Lecture 10: Functional anatomy of CV system Flashcards
How does the heart maintain homeostasis?
Provides potential energy (pressure)
Moves oxygen, nutrients, electrolytes, hormones to meet metabolic requirements
Removes waste (nitrogenous waste and carbon dioxide) that are by-products of oxidation
What is heart failure?
When the heart muscle is unable to contract optimally to pump blood (into systemic and/or pulmonary circulation) at a rate sufficient to meet the requirements of metabolizing organs
What is the incidence of heart disease?
Very high
Complications after onset of heart failure remains a primary cause of death in Canada and other developed nations
Different terminology for heart failure
Acute heart failure = cariogenic shock
Chronic heart failure = heart failure (used to be called congestive heart failure)
High output failure = high CO in heart failure
Symptoms of heart failure
Congestion in lungs results in breathlessness (main symptom of left-sided heart failure)
Fatigue, swelling in ankles and legs, enlargements of liver
Components of the CV system
1) The heart
2) The blood vessels
3) Blood itself
What is the heart?
A pump that serves to pressurize the arterial tree
In what direction does blood move in systemic circulation?
From an area of high pressure (left ventricle) to regions of lower pressure (capillary beds).
Where does a major loss of blood pressure occur?
Points of restriction within the arterial tree (arterioles)
What is the role of the blood vessels?
Serve as the delivery mechanisms for blood as it moves from the heart and for its return to the heart
Arteries: carry blood away from heart
Veins: carry blood to the heart
** naming is independent of the state of blood oxygenation
Two loops of the circulation
Blood travels continuously through the closed circulatory system via two separate loops which begin and end at the heart
Pulmonary circulation
Systemic circulation
Heart works as a dual parallel pump
See figure
Where does pulmonary circulation carry blood?
Between heart and lungs
Where does systemic circulation carry blood?
From the heart to all the peripheral organ systems (brain, liver, skeletal muscles, skin)
Shape and location of heart
Fist sized, hollow muscular organ
Located mid-chest between the sternum and the vertebrae
Left ventricle is a thick spiral and RV is an add on
What is the left side myth of the heart
Heart’s apex thumps the chest wall to the left of the sternum, so people concluded that the whole heart is located there
Chambers of heart
Upper: atria (receive returning blood and transfer to ventricles)
Lower: ventricles (pump blood from the heart)
What separates the two halves of the heart? Role?
Ventricular septum
Part of left ventricle
Directly contributes to cardiac pump function,
Prevents mixing of blood from the low oxygenated blood in the right and the highly-oxygenated blood in left side.
Where does systemic return enter the heart?
Right atrium via vena cava
Blood the flows into RV, which pumps the blood out via the pulmonary artery to the lungs, where it is oxygenated
Which side of the heart pumps blood into pulmonary circulation?
Right side
What happens to blood in the lungs? After the lungs?
Blood loses CO2 and gains O2 before returning to the LA via the pulmonary vein
Blood moves from LA to LV to the aorta and thus into the systemic circulation
See figure
Which loop of circulation does the left side of the heart handle?
Systemic
What does the aorta do?
Carries blood away from the LV.
How is output from the LV distributed?
So the each relevant capillary bed receives a fresh supply of blood.
What happens in the capillary beds?
02 is extracted from the blood, and its then shunted back to the heart via the veins.
How much blood does each side of the heart pump per unit time relative to each other?
Both sides of the heart pump equal volumes of blood per unit time
BUT, the left side does so at high pressure (120 mmHg max systolic pressure), whereas the right side is a low pressure circulation (25 - 30 mm Hg max systolic pressure).
When do the RV and LV contract?
Together
Otherwise, you would have a waggle in the septum
Why does the LV do more work per unit time than the RV?
LV is much more massive than RV
LV is able to easily generate more contractile force. It is required to do so, because its pumping an equal volume of blood into a high resistance system.
What do cardiac valves allow?
Unidirectional movement (forward)
Allows efficient pump action
Behave like one-way doors, open in response to pressure.
Backward pressure forces the valves closed.
Role of atrioventricular valves
Right and left
allow for the movement of blood from the atria to the ventricles during filling.
Keep blood in the ventricles when they contract.
Rising ventricular pressure (blood trying to move backward) forces them closed.
What is the right AV valve called?
tricuspid (three cusps)
What is the left SV valve called?
mitral (two cusps)
What is the role of the chordae tendinae?
AV valves are anchored by fibrous chordae tendinae, which are attached to pupillary muscle
Tendinae prevent leaflet eversion with high pressure.
Where are the semilunar valves? Role?
Opening of aorta
Opening of pulmonary artery
Govern blood flow where major arteries leave the ventricles
How many cusps do the semilunar valves have?
Three
Functioning of semilunar valves
When ventricular pressure exceeds that in the aorta (systole), they are forced open
Close when blood tries to regurgitate
What is the heart wall composed of?
Spirally arranged cardiac myocytes (muscle fibres)
Muscle cells are branched and interconnected, have intercalated discs to allow passage of ions
Form an electrical syncytium
What are the junctions between myocytes called? Function of these junctions?
Intercalated discs
Porous, allow electrical connectivity
Contain gap junctions (low resistance regions, electrical connections) and desmosomes (structural anchoring)
See figure
Layers of the heart
Endocardium (endothelial tissue)
Myocardium (muscle)
Epicardium (thin external membrane)
Electrical connections between atria and ventricles
Electrically isolated
Except for specialized connections or ‘wiring’ (internal pathway)
What separates the atria and ventricles electrically?
non-conductive fibrous skeleton
What does the spiral arrangement of myocytes in the heart allow?
allow the heart to wring blood from ventricular cavities with each contraction.
Difference between cardiac and skeletal muscle contraction
Cardiac contraction uses all or none recruitment of muscle cells
Number of mitochondria in cardiac myocytes
A lot
Heart needs a lot of ATP
Depends on anaerobic metabolism
Generation of new heart cells after birth
Does not happen (although this concept is now challenged)
Increase in heart size occurs due to increase in cell size
What is the pericardial sac?
double walled serous membrane that holds the heart in position.
Removed during cardiac transplantation.
What is secreted by the pericardial sac? Role?
Pericardial fluid
Reduces resistance to movement
What does haemorrhage into the pericardial space cause?
May compress the heart and lead to dysfunction our to insufficient filling
Characteristics of cardiac contractions
Must be coordinated for efficient pumping
The heart functions as two separate pumps in series.
Atrial excitation and contraction normally precedes ventricular contraction.
Contraction of each chamber must occur as a unit.
Contraction of the pair of atria and the pair of ventricles must occur simultaneously.
What does slow conduction through the AV node allow?
Slow conduction through the AV node ensures a delay between contraction of the atria and the ventricles.
Allows the atria to send blood and ventricles to fill properly
How is coordination of contraction achieved?
Structurally
What initiates contraction in the atria?
SA node (pacemaker)
How does excitation occur throughout the atria?
Via cell to cell contact (gap junctions) and the interatrial pathway.
How is the AV node excited?
through the internodal pathway and by cell to cell contact.
Spread of cardiac excitation
An action potential initiated at the SA node first spreads throughout both atria.
Its spread is facilitated by the interatrial and internodal pathways.
The AV node is the only point where an action potential can spread from the atria to the ventricles
From the AV node, the action potential spreads rapidly throughout the ventricles, hastened by a specialized ventricular conduction system consisting of the bundle of His and Purkinje fibers.
Pacemaker activity
The pacemaker of the heart is the structure that is firing the fastest
What would happen if the SA node failed?
AV node would take over and set the rate
What would happen if only the AV node failed?
The atria would fire at the SA node rate
The ventricles would fire at the purkinje fibre rate
What happens if an ectopic focus takes over as the pacemaker?
Ectopic focus goes faster than SA node
Whole heart is driven by rapidly abnormal pacemaker
What is Excitation-contraction coupling
In cardiac muscles, electrical signal is converted into contractile signal
What is the signal for heart contraction?
Elevation of cytosolic Ca2+ derived from ECF and sarcoplasmic reticulum
Creates a plateau in the AP graph
What does elevated Ca2+ lead to?
Cross-bridge cycling between actin and myosin
What does relationship between electrical and mechanical events of heart prevent?
Tetanus
