Chapter 20: Heart Flashcards
(37 cards)
What is the size of the heart and where is it located?
The human heart is roughly the size of a closed fist (300g), is cone-shaped, and resides in the center of the thorax in a cavity called the mediastinum.
Located in the mediastinum in the thoracic cavity within the pericardial cavity surrounded by pericardial fluid
- the cavity is lined by a delicate serous membrane (visceral and parietal)
the fibrous pericardium is a very dense and non-flexible connective tissue that helps protect and anchor the heart
What are the functions of the cardiovascular system?
- Transport gas, nutrients, hormones and metabolic wastes
- Regulation of pH and ion composition of interstitial fluid (neutralizes lactic acid, controls Ca++ and K+ concentrations)
- Restriction of fluid loss at injury sites
- Defense against toxins and pathogens
- Stabilization of body temperature
What are the components of the cardiovascular system?
- Blood: transport medium
- Heart: muscular pump that moves blood around the body
- System of tubes / vessels: arteries, veins, and capillaries
Where is the base of the heart and apex?
Base: top
Apex: bottom
How are blood vessels arranged?
2 circuits:
- Pulmonary: to and from the lungs
- pulmonary veins and arteries
- Systemic: to & from the rest of the body
- systemic arteries and veins
What are the layers of the serous pericardium? What is it?
A serous membrane with two layers
1) visceral pericardium: also called the epicardium, adheres closely to the outer surface of the heart
2) parietal pericardium: lines the inner surface of the pericardial sac
location and function of pericardial fluid: 15-50 mL provides lubrication between parietal pericardium and visceral pericardium
epicardium: visceral mesothelium (simple squamous epithelium and areolar tissue)
parietal mesothelium: epithelial part of the parietal pericardium
What are the layers of cardiac tissue?
1) Pericardium: a loosely fitting connective tissue sac that surrounds the heart. the pericardial cavity contains a small amount of lubricating serous fluid that allows the heart to glide smoothly against the pericardium
- areolar tissue
2) Myocardium: the heart’s muscular layer
- Cardiac muscle tissue
3) Endocardium: a layer of simple squamous epithelial tissue that intimately covers the inner chambers of the heart (atria and ventricles)
- Areolar tissue and endothelium
What is cardiac muscle tissue?
Cardiac muscle, like skeletal muscle is striated. Its fibers are shorter, branch, and typically only have one centrally located nucleus
mitochondria account for 25% of cellular volume
cardiac muscle cells connect to and communicate with neighboring cells forming 2 functional synctiums (atrial and ventricular) through gap junctions in intercalated discs
all or nothing principle
What are the chambers of the heart?
4 chambers, left and right atria and ventricles
Atria
Right atrium: receives deoxygenated blood from the superior vena cava (draws from head and upper limbs) inferior vena cava (draws from trunk and lower limbs) and coronary sinuses (draws from cardiac veins)
Left atrium: receives blood from left and right pulmonary veins that draw from the lungs
Ventricles
Right ventricle: pumps blood into the lungs via the right and left pulmonary arteries
Left ventricles: pumps blood into the body via the aorta
What are the heart valves?
Flaps of dense connective tissue that act as one way valves
(i.e. blood can only flow in one direction). Changes in pressure determine the flow of blood in the heart and cause the valves to open and close. Blood moves along a pressure gradient from high to low
two AV valves:
R. tricuspid valve
L. bicuspid / mitral valve
two semilunar valves:
between left ventricle and aorta
between right ventricle and pulmonary artery
How are the cardiac valves open and close?
Changes in pressure determine the flow of blood in the heart and cause the valves to open and close. Blood moves along a pressure gradient from high to low
Opening the AV valves:
1) blood returning to the heart travels through the atria and force the AV valves to open
2) the atria contract forcing additional blood into the ventricles
Closing the AV valves:
1) the ventricles contract forcing blood against the AV valves
2) the AV vales close (first heart sound)
3) papillary muscles contract to stabilize the valves
Opening the Semilunar valves:
- as ventricles contract and intraventricular pressure rises, blood is pushed up against semilunar valves forcing them open
Closing the Semilunar valves:
- second heart sound
- as ventricles relax and intraventricular pressure falls, blood flows back from arteries, filling the cusps of semilunar valves and forcing the to close
Describe normal valves (flow):
Normal valves result in normal flow, no regurgitation
Normal valves open: laminar flow = quiet
Normal valves close: no flow = quiet
Describe stenotic valves and insufficient valves
Stenotic valves result in reduced blood flow
Stenotic valves open: narrowed valve, turbulent flow = murmur
Insufficient valves result in reduced blood output due to regurgitation. If it folds backwards it is called “prolapse”
Insufficient valves close: leaky volume, turbulent backflow = murmur
the production turbulence by an atherosclerotic plaque deposit, causing increased resistance and slower blood flow
What are the steps of the cardiac cycle?
Step 1: sinoatrial node reaches threshold and generates an action potential and atrial activation begin (60-100 action potentials per minute at rest)
Step 2: depolarization is spread to all the cells of the atria via gap junctions and internodal pathways. A wave of contraction follows the wave of depolarization
Step 3: Depolarization reaches the AV node, which conducts the electrical impulse more slowly: 100 msec delay. This gives atria time to empty into ventricles
Step 4: The action potential is conducted from the AV node through the AV bundle and down the left and right bundle branches
Step 5: The action potential reaches the Purkinje fibers which conduct the impulse through the ventricles where it passes from cell to cell in the contractile fibers via gap junctions
as ventricle depolarizes, ventricles are filled, AV valves close and the Ca++ within each ventricular cardiac muscle cell is released from storage in the sarcoplasmic reticulums, triggering the simultaneous contraction of each ventricular cell
ECG tracing and the cardiac cycle:
P-wave: atrial depolarization
P-R interval: conduction through AV node and AV bundle
Q wave: beginning of ventricular depolarization
QRS complex: completion of ventricular depolarization
What is the cardiac cycle?
Like neurons and skeletal muscle fibers, cardiac cells are capable of generating electrical signals (i.e. excitable tissue)
- these electrical signals, called action potentials, lead to the event of contraction in a similar manner as skeletal muscle fibers (i.e. release of calcium into the cytosol causing the interaction of actin and myosin)
- the cardiac cycle refers to the repetitive contraction and relaxation of the heart of the heart
Systole: a term used to describe the contraction phase of the cardiac cycle
Diastole: a term used to describe the relaxation phase of the cardiac cycle
In order for the cardiac cycle to be effective, proper timing is essential. Time must be permitted for the ventricles to fill, the atria must contract prior to the ventricles, and both ventricles must pump the same amount of blood at the same time
the initiation of the cardiac cycle and the coordination of the complex timing of events is achieved by the cardiac conduction system
What is the cardiac conduction systems?
Cardiac contraction is initiated by a group of specialized cells called the sinoatrial SA node. These myocytes have the ability to spontaneously depolarize
- the electrical signal then travels through the interconnected atrial cells and reaches the atrioventricular node (AV node), where the signal is slightly delayed
- the signal is then delivered to the ventricles by the bundle of His and the Purkinje fibers
What is the bundle of his?
Collection of muscle cells specialized for electrical conduction from the AV node to the point of the apex of fascicular branches
- the fascicular branches lead to the purkinje fibers which excite the ventricles, causing the ventricle muscle to contract at a paced interval
What does the purkinje fibers do?
Excite the ventricles, causing the ventricle muscle to contract at a paced interval
What happens during cardiac cycle (syst/diast)
Start: 0 msec; (AV valves open, semilunar closed)
(a) atrial systole begins: atrial contraction forces a small amount of additional blood into relaxed ventricles
(b) 100 msec atrial systole ends, atrial diastole begins (AV valves closed, semilunar closed)
(c) ventricular systole: first phase: ventricular contraction pushes AV valves closed but does not create enough pressure to open semilunar valves
(d) ventricular systole: second phase: as ventricular pressure rises and exceeds pressure in the arteries, the semilunar valves open and blood is ejected (AV valves closed, Semilunar valves open, ends at 370msec)
(e) ventricular diastole - early: as ventricles relax, pressure in ventricles drops; blood flows back against cusps of semilunar valves and forces them closed. Blood flows into the relaxed atria (Semilunar valves closed, AV closed)
Isovolumetric relaxation
(f) ventricular diastole - late: all chambers are relaxed. Ventricles fill passively (Semilunar valves closed, AV open)
What is SA node and its action potential?
The cells of the SA node are responsible for initiating cardiac contraction. They achieve this by spontaneously depolarizing.
- the plasma membrane of these cells is leaky to Na+. This prevents the cells of the SA node from ever achieving a true resting state. (i.e. as soon as they repolarize, they start depolarizing again)
- the ANS controls the heart rate by influencing the rate of spontaneous depolarization in the SA node
- the SA node naturally discharges 100 times per minute
- therefore at rest we have a predominate PSNS tone to keep the resting heart rate at 60-80 bpm
What are pacemaker cells and contractile cells? Pg. 38
pg.38
Cardiac action potential and the ECG
The action potential in cardiac cells is very similar to that observed in neurons and skeletal muscle, with one exception, cardiac cell membranes contain Ca++ channels, therefore Ca++ moves into the cells during depolarization and creates a plateau phase.
- This influx of Ca++ causes the release of more Ca++ from the sarcoplasmic reticulum, also called the calcium induced calcium release
- when electrodes are placed on the skin, an electrical signal of the entire heart can be observed. (ECG)
- The ECG is an important diagnostic tool as it can detect abnormalities in cardiac conduction
