Question

A person who was slightly dehydrated went to donate blood, but they forgot that they had to go home to feed their mini dinosaur (if you don't feed them regularly, they get really mean), so instead of waiting in the recovery chair, this silly person stood up quickly and walked out the door. As they stood up, they felt dizzy and light headed and almost passed out. a. This person suffered from a quick decrease in MAP, draw the negative feedback loop that was activated by this decrease in MAP and the organs responsible for keeping this person from actually passing out.

b. Choose one arrow in your drawing and explain why the first box causes the next box.

Answer 1

Baroreceptor reflex  is initiated by stretch receptors, called baroreceptors located at specific points in the walls of several large systemic arteries.

It is significant  when a person stands up after having been lying down. Immediately on standing, the arterial pressure in the head and upper part of the body tends to fall, and marked reduction of this pressure could cause loss of consciousness. However, the falling pressure at the baroreceptors elicits an immediate reflex, resulting in strong sympathetic discharge throughout the body. This minimizes the decrease in pressure in the head and upper body.

After the baroreceptor signals  enters   tractus solitarius of the medulla, secondary signals inhibit the vasoconstrictor center of the medulla  and excite the vagal parasympathetic center. The effects are

The sympathetic nervous system's vasoconstrictor and cardioaccelerator functions are stimulated at the same time  and there is reciprocal inhibition of parasympathetic vagal inhibitory signals to the heart. some major changes occur simultaneously, each of which helps to increase arterial pressure, which are;

1. Most arterioles of the systemic circulation are constricted. This increases the total peripheral resistance, thereby increasing the arterial pressure.

2. The veins specially the other large vessels of the circulation are strongly constricted. This displaces blood out of the large peripheral blood vessels toward the heart, thus increasing the volume of blood in the heart chambers. The stretch of the heart then causes the heart to beat with far greater force and therefore to pump increased quantities of blood. This also increases the arterial pressure.

3. Finally, the heart is stimulated by the autonomic nervous system which increases the heart rate and force of contraction.  During strong sympathetic stimulation, the heart  can pump about two times as much blood as under  normal conditions. This causes much needed rise in arterial pressure.