Popular physiology II: A personal ocean
Life arose in the salty waters of the sea, and still very much relies on a saline environment. Land animals have ‘internalised’ some part of the sea inside themselves, an internal ocean of sorts, the ‘extracellular fluid’ (ECF). We must carry this ocean, of about 14 litres, with us all the time. Yet apart from the 5 litres of blood that is plainly visible everytime you get a cut, the rest of it (about 10 litres) is quite hard to find. In fact, this fluid is spaced in droplets and molecules between our cells, filling up minute compartments, bathing our tissues with fresh nutrients and oxygen all the time. These 10 litres of ‘interstitial fluid’ is visible when the droplets merge into larger, visibler drops, such as sweat, saliva and tears. The next time you shed a tear, lick some of it and you can taste the sea.
The composition of ECF must be maintained within a narrow range for us to survive; too much or too little of something — and your cells will perish in minutes. The body is in constant contact with the external environment — alongwith the wind, heat, moisture, dust and micro-organisms that come with it. In addition, every cell is breathing inside, producing minute quantities of toxic material (mostly carbon dioxide and ammonia, none of which is good for your health). All of this will eventually end up in the ECF, and The Body has to find ways and means to buffer any changes that are made to the ECF. For one cannot survive with an internal ocean which is dirty. Just look at an aquarium which has not been cleaned in a week.
The ECF, which consists of blood and interstitial fluid, is 14 litres on average (in adults). Whereas, the average adult weighing 70 kg has much more water in him — in fact, 42 litres of it. This additional 28 litres is completely invisible, because it resides inside each of your cells, and thus is the ‘intracellular fluid’ (ICF). Each cell in your body has its tiny portion of ICF, and is free to experiment with it; intracellular fluids are not as tightly regulated as ECF. When the cell dies, the droplet of ICF is typically grabbed by another nearby cell, and its contents stay private.
Of course, the blood, interstitial fluid and intracellular fluid are not watertight compartments and a steady exchange of materials is always taking place. Cells take up the good stuff (oxygen, glucose and amino acids) from interstitial fluid, and dump the bad stuff (carbon dioxide and ammonia) into it. The interstitial fluid must then find a way to get rid of the waste, and fast. It merges into the flow of blood, and then the lungs, liver & kidney, with their concerted effort, do the cleanup. Specifically, the lung releases the carbon dioxide back into air, the liver converts the ammonia to urea, and the kidney concentrates the urea in urine.
Apart from the nutrients and toxins, the salinity of the ECF (blood and interstitial fluid), i.e. its concentration of minerals, must be maintained at all times. This is because our nerves rely on ions, not electrons, to do the running around. Too much sodium and you might have a seizure, too much potassium and your heart might stop. Add to that the hygroscopic properties of sodium (observe how a pinch of common salt turns wet in the monsoon), and you can easily guess why you might swell up if you have too much sodium inside. The maintenance of a fixed ionic, or ‘electrolyte’ concentration in the ECF, is a critical function of your kidney and the large intestine.
Together, all the activities that go into maintaining the ECF in its pristine condition, is known as ‘homeostasis’, the Greek word for housekeeping. A failure of homeostasis is only too common: dehydration (too little water), diabetes mellitus (too much sugar), chronic kidney disease (too much of everything), malnutrition (too little protein). In fact, all diseases might be traced back to a failure of homeostasis, i.e. an alteration in the personal ocean.
Blood
Evolution works in strange ways; the earth’s atmosphere was not always oxygen rich. The ‘oxygen revolution’ was brought about by primitive algae, through uncontrolled photosynthesis. Many primitive bacteria and protists could not deal with this sudden burst of oxygen (yes, oxygen is toxic, if you didn’t notice — it fires up blowtorches, rusts metals and corrodes all living organisms, including you), and perished. The ones who survived had to cope with, and then become dependent on, oxygen.
Oxygen is readily water soluble, and early animals didn’t have a problem delivering oxygen to each of their cells, because (a) they were small (like a hydra, or a sponge, or jellyfish) and (b) they lived in water, so that water was always in touch with all of their constituent cells. Thus oxygen was the last of their problems.
Once animals grew big and developed tissues like muscle, nerves and intestines (the early flatworms and roundworms) they needed to circulate the water throughout their bodies, as well as carry more oxygen in the same volume of water.
From this point onwards in evolution, two distinct lines seem to have diverged. Insects (and the general phylum of ‘arthropoda’) took the easier route and developed a system of hollow tubes (‘trachea’) throughout their bodies, so that they could breathe air from all over. The flipside of such a system is that a network of air tubes can only get so far before the air runs out, and thus insects never grew beyond a certain size.
The other major phylum, ‘mollusca’ (snails) and ‘chordata’ — to which we belong — took the route of developing specialised molecules to carry oxygen through their body fluids. Oxygen transporter molecules are found in abundance throughout the animal kingdom. Snails and some arthropods (i.e. prawns) use copper. Most vertebrates (fish, birds, reptiles, mammals) use iron to capture oxygen and deliver it to individual cells. Typically, the iron is wrapped in a carrier protein, hemoglobin in our case. Hemoglobin is packaged in tiny corpuscles, the red blood cells (RBCs), which are manufactured in millions every day.
Having all this iron floating inside the ECF makes it dense, necessitating dedicated plumbing. The iron rich part of the ECF, ‘blood’, is segregated from the interstitial fluid, through the means of blood vessels. One particular blood vessel in the body thickens and gains some muscle to become the heart. Together, the heart and blood vessels form the ‘cardiovascular system’, an intricate network of vessels of various calibre throughout the body.
Apart from RBCs, blood also contains a minor fraction of ‘white’ (actually, transparent; they look white when they collect to form pus) blood cells, which patrol the body to protect against invaders (more on this on my series on immunity). And then there are ‘platelets’, little sachets of putty who are always making repairs to blood vessels.
The way it all works is as follows: the right side of the heart pumps the blood into the lungs, where it collects oxygen and gives up carbon dioxide. This ‘oxygenated’ blood then comes back to the heart, and is pumped by the left side of the heart to the rest of the body through ‘arteries’. The arteries become progressively thinner and sparser until only a thin wall remains between blood and interstitial fluid. These ‘capillaries’ let oxygen and food diffuse out, and carbon dioxide and ammonia come into blood. The blood then returns to the right side of the heart through veins, and the cycle continues. In the next cycle, some of this toxin-rich blood reaches kidney, where the toxins are filtered out, and purified blood returns back to heart. This ‘cardio-respiratory-renal’ cycle continues till your last breath, and defines ‘life’ as we know it.
