The pyloric sphincter opens to allow the chyme, the partly digested food into small intestine from the stomach.

 

Suppose a person ingested a balanced meal of carbs, proteins and fats equally with some micronutrients.

 

Body starts to absorb carbs from the saliva itself and the suggestion will continue in the small intestine. Fat has to be digested in the small intestine.

 

But proteins need to broken down into the amino acids by the HCL acid secreted in the stomach.

 

What will happen to the partially digested carbs and the undigested fats in the acidic medium of stomach. Will they just sit tight and wait, till the proteins are fully broken down, or will they (carbs & fats) proceed to small intestine even while the protein is being broken down?

 

Can pyloric sphincter allow chyme selectively?

Protein digestion is not really due to hydrolysis by HCl. It mostly just denatures proteins to some degree. More importantly are the actions of proteases starting with pepsin, which is active in the acidic environment and continues throughout the passage of the gut (secreted by the pancreas) with proteases that work at higher pH.