Her stomach gurgles, her belly quivers,

Her eyes dart round, her body shivers,

Her blood is pumping, pupils dilating,

Her brain firing signals, neurons pulsating.

 

“I’m hungry” she says, stomping her feet,

Her hands curled in fists, her cheeks pink with heat,

Her blood sugars low, her brain senses the threat,

Hypoglycaemic, she’s getting upset.

 

Let’s zoom inside to her stomach, we see

Not much actually, it’s pretty empty,

Let’s go closer still to epithelial cells,

With mechanoreceptors that detect stretch and swell.

 

The cells are secreting, in the absence of food,

Ghrelin, director of hunger and mood.

It travels far to the hypothalamus,

And crosses the barrier to the Arcuate Nucleus.

 

It exerts on the brain orexigenic effects,

Acting on all of the CNS,

As well as controlling energy homeostasis,

It activates signals in other places.

 

It is involved in rewards and incentives,

Activating memory to be more retentive,

In the prefrontal cortex it alters decisions

And signals that food is the brains next mission.

 

AGRP / NPY neurons firing,

She suddenly realises she’s desiring

Foods that will bring comfort and energy,

Turning the hunger to satiety.

 

She glances around, “Is there food in the kitchen?”

“Maybe there’s cake?” she finds herself wishing.

Her brain screaming out “I need sugar and fat!”

“How about this celery stick?”, “No! Not that!”.

 

She settles for leftovers from todays’ lunch,

She prepares the meal and begins to munch,

But before the food even enters her mouth

Her body’s preparing to break it down.

 

The optic nerve signals, olfactory tasting,

Oesophagus contracting, stomach bracing,

Signals firing and cells releasing,

Hormones pumping, saliva secreting.

 

She’s chewing her food, or masticating,

The mechanical movement of teeth activating

Brain pathways and peptides although which? We don’t know,

Undoubtedly appetite suppressing, though.

 

She swallows the food, it enters her gullet,

Peristaltic contractions push it, pull it?

Down to the stomach and through the sphincter,

Exciting the vagus nerve which leads to

 

Acetylcholine release which then acts on

M3 receptors which has an impact on

G cells and Gastrin release which exacts on

Enterochromaffin cells, in fact, on

 

The release of Histamine which causes secretion

Of a Hydrochloric Acid solution

From parietal cells. This breaks down proteins:

Bacterial, viral, it effectively cleans.

 

The stomach defensive to outside threats,

It protects the body from microbe pests,

And starts to get bigger when filling with stuff

Alerting cells that there’s food, sure enough.

 

This is the sign that cells into action,

Satiety hormones promote food satisfaction,

And called to arms they summon their messengers

To zoom round the body, passing on messages.

 

Listen real close and we’ll hear what they’re saying,

Small peptides which act on receptors relaying

Instructions for organs and tissues together,

Discreetly directing digestion, they’re clever!

 

Quick shout out to the second brain,

Enteric Nervous System reigns

Independent of our head,

Taking over when we’re fed.

 

Not consciously controlling food,

But still it can impact on mood.

Example; it can play a role in

Emotion via serotonin.

 

We can’t leave out our microbe friends,

They influence our gut no end,

They breakdown fibre, control immunity,

Live symbiotically with impunity.

 

But more than this they also create

Peptides that mimic our own, although fake

They still can cause the brain to favour

Certain foods and change behaviour.

 

But we’re on a tangent, I transgress,

Trying to unravel the mysteries, the mess,

Let’s turn our attention back to her digestion,

Please excuse this wild digression.

 

The food moves into the small intestine,

Which triggers release of the hormone secretin,

Raising pH, deacidifying the chyme,

Preparing the food for absorption time.

 

Now this is when hormonal magic starts,

They spring into action to play their parts,

All working towards a common goal,

With the finest precision and expert control.

 

Introducing our trio of polypeptides,

Meet P.P, P.P.Y and greet N.P.Y,

Their function is slowing intestinal transit,

Collecting info from gut contents to transmit.

 

There’s cholecystokinin released from the lining

Of epithelial cells, the intestine mining

All of the nutrients. Hormones supplying

Pancreatic enzymes emulsifying.

 

Let’s move to the pancreas, it’s interesting here,

There are many cell types, but you don’t need to fear,

Insulin’s important and so’s glucagon!

Blood glucose levels is what they act on.

 

“Cells, uptake glucose!” ordered insulin.

Encouraged by supporters called incretins,

G.I.P and G.L.P-1 unite

To make sure secretion of insulin’s right.

 

Back to the intestine the food flowing through,

Down to the large one it ends up as poo,

But still there’s absorption before this occurs,

Gradient crossings and nutrient transfers.

 

Her hormones are doing their job quite well,

Signalling enough’s enough for a spell,

We’ll find, if we’re looking, there’s one in particular,

Not quite a satiety hormone, but similar.

 

That is super important in stopping eating,

It acts on the brain to minimise feeding,

Signalling high nutrition, or paucity,

Signalling also body adiposity.

 

It’s Leptin, of course, what else could it be?

Released from adipocytes, completely key.

Directly proportional to adipose stores,

It communicates well, but not without flaws.

 

For when we’re obese, sometimes we’re resistant

To leptins effects, and Ghrelins persistent,

The balance is off and it’s hard to make right

But for now she is healthy, her hormones alright.

 

The food almost finished her plate almost empty,

She’s eaten as much as she can, it was plenty,

She’s starting to feel just a little bit full,

“Delicious! But really I can’t eat it all!”

 

And after a while, digestion is done,

Nutrients absorbed, the waste passed on,

Deposited fat, glucose stores filled,

The body has protein with which to rebuild.

 

She’s feeling much better now that she has eaten,

Happy and comfy, a bit sleepy even,

She washes her dishes and lies on the sofa,

Sinking, so sweetly, into a food coma.