Cardiovascular System Flashcards
What does rate of diffusion depend on?
Area
Diffusion resistance
Concentration gradient
Why do we need a CVS?
Most cells are far away from a source of oxygen and nutrients
Diffusion only occurs over short distances and so is insufficient to supply oxygen and nutrients
So large organisms like humans require a gas exchange and circulatory system with capillaries and blood flow bringing blood to cells deep within the body
How does diffusion depend on area?
Larger the area availble for nutrient/ oxygen exchange the greater the rate of diffusion
Area for exchange between capillaries and tissues is generally very large- depends on capillary density
A tissue which is more metabolically active will have a higher capillary density and thus a larger area and greater rate of diffusion
How does diffusion depend on diffusion resistance?
Diffusion resistance is the difficulty of movement through the barrier
Mostly low, not the factor that limits diffusion
Depends on the nature of the molecule (liphophilic, hydrophilic, size) nature of the barrier (pore size, number of pores for hydrophilic substances) and path length (depends on capillary density and is shortest in the most active tissues)
How does diffusion depend on concentration gradient?
Rate of diffusion is dependent on the concentration gradient - greater the concentration gradient (between capillary blood and tissues) the greater the rate of diffusion
Concentration gradient must be maintained between capillary blood and tissues
A substance which is used by the tissues will have a lower conc in the capillary blood than in the arterial blood (difference depends on the rate that tissues use the substance and the rate of blood flow through the capillary bed)
What is the importance of blood flow and diffusion rate in the CVS
Rate of metabolism is directly proportional to demand for oxygen and nutrients
Increases in metabolism must be met by increases in blood flow
Rate of blood flow is known as the perfusion rate
CVS must supply between 5-25l/min of blood to all the tissues whilst at all times maintaining perfusion to vital organs - brain, kidney and heart
In the cvs what is the pump?
The heart- 2 pumps in series
Left - systemic
Right - pulmonary
In the cvs what is the distribution system?
Vessels and blood
In the cvs what is the exchange mechanism?
Capillaries
In the cvs what is the flow control?
Arterioles and pre capillary spinchters (RESISTANCE)
In the cvs what is the capacitance?
Capacitance- ability to store blood
Veins - total flow in the system has to be able to change- this requires a temporary store of blood which can be returned to the heart at a different rate
Veins have thin walls which can easily distend or collapse enabling them to act as a variable reservoir for blood
What is the mediastinum?
Space between two pulmonary cavities
Central cavity of the chest
Transverse thoracic plane at vertebra 4&5 divides mediastinum into superior and inferior
Superior made up of the oesophagus, branch of the aorta, trachea
Inferior made up of anterior, middle and posterior
The heart and pericardial sac are found in the inferior middle mediastinum
Describe the pericardium
Two layers
Outer fibrous layer - tough and inelastic - attached firmly to the diaphragm via the pericardiocophrenic ligament
Inner serous layer - thin, delicate mesothelium (simple squamous)
–> parietal (more outer; adjacent to fibrous)
Pericardial cavity between (serous fluid)
–> visceral (more inner)
(Parietal layer is reflected onto the heart at the great vessel as the visceral layer)
What are the main nerves that supply the heart?
Vagus nerves CX - parasympathetic innervation to heart, GI and lungs
- left recurrent laryngeal
- right recurrent laryngeal
Phrenic nerves C3/4/5 - sensory supply of pericardium, muscle supply of the diaphragm –> breathing
- left and right
What is the oblique sinus?
Blind ending space inferior to the heart - bound laterally by reflections of parietal and visceral reflections surrounding the IVC and pulmonary veins and posteriorly by the pericardium overlying the anterior aspect of the oesophagus
Between superior vena cava and pulmonary artery
No function
What is the transverse sinus? And its clinical importance?
Between the pulmonary trunk and arch of the aorta leaving and vena cava and pulmonary veins entering
Clamps big vessels in cardiac surgery
What are the main coronary arteries of the heart? And what do they supply?
Left and right coronary arteries are supplied with blood via 2 holes in the aorta
Left coronary artery has 3 main divisions:
-circumflex (left atria)
-left anterior descending / anterior interventricular branch (left and right ventricle)
- left marginal (left ventricle)
Right coronary artery (right atria) (lies in atrioventricular sulcus) has 4 main divisions:
-SANodal branch (SAN)
-right marginal (right ventricle)
-right posterior descending / posterior interventricular branch (right ventricle)
-AVNodal branch (AVN)
What are the main coronary veins?
Coronary sinus Great cardiac vein Middle cardiac vein Small cardiac vein Left marginal vein Left posterior ventricular vein
Drain to a single hole/sinus in the right atrium
When do coronary arteries fill?
When the heart is relaxed in diastole
What is the arrangement of the large elastic artery?
Tunica intima- endothelial cells, sub endothelial layer, discontinuous internal elastic lamina
Tunica media- smooth muscle (40-70 layers of fibroblast in membranes with smooth muscle cells and collagen between) thin external elastic lamina
Tunica adventitia- thin, vasa vasorum (blood vessels, lymphatics and nerve fibres)
What is the arrangement of the medium muscular artery?
Tunica intima- endothelial cells, sub endothelial layer, thicker internal elastic lamina
Tunica media- smooth muscle (40 smooth muscle cells with gap junctions) thicker external elastic lamina
Tunica adventitia- thin, vasa vasorum (blood vessels, lymphatics and nerve fibres) unmyelinated sympathetic nerve fibres- neurotransmitters diffuse through fenestrations in the EEL into the smooth muscle cells- propagate to all cells via gap junctions
What is the arrangement of arteriole?
Tunica intima- endothelial cells, sub endothelial layer, thin internal elastic lamina
Tunica media- smooth muscle (1-3 smooth muscle cells) NO external elastic lamina
Tunica adventitia- scant
What is the arrangement of metarterioles?
Tunica intima- endothelial cells, sub endothelial layer, NO internal elastic lamina
Tunica media- discontinuous smooth muscle layer and NO external elastic lamina
Tunica adventitia- scant/ absent
Individuals muscle cells are spaced apart and each encircles the endothelium of a capillary arising from one metarteriole- pre capillary spinchters (control blood flow into a capillary by opening and closing)
What is the arrangement capillaries?
Tunica intima- endothelial cells and subendothelial layer
Pericytes- branching surface on outer surface of endothelium
Capable of dividing into muscle cells or fibroblasts in angiogenesis, tumour growth and wound healing
What are the three types of capillaries?
Continuous- most common, continuous cells joined by occluding junctions (nerves, muscles, CTs, exocrine glands)
Fenestrated- interruptions across the endothelium- bridges by thin membranes (gut, endocrine glands, renal glomerulus)
Sinsusoidal- larger diameter, slower blood flow, gaps exist in walls allowing whole cells to move between blood and tissue (liver, spleen, bone marrow)
What is the arrangement of the post capillary venules?
Tunica intima- endothelial cells and subendothelial layer
Pericytes- branching surface on outer surface of endothelium
Capable of dividing into muscle cells or fibroblasts in angiogenesis, tumour growth and wound healing
Pressure is lower than that of capillaries or surrounding tissue so that fluid drains into them; except when an INFLAMMATORY RESPONSE IS OPERATING- in which case fluid and leukocytes emigrate; these venules are the preferred location for emigration of leukocytes from the blood
What is the arrangement of venules?
Tunica intima- endothelial cells and subendothelial layer
Tunica media- smooth muscle cells begin to appear
What is the arrangement of medium veins?
Tunica intima- endothelial cells, sub endothelial layer, thin internal elastic lamina
Tunica media- thin smooth muscle and NO external elastic lamina
Tunica adventitia- well developed
What is the arrangement of large veins?
Tunica intima- endothelial cells, sub endothelial layer, thicker internal elastic lamina
Tunica media- thin smooth muscle and NO external elastic lamina
Tunica adventitia- well developed
What are venae comitantes?
Deep paired veins with an artery
All wrapped together in one sheath
Pulsing of artery promotes venous returns within the adjacent parallel paired veins
E.g. Brachial ulnar tibial venae comitantes
What are end arteries?
Terminal arteries supplying all or most of the blood to a body part without significant collateral circulation
Undergo progressive branching without development of channels connecting with other arteries so that if occluded there is insufficient blood supply to dependent tissue
Coronary, renal and splenic nerve
Absolute end arteries: central artery to retina, labyrinthine artery of internal ear
What valves are found in the heart?
Atrioventricular valves ( tricuspid and mitral) Outflow valves ( aortic and pulmonary)
Describe the cardiac muscle
Specialised form- myocardium, individual cells joined by low electrical resistance connections
Striated
T tubules
Diads
Z lines
Gap junctions
3 layers- endocardium (lining membrane that covers internal heart and valves), myocardium (cardiac muscle), epicardium (mesothelium on outer surface of heart= visceral)
Describe the process of contraction in the heart
SAN (pacemaker cells) produces an action potential spontaneously at regular intervals (1AP per second)
Excitation activity spreads over the atria to the atrioventricular node =atrial systole
Reaches AV node and is delayed for 120ms
Spreads down the muscular septum between the ventricles (via the bundle of his) to excite the ventricular muscle from the endocardial side towards the av junction where valves are located and outwards to the epicardium= ventricular systole
Contraction of each cell is produced by a rise in intracellular calcium concentration triggered by an all or nothing action potential
What are some features of the cardiac action potential?
Cardiac ap is very long
1 cardiac ap produces 1 contraction (280ms)
Ap spreads from cell to cell so at each heart beat all the cells in the heart normally contract
Define systole
Period when the myocardium is contracting
Define diastole
Period of relaxation between contractions
How is the ventricle muscle arranged to ensure maximum contraction?
Ventricular muscle is arranged in into figure of 8 bands which squeeze the ventricular chamber forcefully in a way most effective for ejection through the outflow valve - apex of the heart contacts first and relaxes last to prevent back flow
How do you calculate cardiac output?
Heart ejects a stroke volume with each heart beat
Cardiac output= stroke volume x heart rate
What is the normal stroke volume of a 70kg individual?
80 ml
What is the normal resting heart rate of a 70 kg individual?
60
How long does ventricular systole last for?
280 ms
How long does diastole last?
700ms
In diastole where do the ventricles fill from?
Veins
Atrial systole- only forces a small extra amount of blood into the ventricles
What are the main heart sounds?
Two main sounds associated with valves closing
- first sound - lup - closure of av inflow valves (at onset of ventricular systole)
- second sound - dup - closure of outflow valves (end of ventricular systole)
- third sound - early in diastole
- fourth sound - atrial sound
3rd and 4th are normal sounds = gallop rhythm
Murmurs- turbulent flow of blood through a valve (ABNORMAL)
- quality of sounds may change if valves are altered; sounds may split if valves of right and left heart do not close at the same time
What is a heart murmur?
Turbulent flow of blood generates murmurs - narrowed valves (stenosis) or valve not closing properly (incompetence) or absent/permanently fused valves (atresia)
Murmurs occur when blood flow is highest- So we can predict when in the cardiac cycle they should occur (aortic stenosis and incompetence- in rapid ejection phase)
Describe the cardiac cycle
Using poster Isovolumetric relaxation Rapid filling phase Isovolumetric contraction Rapid ejection phase
How frequent are congenital heart defects?
Congenital heart defects are common, with an incidence of 6-8 per 1,000 births
What are the most common congenital heart defects?
The most common heart defects are Ventricular Septal Defects (VSD), followed by Atrial Septal Defects (ASD)
What are the acyanotic heart defects?
Persistence of left to right shunts Atrial septal defects (in particular PFO) Ventricular septal defects Patent ductus arteriosus Aortic stenosis Coarctation Mitral stenosis
What are atrial septal defects?
An ASD is an opening in the septum between the two atria, which persists following birth. They have an incidence of 67 in 100,000 live births.
The foramen ovale exists to prenatally permit right to left shunting of oxygenated blood and is designed to close promptly after birth. Failure of it to close, allows blood to continue to flow between the two atria postnatally. Because left atrial pressure > right atrial pressure, flow will be mainly from left to right, meaning no mixing of deoxygenated blood with the oxygenated blood being pumped around the circulation.
ASDs can occur almost anywhere along the septum, but the most common site is the foramen ovale (Ostium secundum ASD). An ostium primum ASD occurs at the inferior part of the septum, and is less common.
What is a specific example of an ASD?
Patent foramen ovale (PFO)
What is PFO?
PFOs are not a true ASD. PFOs may be present in ~20% of the population and are generally clinically silent, since the higher left atrial pressure causes functional closure of the flap valve.
A PFO may however be the route by which a venous embolism reaches the systemic circulation if pressure on the right side of the heart increases even transiently. This is called a paradoxical embolism.
What is a ventricular septal defect?
VSDs are an opening in the Interventricular Septum. This most commonly occurs in the membranous portion of the septum, but can occur at any point. Since left ventricular pressure is much > than right, blood will flow left to right.
What is patent ductus arteriosus?
The Ductus Arteriosus is a vessel that exists in the foetus to shunt blood from the pulmonary artery to the aorta before the lungs are functioning. This vessel should close shortly after birth as the pressure in the pulmonary artery drops following perfusion of the lungs. Failure to close leads to a PDA. Blood flow through a PDA will be from the aorta to pulmonary artery after birth (High to Low pressure).
What is a mechanical murmur and when is it heard?
A Mechanical Murmur is heard constantly throughout systole/diastole, as pressure in the aorta is always greater than in the pulmonary artery.
If untreated how can conditions of left to right shunting, become problematic?
Although left to right shunting of blood does not cause cyanosis it can be problematic later on if untreated, with the extent of the problems depending on the degree of shunting. Chronic left to right shunting can lead to vascular remodelling of the pulmonary circulation and an increase in pulmonary resistance. If the resistance of the pulmonary circulation increases beyond that of the systemic circulation the shunt with reverse direction as pressures on the right side of the heart increase (Eisenmenger Syndrome).
What is coarctation of the aorta?
Coarctation of the Aorta is a narrowing of the aortic lumen in the region of the ligamemtum arteriosum (former ductus arteriosus). The narrowing of the aorta increases the afterload on the left ventricle and can lead to left ventricular hypertrophy. Because the vessels to the head and upper limbs usually emerge proximal to the Coarctation, the blood supply to these regions is not compromised. However blood flow to the rest of the body is reduced. The extent of the symptoms depends on the severity of the Coarctation.
In very severe cases, an infant may present with symptoms of heart failure shortly after birth. In mild cases, the defect may be detected in adult life. Femoral pulses will be weak and delayed, with upper body hypertension.
What are the cyanosis heart defects?
Persistence of right to left shunts Tetralogy of fallot Tricuspid atresia Transposition of great arteries Hypoplastic great/ left heart Univentricular heart Pulmonary atresia
What is tetralogy of fallot?
The Tetralogy of Fallot is a group of 4 lesions occurring together as a result of a single developmental defect placing the outflow portion of the interventricular septum too far in the anterior and cephalad directions. The four abnormalities are:
- VSD
- Overriding Aorta
- Pulmonary Stenosis (variable degree)
- Right Ventricular Hypertrophy (variable degree)
Pulmonary stenosis causes persistence of the foetal right ventricular hypertrophy, as the right ventricle must operate at a higher pressure to pump blood through the pulmonary artery. The increased pressure on the right side of the heart, along with the VSD and overriding aorta allow right à left shunting and therefore the mix of deoxygenated blood with the oxygenated blood going to the systemic circulation, resulting in cyanosis.
The magnitude of the shunt and level of severity depends on the severity of the pulmonary stenosis. Affected individuals may present with cyanosis in infancy, but mild cases can present in adulthood.
What is tricuspid atresia?
Tricuspid Atresia is the lack of development of the tricuspid valve. This leaves no inlet to the right ventricle (2). There must be a complete Right to Left shunt of all blood returning to the right atrium (ASD or PFO) (1) and a VSD or PDA to allow blood to flow to the lungs (3).
What is the transposition of the great arteries?
Results in two unconnected parallel circulations instead of two in series. In this defect, the right ventricle is connected to the aorta and the left ventricle to the pulmonary trunk. This condition is not compatible with life after birth, unless a shunt exists to allow the two circulations to communicate. A shunt must be maintained or created immediately following birth to sustain life until surgical correction can be made. The ductus arteriosus can be maintained patent and/or an atrial septal defect formed.
What is hypoplastic left heart?
In some cases the left ventricle and ascending aorta fail to develop properly resulting in a condition called Hypoplastic left heart. A PFO or ASD are also present and blood supply to the systemic circulation is via a PDA. Without surgical correction this is lethal.
What is Univentricular heart?
A/v valves and subsequent ventricles do not form Can occur with or without the transposition of the great arteries Results in a unified mass LV (viable) RV (less viable) No RV outlet R to L shunt of entire venous return Blood flow to lungs (PDA)
What is the resting membrane potential largely established by?
Due to the potassium permeability of the cell membrane at rest
- due to the leak of potassium channel
- due to the inward rectifier potassium channels being open
- due to little permeability of other ions
Describe how a resting membrane potential is established
Potassium ions move out of the cells - down their concentration gradient; small movements of ions makes the inside negative wrt the outside
Electrical gradient is set up as charge builds up
Rmp exists when the movement of potassium ions into the cell (electrical gradient) is equal to the movement of potassium ions out of the cell (conc grad) = Eqm K
(Ions are still moving but there is equal movement)
Describe the ventricular cardiac action potential
Steep depolarisation (upstroke)- opening of voltage gated sodium channels Steep and rapid repolarisation- transient outward potassium current; voltage gated potassium channels open very quickly and then inactivate very quickly (like sodium channels) Plateau region with slight repolarisation- influx of calcium ions, but slight depolarisation still occurs because potassium out flux exceeds calcium influx Further steep and rapid repolarisation (hyper too; downstroke)- calcium channels inactivate, voltage gated potassium channels and inward rectifier potassium channels open
Describe the sino atrial node action potential
Slow depolarisation to threshold- PACEMAKER POTENTIAL (funny current) influx of sodium ions via Hyperpolarisation Cyclic Nucleotide (HCN) gated channels (activated by membrane potential more negative than -50mV, and so become inactivated as membrane depolarises) unstable
Steep and Rapid depolarisation (upstroke)- opening of voltage gated calcium channels
Steep and rapid repolarisation- opening of voltage gated potassium channels
Why is the SAN Favoured as the pacemaker of the heart over AVN, atrial muscle, ventricular muscle and PURKINJE fibres?
SAN is the fastest out of the AVN, SAN, atrial muscle, ventricular muscle and PURKINJE fibres to depolarise - automatic
Shortest pacemaker potential
Describe some basic features of cardiac muscle
Striated- actin and myosin filaments Single central nucleus Cells joined at intercalated disks (desmosomes and gap junctions) T tubules (Z lines) are diads Indistinct fibres Branching
What do desmosomes do?
Rivet cells together allowing cardiac cells to contract as one unit
What do gap junctions do?
Permit the movement of ions and electrically couple cells together (spread of depolarisation)
Describe the process of contraction of cardiac muscle
Depolarisation opens the L type calcium channels in the T. Tubule system (25%)
Localised calcium entry opens the calcium induced calcium release (CICR) channels in the SR (75%)
Calcium binds to troponin c causing a conformational change in tropomyosin, shifting it from actin to reveal the myosin binding site on the actin filament
Sliding filament mechanism
Describe the process of relaxation of cardiac muscle
Calcium is pumped back into the SR via SERCA (stimulated by raised calcium)
Or leaves across the cell membrane (sarcolemmal calcium ATPase OR sodium/calcium exchanger NCE)
What control the tone of blood vessels?
Contraction and relaxation of vascular smooth muscle cells
How does the contraction and relaxation of vascular smooth muscle cells control the tone of blood vessels?
Controls the resistance of blood flow
Contracts veins en route to the heart- to increase BP
Increases blood pressure
Where are vascular smooth muscle cells found in arteries, arterioles and veins?
Located in the tunica media
Are actin and myosin still involved in contraction and if so how?
Yes
Diagonal arrangement f acting and myosin filaments
Cells are electrically coupled via gap junctions
Describe the contraction of smooth muscle cells
Depolarisation of cells causes an influx of calcium via the L type voltage gated calcium channels
Inside the cell 4 calciums bind to the calmodulin molecule activating myosin light chain kinase (mlck) by binding to it
Mlck phosphorylates the light myosin chain using ATP phosphate, allowing the interaction of myosin light chain with actin and hence contraction
Describe the relaxation of smooth muscle cells
As calcium levels decline, after contraction mlc phosphatase (which is always active) dephosphorylates the myosin light chain
What inhibits mlck and contraction of smooth muscle cells?
Phosphorylation by protein kinase A
What is vascular smooth muscle contraction dependent on?
Relative activation of mlck and activity of mlcp
What is the autonomic nervous system important for?
Balance- (homeostasis) heart rate, blood pressure, body temperature
Coordinating the body’s response to exercise and stress
What three areas of the body does the autonomic nervous system exert control over?
Smooth muscle (vascular and visceral)
Exocrine secretion
Rate and force of contraction in the heart
What are the 2 divisions of the autonomic nervous system?
Sympathetic and parasympathetic
Describe the arrangement of the sympathetic nervous system
2 neurones
Thoracolumbar outflow from spinal cord
Short preganglionic neurone (ACh –> nicotinic receptors)
Long postganglionic neurone (noradrenaline, norepinephrine –> adrenergic alpha/beta receptors)
Preganglionic neurone synapses at paravertebral origin, above or below on the sympathetic chain or somewhere else (coeliac, superior mesenteric, inferior mesenteric ganglia)
Describe two exceptions to the arrangement of the sympathetic nervous system
Sweat glands, erectile tissue
Short preganglionic neurone (ACh –> nicotinic receptors)
Long postganglionic neurone (ACh –> muscarinic receptors)
Describe the arrangement of chromaffin cells of the adrenal medulla
Sympathetic Preganglionic neurone (ACh ---> nicotinic receptors) Directly int chromaffin cell which releases adrenaline/ epinephrine into the blood stream
When is the activity of the sympathetic nervous system increased?
Increased under stress
What receptors are found on target tissues in the sympathetic nervous system?
Adrenoreceptors - alpha 1&2 beta 1&2 (&3)
Respond to noradrenaline and adrenaline
GPCR (no integral ion channel)
Different tissues can have different types of adrenoreceptors
Why is it important for different target tissues to have different subtypes of adrenoreceptors in the sympathetic nervous system?
Allows for diversity of action- operates through different signalling mechanisms
Selectivity of drug action
Describe the arrangement of the parasympathetic nervous system
2 neurones
Craniosacro outflow from spinal cord
Long preganglionic neurone (ACh –> nicotinic receptors)
Short postganglionic neurone (ACh –> muscarinic receptors)
Ganglion is located near to the target tissue
When is the activity of the parasympathetic nervous system increased?
Increased under basal conditions
What receptors are found on target tissues in the parasympathetic nervous system
Muscarinic receptors
Respond to ACh
GPCR (no integral ion channel)
M1,2 and 3
Regarding the heart what three things is the autonomic nervous system important for?
Heart rate
Force of contraction of the heart
Peripheral resistance of blood vessels (arterioles)
In the heart, what three structures does the sympathetic nervous system innervate?
The sinoatrial node
The atrioventricular node
Myocardium
In the heart in the sympathetic nervous system where does the postganglionic fibre come from?
The sympathetic chain
In the heart what neurotransmitter does the postganglionic neurone release in the sympathetic nervous system?
Noradrenaline
In the heart what receptors on the target tissues does the noradrenaline from the postganglionic fibre act on?
Beta 1 receptors
In what two ways does the sympathetic nervous system affect the heart?
Increases heart rate (positive chronotropic effect)
Increases force of contraction (positive inotropic effect)
How does the sympathetic nervous system increase heart rate?
Noradrenaline binds to the beta 1 receptors
This increases cAMP
Increases the opening of the HCN channels
Speeds up the pacemaker potential
Increases heart rate
How does the sympathetic nervous system increase the force of contraction of the heart?
Noradrenaline binds to beta-1 receptors
Increase in cAMP
Activates protein kinase A
Phosphorylation of calcium channels
Increased calcium entry during the action potential
Increased uptake of calcium in the sarcoplasmic reticulum
Increased sensitivity of contractile machinery to calcium
Increased force of contraction
In the heart what 4 structures are innervated by the parasympathetic nervous system?
Epicardial surface
Within the walls of the heart
SAN
AVN
In the heart where does the preganglionic fibre originate from the spinal-cord in the parasympathetic nervous system?
From the 10th cranial nerve
Vagus nerve
In the heart what neurotransmitter does the postganglionic nerve release in the Parasympathetic nervous system?
Acetyl choline
In the heart what receptors does the acetylene choline from the postganglionic fibres act on in the parasympathetic nervous system?
M2 receptors
In what two ways does the parasympathetic nervous system affect the heart?
Decreases heart rate (negative chronotropic effect)
Decreases AVN conduction velocity
How does the parasympathetic nervous system decrease heart rate?
Acetylcholine binds to the M2 receptors This increases potassium conductance This decreases cAMP Decreases HCN channel activity This slows down the pacemaker potential This decreases the Heart rate
What type of innervation do most vessels receive?
Sympathetic
What is the effect of a decreased sympathetic output on vessels?
Binds to Alpha 1 receptors
Vasodilation
What is the effect of a normal sympathetic output on vessels?
Vasomotor tone
What is the effect of increased sympathetic output on vessels?
Binds to alpha-1 receptors
Vaso constriction
Where can blood vessels which have beta 2 adrenoreceptors and alpha-1 adrenoreceptors be found?
Skeletal muscle
Myocardium
Liver
How does noradrenaline affect blood vessels?
Binds to alpha-1 receptors
Causing vasoconstriction
How does circulating adrenaline affect blood vessels?
Binds to beta-2 receptors
Causing vasodilation
In high amounts, circulating adrenaline can bind to alpha-1 receptors causing vasoconstriction
Does circulating adrenaline have a higher affinity for alpha-1 receptors or beta 2 receptors?
Beta two receptors
In blood vessels how are the beta 2 adrenoreceptors activated?
Increase in cAMP
Opens a type of potassium channel
Causes relaxation of the smooth-muscle
Vasodilation
In blood vessels how are the alpha-1 adrenoreceptors activated?
Increasing calcium from the stores in the sarcoplasmic reticulum and via the influx of extracellular calcium
Causes contraction of smooth smooth muscle
Vasoconstriction
What is the importance of local metabolites in vasodilation and vasoconstriction?
Active tissue produces more metabolites
Local increases in metabolites has a strong vasodilator effect
Local metabolites are more important for ensuring adequate perfusion of skeletal and coronary muscle than the activation of beta 2 receptors
What are baroreceptors?
Nerve endings in the carotid sinus and aortic arch which are sensitive to stretch
How do you baroreceptors work?
Baroreceptors detect stretch in artery walls and hence increased mean arterial pressure
They send signals via afferent Pathways to medulla of the brain
This decreases the heart rate (bradycardia) and causes vasodilation so tired station
Bradycardia and vasodilation counteract the increased mean arterial pressure
What three classes of drugs act on the autonomic nervous system?
Sympathomimetics
Adrenoreceptor antagonists
Cholinergics
How do sympathomimetics work?
They mimic the effects of the sympathetic nervous system
How do sympathomimetics exist?
As alpha adrenoreceptor agonists or beta adrenoreceptor agonists
What is the meaning of flow?
The volume of fluid passing a given point per unit time
Must be the same at all points along the vessel
Flow= volume/time
What is the meaning of velocity?
The rate of movement of fluid particles along the tube
Can vary along the length of the vessel if the radius of the tube changes
Velocity= distance/ time
At a fixed flow, what is the relationship between velocity and cross-sectional area of a vessel?
Velocity is inversely proportional to the cross-sectional area
So the bigger the cross-sectional area – the lower velocity (capillaries)
The smaller the cross-sectional area – the higher velocity (aorta)
What does laminar flow mean?
In laminar flow, there is a gradient of velocity from the middle to the edge of the vessel. Velocity is highest in the centre and fluid is stationary at the edge. The flow in most blood vessels is laminar.
What does turbulent flow mean?
As the mean velocity increases, flow eventually becomes turbulent. The velocity gradient breaks down as layers of fluid try to move over each other faster than physics will allow. The fluid tumbles over, greatly increasing flow resistance.
Turbulent flow has associated sound which is a key feature of clinical examination
What is viscosity?
The extent to which fluid layers resist sliding over one another in laminar flow
Viscosity determines the slope of the gradient of velocity
How does viscosity affect flow?
The higher the viscosity, the slower the central layers will flow, and the lower the average velocity. In a low viscosity fluid, the difference between the centre and edge is large, and in a high viscosity fluid the difference is smaller.
At a constant pressure, what is flow affected by?
Mean velocity which in itself is affected by the viscosity of the fluid and the radius of the tube
What is the relationship between mean velocity and viscosity?
Mean velocity is inversely proportional to viscosity
What is the relationship between mean velocity and cross-sectional area?
Mean velocity is directly proportional to cross-sectional area
How does the radius of the tube affect mean velocity?
At a constant gradient, the wider the Tube the faster the middle layers move
What is Poisseulles law?
Flow is directly proportional to the change in pressure x the radius x the radius squared / (viscosity x length)
What is ohms law?
Pressure = flow x resistance
What is the relationship between resistance and viscosity?
Resistance is directly proportional to viscosity
Resistance increases as viscosity increases
What is the relationship between resistance and radius?
Resistance decreases with fourth power of radius
What is the relationship between resistance and pressure when flow is fixed?
The higher the resistance, the greater the pressure change from one end of the vessel to the other
