I took part in a quiz in which the following question was asked: "Your bodily functions stop when you sneeze. True or false?"

I answered "false", but the quizmaster said it was true. Does he have a case, and if so, what happens physiologically when you sneeze?

Jennifer O'Neill, London, UK

A normal healthy humans can maintain constant blood flow to their brain providing that the average arterial blood pressures is between 50 and 150 mmHg. Above 150 mmHg there is a risk that blood flow becomes too high, which can potentially cause a brain haemorrhage.  Below 50mmHg and there is a risk that blood flow will be inadequate to perfuse the brain.

Giraffes can grow to heights of 5m.  I think that the difference in cerebral blood pressure between standing fully erect and bending down to ground level to drink would be over 360mmHg. 

I have heard that giraffes have a system of one way valves in their neck veins to prevent pressure overload in the venous system when bending down to drink but it would be impossible to achieve this with their arteries because the blood has to travel in the direction of the head.  

How do giraffes maintain constant brain blood flow over such a huge range of blood pressures?

How do their cerebral arteries cope with such high pressures without bursting? 

I can understand that a pulmonary embolism (PE) causes an increase in dead space (an area of lung is no longer perfused but continues to be ventilated and cannot take part in gas exchange).  Increasing dead space alone should not cause hypoxia.

looking at it simplistically, a PE should not cause a shunt.  A shunt happens when an area of lung is perfused but not ventilated (for example in pneumonia).  A shunt causes hypoxia because deoxygenated blood bypasses gas exchange and returns to the left side of the heart.

I have read in a physiology book (West) that the hypoxia in a PE is due to a big increase in the blood flow through healthy parts of the lung, to the extent that gas exchange is limited by the fact that blood passes through the alveolus so quickly that oxygen does not have time to diffuse into the capillaries (diffusion limited).  If this is the case then why don't we all become hypoxic when we exercise?  OUr cardiac output increases dramatically and therefore blood flow through all parts of the lung must also increase dramatically (far more than can be explained by even a PE taking out an entire lung, which would only double blood flow to the healthy lung).

The only explanation that I can come to is that the areas of the lung directly affected by the PE, cause swelling in the adjacent alveoli (which remain perfused) and that the swelling/inflamation limits ventilation in those alveoli causing a shunt.

Does anybody have a better explanation?  I find it strange that hypoxia is one of the signs of a PE and yet there is no obvious explanation as to why it causes this.

most doctors say you need a good 7-8 hours sleep a night (as an adult). but would someone miss out on the "rest and repair" which sleep brings by having a polyphasic sleep cycle?

Having breastfed 2 babies I am interested in finding out where exactly breast milk comes from and how it is made. I can find plenty of information that tells me all about how the production of breast milk is initiated and maintained but none that actually explains where the water, fats, antibodies etc come from and how they are combined to produce milk.

I work in and underground mine here in Australia.  One of my co-workers sneezes whenever he drives out of the entrance to the mine.  Shortly after we travel from the darkness of the underground mine and into bright sunlight.  He's been here for 22 years and says that it happens every time.  Why?

Why and how does hard work cause physical development?

Through physical work, muscles get bigger, tendons develop and bones change.  How does this happen at a cellular level?

Does the same thing happen in all animals & plants?

Is built-in adaptability an evolved trait - if so when in evolutionary history did it begin to emerge?

I have heard on numerous occaisions that it is dangerous to press a weight from behind the head, or pull a resisted bar (e.g. lat pulldown, pullups) behind the head. I have heard a few theories, based on either rotator cuff limitations or the risk of damage to the cervical spine. Anybody know of definite evidence/proof or even with solid reasoning and not just naming a few anatomical structures? Thanks

A friend recently informed me that, if a woman places her palms flat against the inside of the door jambes of a doorway just below shoulder height, she is still able to walk through the doorway with minimal discomfort and without her hands leaving or moving from their position, but that men are simply unable to do this.

Is this true?

Why??