Question:
What creates the feeling of 'excess' blood pressure to an area of the body?
If I hang upside down, and feel blood rushing to my head, what structures are actually responsible for me "feeling" this excess flow of blood? Baroreceptors? Mechanoreceptors? Something else?
Regardless, how does this process actually work? How do I "feel" excess blood pressure to a certain area of my body?

Explanation: We can consider how sensors in our arteries detect changes in pressure and inform our brain.

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Arterial Baroreceptors felling the pressure.

![enter image description here](https://i.sstatic.net/0uHE8.gif)

Arterial blood pressure is normally regulated within a narrow range, with a
mean arterial pressure typically ranging from 85 to 100 mmHg in adults. **It
is important to tightly control this pressure to ensure adequate blood flow to
organs throughout the body** . This is accomplished by negative feedback
systems incorporating pressure sensors (i.e., baroreceptors) that sense the
arterial pressure. The most important arterial baroreceptors are located in
the carotid sinus (at the bifurcation of external and internal carotids) and
in the aortic arch (Figure 1). These receptors respond to stretching of the
arterial wall so that if arterial pressure suddenly rises, the walls of these
vessels passively expand, which increases the firing frequency of receptor
action potentials. If arterial blood pressure suddenly falls, decreased
stretch of the arterial walls leads to a decrease in receptor firing.

The carotid sinus baroreceptors are innervated by the sinus nerve of Hering,
which is a branch of the glossopharyngeal nerve (IX cranial nerve). The
glossopharyngeal nerve synapses in the nucleus tractus solitarius (NTS)
located in the medulla of the brainstem. The aortic arch baroreceptors are
innervated by the aortic nerve, which then combines with the vagus nerve (X
cranial nerve) traveling to the NTS. The NTS modulates the activity of
sympathetic and parasympathetic (vagal) neurons in the medulla, which in turn
regulate the autonomic control of the heart and blood vessels.

![enter image description here](https://i.sstatic.net/Pp9V6.png)

Of these two sites for arterial baroreceptors, the carotid sinus is
quantitatively the most important for regulating arterial pressure. The
carotid sinus receptors respond to pressures ranging from 60-180 mmHg (Figure
2). Receptors within the aortic arch have a higher threshold pressure and are
less sensitive than the carotid sinus receptors. Maximal carotid sinus
sensitivity occurs near the normal mean arterial pressure; therefore, very
small changes in arterial pressure around this "set point" dramatically alters
receptor firing so that autonomic control can be reset in such a way that the
arterial pressure remains very near to the set point. This set point changes
during exercise, hypertension, and heart failure. The changing set point
(response curve shifts to right) explains how arterial pressure can remain
elevated during chronic hypertension.

Baroreceptors are sensitive to the rate of pressure change as well as to the
steady or mean pressure. Therefore, at a given mean arterial pressure,
decreasing the pulse pressure (systolic minus diastolic pressure) decreases
the baroreceptor firing rate. This is important during conditions such as
hemorrhagic shock in which pulse pressure as well as mean pressure decreases.
The combination of reduced mean pressure and reduced pulse pressure amplifies
the baroreceptor response.

![enter image description here](https://i.sstatic.net/1Kza8.png)

Although the baroreceptors can respond to either an increase or decrease in
systemic arterial pressure, their most important role is responding to sudden
reductions in arterial pressure (Figure 3). This can occur, for example, when
a person suddenly stands up or following blood loss (hemorrhage). A decrease
in arterial pressure (mean, pulse or both) results in decreased baroreceptor
firing. Autonomic neurons within the medulla respond by increasing sympathetic
outflow and decreasing parasympathetic (vagal) outflow. Under normal
physiological conditions, baroreceptor firing exerts a tonic inhibitory
influence on sympathetic outflow from the medulla. Therefore, acute
hypotension results in a disinhibition of sympathetic activity within the
medulla, so that sympathetic activity originating within the rostral
ventrolateral medulla increases. These autonomic changes cause
vasoconstriction (increased systemic vascular resistance, SVR), tachycardia
and positive inotropy. The latter two changes increase cardiac output.
Increases in cardiac output and SVR lead to a partial restoration of arterial
pressure.

**It is important to note that baroreceptors adapt to sustained changes in
arterial pressure. For example, if arterial pressure suddenly falls when a
person stands, the baroreceptor firing rate will decrease; however, after a
period of time, the firing returns to near normal levels as the receptors
adapt to the lower pressure. Therefore, the long-term regulation of arterial
pressure requires activation of other mechanisms (primarily hormonal and
renal) to maintain normal blood pressure.**

This information from **PhD Richard E. Klabunde** lecture, that based on his
book **_Cardiovascular Physiology Concepts_ ** (chapter 5, pages 93-123)


Question:
What types of light can't a plant photosynthesize in?
I have a plant on my desk, and it got me to wondering: Can my plant use the light from my monitors to photosynthesize? If so, what light (apart from green light, to a degree) can't plants use to perform photosynthesis?
I know that plants have the photosynthetic pigments to absorb many different wavelengths of light (primarily red and blue) but would there be certain types of light it can't use?
(The specific plant by the way is Schlumbergera truncata)

Explanation: We can explore how the green pigment in plants captures light and how this influences the types of light used in photosynthesis.

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Chlorophyll can use a quite broad range of light for photosynthesis, the only
range where it is not working is approximately between 500 and 620nm. See this
adsorption spectrum of chlorophyll (from the Wikipedia article on [
Chlorophyll ](http://en.wikipedia.org/wiki/Chlorophyll) ):

![enter image description here](https://i.sstatic.net/WAZfw.png)

The lack of chlorophyll to absorb light between 500 and 620nm (roughly)
results in the green color of leafs, because this light is reflected. Besides
the wavelength also the energy of the light is important, so plants will grow
better in sunlight than in the light of your monitor. This [ link
](http://scienceline.ucsb.edu/getkey.php?key=1668) about the maximum
effectiveness of photosynthesis is also interesting.


Question:
Which part of human eyes is getting tired?
It's a thing of common sense that if you read, drive or look at computer screen for too long your eyes will get tired. They burn, itch and try to close all the time.
But I was wondering which part(s) of them need rest so much... Blinking muscles? Lense muscles? Or the eyeball is drying out?

Explanation: We can examine how the muscles responsible for focusing and maintaining eye alignment become fatigued during prolonged visual tasks.

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These symptoms have a name: Computer vision syndrome. Basically our eyes are
made to look at longer distances from 1-6 meters without much accommodation.
Typically computer screens are located at a much closer distance (30-50cm),
which requires constant accommodation by the eye. This leads to high stress on
the muscles in the eye which subsequently get tired. Additionally less
blinking leads to drier eyes.

One of the common recommendations against this is to regularly focus further
located objects to give the eye the chance to relax. This is sometimes called
the "20-20-20" rule, every 20 min focus on a 20 feet (about 6 m) far object
for 20 seconds. See the article in the [ Wikipedia
](http://en.wikipedia.org/wiki/Computer_vision_syndrome) and [ here
](http://www.public.iastate.edu/~rhetoric/105H16/cofp/smkcofp.html) for more
information (and probably also the linked references in the articles).


Question:
How does yeast dissolve in sugar?
There's this fascinating "kitchen experiment". Take a "brick" of baker's yeast (not dry yeast), and a couple teaspoons of sugar, put them together in a cup and stir together. There's no doubt both are solids: yeast is a firm block, you must use quite a bit of force initially to crush it and even then it should remain a rather thick paste. The sugar is crystals, nothing tricky about that. And then, within a minute or so of stirring, your cup contains only liquid - not entirely fluid like water, but not much thicker than cream. Solid+Solid -> Liquid.
What process leads to that? What happens with the baking yeast when it comes in contact with sugar, that it turns from solid to liquid, and releasing enough water to dissolve the sugar?

Explanation: We can look into how certain substances naturally draw water from their surroundings, a process that helps explain the transformation of yeast and sugar from solids to a liquid state.

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The whole process is called [ osmosis ](http://en.wikipedia.org/wiki/Osmosis)
. In it there is the flow of liquid along a concentration gradient. Water then
flows from the side which contains the low concentration of dissolved
molecules (this can be salts or sugar for example) to the side with the higher
concentration until it reached equilibrium. This principle is shown in the
image below (all images from the Wikipeadia article on [ osmosis
](http://en.wikipedia.org/wiki/Osmosis) ):

![enter image description here](https://i.sstatic.net/S9abq.jpg)

The blue dots are the dissolved material, the concentration is high on the
right side and lower on the left. Through the dotted line in the middle (which
is a semi-permeable membrane) only the solvent can pass to reach a
concentration equilibrium. There is a pretty cool osmotic pressure simulator
available on [ Molecular Logic
](http://molo.concord.org/database/activities/72.html) which can also help
understanding the principle.

And this is exactly what happens to the yeast cells. The sugar crystalls get
dissolved (at least partly) by some of the humidity from the yeast block (this
will not work with dried yeast) and form a highly concentrated sugar solution
(also called hypertonic). The sugar concentration inside the cells is
relatively low (called hypotonic). This leads to the effort to balance the
concentration inside the cell and outside of it which then leads to a massive
efflux of water from the cells. This goes so far, that the cell membranes
break and the interior of the cells is released (nucleus, DNA, proteins and so
on) and the sugar is completely dissolved in the released water. The reason
why this mixture is so "gooey" afterwards is the DNA, which forms long, thick
strains.

![enter image description here](https://i.sstatic.net/gv3Y7.png)

This method can be used in the lab to break up cells relatively gently without
applying too much force.

When baking you should make sure, that your yeast is not coming directly into
contact with undissolved sugar (or salt which works the same way), as this
will kill a part of your yeast cells. However, for baking you usually dissolve
the yeast in water or milk, so there is a huge excess of water which dissolves
the sugar (or salt) and protects the cells. Addionally your yeast is much
better distributed in the dough and gives better results.