Archive for the ‘ L1-CVS (Heart) ’ Category

L1- Cardiovascular System (HEART)


It is a closed system composed of the heart and blood vessels.

Blood vessels are

  • arteries
  • arterioles
  • capillaries
  • veins




LV → aorta& branches → arteries → arterioles → capill. → SVC & IVC →RA  → RV.


RV → Pul art. & branches → rt & lt lungs → capill (gas exchange) → 4 pul veins → LA → LV.


—It is a hollow muscular organ.

—Lies in the left side of the thoracic cavity.

—Composed of 4 chambers : 2 atria & 2 ventricles

—Enclosed within a pericardial sac which allows optimal distension  for the heart.


Cardiac muscle fibers

1.Nodal fibers

  1. SAN→initiates the c.impulse
  2. AVN →under conditions

2.Conducting fibers

  1. AV bundle & its branches
  2. Purkinjie fibres

3.Contractile ms fibers

a.Atrial ms

b.Vent. Ms

•thicker than atrial ms

•LV ms is  times thicker than RV for powerful contraction

Function of the Atria

Blood reservoir on both sides

Pumping of 30% of blood from atrium to vent.

Receptors for CV reflexes

AVN & SAN for conduction of cardiac impulses.

Function of the ventricles

—Pumping the blood into the circulation against resistance in big arteries.

Cardiac valves

—2 atrio-ventricular valves AVV:

  1. tricuspid valve between RA & RV
  2. mitral valve between LA & LV

—2 semilunar valves:

  1. aortic valve between LV & aorta
  2. pul valve between RV & pul artery


Cardiac Muscle (Myocardium)

—Cardiac ms is a striated muscle

—Cell membrane separating the cardiac ms cells is called intercalated disc or gap junction

—Electrical resistance through them is very small ;so the cardiac impulse travels easily to all C ms.

All cardiac ms act as one unit (functional syncytium)

  1. The 2 atria → one syncytium
  2. The 2 vent → one syncytium


Cardiac properties

All are myogenic

  1. Rhytmicity
  2. Excitability
  3. Conductivity
  4. Contractility


Ability of the heart to beat regularly on its own

-SAN: 110 bpm

-AVN: 90 bpm

-AV bundle: 45 bpm

-Purkinji fibers: 35 bpm

-Vent.: 25 bpm

—Rhythmicity of the heart is a self excitation process that causes automatic rhythmic contraction.

—SAN has the greatest automatic rhythm

—It is the pacemaker of the heart

—It controls the rate of the whole heart


AP of diff parts of the heart


Is the ability of the cardiac ms to respond to stimuli

Excit. Changes during AP

a.Absolute reractory period(ARP)

Excit of cardiac ms is lost.

No excitation occurs in it.

It coincides with the systole

b.Relative refractory period (RRP):

Excit gradually recovers

A stimulus can produce weaker systole

Coincides with the 1st half of diastole

c.Super normal phase (SNP):

Excit rises above normal

A weaker stimulus is needed .

Occupies the 2nd half of diastole.


It is the ability of the to conduct a cardiac impulse through the heart.

1-—Atrial transmission:

From SAN -à AVN through atrial ms

2-—AV nodal transmission:

Impulse is delayed at the AVN

3-—Purkinji system transmission:

From AVN -à AVB & Purkinji fibers.

Very rapid.

4-—Vent ms transmission:

—To the ventricular ms fibers.



It is the ability of the cardiac ms to contract

Contractility depends on:

A-All or none rule:

cardiac ms contracts maximally or does not contract at all ,provided that all the conditions remain constant.


B-Staircase phenomenon:

Cardiac ms shows gradual increase in contraction following a rapid repeated stimulation


C-Starling law:

Wthin limit, the greater the initial length of the greater the force of cont.


Cardiac cycle

It consists of contraction (systole) followed by relaxation (diastole) of the heart .

1.Atrial systole: 0.1 sec

-atrial p ↑

-is pumping blood from A to V through opened AVV and P retuns to normal


2.Ventricular systole: 0.3 sec

-AVV closed & vent. contract isometrically.

-Vent P ↑

-Ao valve is opened & blood is pumped from LV → aorta at high velocity

3.Ventricular diastole: 0.5 sec

-semilunar valves are closed

-Vent relax isometrically

-Vent P is much decreased to zero.

-AVV are opened

-fall of blood from  A to V by its weight


Remarks on CC:

Duration of CC =0.8 sec

  1. —At syst = 0.1 sec
  2. —At diast = 0.7 sec
  3. —Vent syst = 0.3 sec
  4. —Vent diast = 0.5 sec

—Vent filling

  1. —30% in AT systole – active
  2. —70 % in Vent diastole – passive

Pressures in CC

  1. —Syst BP in LV = 130mmHg
  2. —Dias BP in LV = zero
  3. —Syst BP in RV = 35
  4. —Diast BP in RV = zero
  5. —Syst BP in aorta = 120
  6. —Diast BP in aorta = 80
  7. —Syst BP in Pul art = 30
  8. —Diast BP in Pul art = 10