Spare a thought for the way nature has crafted our body’s physiologic systems. A few of those exquisitely configured systems will serve to illustrate the efficient way in which the body seeks to maintain balance and protect health. Each one of them functions with a cascade of controls, feedback loops, resets and reinforcements to achieve overall balance and stability.
Since Covid-19 has been the dominant theme of recent years, let us look at the immune system first. The ensemble cast includes: rapidly mobilised cytokines as the first responders to a microbial threat, antibodies secreted by the B lymphocytes, which combat the virus circulating in the bloodstream, killer and helper T lymphocytes that provide cellular immunity, memory B and T cells and a diverse population of neutrophils. Some of them play a major role in innate immunity, others in acquired immunity, while some feature in both.
The best of modern military strategists will be unable to match the speed of mobilisation and efficiency of coordinated deployment that the various components of the immune system display in the defence of our body. Yet, we hamper those defences by weakening our immunity by consuming unhealthy diets, alcohol and tobacco, and depriving the body of sleep, sunlight, adequate physical exercise and clean air.
Think of how our body manages the balance of food intake and glucose regulation. Our appetite is stimulated by the hormone ghrelin (hunger hormone) that is secreted by the stomach and a part of the hypothalamus in the brain. It is short-acting and triggered by the brain’s recognition of the need for energy replenishment. As food passes into the small intestine, the brain is signalled again. The satiety hormone leptin is then secreted by the small intestine and adipose tissue of the body. That suppresses the appetite. It is not surprising that both platelet activator and inhibitor are derived from a common parentage – the cell signalling molecular family of eicosanoids that are derived from polyunsaturated fatty acids.
Fine-tuning of the balance between clotting and bleeding can, thus, be efficiently managed by the body till we distort the balance through our behaviours. It is also worth noting that the sub-families of eicosanoids are also involved in the regulation of blood pressure, inflammation, allergy, fever, regulation of childbirth, and cell growth. Isn’t it remarkable as to how so many of our body’s functions are so well coordinated through interlinked regulatory systems?
The genius of nature is amazing, in terms of the connectivity it provides within our bodies and to the nature outside. This is an orchestra that will make beautiful music for our health, till we strike the wrong keys or sing discordant notes. As the digested food releases glucose, which passes the blood-brain barrier, the body modulates the blood levels of glucose that is needed as fuel by the body’s cells. Insulin is secreted by the beta cells of pancreas and lowers blood glucose by enabling glucose to enter the cells for utilisation.
The alpha cells, also in the pancreas, secrete glucagon, when blood sugar goes down. This hormone helps to raise blood sugar levels by converting glycogen stored in the liver to glucose (glycogenolysis) and amino acids from proteins into glucose (gluconeogenesis). Glucagon secretion is stimulated by low blood sugar levels, protein-rich meals and adrenaline. Its secretion is reduced by carbohydrate-rich meals.
The food that passes from the stomach to the small intestine is not just a speeding sugar express. It contains many other components that influence the speed and level of nutrient absorption, satiety and the rate of blood sugar rise. It carries the nutrients we need and those that our gut bacteria relish. Proteins, fat and fibre (soluble and insoluble) play a role in these processes. They are dealt with by different enzymes and evoke several other physiological effects than sugars do.
Our body deals with all that complexity without us being aware of it. The brain silently monitors these processes through its sensors and ensures order through its neuro-hormonal controls and feedback loops. Yet, even the rational part of the brain succumbs to temptations of unhealthy foods, which increase the level of dopamine (the “pleasure hormone”) in our brain. This pleasure creates a craving for foods rich in sugar, fat and salt, and overrides the signals of satiety.
The manufacturers and marketers of unhealthy food products and beverages exploit this chink in the rational armour of our brain. Obesity and related diseases are not because our physiology has failed us. It is because sections of the food industry have found a trojan horse to breach its defences. A life-threatening manifestation of Covid-19 has been the increased tendency for blood clots to form in both arteries and veins. When we examine the mechanisms related to the clotting of blood, we again see a balance of forces that cause or prevent it in our body. There are two pathways to initiate blood clots.
One proceeds through a set of circulating blood proteins produced by the liver. These coagulation factors, when activated, end with a fibrin mesh that traps the blood cells and creates a clot. The other pathway is via blood cells called platelets. When activated, the platelets produce a chemical called thromboxane A2. This stimulates clotting. Another chemical called prostacyclin, generated by the blood vessel wall, prevents clotting by countering the action of thromboxane A2. Both these chemicals are generated from a common precursor, the eicosanoids lipids (prostaglandins), and are structurally very similar. This yin and yang of coagulation is another fascinating reminder of how delicately the body creates a balance to keep us alive and healthy. Unfortunately, we disturb that balance when we eat foods that have loads of unhealthy fats or smoke or become very sedentary.
Consider the way the strength and suppleness of the bones in our body is regulated. Calcium levels in our blood are elevated by the action of the parathyroid hormone secreted by the parathyroid gland that adjoins the thyroid gland in the front of our neck. The calcium helps bone formation, which is engineered by cells called the osteoblasts. When osteoblasts are surrounded by their secretions, they are called osteocytes. These make a hormone called sclerostin that inhibits bone formation. Sclerostin acts as a mechanosensor. When osteocytes sense bone-loading activities performed by a person, sclerostin secretion is reduced. This helps in stress-bearing bone formation. The calcium levels in the blood are lowered by a hormone called calcitonin; it is secreted by the thyroid gland. Calcitonin acts to reduce the activity of osteoclasts that break down the bone, so that the blood calcium levels do not increase too much. But osteoclasts serve a very important function in normal conditions. They eat the bone, to reshape it into a stronger and more resilient structure that is better at bearing loads than before.
This constant remodelling goes on, with the osteoblasts laying the bone like masons and osteoclasts reshaping it like carpenters chipping away, with the parathyroid hormone and calcitonin helping by correctly rationing the calcium supply. How do the osteoblasts and osteoclasts know how much bone to lay and how to reshape it? They get their signals from piezoelectric currents generated by bone stress. Every time you twist or turn in a chair or in a dance, those signals go to these bone-sculpting cells! We do them no favour when we adopt wrong postures in office chairs, in vehicles or pour over computers and cell phones for long periods. Also, when we are sedentary, we do not give our bones the stimulus to stay strong. Our nutrition and physical activity determine our bone health. Apart from personal choices, our social conditions determine our diets and the levels of physical activity. The living conditions we create in a changing society reflect on our individual bodies.
Excerpted with permission from Pulse to Planet: The Long Lifeline of Human Health, K Srinath Reddy, HarperCollins India.
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