Q4. Assertion: the heart keeps beating even without signals from the nerves. Reason: the nodal tissue of the heart is autoexcitable. Are both true, and does the reason explain the assertion?
Both are true, and the reason explains the assertion: the nodal musculature generates action potentials without any external stimuli (NCERT page 199), so the heart is myogenic and the nerves only moderate it (page 202).
Q5. The sympathetic nerves raise the cardiac output in two ways. What are they?
They increase the rate of the heart beat and the strength of ventricular contraction (NCERT page 202); since cardiac output = stroke volume × heart rate (page 200), these usually raise output when stroke volume is maintained or increases. Excessively high rates can instead reduce filling.
Q6. Why are adrenaline and noradrenaline called emergency hormones?
They are secreted rapidly in response to stress of any kind and during emergencies (NCERT page 244).