Week 1 + 2 - intro to CVS and histology Flashcards Preview

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Flashcards in Week 1 + 2 - intro to CVS and histology Deck (83):

How does diffusion of substances within the blood take place?

Between the blood and the tissues at the capillaries
- Some molecules are lipophilic and can diffuse directly through the lipid bilayer (e.g. CO2, O2)
- Other molecules are hydrophilic and diffuse though small pores in the capillaries (e.g. glucose, lactate)
- All molecules will move down their concentration gradient


What does the rate of diffusion depend on?

- Area: the area available for exchange; usually very large between capillaries and tissues; depends on capillary density
- Diffusion resistance: small molecules diffuse through pores more easily than large molecules
- Concentration gradient: the greater the concentration gradient, the greater the rate of diffusion; must be maintained for exchange to continue; depends on rate of use by tissues; the lower the blood flow, the lower the capillary concentration


What does diffusion resistance depend on?

- Nature of the molecule (e.g. lipophilic/hydrophilic)
- Nature of the barrier (e.g. pore size and number of pores)
- Path length (depends on capillary density)


What is 'perfusion rate'?

The rate of blood flow


What is the normal cardiac output for an average adult male at minimum and maximum blood flow?

- Brain: 0.75 l/min min and max (metabolic needs are constant, extremely intolerant of flow interruption)
- Heart: 0.3 l/min min and 1.2 l/min max (increases during exercise)
- Kidney: 1.2 l/min min and max (requires a constant high blood flow to maintain its function)
- Gut: 1.4 l/min min and 2.4 l/min max (digestion of a meal generates a substantial increase in flow; can be decreased during exercise)
- Skin: 0.2 l/min min and 2.5 l/min max (not metabolically very active, flow can increase for thermoregulation)
- Rest of body: 0.2 l/min
So in total: 5.0 l/min minimum and 24.5 l/min maximum


What are the major components of the circulation?

- Pump = the heart
- Distribution system = vessels and blood
- Exchange mechanism = capillaries (via diffusion)
- Flow control = arterioles and pre-capillary sphincters
- Capacitance = veins


What is capacitance?

The ability to cope with changes in the cardiac output
- A store of blood that can be called upon to cope with temporary imbalances between the amount of blood returning to the heart and the amount that it is required to pump out
- Veins have thin walls which can easily distend or collapse, enabling them to act as a variable reservoir for blood


Why is flow control required?

The output of the pump must be distributed appropriately - - By restricting flow to those parts of the body which are easy to perfuse
- This drives blood to those parts which are not so easy to get blood to
- Uses resistance vessels
- Arterioles have lots of smooth muscle in their walls, which can contract


What is the distribution of blood in the CVS?

- Veins: 67%
- Heart and lungs: 17%
- Arteries and arterioles: 11%
- Capillaries: 5%


How does blood travel around the body? (vessels)

- From the heart
- Through large arteries
- Medium (muscular/distributing) arteries
- Arterioles
- Metarterioles
- Capillaries
- Post-capillary venules
- Venules
- Medium veins
- Large veins
- Then back to the heart


What are arteries?

Vessels that carry blood away from the heart to the capillary beds
- Different arteries contain varying amounts of elastic fibres and smooth muscle fibres in their walls
- Named elastic (conducting) or muscular (distributing) arteries


What is the difference between elastic and muscular arteries?

E: more elastic fibres than smooth muscle fibres in their walls, expand slightly with each heartbeat
M: more smooth muscle fibres than elastic fibres in their walls, branch into arterioles


What do arterioles do?

- Regulate the amount of blood reaching an organ/tissue
- Regulate blood pressure


What controls the diameter of the muscular arteries and arterioles?

The autonomic nervous system


What are metarterioles?

Small branches of the arterioles
- They carry blood into the capillaries


What layers are found in the walls of elastic arteries?

Tunica intima:
- Next to the lumen
- Contains the endothelium, a subendothelial layer and internal, discontinuous elastic lamina

Intermediate tunica media:
- Thicker in arteries
- 40-70 fenestrated elastic lamellae
- Thin external elastic lamina may be present
- Smooth muscle cells and collagen between the lamellae
--- The smooth muscle cells produce the elastin, collagen and matrix

Tunica adventitia
- Outer layer
- Thin layer of fibroelastic connective tissue containing vasa vasorum (vessels of vessels), lymphatic vessels and nerve fibres


Describe the walls of muscular arteries

Same as elastic arteries
- Except the tunica media has 40 layers of smooth muscle cells (rather than 40-70 fenestrated elastic lamellae) which are connectedly gap junctions for coordinated contraction
- The external elastic lamina is also more prominent


How does vasoconstriction occur?

- Stimulated by sympathetic nerve fibres
- Noradrenaline is released at nerve endings
- It diffuses through fenestrations in the external elastic lamina into the external tunica media
- It can depolarise some of the superficial smooth muscle cells
- Depoarisation is propagated to all cells of the tunica media via gap junctions


What is an end artery?

A terminal artery supplying all or most of the blood to a body part without significant collateral circulation
- Undergo progressive branching without the development of channels connecting with other arteries
- If occluded, there is insufficient blood supply to the dependent tissue
- E.g. coronary artery, renal artery, splenic artery


Describe the arteriole walls

- Have only 1 to 3 layers of smooth muscle in their tunica media
- The thin internal elastic lamina is present in larger arterioles only
- In small arterioles, the tunica media is composed of a single smooth muscle cell that completely encircles the endothelial cells
- The external elastic lamina is absent
- The tunica adventitia is negligible


Describe metarteriole walls

- Differ from arterioles in that the smooth muscle layer is not continuous
- The individual muscle cells are spaced apart and each encircles the endothelium of a capillary arising from the metarteriole (this is a precapillary sphincter)


What do lymphatic capillaries do?

Drain away excess extracellular fluid, returning it to the blood at the junctions of the internal jugular and subclavian veins


What are some characteristics of capillaries?

- Present by far the largest surface area for agas and nutrient exchange
- During passage through the capillaries, blood velocity is at its lowest (allows time for gas and nutrient exchange with surrounding tissues)
- 7-10 um in diameter
- Less than 1 nm long
- Made of a single layer of endothelium and its basement membrane
- Passing RBCs fill virtually the entire capillary lumen, minimising the diffusion path to adjacent tissues


What are the 3 types of capillaries?

- Most common type
- Located in nervous, muscle and connective tissues, exocrine glands and the lungs
- Cells joined by tight junctions
- Pericytes form a branching network on the outer surface of the endothelium
- Continuous endothelial layer

- Seen in parts of gut, endocrine glands and renal glomerulus
- Interruptions exist across thin parts of the endothelium (bridged by a thin diaphragm)
- 4 possible routes of transport across the endothelial wall

- Larger diameter
- Slower blood flow
- Seen in spleen, liver and bone marrow
- Gaps exist in the walls allowing whole cells to move between blood and tissue


What are the 4 routes of transport across the endothelial wall of a fenestrated capillary?

- Direct diffusion
- Diffusion through intercellular cleft
- Diffusion through fenestration
- Pinocytosis


What are pericytes?

Cells that are capable of dividing into muscle cells or fibroblasts, during angiogenesis, tumour growth and wound healing


Describe postcapillary venules

- Wall = similar to that of capillaries (endothelial lining with associated pericytes
- Receive blood from capillaries
- Have a diameter of 10-30 um
- Even more permeable than capillaries
- Their pressure is lower than that of capillaries or the surrounding tissue, so fluid tends to drain into them


Describe venules

- Can have a diameter of up to 1mm
- Smooth muscles begin to be associated with the endothelium when the diameter increases to more than 50um
- Endothelium is associated with pericytes or thin smooth muscle cells to form a very thin wall
- Valves can press together in venules to restrict retrograde transport of blood


Describe veins

- Have a larger diameter than any accompanying artery
- Has a thinner wall that has more connective tissue and fewer elastic and muscle fibres
- Small and medium sized veins have well developed adventitia and thin tunica intima + tunica media
- Large veins have diameters > 10mm, thicker tunica intima, no prominent tunica media and a well-developed tunica adventitia


What are venue comitantes?

Deep, paired veins
- In certain anatomical positions they accompany 1 of the smaller arteries on each side of the artery
- 3 vessels are wrapped together in 1 sheath
- The pulsing of the artery promotes venous return within the adjacent, parallel, paired veins


Where does blood flow fastest?

Where the total cross-sectional area is least


Where does the heart lie?

In the middle mediastinum


What is the mediastinum?

The central compartment of the thoracic cavity
- Contains all the thoracic viscera and structures except the lungs
- Covered on each side by mediastinal pleura
- Highly mobile region because it consists of hollow visceral structures, united only by loose connective tissue
- The major structures in the mediastinum are also surrounded by blood and lymphatic vessels, lymph nodes, nerves and fat
- Divided into superior and inferior parts
- The inferior mediastinum is further subdivided by the pericardium into anterior, middle and posterior parts
- The pericardium and its contents constitute the middle mediastinum


What is the surface anatomy of the heart?

- A 'pyramid that has fallen over' with the apex of this pyramid pointing in an anterior-inferior direction
- Has 5 surfaces: anterior (right ventricle), posterior (left atrium), inferior (right + left ventricles), right pulmonary (right atrium) and left pulmonary (left ventricle)
- Borders: right (right atrium), inferior (left + right ventricle), left (left ventricle + some of left atrium) and superior (right + left atrium + great vessels)
- Pericardial sinuses


What are the pericardial sinuses?

Passages formed by the unique way in which the pericardium folds around the great vessels
- Oblique and transverse


What is the oblique pericardial sinus?

A blind ending passageway located on the posterior surface of the heart
- Put hand underneath then move it up towards the head until you reach a stop


What is the transverse pericardial sinus?

Found superiorly on the heart, located:
- Posteriorly to the ascending aorta and pulmonary trunk
- Anteriorly to the superior vena cava
- Superior to the left atrium


What is the pericardium?

- A fibroserous membrane that covers the heart and the beginning of its great vessels
- Closed sac
- Composed of 2 layers:
--- Fibrous pericardium: tough, external layer continuous with the central tendon of the diaphragm
- Serous pericardium: the parietal layer lines the heart and great vessels, mainly composed of mesothelium


What is the pericardial cavity?

A gap between the outer and inner serous layers
- Contains a small amount of lubricating serous fluid
- Minimises friction generated by the heart as it contracts and moves about


What are the functions of the pericardium?

- Fixes the heart in the mediastinum and limits its motion (as it attaches to the diaphragm, sternum and tunica adventitia)
- Prevents overfilling of the heart (relatively inextensible fibrous layer of the pericardium prevents the heart from increasing in size too rapidly)
- Lubrication (a thin film of fluid between the 2 layers of the serous pericardium reduces the friction generated by the heart)
- Protection from infection (fibrous pericardium serves as a physical barrier between the muscular body of the heart and adjacent organs prone to infection)


What does the phrenic nerve do?

- Innervated the pericardium
- Provides motor and sensory innervation to the diaphragm
- Originates in the neck and travels down through the thoracic cavity


What is pericarditis?

- Inflammation of the pericardium
- Main symptom = chest pain
- Has many causes, including bacterial infection and MI
- Can cause acute cardiac tamponade due to accumulation of fluid in the pericardial cavity


What is cardiac tamponade?

- The relatively inextensible fibrous pericardium can cause problems when there is an accumulation of fluid within the pericardial cavity
- The rigidity means that the heart is subjected to the resulting increased pressure
- The heart chambers can become compressed, thus compromising cardiac output
- Causes are many and varied, including pericarditis and haemopericardium


What are coronary arteries?

The end arteries that supply the whole heart
- They are branched
- Arise from the right and left aortic sinuses within the aorta (small openings within the aorta behind the left and right flaps of the aorta valve. When the heart if relaxed, the back flow of blood fills these valve pockets, therefore allowing blood to enter the coronary arteries)
- Left coronary artery initially branches to yield the left anterior descending artery
- LCA can then progress to become the left marginal artery and the left circumflex artery
- Right coronary artery branches to form the right marginal artery anteriorly and the posterior inter ventricular artery posteriorly


Describe the venous drainage of the heart

The heart is drained by a series of cardiac veins which in turn drain into the coronary sinus (main vein)
- The coronary sinus then drains into the right atrium


What region does the right coronary artery supply?

- Right atrium
- SA and AV nodes
- Posterior part of inter ventricular septum
Veins that drain region = small and middle cardiac veins


What region does the right marginal artery supply?

- Right ventricle
- Apex
Veins that drain region = small and middle cardiac veins


What region does the posterior interventricular artery supply?

- Right ventricle
- Left ventricle
- Posterior third of inter ventricular septum
Vein that drains region = left posterior ventricular vein


What region does the left coronary artery supply?

- Left atrium
- Left ventricle
- Interventricular septum
- AV bundles
Vein that drains region = great cardiac vein


What region does the left anterior descending artery supply?

- Right ventricle
- Left ventricle
- Anterior 2 thirds of inter ventricular septum
Vein that drains region = great cardiac vein


What region does the left marginal artery supply?

- Left ventricle
Veins that drain region = left marginal and great cardiac vein


What region does the circumflex artery supply?

- Left atrium
- Left ventricle
Vein that drains region = great cardiac vein


Describe the basic structure of the heart

- 2 pumps in series
- Each side consists of a thin walled atrium which acts as reservoirs to supply the muscular ventricle
- Flows into and out of the ventricle is controlled by valves:
--- Atrioventricular: mitral and tricuspid
--- Outflow: aortic and pulmonary


Describe heart muscle

- Specialised form of muscle
- Discrete cells connected electrically
- Cells contract when action potential arrives in the membrane
- Action potential is long
- They are triggered by spread of excitation from cell to cell, so all cells contract


Where do the right and left side of the heart pump blood to?

Right: lungs (pulmonary circulation)
Left: body (systemic circulation)


How does excitation spread over the heart?

- Over the atria to the AVN
- Delayed here for about 120 ms
- Down the muscular septum (via the bundle of His) between the ventricles to excite the ventricular muscle from the endocardial side to the epicardial surface
- Contraction spreads through the ventricular myocardium and up towards the AV junction where the valves are located


How does contraction of the heart occur?

- The apex of the heart contracts first and relaxes last to prevent back flow
- Contraction of the atria is not forceful
- The ventricular muscle is organised into figure of 8 bands which squeeze the ventricular chamber forcefully in a way most effective for ejection through the outflow valve


Why is the left ventricle more muscular than the right?

It is at a higher pressure to pump blood all the way round the body, so muscle must be thicker in order to withstand the pressure


What do ventricles do with blood?

- Fill from the veins during diastole
- Pump blood into the arteries in systole


What are pacemaker cells?

A small group of cells that generate an action potential which spreads over the whole heart producing a coordinated contraction
- They generate 1 action potential at regular intervals (1 a second)
- Each action potential produces 1 beat (systole, lasts about 280 ms)
- The interval between the beats is known as diastole (lasts about 700 ms)


How does the cardiac cycle start?

Towards the end of ventricular systole:
- Ventricles contracted
- Intraventricular pressure is high
- Outflow valves are open
- Blood is flowing into the arteries
- Ventricular pressure > atrial pressure so the atrioventricular valves are closed


What happens during diastole?

Ventricles begin to relax
- Intraventricular pressure falls, becomes < arterial
- Brief backflow (regurgitation of blood) closes the outflow valves
- All valves are now closed
- Isovolumetric relaxation occurs (so pressure falls)


What happens during diastole?

Blood has continued to return to the atria during systole:
- Atrial pressure is relatively high
- As intraventricular pressure falls, eventually, atrial pressure > intraventricular pressure
- So the atrioventricular valves open
With the a/v valves open, ventricles:
- Fill rapidly (rapid filling phase)
- Lasts about 280 ms
- Most filling of ventricles occur in this phase
The ventricles fill more slowly as diastole continues:
- Since they are almost full with blood
- Intraventricular pressure rises as the ventricular walls stretch
- This continues until intraventricular pressure matches atrial, and filling stops


Why does the rapid filling phase occur?

As the atria have been distended by continuing venous return during the preceding systole, the blood is initially forced rapidly from the atria to the ventricles, hence the rapid filling phase


What happens during atrial systole?

- Contraction of the atria
- Forces a small amount of blood into the ventricles
- Delay of 100-150 ms before the ventricles contract


What happens during ventricular systole?

- Intraventricular pressure rises very quickly
- Quickly exceeds atrial pressure
- So after brief back flow (regurgitation) the a/v valves close
- All valves are now closed
- The ventricles then contract isovolumetrically and intraventricular pressure rises rapidly
- Until intraventricular pressure > arterial pressure
- The arterial pressure has fallen during diastole
- The outflow valves are open
- Blood is ejected rapidly into the arteries (rapid ejection phase)
- Arterial pressure rises rapidly


What happens when arterial pressure rises during ventricular systole?

- The rate of ejection of blood falls
- Both arterial and intraventricular pressures peak towards the end of systole
- Outflow eventually ceases, with some blood still in the ventricle


Explain the origins of the 1st and 2nd heart sounds

Sound is produced by sudden acceleration and deceleration of structures or by turbulent flow
- 1st heart sound: as the a/v valves close, oscillations are induced in a variety of structures, producing a 'lup' sound
- 2nd heart sound: as the semi-lunar outflow valves close, oscillations are induced in other structures (including the column of blood in the arteries), producing a 'dup' sound


Explain the origins of other heart sounds

- 3rd sound: may be heard early in diastole
- 4th sound: sometimes associated with atrial contraction (usually in children)


Why can heart sounds split?

If valves of the left and right heart do not close at the same time:
- Due to faulty valves


What causes heart murmurs?

Turbulent flow of blood
- May be due to a narrowed valve or the valve not closing properly
- Occur when blood flow is highest
- Lup-whoosh-dup


What is the cardiogenic field?

The precursor to the heart, blood vessels and blood
- Created during gastrulation
- At first, it lies at the cranial end of the embryo before folding occurs


What does lateral folding do?

Creates a heart tube


What does cephalocaudal folding do?

Brings the tube into the thoracic region


What are the endocardial tubes?

A pair of tubes that develop within the cariogenic field as the CVS develops
- During 3rd week of development
- The tubes are brought together during embryonic folding
- They fuse in the midline to create the primitive heart tube
- The primitive heart tube is linear at first, receiving blood at its caudal pole and pumping blood from its cranial pole


What does looping of the heart do?

- Places both the inflow and outflow of the primitive heart tube cranially, with the inflow dorsal to the outflow
- Begins around day 23 and finishes around day 28
- The cephalic, cranial portion bends ventrally, caudally and to the right
- The caudal portion bends dorsally, cranially and to the left


How does the heart tube lie within the pericardial cavity?

Suspended by a membrane
- This membrane subsequently degrades


What is the role of folding?

- Puts the primordium of the right ventricle closest to the outflow tract
- Puts the primordium of the left ventricle closest to the inflow tract
- Puts the atrium dorsal to the bulbus cords (inflow dorsal to outflow)
- Creates the transverse sinus


How do the atria and ventricles communicate after looping?

Via the atrioventricular canal


How do the great vessels develop?

Sinus venosus:
- The embryo collects blood from the placenta, yolk sac and the body, all of which goes to the sinus venosus
- Right and left sinus horn are of equal horn
- Venous return shifts to the right-hand side, left sinus horn recedes
- Right sinus horn is absorbed by enlarging right atrium


What are the aortic arches?

Early arterial system
- Begins as a bilaterally symmetrical system of arched vessels
- They undergo extensive remodelling to create the major arteries leaving the heart
- Each arch has a corresponding nerve
- The course of this nerve is influenced by:
--- Caudal shift of the developing heart and expansion of the developing neck region
--- The need for a foetal shunt between pulmonary trunk and aorta


What derivatives are there of the aortic arches?

4th arch:
- R = proximal part of right subclavian artery
- L = arch of aorta
6th arch:
- R = right pulmonary artery
- L = left pulmonary artery and ductus arteriosus
No human derivative from 5th arch


How do the atria develop?

Right atrium develops from:
- Most of the primitive atrium
- Sinus venosus (right horn)
- Receives venous drainage from the body (venae cava) and the heart (coronary sinus)
Left atrium develops from:
- A small portion of the primitive atrium
- Absorbs proximal parts of the pulmonary veins
- Receives oxygenated blood from the lungs
- As the left atrium expands and absorbs the pulmonary veins, the oblique pericardial sinus is formed