cells, whereas those in direct contact with lymph are known as lymphatic endothelial cells.

Vascular endothelial cells line the entire circulatory system, from the heart to the smallest capillaries. These cells have unique functions in vascular biology. These functions include fluid filtration, such as in the glomerulus of the kidney, blood vessel tone, hemostasis, neutrophil recruitment, and hormone trafficking. Endothelium of the interior surfaces of the heart chambers is called endocardium.

Hemangioma

Hemangioma is a benign tumor derived from blood vessel cell types, most commonly infantile hemangioma, a common benign tumor of infancy. Infantile hemangiomas, known colloquially as strawberry marks and seen at birth or in the first weeks of life, are most commonly seen on the skin. A hemangioma can occur anywhere on the body, but most commonly appears on the face, scalp, chest or back. Treatment of a hemangioma usually is unnecessary, unless the nodule interferes with vision or breathing.

Intracranial aneurysm

Intracranial aneurysm, also known as brain aneurysm, is a cerebrovascular disorder in which weakness in the wall of a cerebral artery or vein causes a localized dilation or ballooning of the blood vessel.

Aneurysms in the posterior circulation (basilar artery, vertebral arteries and posterior communicating artery) have a higher risk of rupture. Basilar artery aneurysms represent only 3%-5% of all intracranial aneurysms but are the most common aneurysms in the posterior circulation.

Ligature (medicine)

In surgery or medical procedure, a ligature consists of a piece of thread (suture) tied around an anatomical structure, usually a blood vessel or another hollow structure (e.g. urethra) to shut it off. With a blood vessel the surgeon will clamp the vessel perpendicular to the axis of the artery or vein with a hemostat, then secure it by ligating it; i.e. using a piece of suture around it before dividing the structure and releasing the hemostat. It is different from a tourniquet in that the tourniquet will not be secured by knots and it can therefore be released/tightened at will.

The principle of ligation is attributed to Hippocrates and Galen, later reintroduced some 1,500 years later by Ambroise Paré, and finally it found its modern use in 1870–80, made popular by Jules-Émile Péan.

Myogenic mechanism

The myogenic mechanism is how arteries and arterioles react to an increase or decrease of blood pressure to keep the blood flow within the blood vessel constant. Myogenic response refers to a contraction initiated by the myocyte cell itself instead of an outside occurrence or stimulus such as nerve innervation. Most often observed in (although not necessarily restricted to) smaller resistance arteries, this 'basal' tone may be useful in the regulation of organ blood flow and peripheral resistance, as it positions a vessel in a preconstricted state that allows other factors to induce additional constriction or dilation to increase or decrease blood flow.

The smooth muscle of the blood vessels reacts to the stretching of the muscle by opening ion channels, which cause the muscle to depolarize, leading to muscle contraction. This significantly reduces the volume of blood able to pass through the lumen, which reduces blood flow through the blood vessel. Alternatively when the smooth muscle in the blood vessel relaxes, the ion channels close, resulting in vasodilation of the blood vessel; this increases the rate of flow through the lumen.

This system is especially significant in the kidneys, where the glomerular filtration rate (the rate of blood filtration by the nephron) is particularly sensitive to changes in blood pressure. However, with the aid of the myogenic mechanism, the glomerular filtration rate remains very insensitive to changes in human blood pressure.

Myogenic mechanisms in the kidney are part of the autoregulation mechanism which maintains a constant renal blood flow at varying arterial pressure. Concomitant autoregulation of glomerular pressure and filtration indicates regulation of preglomerular resistance. Model and experimental studies were performed to evaluate two mechanisms in the kidney, myogenic response and tubuloglomerular feedback. A mathematical model showed good autoregulation through a myogenic response, aimed at maintaining a constant wall tension in each segment of the preglomerular vessels. Tubuloglomerular feedback gave rather poor autoregulation. The myogenic mechanism showed 'descending' resistance changes, starting in the larger arteries, and successively affecting downstream preglomerular vessels at increasing arterial pressures. This finding was supported by micropuncture measurements of pressure in the terminal interlobular arteries. Evidence that the mechanism was myogenic was obtained by exposing the kidney to a subatmospheric pressure of 40 mmHg; this led to an immediate increase in renal resistance, which could not be prevented by denervation or various blocking agents.

Percutaneous

In surgery, a percutaneous procedure is any medical procedure or method where access to inner organs or other tissue is done via needle-puncture of the skin, rather than by using an "open" approach where inner organs or tissue are exposed (typically with the use of a scalpel).

The percutaneous approach is commonly used in vascular procedures such as angioplasty and stenting. This involves a needle catheter getting access to a blood vessel, followed by the introduction of a wire through the lumen (pathway) of the needle. It is over this wire that other catheters can be placed into the blood vessel. This technique is known as the modified Seldinger technique.

More generally, "percutaneous", via its Latin roots means, 'by way of the skin'. An example would be percutaneous drug absorption from topical medications. More often, percutaneous is typically used in reference to placement of medical devices using a needle stick approach.

In general, percutaneous refers to the access modality of a medical procedure, whereby a medical device is introduced into a patient's blood vessel via a needle stick. This is commonly known as the Seldinger technique named after Sven Ivar Seldinger. The technique involves placing a needle through the skin and into a blood vessel, such as an artery or vein, until bleedback is achieved. This is followed by introduction of a flexible "introducer guide wire" to define the pathway through the skin and into the passageway or "lumen" of the blood vessel. The needle is then exchanged for an "introducer sheath" which is a small tube that is advanced over the introducer guide wire and into the blood vessel. The introducer guide wire is removed, and exchanged for a catheter or other medical device to be used to deliver medication or implantation of a medical implant such as a filter or a stent into the blood vessel.

The benefit of a percutaneous access is in the ease of introducing devices into the patient without the use of large cut downs, which can be painful and in some cases can bleed out or become infected. A percutaneous access requires only a very small hole through the skin, which seals easily, and heals very quickly compared to a surgical cut down.

Percutaneous access and procedures frequently refer to catheter procedures such as percutaneous transluminal angioplasty (PTA) ballooning, stent delivery, filter delivery, cardiac ablation, and peripheral or neurovascular catheter procedures but also refers to a device that is implanted in the body, such as a heart pump (LVAD), and receives power through a lead that passes through the skin to a battery pack outside the body.

Pulmonary circulation

The pulmonary circulation is the portion of the circulatory system which carries deoxygenated blood away from the right ventricle of the heart, to the lungs, and returns oxygenated blood to the left atrium and ventricle of the heart. The term pulmonary circulation is readily paired and contrasted with the systemic circulation. The vessels of the pulmonary circulation are the pulmonary arteries and the pulmonary veins.

A separate system known as the bronchial circulation supplies oxygenated blood to the tissue of the larger airways of the lung.

The earliest human discussions of pulmonary circulation date back to Egyptian times. Human knowledge of pulmonary circulation grew gradually over centuries, and scientists Ibn al-Nafis, Michael Servetus, and William Harvey provided some of the first accurate descriptions of this process.

Sinusoid (blood vessel)

A sinusoid is a type of capillary that is similar to a fenestrated capillary. Sinusoids are classified as a type of open pore capillary or discontinuous capillary as opposed to the continuous types. Fenestrated capillaries have diaphragms that cover the pores whereas sinusoid capillaries lack a diaphragm and just have an open pore. The open pores of endothelial cells greatly increase their permeability. In addition, permeability is increased by large inter-cellular clefts and fewer tight junctions. The level of permeability is such as to allow small and medium-sized proteins such as albumin to readily enter and leave the blood stream.

Sinusoids are found in the liver, lymphoid tissue, endocrine organs, and hematopoietic organs such as the bone marrow and the spleen. Sinusoids found within terminal villi of the placenta are not comparable to these because they possess a continuous endothelium and complete basal lamina. This word was first used in 1893.

Stenosis

A stenosis (from Ancient Greek στενός, "narrow") is an abnormal narrowing in a blood vessel or other tubular organ or structure. It is also sometimes called a stricture (as in urethral stricture).Stricture as a term is usually used when narrowing is caused by contraction of smooth muscle (e.g. achalasia, prinzmetal angina); stenosis is usually used when narrowing is caused by lesion that reduces the space of lumen (e.g. atherosclerosis). The term coarctation is another synonym, but is commonly used only in the context of aortic coarctation.

Restenosis is the recurrence of stenosis after a procedure.

Thrombosis

Thrombosis (from Ancient Greek θρόμβωσις thrómbōsis "clotting”) is the formation of a blood clot inside a blood vessel, obstructing the flow of blood through the circulatory system. When a blood vessel (a vein or an artery) is injured, the body uses platelets (thrombocytes) and fibrin to form a blood clot to prevent blood loss. Even when a blood vessel is not injured, blood clots may form in the body under certain conditions. A clot, or a piece of the clot, that breaks free and begins to travel around the body is known as an embolus.Thrombosis may occur in veins (venous thrombosis) or in arteries. Venous thrombosis leads to congestion of the affected part of the body, while arterial thrombosis (and rarely severe venous thrombosis) affects the blood supply and leads to damage of the tissue supplied by that artery (ischemia and necrosis). A piece of either an arterial or a venous thrombus can break off as an embolus which can travel through the circulation and lodge somewhere else as an embolism. This type of embolism is known as a thromboembolism. Complications can arise when a venous thromboembolism (commonly called a VTE) lodges in the lung as a pulmonary embolism. An arterial embolus may travel further down the affected blood vessel where it can lodge as an embolism.

Thrombus

A thrombus, colloquially called a blood clot, is the final product of the blood coagulation step in hemostasis. There are two components to a thrombus: aggregated platelets and red blood cells that form a plug, and a mesh of cross-linked fibrin protein. The substance making up a thrombus is sometimes called cruor. A thrombus is a healthy response to injury intended to prevent bleeding, but can be harmful in thrombosis, when clots obstruct blood flow through healthy blood vessels.

Mural thrombi are thrombi that adhere to the wall of a blood vessel. They occur in large vessels such as the heart and aorta, and can restrict blood flow but usually do not block it entirely. They appear grey-red with alternating light and dark lines (known as lines of Zahn) which represent bands of entrapped white blood cells and red blood cells (darker).

Vascular permeability

Vascular permeability, often in the form of capillary permeability or microvascular permeability, characterizes the capacity of a blood vessel wall to allow for the flow of small molecules (drugs, nutrients, water, ions) or even whole cells (lymphocytes on their way to the site of inflammation) in and out of the vessel. Blood vessel walls are lined by a single layer of endothelial cells. The gaps between endothelial cells (cell junctions) are strictly regulated depending on the type and physiological state of the tissue.

There are several techniques to measure vascular permeability to certain molecules. For instance, the cannulation of a single microvessel with a micropipette, the microvessel is perfused with a certain pressure, occluded downstream and then the velocity of some cells will be related to the permeability. Another technique uses multiphoton fluorescence intravital microscopy through which the flow is related to fluorescence intensity and the permeability is estimated from the Patlak transformation of the intensity data In cancer research, the study of permeability of the microvasculature that surrounds tumours is of great interest as the vascular wall is a barrier of large molecules into the tumours, the vessels control the microenvironment which affect tumour progression and changes to the permeability may indicate vascular damage with drugs.An example of increased vascular permeability is in the initial lesion of periodontal disease, in which the gingival plexus becomes engorged and dilated, allowing large numbers of neutrophils to extravasate and appear within the junctional epithelium and underlying connective tissue.

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