#13343 - Histology Of Blood Vessels - Organisation of the Body

Circulatory system

Overview

-there are two separate but connected components: cardiovascular system, transports blood around the body lymphatic vascular system , collects excess extracellular fluid and returns it to the cardiovascular system

-cardiovascular system: made of a heart which pumps blood and blood vessels which transport it around the body.

-Heart pumps blood at high pressure- transmitted through large elastic arteries that carry blood away from the heart- smaller muscular arteries- thin walled- capillary beds- drained by venules

-the cardiovascular system is divided into the pulmonary circulation- deoxgynated blood pumped from the right side of the heart to the lungs where it is oxygenated and returned to the left side of the heart. This circulation is low resistance and low pressure Systemic circulation: oxygenated blood pumped by the left side of the heart and returns to the right after completing a circuit- high resitance and high arterial pressure

-portal system: Blood is carried from one site to another without directly involving the heart: hepatic portal system, hypothalamic pituitary portal system

Blood vessels

Functions:

-O2, co2 delivery

-Nutrient delivery

-Waste removal

-Delivery of hormonal signals

-control blood flow

Structure of vessels

-all arteries are composed on three concentric layers

i) Tunica intima:

-continuous sheet of flattened simple squamous endothelial cells that lines the lumen – provides a smooth surface for flow

-Supporting this is sub endothelial connective tissue- loose connective tissue

-lined with internal elastic lamina (thin band of elastic fibres)

d) initiation of coagulation: when endothelial cells are damaged they release von Willebrand factor-these attach to the underlying collagen. Glycoprotein 1b receptors on platelet membrane bind to vWf, other glycoprotein receptors bind to collagen. This leads to platelet adhesion, stimulation of the glycoprotein receptors triggers the platelet activation pathway- coagulation plug

Von willebrand’s disease: genetic disorder- individual is inacapable of producing a normal amount of Von Willebrand’s factor, problems with blood clotting- bruising easily, longer bleeding time, excessive bleeding from tooth extraction/ excessive menstrual bleeding and bloody mucous membranes

e) angiogenesis: occurs in response to repair damaged vessels, or form new vessels in repairing injuries or subsequent to menstruation. New vessels arise from existing vessels due to interaction of signalling molecules angiopoietins 1 + 2 with specific receptors on endothelial cells that induce mitotic activity in pre existing endothelial cells and recruit smooth muscle cells to form the tunica media

f) synthesis of growth factors: colony stimulating factors- these induce cells of the blood lineage to undergo mitosis and produce various blood cells, growth inhibitors

oxidation of lipoproteins: endothelial cells also oxidise high cholesterol containing low density lipoproteins so that the oxidised by product can be phagocytosed by macrophages

ii)Tunica media:

thickest of three layers, circularly arranged smooth muscle cells, replaced by elastic in elastic arteries

-interspersed within the layers are elastic fibres and type 3 collagen. The collagen provides resistance and if not made properly leads to aneurysm. Ehlers Danlos syndrome 4: autosomal dominant defect in type 3 collagen- blood vessels are fragile and prone to rupture

-external elastic lamina

iii) Tunica adventitia: outermost layer of the vessel wall- fibroelastic connective tissue arranged longitudinally with adipose tissue. There is mostly fibroblasts, type 1 collagen fibres, elastic fibres, adipose tissues.

Larger vessels there is also the vaso vasorum; small blood vessels that supply the tunica adventitia and the tunica media -prevalent in veins as venous blood contains less oxygen and nutrients

ARTERIES

-transport blood away from the heart and the relative proportions of the three layers varies in different types of arteries:

-Can be divided into three categories according to size and morphological characteristics

a) Elastic arteries (conducting arteries)

E.g major distribution vessels: Aorta, brachiocephalic trunk, common carotid, subclavian arteries, most of the large pulmonary arteries- largest of arteries, arises directly from the heart – have to withstand pressure

Structure: walls have a thick layer of elastic tissue that occupy most of the tunica media, walls are yellow due to abundance of elastin- the sheets of elastin are separated by smooth muscle fibres, collagen

-Elastin-insoluable protein polymer, synthesised from tropoelastin-linear polypeptide, made of small non polar amino acids, also contains proline and lysine. Tropoelastin is secreted into the extracellular space and interacts with glycoprotein microfibrils-fibrillin-this functions as a scaffold onto which tropoelastin is deposided, cross links form between the tropoelastin-elastin-interconnected rubbery network that can stretch

Function: When blod expelled from the heart during systole walls need to withstand pressure- elastic fibres allow distension of arteries, it also stabalises blood flow and maintains arterial pressure during diastole- blood flow is highly pulsatile

Pathology: a) Ageing: loss of elasticity- increases peripheral resistnace- increase in arterial blood pressure-heart failure due to increased afterload b) Marfans : genetic disease, misfolding of fibrillin 1 protein- fibrillin is key glycoprotein that forms the elastin microfibril that forms the scaffold which organises deposition of protein elastin. Elastin is formed by the crosslinking soluable tropoelastin monomers-dilated aorta/aortic aneurysm

-further from the heart the amount of elastin decreases and media becomes more muscular

b)Muscular arteries

-make up most of the named arteries – E.g Femur, carotid arteries

Structure: Thick Tunica media is made up mostly of many layers of smooth muscle cells- some have 3/4 layers of smooth have 40. There are very few elastic fibres between the concentrically arranged smooth muscle fibres. Tunica intima is thinner than in elastic arteries. Elastic tissue concentrated as 2 defined elastic sheets, internal elastic lamina (Very prominent)- between tunica intima and tunica media and external elastic lamina (media, adventia

Function: maintains pressure, allows selective distribution of blood to different organs

Atheroscelerosis: chronic disease, thickening, hardening, loss of elasticity and occlusion in arterial walls- deposition of fatty material within walls of large and medium sized arteries

-Initiates with endothelial damage, LDL cholesterol invades endothelium and becomes oxidised- this leads to macrophage invasion which take up modified LDL cholesterol- accumulations of cholesterol, engorged macrophages called foam cells

-WBC stimulate proliferation of fibroblasts- increased collagen deposition forms a plaque- thickens the tunica intima

-the roughened lining of the lumen- accumulation of platelets and fibrinin- thrombus-Narrowing, clot formation, weakness in vessel wall

-severe atheroma-muscle cells in tunica media are replaced by non-contractile, non elastic collagen- aneurysm

-it is caused by hypertension, ageing, diabetes, smoking

-If this condition involves coronary arteries, decreased blood flow to the myocardium causes coronary heart disease

-chest pain/ shortness of breath/ sweating/ palpitations/ weakness/ fainting spells/ irregular heart beat

Stoke: Ischemic stroke- blood vessel supplying the brain becomes blocked or hemorrhagic stroke- blood vessel bursts, lack of blood flow causes anoxia of the affected region- death of neurons and results in weakness, paralysis, sensory loss

Aneurysm: damaged vessel may become weakened and begin to enlarge and form a bulging defect- aneurysm. Left untreated it may rupture without warning- internal bleeding with fatal consequences

Arterioles

-smallest arteries

Structure: Diameter of less than 0.1mm , width of the wall is equal to the diameter, just one or two smooth muscle layers

Function: these are the principal resistance vessels, so are responsible for controlling local blood flow through changing their radius- tonically constricted by the sympathetic vasoconstrictor nerves, but factors released from endothelial cells/paracrine agents from the local tissue change their radius- control arterial blood pressure

Function of the endothelial cells: Regulates vascular tone

vasoconstriction: endothelial cells synthesise and secrete endothelin 1

vasodilation: parasympathetic nerve fibres act on endothelial cells, release of NO, prostacyclin- induce the cyclic AMP in smooth muscle cells- relaxation ,

-controls distribution of blood to whole capillary beds: Incomplete rings of smooth muscle cells precapillary sphincters, encircle the origins of the capillaries- contraction of precapillary sphincters of the arteriole shunts blood into capillaries= also controlled by sympathetic autonomoic nerves

CAPILLARIES

Structure: thin walled networks that are supplied by arterioles and drained by venules,

-single layer of endothelial cells- narrow vascular channels- 8-10 micrometers diameter, less than 1 micrometer long

-associated with capillaries are basal laminae and pericytes – these support the vessels but can differentiate into fibroblasts, smooth muscle cells or macrophages. Pericytes play an important role in angiogenesis

Control of blood flow:

-No tunica media, so no smooth muscle cells, so no vasomotor activity – control of blood flow through the capillaries achieved by...