Bodily logistics
How substances move around, and out of, your body.
I want to help you understand more about the substances (like medicines) you put into your body. To do that, we need to make sure we’ve got the some of the basics covered first.
In this article we’re going to focus on what I consider the key logistics systems when it comes to processing the things we put into our mouths. And to help you think about what this involves I’d like to kick off with a bit of history.
Logistics are critical to survival
My father was born in Malta, a small island in the Mediterranean Sea. Think of a craggy limestone rock type island, not lush green foliage and flowing streams.
Its location between Europe and Africa has seen Malta play a strategic role in several big battles over the centuries. One of the biggest being World War II.
Malta, a member of the British Commonwealth, was used as a military base. Its strategic importance increased in 1940 when Italy entered the war and the Nazis started making moves into North Africa.
For two and a half years Malta withstood an onslaught of bombs. It’s a bit hard for me to determine the line between reality and folklore here (I’ve heard a lot of stories over the years) but it’s pretty clearly documented that it was one of the most heavily bombed locations in WWII.
In November 1941 Hitler turned up the heat even further, intent on closing down British shipping in the Mediterranean. Malta was under siege. By April 1942 the situation was dire. For around four months, every ship carrying food and supplies to the island was destroyed. The people were being starved and bombed into submission. Combine malnutrition with a lack of water, medicines shortages and poor living conditions and you have a recipe for a public health crisis.
But the Maltese people were resourceful and worked together to survive. They sheltered underground, utilising an abandoned underground railway with further shelters dug out by those that were willing and able. A Communal Feeding Department was established to distribute rationed supplies to the people. Victory kitchens were set up to make what little food was available go further. Wood from the bombed buildings was scavenged to be used to fuel the fires of the kitchen.
Operations to resupply the island were attempted, but few ships got through. Great sacrifices were made by both military personnel and civilians to secure enough supplies to keep the country alive and refuel the aircraft to mount an offensive and enable the people to outlast the enemy.
In a small country without plentiful resources, logistics were critical to surviving the war. Securing what few supplies were available, distributing them to those with need, and maintaining standards of living as best they could was critical for keeping the people alive.
The same types of logistics systems can be seen within our bodies. The digestive system receives and processes the cargo to ensure the body has access to an adequate supply of nutrients. The cardiovascular system serves as the distribution network, enabling access to supplies and removal of waste products. While the urinary tract serves as the wastewater treatment plant, critical to maintain healthy conditions.
Understanding these systems is foundational to learning about how medicines work. So let’s take a closer look at each of these three systems and how they work together to process, distribute and dispose of the substances we put into our bodies. We’ll concentrate on the substances everyone on the planet puts in their mouths each day from the moment we’re born -food and drink.
Receiving and processing cargo in the gut
The digestive system, or gut, is where we bring in the cargo and prepare the supplies for distribution. But just as the arrival of a cargo ship in the port requires work to make those supplies accessible, so too does the gut have to put in effort to unlock the nutrients contained within our diet.
The upper part of the gut can be thought of a bit like the dockyards. Cargo needs to be offloaded from the ship and broken down into packages more suitable for road transport.
Processing our food begins in the mouth through chewing, movement and saliva which break solids down into smaller pieces. The chunks of solid and semisolid foods, liquids we drink and whatever else we swallow then make their way along the tongue down the oesophagus to the stomach where the acidic digestive juices and movement break things down further.
Here it doesn’t really resemble food you’d like to eat anymore, it’s a semi-solid slurry type state of everything that was in the stomach mixed together. I’m sure you’ve seen this on more than one occasion in the form of vomit.
The slurry of food can now pass into the first part of small intestine (the duodenum). Here enzymes from the pancreas and bile from the gallbladder in the liver come into the mix. The food exits the duodenum as a liquid which moves through the remainder of the small intestine (the jejunum and the ileum). The food and gut contents is now a mix of nutrients including fats, fibres, sugars, proteins, vitamins and minerals. There are probably also some manufactured chemicals like artificial colours, flavouring agents, preservatives and medicines in the mix.
It’s here in the small intestine where the cargo processing facility of the gut meets the distribution network of the cardiovascular system. It’s like the supplies are now ready to get packed into vehicles for road transport. Nutrients and other compounds that are able to physically move from the gut to the bloodstream do so through a process called absorption.
Just like there’s packaging and rubbish leftover when you breakdown a pallet of supplies into smaller packages, so too will there be elements of the food that isn’t absorbed, like fibre. These leftovers stay in the gut where they are moved on to the colon- the body’s solid waste processing facility.
Here the slushy mix of leftovers is transformed into a solid form of poo. Once the poo has formed it sits in the rectum, the final compartment of the digestive system, until you’re ready to poop it out of your body through your butt hole (the anus) and be rid of it forever.
The distribution network of the cardiovascular system
Supplies don’t do much good sitting in a storage facility, they need to get out to the people that need them. This is the role of the blood vessels and the cardiovascular system. It provides the road transport network for the body.
The network of roadways within the body is extensive. While the features of the infrastructure may vary, there’s one thing that all blood vessels have in common- it’s strictly one way traffic all the way.
All the nutrients and other compounds that were absorbed from the gut are taken to the liver for final processing. This is where a process called metabolism takes place.
Metabolism is the body’s opportunity to catch any toxins and break them down to be ready for disposal before letting them pass into the general circulation. Any waste products are sorted like recycling. In this case they’re either moved into the bile to be pooped out, or into the bloodstream to be dealt with by the kidneys.
The liver also does some final processing of nutrients to make sure they’re suitable for immediate distribution or storage for later use.
Once the nutrients and any other compounds have made it past the clearing house that is the liver, they’re on the open road. They make their way to the right side of the heart through the vena cava where they’re pumped through the pulmonary circulation. This is where the red blood cells pick up oxygen and carbon dioxide is sent to the lungs to get rid of. This freshly oxygenated blood comes back to the left side of the heart where it can take the oxygen, nutrients, and anything else that’s made it this far onto the highway system known as the arteries.
Every now and then some of the supplies will be sent down the exit ramp of the arterioles toward a tissue destination. Here, the supplies move into the side streets of the capillaries. These roadways are really small which means the traffic is moving slow enough for the supplies to be offloaded to the tissues.
The tissues don’t just take supplies out of the blood, they also deposit things into it. Waste products and other things like hormones and electrolytes move into the bloodstream for further distribution around the body.
Now, it’s worth noting that just like within a road based distribution network, the efficiency of deliveries depends on the integrity of the infrastructure. If there’s major damage or blockage of blood vessels it makes it hard for the supplies to get through to where they’re needed. It also makes it harder to clear out the waste, putting the health of the tissues relying on that blood supply at risk. This is how things like high cholesterol, smoking, and diabetes cause cardiovascular disease- they all damage the blood vessels which can cut off supplies to areas like the brain (stroke), heart (heart attack), and limbs (peripheral vascular disease).
The waste products and anything else offloaded by the tissues gradually make their way back to an on-ramp (venule) and onto the highway of the vein headed back to the heart where the blood can get topped up with oxygen once more. Then it’s back to the arterial highway for the waste products until they make it onto the exit ramp that heads to the kidneys.
The wastewater system of the urinary tract
The kidneys and urinary tract are like the wastewater management plant for your body.
Supplies and waste products for the kidney make their way through the arteries to the arterioles and eventually capillaries, just like with any other tissue’s blood supply. But in the kidney these capillaries form part of tiny filtration units called the glomerulus which are found in the nephrons.
Within these glomerular filtration units the waste products are moved out of the blood into the urine for disposal. The ‘cleaned up’ blood can then continue on in the distribution network that is the circulation system.
The filtration process in the kidneys also helps to balance the water and salt in your body, and manage the load on your blood vessels which we call blood pressure. If the pressure in the blood vessels is too high it can damage small blood vessels like capillaries. In the kidneys, this can cause damage to the filtration units, making it harder to clear the waste from your bloodstream.
If waste can’t be removed it can cause big problems. During the siege of Malta a lack of water and cramped living conditions in air raid shelters meant sanitation wasn’t as good as it should have been. This, combined with a population that was malnourished, resulted in rapid spread of communicable diseases like dysentery. So too can an inability to remove waste put your health at risk. Now if you’re otherwise healthy with good access to food and water you might not notice much difference. However, if you were to become acutely unwell, dehydrated or have other health problems that make you vulnerable your reduced kidney function could mean you get very unwell and need to go to hospital.
As the urine forms it flows into the bladder for storage. This is where it stays until a large enough volume has collected it travels through your urethra as you pee it out, eliminating the waste from the body.
So there you have it, a brief overview of the key logistical systems that allow your body to absorb, distribute and eliminate the substances you put into your body via the mouth. This article has focused on just three of the systems, we haven’t really even considered all the other systems that are also essential to survival. But I hope it has illustrated how our bodies can be thought of as a system of systems, all interconnected and working together.
Now that we’ve established this foundation we can go on to explore how substances like medicines work in a bit more detail. In further topics we can also look at the roles of the other body systems that weren’t covered here.
This is a great article, thanks Lauren.