CVS Session 1: anatomy and histology Flashcards
What are the factors that affect diffusion?
- Area for exchange: determined by capillary density and generally not limiting as area usually very large
- Diffusion resistance: affected by nature of molecule, nature of the barrier and the diffusion distance. For small molecules this is usually not limiting
- Concentration gradient: between capillary blood and tissues. Must be maintained at a high concentration to have high diffusion rate, by rate of flow
How does rate of blood flow (perfusion rate) vary between tissues?
Increased metabolism must be met by increased blood flow to ensure adequate oxygen and nutrients
Brain-high constant flow
Heart-flow increases during exercise
Kidneys-high constant flow
Skeletal muscle-very high during exercise
Gut-high after a meal
Skin-increases to control temperature
What is normal cardiac output at rest and how high can it rise during exercise?
5L per minute, can increased to a maximum of 25L per minute (for athletes this could be a few litres higher)
What is the distribution of blood in the vascular system?
67% veins
17% heart and lungs
11% arteries and arterioles
5% capillaries
Describe the main components of the cardiovascular system
Pump-heart
Distribution system-arteries and blood
Exchange mechanism-capillaries
Flow control-arterioles and pre-capillary sphincters
What is resistance?
Reduces the ease with which some regions are perfused in order to direct flow elsewhere (e.g. brain is harder to perfuse because of gravity)
Arterioles and pre-capillary sphincters add it to the CVS
What is capacitance?
A temporary blood store that can be returned to the heart at a different rate in order to control the total flow in a system.
Veins provide it as thin walls with smooth muscle can easily distend or collapse
Why is the heart 2 pumps in series?
Left pumps blood around systemic circulation, right pumps blood around pulmonary circulation. Circulation is from high to low pressure then back to heart.
The heart lies in the middle mediastinum. What is this?
The region in the thoracic cavity between the right and left pleural cavities. Contains the pericardial sac
What does the pericardium contain?
The heart and the beginning of the great vessels
Describe the fibrous pericardium
Tough external layer. Continuous with the central tendon of diaphragm, relatively non-distensible, prevents overfilling
What is cardiac tamponade caused by?
Pericardial effusion or haemopericardium (due to perforation of a weakened area of heart from MI, surgery or stab wound) causes an accumulation of fluid, compressing the heart because of the non-distensibility of the fibrous pericardial sac.
Quickly becomes lethal as circulation fails due to the backup of blood, beginning where the SVC enters the pericardium
Describe the serous pericardium
Inner layer split itself into two layers, separated by the pericardial cavity which contains a small amount of lubricating fluid to minimise friction.
Outer parietal layer: lines internal surface of fibrous pericardium
Inner visceral layer: forms epicardium (shiny outer layer of the heart)
Functions of the pericardium?
- Holds heart in place as is attached to diaphragm, sternum and tunica adventitia of great vessels
- Prevents overfilling
- Lubrication-reduces friction as heart beats
- Protection from infection (e.g. from lungs)
What is the innervation of the pericardium?
Phrenic nerve-C3,4,5
Also provides motor and sensory innervation to diaphragm
Source of referred pain e.g. shoulder pain in pericarditis
Describe the main features of the transverse pericardial sinus
Location: posterior to ascending aorta and pulmonary trunk, anterior to SVC, superior to left atrium
Formed during embryonic folding
Function: separates arterial and venous vessels
Clinical use: to identify and ligate arteries during coronary artery bypass grafting
What is pericarditis?
Inflammation of the pericardium, may make serous pericardium causing friction audible on auscultation.
Symptoms/signs: chest pain, pericardial friction rub on auscultation
Chronic cases may calcify, reducing cardiac efficiency
What is pericardial effusion?
Fluid from pericardial capillaries enters pericardial cavity, or pus.
Heart compressed and ineffective
Due to inflammation or congestive heart failure (where venous return is greater than cardiac output due to right heart hypertension)
Describe the main features of the oblique pericardial sinus
A pericardial reflection surrounding the veins of the heart. A pocket-like space posterior to the base of the heart (at top), formed by the left atrium.
Bounded laterally by reflection surrounding pulmonary veins and IVC, and posteriorly by the pericardium overlying the anterior oesophagus
Can be felt inferiorly but is a blind sac
What makes up the endocardium?
Thin endothelium and connective tissue, covers valves
What makes up the myocardium?
Thick, helical cardiac muscle forming the main walls
What makes up the epicardium?
Thin mesothelium formed by the visceral layer of the serous pericardium
Describe the orientation of the heart, a ‘pyramid that’s fallen over’
Apex-pounts in an anterior-inferior direction
Anterior/sternocostal: right ventricle
Posterior/base: left atrium
Inferior/diaphragmatic: left and right ventricles
Right pulmonary: right atrium
Left pulmonary: left atrium
What are the borders of the heart?
Right-right atrium
Left-left ventricle and part of left atrium
Inferior-right ventricle (v. small contribution from left)
Superior: right and left atria and the great vessels
What are cardiac sulci?
Grooves on the surface created by the chambers.
What is the coronary sulcus?
AKA atrioventricular groove. Runs transversely around the heart
What is the name of the sulci which run vertically to separate the ventricles?
Anterior and posteriro interventricular
Through which structures does blood pass to enter the right atrium?
IVC, SVC and coronary sinus
Describe the passage of blood around the heart and great vessels
- Enters RA through IVC, SVC and coronary sinus (from right and left brachiocephalic veins)
- Goes through tricuspid valve into right ventricle
- Through pulmonary semilunar valve to pulmonary trunk
- Moves into right (sup and inferior) and left pulmonary arteries to travel to lungs
- Oxygenated blood returned to heart through pulmonary veins
- Into left atrium
- Through mitral valve into left ventricle
- Through aortic semilunar valve into aorta, distributed to systemic circulation through its branches
What do the right and left aortic sinuses give rise to, respectively?
Right and left coronary arteries
What are the branches of the left coronary artery?
Anterior interventricular, left marginal and left circumflex
What are the branches of the right coronary artery?
Right marginal anteriorly, posterior interventricular posteriorly
What is the role of the coronary sinus?
Cardiac veins which drain into a coronary sinus (the main cardiac vein, located on the posterior surface identify on prosection
Sinus drains into right atrium (opening is between the right AV orifice and the IVC orifice)
Which tributaries drain into the coronary sinus?
Great cardiac vein: from apex, follows anterior IV groove into coronary sulcus, around left heart to joint coronary sinus identify on prosection
Small cardiac vein: anterior surface, passes around right heart to join coronary sinus
Middle cardiac vein: posterior heart, drains into right heart
Left marginal vein: left posterior side
Left posterior ventricular vein: centre posterior, runs along posteriror IV sulcus to join coronary sinus
Right coronary artery?
Supplies: Right atrium, SAN, AVN, posterior IVS
Vein: small and middle cardiac veins
Right marginal artery?
Supplies: right ventricle, apex
Vein: small and middle cardiac veins
Posterior interventricular artery?
Supplies: RV, LV, posterior 1/3 of IVS
Vein: left posterior ventricular vein
Left coronary artery?
Supplies: left atrium, right atrium, IVS, AV bundles
Vein: great cardiac vein
Anterior interventricular artery? (=LAD: left anterior descending)
Supplies: RV, LV, anterior 2/3 of IVS
Vein: great cardiac vein
Left marginal artery?
Supplies: left ventricle
Vein: left marginal and great cardiac veins
Circumflex artery?
Suplies: LA, LV
Vein: great cardiac vein
Describe the structure of cardiac muscle
Striated
Intercalated discs: strong horizontal components at Z band level. Between cells mechanical coupling.
Gap junctions: weaker vertical components. Allow electrical coupling
Central nucleus: one or two per cell
T tubules at Z band (not A-I band as in skeletal)
Myofibrils: not distinct, cross-striations less obvious, less enclosed in SR, actin and myosin continuous masses in cytoplasm, adherens-type junctions to anchor cells to actin
Diads (not triads) of T tubule system
What are purkinje fibres?
Modified monocytes. Have abundant glycogen, sparse myofilaments, extensive gap junctions, few intercalated discs. Located between the endocardium and myocardium
Carry impulses from SAN to ventricular muscle very quickly to allow synchronous ventricular contraction
How does blood get from the right ventricle to the lungs?
RV–>pulmonary trunk–>R/L pulmonary artery–>lung
Pulmonary arteries are major elastic arteries
What are the major elastic arteries that transport blood from the left ventricle?
Into aortic arch then into brachiocephalic, left common carotid or left subclavian arteries
Branches throughout abdomen, terminate by bifurcating into the left and right common iliac arteries in the pelvis
State the main features of elastic conducting arteries
Conduct blood away from the heart and act as pressure reservoirs. E.g. aorta, subclavian
In systole, pressure rises so walls stretch and in diastole walls recoil to maintain pressure.
Walls may be yellow when fresh due to the high elastin content
Describe the structure of elastic conducting arteries
Tunica intima: endothelial cells with long axes orientated parallel to the long axis of the artery. Narrow, discontinuous elastc lamina. Next to lumen
Tunica media: fenestrated elastic membranes with smooth muscle and collagen between. May have thin external elastic lamina. Thick
Tunica adventitia: thin fibroelastic connective tissue with vaso vasorum (vessels of vessles), lymph vessels and nerve fibres
What is an aortic dissection?
A break in the tumour intima causing blood to enter the tunica media between the elastin. Painful and serious, happens in Marfan’s syndrome due to the defective elastin
What are the main features of muscular (distributing) arteries?
Intermediate diameter. Most of the ‘named’ arteries.
Branch into arterioles whose function is to regulate amount of blood reaching a tissue or regulate blood pressure
Describe the structure of muscular (distributing) arteries
Tunica intima: endothelium, thick internal elastic lamina
Tunica media: layers of circular smooth muscle cells connected by gap junctions for coordinate contraction. Thick external elastic lamina
Tunica adventitia: thin fibroelastic connective tissue with thin vaso vasorum, lymphatic vessels and nerve fibres
How does vasoconstriction occur?
Sympathetic nerve fibres stimulate release of noradrenaline. This diffuses through fenestrations in the external elastic into the TM to depolarise superficial smooth muscle cells, and gap junctions propagate this to all cells of the TM
What are end arteries?
Terminal arteries supplying all/most of blood to a particular area without significant collateral circulation. E.g. coronary artery, splenic and renal arteries
If occluded blood supply to that area is cut off
Absolute end arteries are the central artery to the retina and labyrynthine artery of the internal ear
What is bridging?
Compression of a segment of a coronary artery during systole, causing narrowing that reverses during diastole
What are the main features of arterioles?
Arteries with a diameter less than 0.1mm
1-3 layers of smooth muscle in TM
thin external elastic lamina only in large arterioles
in small arterioles TM is a single smooth muscle layer around endothelium with no elastic lamina
very thin adventitia
What are metarterioles?
Arteries that supply blood to capillary beds
Smooth muscle layer is discontinuous
Individual muscle cells are spaced apart
What is a pre-capillary sphincter?
Each smooth muscle cell of metarterioles acts as a sphincter by controlling blood flow into capillaries during contraction
General features of capillaries
Lowest blood velocity so gas and nutrient exchange can occur
About 10 um diabeter and less than 1mm long
Single layer of endothelium plus a basement membrane
What are continuous capillaries?
Most common type. In neurones, muscle, connective tissue, exocrine glands and lungs
Continuous endothelial layer with tight junctions
What are fenestrated capillaries?
Found in the gut, endocrine glands and renal glomerulus. Interruptions in endothelium bridged by a thin diaphragm
What are sinusoids?
Discontinuous capillaries of about 40um diameter. Found in liver, spleen and bone marrow. Gaps in the wall allow whole cells to move between blood and tissue
What are pericytes?
Branching network on the outside of capillary endothelium. Can divide into muscle cells or fibroblasts during angiogenesis, tumour growth and wound healing
General features post-capillary venules
Endothelial lining with pericytes. Receive blood from capillaries and are even more permeable.
Diameter ~20um
Lower pressure than capillaries and surrounding tissue so fluid drains into them
In inflammation however fluid and leukocytes move to venules
General features-venules
Diameter about 1mm
Smooth muscle and TM appear
Can have valves-thin extensions that press together to restrict retrograde blood transport
General features-veins
Usually larger diameter than accompanying artery and thinner wall with more CT, less elastic and muscle fibres. Valves act with muscle contraction to propel blood to the heart
Small veins: developed TA, thin TI and TM
Large veins: >10mm. Thicker TI, developed TA, not prominent TA except legs which have thick. E.g. vena cavae, pulmonary vein, portal vein renal vein, internal jugular vein, iliac vein, azygous vein
What are venae comitantes?
Deep paired veins.
In some positions accompany smaller artery on each side, 3 vessels wrapped in one sheath.
The artery pulsing promotes venous return in the adjacent parallel paired veins.
E.g. brachial, ulnar and tibial venae cominantes
