CARDIOVASCULAR SYSTEM (CVS) l Physiology MCQs (multiple choice question) for dental students
INTRODUCTION in CARDIOVASCULAR SYSTEM (CVS)
- The cardiovascular, endocrine, and nervous systems constitute the principal coordinating and integrating systems of the body.
- Whereas the nervous system is primarily concerned with communication and the endocrine glands with regulation of certain body functions, the cardiovascular system serves to transport and distribute essential substances to the tissues and to remove metabolic end products.
- The cardiovascular system
also shares in many homeostatic mechanisms as regulation of body temperature, humoral communication throughout the
body, and adjustments of O2 and nutrient supply in different physiologic
states.
- The primary purpose of studying the cardiovascular system is to value its function and to describe the control mechanisms that are responsible for alteration of cardiac pump and blood distribution necessary to meet the changing requirements of different tissues in response to a wide spectrum of physiological and pathological conditions.
- The heart consists of two pumps:
- the right ventricle to that pumps blood through the lungs for O2 and CO, exchange of (the pulmonary circulation)
- the left ventricle that pumps blood to all other tissues of the body (the systemic circulation).
- So the right heart pump provides the energy necessary to move blood through the pulmonary vessels, and the left heart pump provides the energy to move blood through the systemic organs.
- Unidirectional flow through the heart is achieved by the presence of cardiac valves (semilunar and atrioventricular).
CHARACTERISTIC (PROPERTIES) OF THE CARDIAC MUSCLE
I] Cardiac Automaticity (Rhythmicity):
Definition:
- Automaticity is the ability of certain cardiac cells "pace maker" to spontaneously depolarize and generate an action potential.
N.B.
- Efficient pumping action of the heart requires a precise coordination of the contraction of millions of individual cardiac muscle cells.
- Contraction of each cell is triggered when an electrical excitatory impulse (action potential) spreads over its membrane.
- Simultaneous activation of the whole cardiac muscle cells is achieved primarily by the conduction of action potentials from one cell to the next.
- This conduction occurs via the gap junctions that connect all cells of the heart into a functional syncytium (i.e., acting as one synchronous unit).
- In addition, the pace maker, and conductive system of the heart are specifically adapted to control the frequency of cardiac excitation, the pathway of conduction, and the rate of the impulse propagation through the heart.
- The heart is a myogenic organ; it does not need nerve stimulation to initiate its contraction
Origin:
- The heart is a myogenic
organ; it does not need nerve
stimulation to initiate its contraction. The heartbeat originates in the
Sino-atrial node (SA node).
- SAN:
- Site: located at the junction of the superior vena
cava with the right atrium.
- Frequency of discharge: It contains specialized cells,
which discharge spontaneously and rapidly at a frequency of 60-100 per minute
(automaticity or rhythmicity).
- Excitability:
- It is
the most excitable among the myocardial cell
- Why? Because it has less negative potential than the
rest.
- Unstable RMP: It spontaneously depolarized during the diastole causing what is called pre-potential or diastolic depolarization (see later – slow action potential).
- Therefore it is the normal cardiac pacemaker: Its rate of discharge determines the heart rate (HR) (see later – slow action potential).
The cardiac action potentials:
The principal types of cardiac action potentials are the slow and fast types:
1- The slow type occurs in
nodal system:
- the Sino-atrial (SA) node "pacemaker
potential" which is responsible for cardiac automaticity.
- Atrio-ventricular
(AV) node.
2- The fast type occurs in the
cardiac muscle:
- atrial myocytes.
- ventricular
myocytes.
1- Slow action potential (for pacemaker cell) and its Ionic bases:
The slow response depends
mainly on voltage gated L-type Ca channels and consists of three phases:
● Phase 1 – Initiation and Pacemaker prepotential (funny current):
- Initiation of slow action potential (hyperpolarization):
- At the end of repolarization of the pacemaker K
outflux is maintained and causes hyperpolarization of the SAN (-60mv) → this
triggers Na channel to be activated and depolarization starts.
- Because this Na channel is activated during hyperpolarization rather than depolarization, it has termed the funny “f” current.
- Pacemaker prepotential:
- The Na entry drives the membrane potential towards more positivity → this potential opens of T-type Ca channels (T for transient i.e. open transiently) → both Na and Ca cause slow depolarizing baseline (which is the key to automaticity) → drive the membrane potential to the firing (-40mV).
● Phase 2 – Depolarization after firing level:
- By reaching the firing level, the membrane potential is suitable to open the L-type Ca channels (L for long-lasting) → causing depolarization.
- N.B: Ca channels:
- The calcium
entry due to opening of T-channels completes the prepotential.
- The calcium
entry due to opening of L-channels produces impulse (depolarization).
- The slow response depends mainly on voltage gated
L-type Ca channels.
● Phase 3 – Repolarization and hyperpolarization:
Repolarization:
- At the
peak of depolarization: L-type Ca channels close and voltage gated K channels
open →membrane repolarization occurs → membrane potential declines to the
firing level (-40 mV).
Hyperpolarization:
- After the membrane potential reaches the firing level
(-40 mV) → more K is outfluxed after → causes hyperpolarization to the
pacemaker cells → this hyperpolarization activates Na channels starting the
current again “funny current”.
- Without
hyperpolarization, the Na channels cannot be opened and the pacemaker would
cease.
2- Fast action potential (for myocardial cell) and its Ionic bases:
- The fast response depends mainly on voltage gated Na channels and consists of five phases:
● Phase O: Initial Rapid depolarization & overshoot (a fast response action potential):
- caused by: opening of voltage-gated
- Na+ channels leading to rapid Na influx.
- the cell membrane rapidly depolarizes and the potential
- difference reverses (positive overshoot, upstroke), such that the potential of the interior of the cell exceeded that of the exterior by about 20 mV (i.e. +20mv)
● Phase 1: The early partial repolarization:
- caused
by activation of voltage-gated K+ channels producing a transient outward K+
● Phase 2: prolonged sustained depolarization (The plateau):
- the flat portion of the curve persists for about 0.1
to 0.2 seconds.
- caused by: influx of Ca through voltage gated L- type
Ca++ channels, that is balanced by the efflux of an equal amount of positive
charge carried by K+.
- N.B.
- L- type Ca++ channels is inactivated much more slowly than fast Na channels.
- Ca entery during this phase is involved in excitation
contraction coupling (see later - contractility)
● Phase 3: Repolarization phase:
- efflux of K+ from the cardiac cell begins to exceed
the influx of Ca', until the resting state of polarization is regained.
- N.B.
Repolarization (phase 3) is a much slower process than depolarization (phase 0)
● Phase 4: Complete repolarization:
- The interval from the end of repolarization until the
beginning of the next action potential is designated.
- when the membrane potential goes back to the resting level (-90 mv) → Na-K pump works to drive the excess Na+ out and the excess K+ in.
● Restoration of Ionic Concentrations
- The excess Na that entered the cell rapidly during
phase 0 and more slowly throughout the action potential is removed from the
cell by the action of the enzyme Na+/K+-ATPase.
- Ca that
had entered the cell during phase 2 is eliminated by:
- Most of this
Ca is eliminated by Na/Ca exchange, which exchanges three Na+ for one Ca.
- Small fraction of the Ca is eliminated by an adenosine
triphosphate (ATP)- driven Ca' pump.
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