Week 9 Flashcards
When do papillary muscles contract
When the valve flaps are closed at the same time that the ventricles contract. They serve to keep the valve flaps tightly closed and avoid them flipping in the wrong way. Chordae Teninaea connect papillary muscles to valves and are pulled tight
What are the semilunar valves? Name the two types and what their purpose is
Pulmonary semilunar valve: located between the right ventricle and the pulmonary artery (pulmonary trunk)
Aortic semilunar valve: located between the left ventricle and the aorta. This valve has two little holes, otherwise looks the same as pulmonary.
These valves open during ventricular contraction to allow blood to enter circulation. They don’t deal with very much pressure so are not supported by chordae tendineae. They snap shut as ventricles relax to prevent back flow of blood into ventricles.
Why are there no one-way valves for blood flow into the atria? what other features of the atria create this effect (four total)
The atria has continuous, uninterrupted blood flow! Four characteristics make this possible:
- No valves to interrupt blood flow
- Atrial systole contractions do not contract to the extent that would block blood flow, blood continues to flow into atria during atria systole
- Atrial contractions must be gentle enough to not exert any back pressure
- Atria relaxation is timed to relax before the start of ventricular contraction
Overall this allows atria 70% more cardiac output.
Which valve is most likely to fail?
Left / mitral / bicuspid. Left side of heart has way more pressure and so is more prone to failure.
What is heard during auscultation on the chest
First sound or Lub or S1: produced by closing of AV valves during ventricular contraction or Systole
Second sound or Dub or S2: produced by closing of semilunar valves when ventricles relax or Diastolate. Louder than the first sound.
Name common causes of heart murmurs*
- Defective heart valves: for example aortic valve stenosis (narrowing) due to calcium deposits produces mid-systolic heart murmur
- Rheumatic Endocarditis: streptococcal bacterial infection causes antibodies to attack heart valves “Rheumatic fever”
- Mitral Stenosis: mitral valve thickens and calcifies causing a rise in left atrial and pulmonary vein pressure resulting in hypertension
- Mitral Valve Prolapse: most common cause of mitral regurgitation (blood flows backward into atrium). Both congenital (excess leaflet material) and acquired. Most people lack symptoms and have normal lifespan. Chordae Tendinae rupture can worsen regurgitation and replacement with mechanical or biological valve may be needed
- Septal defects: holes in the septum between right and left sides. Congenital. May lead to hypertension and edema
Describe all the steps in the cardiac cycle. *Which step contributes the most blood volume to the ventricles?
- Quiescent period (relaxation phase): When atria and ventricles are both relaxed, venous blood fills the atria. When atria pressure is greater than ventricular, the AV valves open allowing blood to flow from atria to ventricles. While all 4 chambers are relaxed is when 80% of the blood fills the ventricles (rapid filling stage)
- Atrial Contraction (systole): Contraction of the atria adds the final 20% volume to the End-Diastolic Volume
- Isovolumetric contraction: Ventricles begin to contract (systole) and AV valves shut (produce S1 lub). Both AV and semilunar valves are closed
- Ejection: Ventricle pressure becomes greater than aorta and opens semilunar valves to eject two thirds (ejection fraction) of the blood they contain. This is the Stroke Volume. The one third that remains is the End-Systolic volume
- Isovolumetric Relaxation: ventricle pressure falls below artery pressure and semilunar valves close (produce S2 dub). Both AV and semilunar valves are closed and heart is in diastole
what is contraction and relaxation of the heart called
Systole = contraction Diastole = relaxation
When the words are used without reference to specific chambers they usually refer to the ventricles. However atria do have systole and diastole, we just care more about the ventricles
What happens to the function of the heart if the atria don’t contract? why?
The heart still pumps and people can survive with an atria that does not contract. They are more at risk for blood clots leading to stroke, however. This is because most of the blood flow from atria to ventricles is accomplished passively and atrial contraction only contributes about 20% to the final ventricular volume. So the heart is still functioning at like 80% capacity
What blood pressure measurements occur at each state of the cardiac cycle? Mostly focus on left ventricle, short description of right ventricle. Where do we measure blood pressure?
In diastole: arteries 80 mmHg
Ejection (when left ventricle pressure is greater than aorta): both ventricle and aorta 120 mmHg
Isovolumetric relaxation (when ventricle pressure is below arteries pressure): left ventricle 0 mmHg and aorta 80 mmHg
Similar events occur in right ventricle (pulmonary circulation) but at lower pressure which a maximum of 25 mmHg during systole and 8 mmHg at diastole
*A persons blood pressure is measured at the brachial artery
What is the dicrotic notch
On a graph of arterial pressure, there is a short inflection in the descending portion that is produced by the closing of the elastic aortic and pulmonic semilunar valves. The closed semilunar valves are stretched and lead to a slight drop in blood pressure, then they recoil which leads to a quick upsurge in blood pressure. This dicrotic notch is associated with the second heart sound S2 created by closing of the semilunar valves
Review the electrical conduction of the myocardium. What is a term to describe the single functioning unit?
Myocardial cells are interconnected by gap junctions which are electrical synapses. The entire mass of interconnected cells is the myocardium and it can be called a Functional Syncytium because any action potentials that originate in any cell in the mass can be transmitted to all the other cells.
What creates the automaticity or intrinsic rhythmicity of the heart? Name the cells, not the mechanism
Pacemaker cells
- Sinoatrial node (the pacemaker): located in right atrium near superior vena cava, serves as the primary pacemaker.
- AV node: normally suppressed by SA node. Has slower diastolic depolarization and action potential production. Also called an ectopic pacemaker or ectopic focus because it is abnormal to use
- Purkinje fibers: normally suppressed by SA node. Has slower diastolic depolarization and action potential production. Also called an ectopic pacemaker or ectopic focus because it is abnormal to use
*Describe the mechanism that creates intrinsic rhythmicity in the heart
Sinoatrial Node cells exhibit a slow spontaneous depolarization called the Pacemaker Potential, which occurs during diastole (also called diastolic depolarization). This is created by ion channels:
- HCN channels: open in response to HYPERpolarization to allow Na+ to enter and cause depolarization (“Funny Current”). They also open in response to cAMP (from beta adrenergic signaling nor/epinephrine)
- Dihydropyridine Receptors: once diastolic depolarization reaches threshold of -40 mV, DHP receptors open and allow Ca2+ to flow inside the cell. The Ca2+ binds RyR2 ryanodine receptors to cause huge calcium increase to depolarize cell. (Ca2+ induced Ca2+ release mechanism)
- Voltage gated K+ channels: open after depolarization max is reached and repolarize cell by outward K+ migration
How is heart rate increased or decreased? What specific molecules are involved and what do they bind to?
Increase: sympathoadrenal stimulation produces (nor)epinephrine which causes cAMP production in pacemaker cells which opens HCN channels and promotes entry of Ca2+. Causes an increased diastolic depolarization which produces a faster cardiac rate and strength of contraction
Decrease: parasympathetic vagus nerve (X) releases acetylcholine which bind muscarinic receptors and (through G proteins) open K+ channels. This outward diffusion of K+ slows diastolic depolarization and slows cardiac rate
Is the SA node a single uniform structure? What is the name for the normal rhythm produced by the SA node and how does it time the atrial and ventricular contractions appropriately?
Not a uniform structure. Consists of different pacemaker regions around the right atrium that are electrically separated and communicate through Sinoatrial Conduction Pathways. Action potentials spread through these pathways to depolarize atria and ventricles, but path to ventricles is longer (AV node, bundle of His, Purkinje fibers) so that ventricles will contract slightly after atria. This is how the sinoatrial node produces a Normal Sinus Rhythm.
*Describe the unique feature of a myocardial action potential
NOTE: there are 2 separate action potentials required for a heart: the sinoatrial node produces the pacemaker potential which THEN stimulates the myocardium to produce its own action potential
Myocardial cells are stimulated by pacemaker potentials that depolarize to a threshold where Fast Na+ Channels open. The level of depolarization is maintained during a Plateau Phase before repolarization! Plateau phase is a result of a slow inward diffusion of Ca2+ through Slow Ca2+ Channels (dihydropyridine receptors) which balances a slow outward K+ diffusion. Rapid repolarization at the end of the plateau phase is achieved by opening of voltage gated K+ channels
The plateau phase is unique and needed to give time for contraction to occur! The Ca2+ that enters during this phase signals contraction (RyR2) so that contraction is complete before the membrane recovers from refractory period. This prevents summation from occurring
How are action potentials conducted from the atria to the ventricles (through the nonconductive fibrous skeleton)
Specialized conducting tissue is required: the Atrioventricular (AV) Node, the bundle of His, and the Purkinje Fibers.
Action potentials generated by the SA node spread through atria into the AV node (located on interatrial septum). After a brief delay, the impulse continues through the bundle of His (atrioventricular bundle) at the top of the inter ventricular septum. This descends through the fibrous skeleton and divides into left and right bundle branches, which are continuous with the Purkinje fibers in the ventricle walls. The delay at the AV node is important for timing ventricular contraction 0.1-0.2 seconds after atrial contraction
Review the excitation-contraction coupling mechanism of the myocardial cells
Ca2+ induced Ca2+ release is stimulated by action potentials which open Dihydropyridine voltage gated Ca2+ channels in the plasma membrane and allow a puff of calcium into the cytoplasm. This calcium binds the ryanodine RyR2 receptors in the sarcoplasmic reticulum and opens them, allowing a large influx of calcium into the cytoplasm. The calcium binds troponin C and allows contraction.
These events occur at Signaling Complexes, where sarcolemma is close to sarcoplasmic reticulum.
How is relaxation of the muscle during repolarization of the myocardial cells achieved?
Ca2+ concentration is lowered by Sarcoplasmic Reticulum Ca2+ ATPase (SERCA) pumps which actively transport calcium into the SR. Calcium is also extruded into the extracellular fluid by the Sodium Calcium Exchanger (NCX) via secondary active transport and the Ca2+ ATPase pump via primary active transport. These ensure myocardium relaxes during repolarization during diastole.
What prevents the heart from sustaining a contraction? Why is this important?
Because of the plateau phase which creates a long refractory period where the heart cannot be stimulated until it has relaxed from the previous contraction. This ensures rhythmic pumping of the heart
Describe the normal ECG waves. How does the device record an ECG? What action of the heart produces each wave?
Electrocardiograph’s record ion flow (not action potentials directly) to produce an electrocardiogram (ECG or EKG).
1. P wave: depolarization of the atria. When about half of the atria is depolarized the upward deflection is at max value. When the entire atria is depolarized the ECG returns to baseline.
(atria repolarization is covered up by the QRS wave)
2. QRS wave: depolarization of the ventricles. Same as with atria, it peaks at half depolarized and returns to baseline at full depolarization.
(S-T segment is the plateau phase)
3. T wave: repolarization of the ventricles. Note that the wave points in the same direction as QRS despite being opposite potential changes, this is because depolarization occurs from endocardium to epicardium while repolarization spreads in the opposite direction.
Which sounds are associated with each wave on and ECG
QRS wave occurs at systole (depolarization) so the shutting of the AV valves occurs immediately after the QRS wave. This is the first heart sound (S1 or lub)
T wave occurs during diastole (repolarization) of ventricles so the shutting of the semilunar valves occurs shortly after the T wave. This is the second heart sound (S2 or dub)
What are the three coats that compose the walls of arteries and veins (not capillaries!)
Tunica externa: connective tissue Tunica media: smooth muscle Tuncia interna (intima): has three parts, an innermost epithelium (endothelium) which lines all vessel lumina, a basement membrane (glycoproteins0, and an internal elastic lamina formed from elastin fibers.
