Phase 3 - Week 2 (Blood vessels, Blood Pressure, Postural Hypotension) Flashcards
List the types of blood vessels
- Arteries
- Arterioles
- Capillaries
- Venules
- Veins
List the layers of all blood vessels
From outside -> lumen
- Tunica adventitia
- Tunica media
- Tunica intima
Describe the structure of the tunica adventitia
- Outer layer
- Loose, thick layer of connective tissue
- Consists of elastic + collagen fibres
- Network of tiny nerves, lymphatics + capillaries that supply the vessel wall
What is the function of the tunica adventitia?
- Anchors vessel wall to surrounding structure
- Gives some protection
Tunica media
- Middle, muscular + connective tissue layer
- Consists mainly of muscle cells + elastic fibres
- External elastic lamina - helps artery recoil after it has stretched due to increase in BP during each heartbeat
- Smooth muscle - arranged concentrically around lumen, adjust vessel diameter through contraction (vasoconstriction) and relaxation (vasodilation), regulates blood pressure + blood flow
Tunica Intima
- Innermost epithelial lining, forms perimeter of lumen
- 4 components - internal elastic lamina, lamina propria, basement membrane, endothelium
- Composition of layers depends on vessel size and position in circulatory system
Internal elastic lamina of BVs
Thin sheet of elastic fibres to help vessel recoil after it has been stretched by increased BP. Contains window-like openings that facilitate diffusion between tunica intima and tunica media
Lamina propria of BVs
Outer layer of elastic connective tissue that contains capillaries
Basement membrane of BVs
Framework of collagen fibres deep to endothelium. High tensile strength provides firm supportive base + anchorage for the endothelial lining to lamina propria, while retaining elasticity enough to ensure vessel’s ability to stretch + recoil. Regulates molecular movement, important role in tissue repair of BV walls.
Endothelium of BVs
Layer of simple squamous epithelial cells in direct contact with blood. Permeable to certain materials, regulates diffusion of substances, prevents cells sticking to its walls, contracts to prevent blood flow.
Describe the function of arteries
- Carry oxygenated blood away from heart to organs
- Blood is under high pressure so have strong, muscular walls to cope with surge of blood
List the types of arteries
- Elastic arteries
- Muscular arteries
- Arterioles
Elastic arteries
Aka conducting arteries - conduct blood from heart into muscular arteries
Describe the general structure of arteries
- Have all three layers of a typical BV
- Tunica media exhibits greater muscular + elastic thickness than veins, enabling walls to stretch easily with small increase in BP
Describe the structure of elastic arteries
- Well defined internal + external elastic laminae
- Tunica media is thick and full of elastic fibres, enabling walls to stretch easily with increase in BP, as seen during systole
Give examples of elastic arteries
- Aorta
- Subclavian
- Common iliac
- Common carotid
Describe the function of elastic arteries
- Propel blood from heart during ventricular diastole
- Elastic properties are essential to accommodate the volume of blood created when blood is expelled from the heart
- Elastic fibres convert mechanical -> kinetic energy as they recoil, force blood away from the heart
Muscular arteries
- Aka distributing arteries, repeatedly branch until reaching target organs
- Less elastic than conducting arteries, don’t have to deal with the same degree of pressure changes
Describe the structure of muscular arteries
- Well-defined internal but thin external elastic laminae
- Thick tunica media, concentrically arranged layers smooth muscle cells
- Tunica adventitia often thicker than tunica media - longitudinally oriented fibroblasts, collagen fibres + elastic fibres. Loose arrangement of cells enables arteries to alter diameter
Give examples of muscular arteries
- Renal
- Splenic
- Internal carotid
- Femoral
- Popliteal
- Axillary
- Radial
- Ulnar
- Smaller brachial arteries
Describe the function of muscular arteries
- Lack of elastic fibres in walls means that recoil doesn’t propel blood
- Maintain a state of partial contraction or vascular tone - ensuring that vessel pressure and efficient blood flow are sustained + enable efficient adjustment of rate of blood flow by vasoconstriction + vasodilation
Arterioles
- Aka resistance vessels
- Numerous, microscopic arteries that feed blood into capillary networks
Describe the structure of arterioles
- Tunica intima is thin with fenestrated (porous) internal elastic lamina that diminishes as the arteriole tapers towards its terminal end and continues as a capillary - region referred to as the metarteriole
- Muscular tunica media made of 1-2 concentrically arranged layers of smooth muscle cells
- Smooth muscle cell forming the precapillary sphincter demarcates, controls flow of blood between metarteriole and its adjoining capillary
- Tunica adventitia contains unmyelinated sympathetic nerves + loose connective tissue
Describe the function of arterioles
Local chemical mediators and sympathetic nerve supply of arteriole triggers vasoconstriction + vasodilation, regulating rate of blood flow, BP and vascular resistance
Capillaries
- Short, branched, interconnecting vessels that form networks in every structure of the body
- Bridge gap between arterioles and venules
- Microcirculation - blood flows from a metarteriole, through capillaries into a post-capillary venule
- Lack tunica media and tunica adventitia
- Thin, one-layer epithelial structure and huge surface area allows rapid exchange of small molecules e.g. glucose, and diffusion of gases e.g. oxygen/carbon dioxide between blood and cellular interstitial fluid
- Aka exchange vessels
What is the function of capillaries?
Provide large surface area in contact with tissues throughout the body for exchange of nutrients/waste products
Describe the function of veins
Carry blood back to the heart from extremities and organs of the body. More numerous than arteries, often appear as pairs.
Describe the structure of veins
- Generally have very thin walls in relation to their diameter, with large lumen
- Form vascular/venous sinuses in certain regions - widened areas which are structurally different to regular veins
- Thin endothelial walls, with complete lack of smooth muscle - reliant on surrounding connective tissue for support
Where are veins located?
- Superficial veins spread throughout subcutaneous layer, deep to skin and deep veins run between skeletal muscles
- Connections between deep and superficial called anastomotic veins
Venules
- Like arterioles - numerous + microscopic, walls are much thinner
- Post-capillary venules are continuous with capillary networks - small diameter, gaps between endothelial cells in walls
- As venules continue further from capillary diameter increases, become more muscular with more layers of concentrically arranged smooth muscle cells
- Function = drain blood from capillary networks and feed the return flow of blood to the veins. Post-capillary venules function as an exchange unit, larger thicker walled venules don’t
Medium veins
- Have all 3 layers of typical BV - tunica adventitia is thickest layer, consisting of collagen and elastic fibres, tunica media, thin and lacking in smooth muscle, elastic fibres and tunica intima
- Do not have internal or elastic laminae, not able to withstand high pressure
- BP in veins is relatively low, rely on contraction of surrounding skeletal muscle and pumping action of heart to boost venous return
- Many contain valves, small extensions/infoldings of tunica intima into the lumen, creating flap-like cusps
Give examples of medium veins
- Renal
- Internal carotid
- Ulnar
- Splenic
- Popliteal
Describe the structure of large veins
Thick tunica adventitia, similar to that of medium veins, but lack valves
Give examples of large veins
- Superior vena cava
- Internal jugular veins
- Common iliac veins
Describe the function of large veins
Drain from their tributaries into the heart
List the types of capillaries
- Continuous capillaries
- Fenestrated capillaries
- Sinusoids
Continuous capillaries
- Majority of capillaries
- E.g. in brain, lungs, smooth muscle, skeletal muscle and connective tissue
- Walls made of continuous layer of endothelial cels with small gaps between (intercellular clefts)
- Have a complete outer basement membrane
Fenestrated capillaries
- In kidneys, villi of small intestine, choroid plexus of brain and endocrine glands
- Walls are fenestrated (porous) - plasma membranes of endothelial cells in walls covered in small holes (fenestrations)
- Have a complete outer basement membrane
Sinusoids
- In bone marrow, liver, spleen, anterior pituitary gland, parathyroid gland and lymph nodes
- Wide and meandering, large fenestrations in endothelium
- Have large intercellular clefts + incomplete outer basement membrane
- Allow larger structures e.g. complex proteins and blood cells to pass from tissue -> bloodstream
- Specialised lining cells extend across lumen - function differs depends on tissue e.g. in liver they are phagocytic, removing bacteria/foreign bodies from blood
Vasomotion
Process by which blood passes from arterioles -> capillaries or capillaries -> venules. Intermittent contraction and relaxation of pre-capillary sphincters that pushes blood from a metarteriole to a capillary. Controlled by chemicals released by endothelial cells.
How is blood hydrostatic pressure generated?
Pumping action of heart as ventricles contract, results in pressure exerted by blood on walls of vessels
List the physiological factors which effect blood pressure
- Cardiac output
- Blood volume
- Vascular resistance
When/where is blood pressure highest?
In the aorta during ventricular systole
When/where is blood pressure lowest?
During ventricular diastole in the right ventricle (reaches 0 mmHg)
Normal blood pressure
120/80 mmHg
Mean arterial pressure
Average pressure of blood in all arteries
How is blood pressure measured?
Using a sphygmomanometer, in mmHg
How is MAP calculated?
MAP = cardiac output x resistance
What factors effect the MAP?
- Cardiac output increases, MAP increases (if resistance is constant)
- Stroke volume/heart rate increases, MAP increases (if resistance is constant)
- If total blood volume falls >10%, MAP decreases
List the factors effecting vascular resistance
- Lumen diameter
- Blood viscosity
- Blood vessel length
Explain how lumen diameter effects vascular resistance
- Smaller lumen diameter greater vascular resistance, higher BP
- Vasoconstriction increases resistance + therefore BP
- Vasodilation decreases resistance + therefore BP
Explain how blood viscosity effects vascular resistance
- Determined by number of RBCs per volume blood plasma + concentration of circulating plasma proteins
- Higher blood viscosity, greater resistance + BP
- Decrease in water intake (dehydration), increased blood viscosity, increased BP
- Increase in RBC, increased blood viscosity, increased BP
- Increase in plasma proteins per volume of plasma, increased blood viscosity, increased BP
Explain how blood vessel length effects vascular resistance
Longer blood vessels, greater resistance, greater BP
Systemic vascular resistance
Aka total peripheral resistance, combined effect of the vascular resistance of all systemic vessels
List the ways in which blood pressure is regulated
- Autonomic nervous control
- Hormonal control
- Mediated by local physical/chemical changes
Describe the autonomic control of the heart
- Cardiovascular centre in medulla
- Receives input from sensory receptors/higher brain centres
- Acts by increased or decreasing sympathetic (stimulatory) and parasympathetic (inhibitory) innervation of the heart via the cardiac accelerator nerves (sympathetic) and vagus nerve (parasympathetic)
- Regulates heart rate and contractility of ventricles
Vasomotor centre
- Part of medulla
- Has vasoconstrictor and vasodilator centres
- Controls blood vessel diameter by causing constriction or dilation
Describe the autonomic reflex which controls blood pressure
1 .Sensory receptors - specialised sensory nerve endings located throughout the body that detect changes in the state of the system and send afferent nerves to the brain
- Cardiovascular centre in medulla of brain processes information, integrated with information from other sensory receptors
- Reflex occurs as brain alters activity of efferent sympathetic adrenergic + parasympathetic cholinergic nerves controlling cardiovascular function - elicit a positive or negative response, reversing the change in state
List the 3 main reflexes which control blood pressure
- Baroreceptor reflex
- Proprioreceptor reflex
- Chemoreceptor reflex
Baroreceptors
Mechanoreceptors in walls of carotid sinus and aortic arch. Sensory (afferent) nerve endings which detect changes in wall stretch from changes in arterial BP.
Baroreceptor reflex
- Fall in arterial BP, decrease in frequency of impulses from baroreceptors to cardiovascular centre through afferent fibres in glossopharyngeal and vagus nerves
- Cardiovascular centre reduces parasympathetic vagal stimulation to SAN and increases sympathetic stimulation through the cardiac accelerator nerves
- Result is that auto-rhythmic cells increase spontaneous depolarisation, heart rate is raised for BP increases
- Negative feedback mechanism - normal wall stretch leads to normal frequency of baroreceptor impulses so reduction in stimulus from cardiovascular centre
Proprioreceptors
Monitor position of limbs, detect changes in joint angles and muscle length/tension at onset of exercise
Proprioreceptor reflex
- Proprioreceptor detects change which indicates start of exercise
- Info sent to brain through afferent nerves
- Cardiovascular centre stimulates increase in sympathetic stimulation to heart
- Triggers increase in heart rate, so BP increases
Chemoreceptors
- Sensory receptors close to baroreceptors of carotid sinus and aortic arch, bundled into small structures called carotid + aortic bodies
- Monitor chemical changes in blood, stimulated by changes in partial pressures of oxygen + carbon dioxide and hydrogen ion concentration
Chemoreceptor reflex
- When change in chemical composition of blood occurs, increase in frequency of impulses from chemoreceptors via afferent nerves to cardiovascular centre in brain
- In response to change cardiovascular centre increases sympathetic stimulation to arterioles, veins and heart
- Vasoconstriction of arterioles mainly in skeletal muscle + rise in heart rate/arterial blood pressure reverses change in blood composition
- Also alters breathing, further regulating blood composition
Describe the renin-angiotensin-aldosterone system
- Fall in blood volume/decreased blood flow/reduction in afferent arteriole pressure
- Juxtaglomerular cells of kidneys secrete enzyme renin into bloodstream
- Renin acts on circulating angiotensin, undergoes proteolytic cleavage to form angiotensin I
- Vascular endothelium (esp. in lungs) has angiotensin converting enzyme (ACE) that converts angiotensin I to angiotensin II
List the functions of angiotensin II
- Constricts resistance vessels increasing systemic vascular resistance and arterial pressure, increasing BP
- Stimulates sodium reabsorption at renal tubular sites, increasing sodium and water retention by the body, increasing blood volume, increasing BP
- Acts on renal cortex to release aldosterone, which acts on kidneys to increase sodium and water retention, increasing BP
- Stimulates release of ADH from posterior pituitary which increases fluid retention in kidneys, increasing BP
- Stimulates thirst centres in brain
- Facilitates noradrenaline release from sympathetic nerve endings + inhibits noradrenaline re-uptake by nerve endings - enhances sympathetic adrenergic function
- Stimulates cardiac/vascular hypertrophy
List the hormones/hormonal pathways which control blood pressure
- Renin-angiotensin-aldosterone system
- Adrenaline and noradrenaline
- Anti-diuretic hormone (ADH)
- Atrial natriuretic peptide (ANP)
- Erythropoietin
Describe the role of adrenaline and noradrenaline in regulating blood pressure
- During stress, in response to sympathetic stimulation, adrenal medulla releases adrenaline and noradrenaline into bloodstrean
- Both increase heart rate and contractility, increasing BP
- Forms negative feedback loop
- Also cause generalised vasoconstriction but vasodilation in skeletal muscle
Describe the role of ADH in regulating blood pressure
- Peptide hormone produced in hypothalamus, transported to posterior pituitary which secretes it into blood
- Two sites of action - kidneys and BVs
- Primary function = regulate extracellular fluid volume by regulating renal handling of water. Acts on renal collecting ducts via V2 receptors to increase water permeability (cAMP-dependent mechanism) which leads to decreased urine formation - increases blood volume, cardiac output + arterial pressure
- Secondary function = vasoconstriction
- Cardiopulmonary baroreceptors, osmoreceptors + angiotensin II receptors regulate the release of ADH through afferent nerve firings to brain
Describe the role of ANP in regulating blood pressure
- Released from atrial cells
- Causes vasodilation, decreases BP
- Reduces total blood volume by promoting loss of salt and water in the urine
Describe the role of erythropoietin in regulating blood pressure
Released in response to low oxygen levels in blood, stimulates bone marrow to produce RBC, increases blood viscosity, increases BP
Define postural/orthostatic hypotension
Low BP when standing from sitting/lying down, occurs when mechanisms for regulation of orthostatic BP control fails. Fall in systolic pressure of at least 20mmHg or fall of diastolic pressure of at least 10mmHg within 3 mins of standing
How is orthostatic pressure normally regulated?
Baroreflexes, normal blood volume and defences against excessive venous pooling
Orthostatic intolerance
Development of symptoms e.g. lightheadedness and blurred vision when standing. Symptoms due to cerebral hypoperfusion.
How is postural normotension usually maintained?
- Normally able to maintain stable BP when supine and standing
- Depends on normal plasma volume, intact baroreflexes and reasonable venomotor tone
- Normally - standing results in fall in blood and pulse pressure which is sensed by baroreceptors in carotid sinus and aortic arch
- Causes increase in adrenergic pathway - increases heart rate to correct initial fall in BP and total systemic resistance
List the causes of postural hypotension
- Hypovolaemia (reduced blood volume) - loss of blood, loss of plasma (burns etc.), loss of body sodium and consequent intravascular water
- Excessive venous pooling - due to impaired venous tone
- Splanchnic- mesenteric bed - volume increases after meal, increased venous capacitance causes venous pooling
- Neurological failure - defect in baroreceptor/adrenergic pathway - Guillain-Barre syndrome, acute autoimmune autonomic neuropathy, Parkinson’s disease
- Heart problems e.g. bradycardia, valve problems, MI
- Endocrine problems e.g. diabetes, thyroid conditions
List the risk factors associated with postural hypotension
- Old age - common in those >65
- Medications - to treat hypertension, Parkinson’s, antidepressants, antipsychotics, muscle relaxants, medications to treat erectile dysfunction + narcotics
- Certain diseases - heart conditions, nervous disorders, diabetes
- Heat exposure - dehydration
- Bed rest
- Pregnancy - circulatory system rapidly expands, blood pressure likely to drop
- Alcohol
List the symptoms of postural hypotension
- Lightheadedness/dizziness after standing - presyncope
- Blurry vision
- Weakness
- Fainting (syncope)
- Confusion
- Nausea
List the complications associated with postural hypotension
- Falls
- Stroke
- Cardiovascular disease
Describe how postural hypotension is diagnosed
- Measure BP with person supine/seated
- Measure BP standing for at least 1 minute prior to measurement
- If systolic BP falls by 20mmHg or more when standing - review medication, measure subsequent BP with person standing, consider referral to specialist care if symptoms persist
Define hypertension
Persistently high arterial blood pressure (140-200 mmHg systolic, 90-110 mmHg systolic)
Describe the epidemiology of hypertension
- One of the most important preventable causes of premature morbidity/mortality in the UK
- Very common in the UK, prevalence strongly influenced by age
- 1/4 of all adults
- 1/2 of those >60
List the types of hypertension
Essential/idiopathic/primary hypertension = no known cause
Secondary hypertension = resulting from primary disease (e.g. renal, vascular, neural)
Describe the causes of primary essential hypertension
Multifactoral -
- Polygenic - genetic
- Environmental - dietary, social influences
- Structural factors - vascular remodelling
Describe the causes of secondary hypertension
- Coarctation of the aorta
- Pheochromocytoma
- Hyperaldosteronism
- Cushing’s syndrome
- Diabetic nephropathy
- Polycystic kidney disease
- Renovascular disease
- Thyroid problems
- Hyperparathyroidism
- Sleep apnea
- Obesity
- Pregnancy
- Medications + supplements
List the symptoms of hypertension
- Headache
- Dysponea
- BP >140/90
- Retinopathy
- Chest pains
- Sensory or motor problems
- Often asymptomatic
- Signs of end organ damage if untreated + severe
List the risk factors associated with hypertension
- Obesity
- High alcohol intake
- Metabolic syndrome
- Diabetes
- Dyslipidaemia
- High sodium intake
- Sleep apnoea
- Old age
- Family history - genetic
- Smoking
List the classifications of hypertension
Stage 1 = BP 140/90mmHg in clinic, subsequent ambulatory average/home BP 135/85mmHg or higher
Stage 2 = BP 160/100mmHg clinic, subsequent daytime average/home Bp 150/90mmHg or higher
Severe hypertension = 180mmHg systolic BP clinic or higher or clinic diastolic 110mmHg or higher
List the effects of untreated hypertension
- Damage to arteries
- Damage to heart
- Damage to brain
- Damage to kidneys
- Damage to eyes
- Sexual dysfunction
- Other
Describe the damage to arteries caused by hypertension
- Hypertension can damage endothelial cells of arteries - allows from atherosclerotic plaques to form - limits blood flow, less elastic
- Risk of aneurysm - constant pressure on weakened artery
Describe the damage to the heart caused by hypertension
- Coronary artery disease - angina, MI or arrhythmias
- Left ventricular hypertrophy - increased risk of MI, heart failure + sudden cardiac death
- Heart failure - strain due to hypertension can weaken cardiac muscle, heart failure
Describe the damage to the brain caused by hypertension
- Transient ischemic attack - brief, temporary disruption of blood supply to brain. Often caused by atherosclerosis/thrombus which can arise from hypertension
- Stroke - part of brain is deprived of oxygen/nutrient causing cell death in brain
- Vascular dementia
- Mild cognitive impairment
Describe the damage to the kidneys caused by hypertension
- Kidney failure - damage to large arteries that supply kidneys + glomeruli, dangerous levels of fluid and waste can accumulate
- Glomeruloscelorsis - scarring of glomeruli, can lead to kidney failure
- Kidney artery aneurysm
Describe the damage to the eyes caused by hypertension
- Retinopathy
- Choroidpathy - fluid build-up under retina
- Optic neuropathy
Describe how hypertension can cause sexual dysfunction
- Reduced blood flow to penis due to atherosclerosis caused by hypertension - inability to achieve and maintain erection
- Reduced blood flow to vagina due to atherosclerosis caused by hypertension - decrease in sexual desire/arousal, vaginal dryness
Describe other complications of hypertension
- Osteoporosis - increased calcium elimination leading to loss in bone density
- Obstructive sleep apnoea - may be triggered by hypertension
List the lifestyle interventions introduced to treat hypertension
- Healthy diet
- Regular exercise
- Relaxation therapies
- Reduce alcohol intake
- Discourage excessive consumption of caffeine-rich products
- Reduce dietary sodium intake
- Stop smoking
List the types of antihypertensive drugs
- Vasodilator antihypertensive drugs
- Centrally acting antihypertensive drugs
- Adrenergic neurone blocking drugs
- Alpha-adrenoceptor blocking drugs
- Diuretics
- Beta blockers
- Angiotensin-converting enzyme (ACE) inhibitors
- Calcium channel blockers
- Renin inhibitors
Give examples of vasodilator antihypertensive drugs
- Hydralazine hydrochloride
- Sodium nitroprusside
- Minoxidil
Give examples of centrally acting antihypertensive drugs
- Methyldopa
- Clonidine hydrochloride
- Moxonidine
Describe the mechanism of action of adrenergic neurone blocking drugs
- Prevent the release of noradrenaline from postganglionic adrenergic neurones
- Do not control supine blood pressure and may cause postural hypotension
- E.g. Guanethidine monosulfate
Give examples of alpha-adrenoceptor blocking drugs
- Prazosin
- Doxazosin
- Indoramin
Explain the mechanism of action of diuretics in treating hypertension
- Act on the kidneys to increase sodium (and therefore water) eliminated from the body
- Reduces blood volume, reducing BP
Give examples of diuretics used to treat hypertension
- Hydrochlorothiazide (Microzide)
- Chlorthalidone
Explain the mechanism of action of beta blockers in treating hypertension
Block sympathetic effects of adrenaline - reduces heart rate and contractility, reducing BP
Give examples of beta blockers used to treat hypertension
- Acebutolol
- Atenolol
Explain the mechanism of action of ACE-inhibitors in treating hypertension
Inhibit the activity of angiotensin-converting enzyme (ACE), preventing the conversion of angiotensin I to angiotensin II - results in widespread vasodilation, reducing BP
Give examples of ACE-inhibitors used to treat hypertension
- Lisinopril
- Benazepril
- Captopril
Explain the mechanism of action of calcium channel blockers in treating hypertension
- Block the entry of calcium ions to cardiac myocytes
- Reduces heart rate/contractility, reducing BP
Give examples of calcium channel blockers used to treat hypertension
- Amlodipine
- Diltiazem
Explain the mechanism of action of renin inhibitors in treating hypertension
- Inhibit the binding of renin to angiotensinogen - prevents the formation of angiotensin I and angiotensin II
- Causes widespread vasodilation and therefore reduced BP
Give examples of renin inhibitors used to treat hypertension
- Aliskiren
List common side effects of antihypertensive drugs
- Cough
- Diarrhoea or constipation
- Dizziness or lightheadedness
- Erectile dysfunction
- Anxiety
- Feeling tired, weak, drowsy etc.
- Headache
- Nausea or vomiting
- Skin rash
- Weight loss or gain
