Cardiac cycle 3- Atrial systole

Cardiac cycle 3- Atrial systole


– AT THE END OF ATRIAL
AND VENTRICULAR DIASTOLE, RELAXATION, WE HAD THE FIRING
OF THE SA NODE. THE SA NODE PASSED
THE ACTION POTENTIAL ALL THROUGH THE MYOCARDIUM
OF THE ATRIA. AND THAT IS RECEIVED, THAT INFORMATION,
THAT ACTION POTENTIAL, IS RECEIVED BY THE CONTRACTILE
CELLS AND NOW THEY GO. WE HAVE ATRIAL CONTRACTION,
OR ATRIAL SYSTOLE. NOTICE WHAT IT LOOKS LIKE. WE HAVE LITTLE RED
ARROWS HERE INDICATING THAT WE’RE DECREASING
THE VOLUME OF THE ATRIA. WHAT’S GOING TO HAPPEN? WE ARE NO LONGER PASSIVELY
FILLING OUR VENTRICLES. BLOOD IS NOW FORCED
INTO THE VENTRICLES. AND THIS IS GOING
TO DO TWO THINGS: IT’S GOING TO INCREASE
THE PRESSURE, AND IT’S GOING
TO INCREASE THE VOLUME. REMEMBER THAT WHEN WE
WERE PASSIVELY FILLING OUR VENTRICLES,
WE WERE INCREASING THE VOLUME. BUT WE HAD NO CHANGE
IN THE PRESSURE, BECAUSE IT WAS JUST
A PASSIVE PROCESS. THE VENTRICLES WERE RELAXING
TO ACCOMMODATE THE INCREASE OF VOLUME THAT WAS COMING IN, SO THERE WAS NO CHANGE
IN THE PRESSURE. 80 PERCENT OF VENTRICULAR
FILLING HAPPENS THIS WAY– PASSIVELY. SO MOST OF THE BLOOD GETS MOVED
INTO THE VENTRICLE IN THAT PASSIVE PERIOD. HOWEVER, THE LAST 20 PERCENT IS
SMASHED INTO THE VENTRICLES WHEN THE ATRIA CONTRACT. NOW YOU CAN IMAGINE THAT,
YEAH, THAT’S TOTALLY GOING
TO INCREASE THE PRESSURE. THE CONTRACTION IS ELECTRICAL. SO KEEP IN MIND THAT WE
HAVE ACTION POTENTIALS RUNNING THROUGH CONTRACTILE CELLS AND THAT’S WHERE WE
GET THIS P-WAVE. THE P-WAVE IS ASSOCIATED
WITH ATRIAL CONTRACTION. YEAH, YEAH, YEAH.
NOW. WE HAVE TO ACCOMMODATE, OR WE HAVE TO ADDRESS
THE FACT THAT, YEAH, WE FIRED THE SA NODE AND
THE MESSAGE GOES WHERE, NEXT? YOU KNOW IT’S TRUE. THE MESSAGE IS GOING
TO GO TO THE AV NODE. NOW:
AV NODE IS, SOMEHOW WE HAVE TO GET
THE MESSAGE OF THE ORDER TO CONTRACT TO THE VENTRICLES
IF, WOULDN’T YOU EXPECT THAT VENTRICULAR CONTRACTION IS
GOING TO HAPPEN NEXT? AND REMEMBER THE ANATOMY. WE HAVE TO GO
THROUGH THE AV BUNDLE, WHICH, REMEMBER,
THIS ISN’T IN THE CHAMBER, IT’S ACTUALLY
IN THE INTERVENTRICULAR SEPTUM, AND INTO MY BUNDLE BRANCHES AND
DOWN INTO THE PURKINJE FIBERS. AND WHILE THE ATRIA CONTRACT, THE AV NODE HANGS
ONTO THE MESSAGE. THAT’S WHY
THE AV NODE SLOWS IT DOWN. BECAUSE IMAGINE THAT
IF THE ATRIA ARE CONTRACTING AND THE VENTRICLES ARE
CONTRACTING AT THE SAME TIME, CAN YOU IMAGINE HOW BASICALLY
THERE’S GOING TO BE, LIKE, A PUSH-PULL, LIKE,
WHO’S GOING TO WIN? I’M VOTING ON THE VENTRICLES, THEY’RE A LITTLE BIGGER
AND TOUGHER THAN THE ATRIA, IF YOU KNOW WHAT I’M SAYING. SO IT’S NOT A GREAT STRATEGY. LET’S GO AHEAD
AND HAVE THE ATRIA CONTRACT FIRST SO THEY CAN SQUEEZE ALL
THAT BLOOD INTO THE VENTRICLES, HOLD ONTO THE MESSAGE– WAIT FOR IT, VENTRICLES,
WAIT FOR IT. AND THEN WE’LL SEND THE MESSAGE. NOW THE MESSAGE IS REALLY FAST. BUT, AGAIN, ANATOMICALLY ALL THE ACTION POTENTIAL IS HELD
IN A CERTAIN SPACE. SO THE ACTION POTENTIAL ISN’T
ALLOWED TO SPREAD RIGHT AWAY. IT ISN’T ALLOWED TO SPREAD UNTIL IT GETS TO THE BOTTOM
OF THE VENTRICLE. THIS RIGHT HERE
IS VENTRICULAR—. THE MESSAGE IS PASSING DOWN
INTO THE VENTRICLES, THE ACTION POTENTIAL, AND WE’RE GETTING READY
TO DEPOLARIZE THE VENTRICLES. WE’RE GETTING READY
FOR THIS STAGE OF THE EKG. WE’RE GOING TO SUM IT ALL UP
WITH THE QRS COMPLEX. BUT WE’RE NOT QUITE THERE YET, WE’RE JUST SPREADING
THE ACTION POTENTIALS. AND WE’RE GETTING READY.