Heart 1 Flashcards
What does the heart do?
Circulates blood, about the size of a fist, enclosed by pericardium
3 layers of the heart:
- Epicardium: inner layer of pericardium
- Myocardium: muscle
- Endocardium: thin sheet of endothelium
Myocardium:
-cardiac muscle (arranged in spirals)
-fibrous skeleton (collagen and elastin, provides support, electrical insulation)
Anatomy of the heart:
4 chambers:
-Right and left ratio (RA,LA)
top receiving chambers
-Right and left ventricles (RV, LA)
bottom larger pumping chambers
separated by septa:
-interatrial septum
-interventricular septum
Valves:
-Atrioventrcular (AV) valves
between atria and ventricles
open when pressure is greater in atria
close when pressure is greater in ventricles
-Semilunar valves
between ventricles and large arteries
open when pressure is greater in ventricles
close when pressure is greater in arteries
Heart valve defects cause…
increased workload of heart
Pulmonary vs systemic circuit
-capillary beds of lungs where gas exchange occurs
-capillary beds of all body tissues where gas exchange occurs
Cardiac muscle fibers:
-striated (contain sarcomeres)
-single nucleus
-short, fat, branched
-large and numerous mitochondria
adjacent cells are connected by intercalated discs (allows AP to spread easily between cells, acts as single coordinated unit called syncytium)
Syncytiums:
- Atrial syncytium: walls of two atria
- Ventricular syncytium: walls of two ventricles
separated by fibrous tissue
APs are conducted between the two through the AV node
Cardiac muscle vs skeletal muscle
-in both: contraction in stimulated by an action potential
differ from skeletal muscle contraction in 3 ways:
1.means of stimulation
in cardiac muscle, 1% of cells are self-excitable
2. organ vs motor unit contraction
in skeletal muscle; not all fibers contract with each impulse, only the motor unit that is stimulated
in cardiac muscle, all cardiac fibers contract as unit, or none do
3. length of absolute refractory period
250ms in cardiac cells vs 15ms in skeletal fibers
Cardiac muscle AP:
- depolarization
action potential opens voltage gated Na+ channels, Na+ flows into cells (3-5ms)
2.Plateau
slow Ca2+ channels open in sarcolemma, Ca2+ flows into cell, triggers opening of ligand gated Ca2+ channels on SR, few K+ channels open(~175ms) - Depolarization
slow Ca2+ channels close, Ca2+ is pumped back out, voltage gated K+ channels open, K+ flows out go cell (~75ms)
-longer AP leads to longer contraction
-longer refractory period prevents tetany: condition in which muscle remains involuntarily contracted
Sequence of events for contraction:
- depolarization
stimulus causes fast Na+ channels to open, Na+ enters cell, reverses membrane potential, rising phase of AP - depolarization wave opens calcium channels
Ca2+ enters sarcoplasm, plateau phase of AP - excitation-contraction coupling
cross-bridges can form as Ca2+ binds to troponin
Calcium and muscle contraction: Ca+
-like in skeletal muscle, calcium is required to bind to troponin, moves tropomyosin and allows cross-bridge formation between actin and myosin
-10-20% of Ca2+ comes from the extracellular fluid (channels open due to depolarization)
-remaining Ca2+ is released from SR (releases triggers by extracellular Ca2+ binding to ligand gated calcium channels)
Calcium and muscle contraction: strength
-strength of contraction depends on concentration of Ca2+ in ECF
heart placed in calcium-free solution will stop beating
skeletal muscle is not affected the same way, as Ca2+ comes entirely from the SR, not ECF
Calcium and muscle contraction: where does it go
-during repolarization, Ca2+ is pumped back into the SR or extracellular space which leads to muscle relaxation and requires ATP
