Principles Of Blood Vessels
Cardiovascular system is… A closed system. A double system – pulmonary and systemic circulations.
Arterial system
Artery Vascular tube carrying blood away from the heart → progression of smaller branches → flow towards capillary bed. Systemic arteries transport oxygenated blood. Pulmonary arteries transport deoxygenated blood. Umbilical arteries in foetus carry deoxygenated blood to placenta.
Structure of arteries Structure Layer Lumen Central channel Filled with blood in living. Filled with air in the dead. Intima Inner layer Connective tissue lined by endothelium. Media Middle layer Concentrically arranged smooth muscle fibres and elastic fibres. Adventitia Outer layer Primarily composed of collagen fibres. Contains vasomotor nerve fibres. Contains vasa vasorum – ‘vessels of vessels’.
Types of arteries Elastic arteries (e.g. aorta) Largest and closest to heart. Abundant elastic tissue (appear yellow) in media. Act as conducting vessels – large lumen allows them to serve as low-resistance pathways that conduct blood from the heart to medium-sized arteries. Elastic recoil prevents sudden drop in blood pressure during ventricular filling. Muscular arteries Predominantly smooth muscle in media. Form majority of named arteries. Act as distribution vessels – branch extensively and progressively reducing their calibre.
Where arteries divide into terminal branches, the larger branch tends to be more directly in line with the main trunk, with the smaller at a greater angle.
Arterial branches in trunk: parietal (to body wall) or visceral (to viscera). Arterial branches to somatic structures (in limbs and body wall): cutaneous, muscular, arteriae nervorum (to nerves), nutrient (to long bones), articular (around joints).
Arterioles Small branches that feed the capillary bed. Largest ration of wall thickness to lumen. Principles Of Blood Vessels
Calibre of lumen maintained by smooth muscle tone. Smooth muscle tone controlled by sympathetic vasomotor nerves. Erectile tissue arterioles supplied by parasympathetic nerves. Act as resistance vessels – changes in their calibre regulates blood flow and blood pressure. The greatest drop in blood pressure occurs across arterioles.
Capillaries Smallest vessels at tissue level. Thinnest walls – single layer of endothelial cells + basement membrane. Permeable to water, electrolytes, gases, cellular nutrients and wastes. Permeability varies according to sites. Permeability can dramatically increase during inflammatory response. Sites that are often subjected to great hydrostatic pressure (e.g. in limbs) less permeable → minimise fluid leaking out. Organs where capillary membrane is involved in transport of large molecules are more permeable. E.g. transport of hormones in endocrine glands. Act as exchange vessels – microcirculation of fluid movement. Water movement as a result of opposing hydrostatic pressure and osmotic pressure. Net water movement out of arterial end. Net water movement into venous end.
Sinusoids Specialised capillaries with larger calibre and more sluggish flow. Discontinuous or absent basement membrane → greater permeability. Modified endothelium – has phagocytes → scavenge particles, including old blood cells. Found particularly in organ components with haemopoietic and defence functions. Examples: liver, spleen, bone marrow. Allows newly formed red and white blood cells to enter vascular system. Allows certain white blood cells to exit vascular system.
Development of arteries Derived from mesoderm. Commence in embryo as capillary networks → preferred channels become major arterial pathways while others narrow. Narrower paths can be opened after occlusion of preferred channel. Axial artery runs along centre of limb rather than at the periphery → conserve heat. Arteries do not run superficially for long distances.
Avascular structures Structures not derived from mesoderm are avascular. Epidermis – ectoderm derived. All other surface epithelia – primarily endoderm derived. Blood vessels to CNS are extrinsically derived → blood brain barrier. Articular cartilage is mesoderm derived but subject to continuous compression → avascular.
Principles Of Blood Vessels
Arterial blood pressure Lateral pressure on vessel wall = pressure wave (from heart) + hydrostatic pressure (gravity on column of blood) Stand upright → increase length of blood column above specified level → accentuate hydrostatic component. Arterial pressure >> venous pressure. Blood flows down pressure gradient. Systolic pressure: due to propulsion of blood during ventricular contraction. Diastolic pressure: due to propulsion of blood by elastic arteries while heart refills. Pulse pressure: difference between systolic and diastolic pressures. Detected clinically by palpation of radial artery.
Arterial flow in elastic and muscular arteries is pulsatile – reflecting systole and diastole. Capillary and venous flow is continuous and low pressure.
Arterial haemorrhage Due to rupture of weakened vascular wall or external injury of vessel wall. Vessels constrict, platelet aggregation plug wall defect → minimise blood loss. Predisposing factors: platelet defects, high blood pressure – also accentuates bleeding. Blood loss from large veins can be as severe as from arteries. Blood loss → drop in blood pressure → symptoms such as fainting. Internal haemorrhage → pressure in compartment → pain. Large compartments can accumulate large dangerous volumes of blood without significant rise in local pressure and pain. Treatment: Ligation or surgical repair of damaged vessel. RICE – rest, ice, compression, elevation.
Anastomoses Links between arteries, between arterioles, or between arterioles and venules without intervening capillary bed. Adjacent branches of arteries tend to anastomose with each other. Offers alternative route – collateral circulation. True anastomoses: Links directly between branches of large calibre. Midline incision → arterial blood spurts from both sides. Examples: labial branches of facial artery, circle of Willis, palmar arches, plantar arch. Potential anastomoses: Links directly between branches of small calibre (arterioles). Have the (potential) capacity to enlarge their calibre. Skeletal muscles receive the most arterial branches and contain the majority of anastomoses. Anastomoses occur around joints but are significant only within muscle bellies that cross a joint. Main artery is kinked by joint circulation → collateral circulation. Arteriovenous (AV) anastomoses: Direct communications between small arteries and veins. Lumen diameter can be varied via neural control of smooth muscle tone. Located where there is intermittent blood flow. Occur in exposed parts – involved in thermoregulation. In skin of nose, lips, ears. Principles Of Blood Vessels
Shunts blood between superficial and deeper cutaneous vessels.
Anatomical end artery Does not form anastomoses with another artery. Occlusion → compromise entire territory of distribution. Examples: branches of cerebral arteries, arterial branches to kidney segments and liver segments, central retinal artery. Functional end artery Take part in potential anastomoses but not true anastomoses. Gradual occlusion → time for collaterals to dilate. Sudden occlusion → part of territory of distribution becomes unviable. Examples: right and left coronary arteries anastomose with each other at arteriolar level.
End organ Isolated body part or organ that tends to be supplied by single avenue of arterial supply → particularly vulnerable to having its arterial supply cut off. End tissues: area furthest from arterial source; tends to be supplied by terminal arterial branches → end tissue of end organs most vulnerable to interruption of arterial supply. Beware of injecting vasoconstrictor into terminal body parts – digits, penis. Blind ending organs project into or are suspended within a cavity. Examples: appendix, gall bladder, spleen, brain, spinal cord, heart, retina (‘end organ’s end organ’). Cartilaginous epiphyseal plate in growing long bone is avascular → forms barrier preventing communication between epiphyseal and metaphysical arteries – end arteries. End arteries supply solid viscera that divide into separate vascular segments. Examples: kidney, liver, lungs.
Terms Ischaemia: Decreased blood supply. Infarction: Tissue death from complete loss of blood supply. Thrombus (‘clot’): Blood clot formed in vascular system of living system. Not the same as post-mortem clot or haematoma (outside vascular system). Embolus (‘plug’): A substance transmitted by the blood stream that lodges in a vessel. Thromboembolus: Thrombus or part of thrombus that dislodges and is transmitted by blood stream to a distant site.
Arterial occlusion Internal – within lumen By thrombus or embolus. Intramural – within wall. Thickening of intima by atherosclerosis or spasm of smooth muscle in media. External (extramural) By compression or ligation.
Principles Of Blood Vessels
Arterial emboli An embolus within an artery tends to lodge immediately distal to a branch point, where the main artery narrows. Pulmonary embolus Originate from systemic veins (e.g. deep veins in calf) → right atrium → right ventricle → pulmonary artery. May also originate in right side of heart. Symptoms range from none to sudden death. Life threatening if large → occlude major or multiple branches or pulmonary artery. Area of pulmonary infarction → causes breathlessness and chest pain. Continued arterial supply unable to keep lung tissue alive → bleeds → haemorrhagic infarct → appears red. Systemic arterial embolus Originate from left side of heart or associated valves. May also arise from systemic arteries. Potentially more harmful if involves end arteries.
Nerve supply of blood vessels Vasomotor nerves: Motor nerves that supply blood vessels. Sympathetic nerve fibres to all arterioles. Arterioles of erectile tissue also receive parasympathetic fibres.
Vascular supply of vessels Endothelium and intima receive nutrition directly by diffusion from blood in lumen. Vasa vasorum supply media and adventitia. Include arteriae vasorum and venae vasorum.
Venous system
Venous system composed of 3 separate major systems. Pulmonary venous system Drains right and left pulmonary veins into left atrium. Systemic venous system Has 4 subsystems – superior vena caval, inferior vena caval, azygos, vertebral. Tributaries of SVC and IVC drain deoxygenated blood into right atrium. Azygos (‘unpaired’) system Drains the vertebral system into SVC. Drains posterior abdominal and thoracic wall. Partial bypass between SVC and IVC → equilibrate pressure between them. Vertebral veins – internal and external vertebral venous plexuses. Where blood cells produced in vertebral column enter circulation. Portal venous system – veins that link two capillary beds. Hepatic veins drain into IVC.
Principles Of Blood Vessels
Portal venous system: Low pressure. Hepatic portal venous system – drains GI tract, pancreas and spleen. Portal vein enters liver at porta hepatis. Portal venous blood: Deoxygenated. Transports absorbed nutrients from small intestine and hormones secreted from pancreas to liver. Veins branch and terminate in another capillary bed – hepatic sinusoids → drained by tributaries of hepatic vein (belonging to IVC system). Hypophyseal portal venous system – tiny veins linking capillary beds between hypothalamus and hypophysis (pituitary gland). Transport special hormones to regulate anterior pituitary hormone production.
Veins Vascular tubes that carry blood away from tissues → progressively larger tributaries → towards heart. Systemic venous blood typically deoxygenated. Pulmonary venous blood oxygenated. Umbilical vein in foetus carries oxygenated blood from placenta to inferior vena cava → mix with deoxygenated blood in heart → exits via aorta. Bypasses liver and lungs.
Structure of veins Lower pressure and greater volume → thinner walls and larger lumens than arteries. Act as capacitance vessels – contain most of blood volume. Venous tributaries correspond to arterial branches. More numerous, many unnamed. Venules – correspond to arterioles. Structure Layer Lumen Central channel Filled with blood in living. Filled with blood clot in the dead. Intima Inner layer Lined by endothelium. Media Middle layer Contains layer of connective tissue with elastic fibres. More fibrous and less elastic than in arteries. Contains less smooth muscle than in arteries. Adventitia Outer layer Primarily composed of collagen fibres.
Venous valves: Folds of endothelium lining veins, typically with a pair of cusps. Direct venous return towards heart. Prevent pooling of blood distally in limbs. A valve is typically located at the termination of a vein. Valves are particularly numerous in long veins → breakup into shorter units. Valves present in perforating veins (connect superficial and deep systems) → direct flow from superficial to deep. Most important in calves. Principles Of Blood Vessels
The veins of the vena caval systems traversing body cavities of the trunk, together with the entire vertebral and azygos systems of veins, are valveless. Valveless communicating veins – flow may occur in either direction between vertebral and caval systems.
Venae comitantes: Pair of companion veins wrapped around artery. Primarily located in limbs, particularly distally. Veins can intercommunicate. Heat transfer from warm arterial blood to cool venous blood returning from periphery → conserve heat. Arrangement assists venous return.
Venous flow. Vascular venous pump Aids venous flow in periphery. Vascular bundle resists expansion associated with arterial pulsation → compresses blood within venae comitantes. Valves direct flow proximally. Muscular venous pump Main factor responsible for flow from peripheral veins. Contraction of skeletal muscle belly → compresses deep veins within compartment. Thoracic venous pump Venous return in trunk is via double pump mechanism coupled to respiration. Inspiration → diaphragm descends → negative intrathoracic pressure, shortens IVC, lengthen SVC → empties IVC, fills SVC. Converse occurs during expiration. Venous return from both caval systems – from below in inspiration, from above in expiration.
Varicose vein – abnormal dilation of a vein, which may become elongated and tortuous. More prominent with prolonged elevation of venous pressure. Portal hypertension → oesophageal varices under surface of lower oesophagus → bulge into lumen → may rupture → catastrophic bleeding. Chronic straining → haemorrhoids – dilated veins under surface lining of anal canal → bulge into lumen. May bleed, prolapse beyond external anal sphincter, or thrombose.
Venous valve incompetence → reflux of blood. Structural damage to valve cusps or dilation of vein at site of valve. Incompetent valves of perforating veins in leg – muscular pump shunts blood back under pressure from deep veins → blood pools in superficial veins. Circulation to skin impaired by high venous blood pressure → skin ulcerates and heals poorly. Domino effect of dilation – varicose veins may be both a cause and effect of valve incompetence.
Perforating veins: Venous communications between superficial and deep veins in lower limbs, perforating deep fascia. Presence of valves → direct flow from superficial to deep. Principles Of Blood Vessels
Dural venous sinuses: Endothelial-lined spaces within cranial cavity, between outer and inner layers of dura mater. Enclosed by dense connective tissue – tension in fibrous walls prevent collapse despite negative intracranial pressure. Drain blood from brain. Superior sagittal sinus drains CSF from subarachnoid space via arachnoid granulations. Emissary veins – communications between intracranial and extracranial veins.
Portal-systemic anastomoses Portal venous system is valveless → flow in either direction. Major sites – lower end of oesophagus and anal canal.
Venous plexuses – intercommunicating networks of tributaries of veins. Found around pelvic viscera → accommodate to changes in shape of organs. Extensive in sole of foot and within vertebral canal → act as cushion.
Deep vein thrombosis Deep veins of calf predisposed to thrombosis if surrounding muscles not contracting regularly. Stasis and pooling of blood in soleal venous sinuses. DVT in calf can cause pulmonary thromboembolism – occlusion of pulmonary artery or its major branch in the lung by thromboembolus. Life threatening. Thromboemboli → IVC → right side of heart → pulmonary arterial system. Thromboemboli more common in veins than in arteries due to sluggish flow. Usually small and filtered by lungs without damage. Venous blood passes through pulmonary capillary bed before reaching systemic arteries → organs supplied by systemic arteries protected.