Question

during hemorrhagic shock in which blood pressure is decreased, which of the regulatory mechanisms is most important is increasing cardiac output to help maintain blood pressure?
multiple choice
increase in parasympathetic stimulation of the heart
increase venous return
increase in the amplitude of the heart sounds
increase vagal stimulation of the heart
increased sympathetic stimulation of the heart

Explanation:

• Parasympathetic stimulation (including vagal stimulation) generally slows the heart rate and would not help increase cardiac output. So "Increase in parasympathetic stimulation of the heart" and "Increase vagal stimulation of the heart" are incorrect.
• The amplitude of heart sounds is related to factors like the force of valve closure and the structure of the heart and surrounding tissues, not directly to increasing cardiac output to maintain blood pressure. So "Increase in the amplitude of the heart sounds" is incorrect.
• Cardiac output ($CO$) is calculated by the formula $CO = heart\ rate\times stroke\ volume$. Sympathetic stimulation increases heart rate and also can increase the force of contraction (which affects stroke volume). An increase in venous return (pre - load) can increase stroke volume (up to a point, according to the Frank - Starling law). But in the context of regulatory mechanisms to increase cardiac output quickly in hemorrhagic shock, the nervous system (sympathetic stimulation) acts more rapidly. Sympathetic nerves release neurotransmitters that bind to receptors on the heart to increase heart rate and contractility.

Answer:

Increased sympathetic stimulation of the heart