Profitable Wonders
There is nothing like time spent working on a kidney transplant unit for heightening one’s awareness of the remarkable properties of this indispensable organ. Or indeed for appreciating the ineffable joys of peeing, as emotionally described by the recipient of a new kidney, following several irksome years being hitched up to a haemodialysis machine.
He had, he told me, recently had the urge ‘to go’ when driving through the countryside and duly relieved himself by the side of the road. This experience of passing urine alfresco under the silvery light of the moon had moved him to tears: ‘You can never know the ecstasy of it.’
Forty years on, I still find myself re-imagining his exhilaration when, in Barry Humphries’s immortal Barry Mckenzie phrase, ‘pointing Percy at the porcelain’.
The fundamental units of the kidney, the minuscule nephrons – two million of them in all – are collectively the most efficient filters in the universe, cleansing the entire volume of blood (five litres) of its impurities an astonishing 20 times every day.
The principal waste product is urea, composed of toxic nitrogen atoms – the by-product of that constant renewal of the body’s tissues where, day in, day out, 70 billion cells (of the gut, liver, bone etc) are destroyed, to be replaced by new ones.
The total amount of urea so produced is around 20 grams which, unless excreted, accumulates to cause the symptoms of kidney failure: progressive weakness, fatigue, constant nausea and intractable itching, culminating in stupor and death.
The process of filtration is analogous, if not strictly so, to the making of an espresso, where the blood plasma is forced under pressure through the thin walls of the blood vessels at the head of the nephron into a convoluted collecting duct.
The filtering of the total volume of blood 20 times every day produces 45 gallons of plasma, 99 per cent of which will be reabsorbed, leaving just one per cent containing waste products such as urea to be excreted as urine.
This might seem a bit odd. Why should the nephrons filter such prodigious amounts of plasma only for virtually all of it to be reabsorbed? This brings us to the further crucial function of the kidneys: their role in homeostasis, ensuring the amounts of water, salt, potassium and glucose in the body’s tissues remains within the same narrow limits on which their efficient functioning depends.
Sticking with salt, our freedom to consume as much, or as little, salt as our tastebuds dictate is due to reabsorption being an active process.
Specialised proteins (or ‘transporters’) in the lining of the nephron grab on to the molecules in the filtered plasma, conveying them back into the bloodstream. When, after we have scattered salt on our potatoes, the concentration is high, those transporting proteins will, as it were, be overwhelmed, the excess excreted unchanged in the urine. And vice versa: too little salt in the diet, and the transporters voraciously reabsorb it – so only minuscule amounts are present in the urine.
The situation with water is rather different. Water is much the most important constituent of the body – we are two-thirds water; over 80 per cent of the grey matter of the brain and almost 90 per cent of the blood in the circulation is water. It is a major component of saliva, essential for the regulation of the body temperature, lubricating the joints and much else besides. Again, too much and the tissues become swollen and bloated; too little and we suffer the dire effects of dehydration.
Most of the water filtered through the kidneys is reabsorbed by the universal process of osmosis, being sucked through the walls of the nephron from the dilute plasma back into the ingeniously ever more concentrated tissues of the kidney.
Meanwhile, the pituitary gland, at the base of the brain, monitors the consistency of the blood and, if it is too ‘thick’, secretes the hormone ADH (antidiuretic hormone) which further accelerates the amount of water passing back into the circulation.
These two mechanisms combined are quite extraordinarily efficient – vividly demonstrated when the physiologist J B S Haldane forced himself to drink six litres of water in just six hours. His kidneys responded to this extraordinary feat of more than doubling his blood volume by producing copious amounts of urine which, when measured, came to precisely the volume of water he had drunk.
For these and many other reasons, those unromantic kidneys – and the urine they produce – deserve our heartfelt thanks and praise.
‘We are two-thirds water; over 80 per cent of the brain’s grey matter is water’