Lecture 19: The Cardiovascular System Part 2

Question 1

What is the sequence for excitation of the heart

Answer 1

1. Sinoatrial (SA) node (pacemaker)
   - generates impulses about 75 times/minute (sinus rhythm)
2. Atrioventricular (AV) node
   - smaller diameter fibres, fewer gap junctions
3. Atrioventricular (AV) bundle
   - only electrical conduction between the atria and ventricles
4. Right and left bundle branches
   - two pathways in the interventricular septum that carry the impulses towards the apex of the heart
5. Purkinje fibers:
   - complete the pathway into the apex and ventricular walls

Question 2

Defects in the intrinsic conduction system may result in?

Answer 2

1. Arrhythmias: irregular heart rhythms
2. Uncoordinated atrial and ventricular contractions
3. Fibrillation: rapid, irregular contractions; useless for pumping blood

Question 3

Extrinsic innervation of the heart:

Answer 3

Heart beat is modified by the ANS
Cardiac centres are located in the medulla oblongata
-cardioacceleratory centre inner sites SA and AV nodes, heart muscle, and coronary arteries through sympathetic neurons
-cardioinhibitory centre inhibits SA and AV nodes through parasympathetic fibres in the vagus nerves

Question 4

Heart sounds:

What are the two sounds (lub-dub) associated with?

Answer 4

Associated with closing of heart valves
-first sound occurs as AV valves close and signifies beginning of systole
-second sound occurs when SEmilunar valves close at the beginning of ventricular diastole
Heart murmurs: abnormal heart sounds which indicates valve problems

Question 5

Phases of the cardiac cycle:

1. Ventricular filling
2. Ventricular systole
3. Isovolumetric relaxation

Answer 5

1. Ventricular filling: takes place in mid-to-late diastole
   - AV valves are open
   - 80% of blood passively flows into ventricles
   - atrial systole occurs, delivering the remaining 20%
   - end diastolic volume is the volume of blood in each ventricle at the end of ventricular diastole
2. Ventricular systole
   - atria relaxes
   - rising ventricular ventricular pressure results in AV valves closing
   - in ejection phase, ventricular pressure exceeds pressure in the large arteries, forcing the semilunar valves open
   - each systolic volume: is volume of blood remaining in each ventricle
3. Isovolumetric relaxation occurs in early diastole
   - ventricles relax
   - backfow of blood in aorta and pulmonary trunk closes semilunar valves

Question 6

Autonomic nervous system regulation:

Sympathetic and parasympathetic

Answer 6

1. Sympathetic nervous is activated by emotional or physical stressors
   - norepinephrine causes the pacemaker to fire more rapidly (increasing conactility)
2. Parasympathetic nervous system opposes sympathetic fibres.the heart at rest exhibits Vagal tone (parasympathetic)
3. Atrial reflex: a sympathetic reflex initiated by increased venous return
   - stretch of atrial walls stimulates the Sa node
   - also stimulates atrial stretch receptors activating sympathetic reflexes

Question 7

Chemical regulation of Heart rate:

Hormones

Answer 7

Epinephrine from adrenal medulla enhances heart rate and contractility
Thyroxine increases heart rate and enhances the effects of norepinephrine and epinephrine

Question 8

Other factors that influence heart rate

Answer 8

Age
Gender
Exercise
Body temperature

Question 9

Homeostatic imbalances: tachycardia, bradycardia

Answer 9

Tachycardia: abnormally fast heart rate (>100 bpm)
-if persistence, may lead to fibrillation
Bradycardia: heart rate slower than 60bpm
-may result in grossly inadequate blood circulation
-may be desirable result of endurance training

Question 10

Congestive heart failure:

Answer 10

Progressive condition where the cardiac output is so low that blood circulation is inadequate to meet tissue needs
Caused by:
-coronary atherosclerosis 
-persistent high blood pressure 
-multiple myocardial infarcts
-dilated cardiomyopathy

Question 11

Developmental aspects of the heart:

what are the fetal heart structure ps that bypass pulmonary circulation?

Answer 11

Foremen ovale: connects the two atria

Ductus arteriosus: connects the pulmonary trunk and the aorta

Question 12

Congenital heart defects of infant

Answer 12

Leads to mixing of systemic and pulmonary blood
Involve narrowed valves or vessel that increase the workload of the heart
1. Ventricular septal defect: the superior part if the inter ventricular septum fails to form; thus blood mixes between the 2 ventricles.
2. Coarctation of the aorta: a part if the aorta is narrowed, increasing the workload of the left ventricle
3. Tetralogy of fallout: multiple defects-
A) pulmonary trunk too narrow and
B)resulting in hypertrophied right ventricle
C) ventricular septal defects
D) aorta opens from both ventricles

Question 13

Age related changes affecting the heart:

Answer 13

Sclerosis and thickening of valve flaps
Decline in cardiac reserve
Fibrosis of cardiac muscle
Atherosclerosis