Sunday, August 12, 2012

Getting to the heart of a stroke - Part I

Based on a talk at Dr. Purser's wellness group, July 2012.

What do economists and physicians have in common?  The world we now live in is straddled with economic woes. In the eyes of an economist, there is a sickness going around and that illness is ‘debt’. Economists make forecasts and propose fixes for the monetary structure. They look at debt from this perspective. Everyone would agree that economic growth is the best way to get out of debt. In politics, the word debt is looked at differently depending on who you talk to. One end of the spectrum, debt stands for ‘Do Everything But Tax’ and at the other end of the spectrum, ‘Do Everything By Tax’.  

The word debt has also crept into human physiology. This is an entirely different kind of debt, the kind that draws the line between health and disease, life and death. In  the human body, the stakes are a lot higher. This debt is called the oxygen debt. In simple terms, cells use oxygen. If the supply of oxygen to individual cells is interrupted, the cells don’t immediately die. They survive by other means and the cells go into debt, called the oxygen debt. If the cells don’t repay this debt by replenishing itself with fresh oxygen, cellular ‘default’ occurs. The cellular equivalent of bankruptcy called cell death then slowly sets in. Depending on the location in the body, cellular death is called different names. For, example, if cells in the heart are deprived of oxygen, a heart attack ensues and similarly if oxygen supply to the cells of brain is interrupted, brain cells die resulting in a stroke. 

The main engine of the cellular economy is an oxygen molecule. Just as we breathe in oxygen through the lungs, the cells also breathe in oxygen. This is also called cellular respiration. Every living cell in the human body does this. What do cells do with oxygen? Oxygen allows cells to release energy stored in glucose. This energy that is released in the form of a molecule called the ATP. This ATP is the currency of the cellular economy. All cellular functions are carried out with payment in the form of ATP. From a tiny hair follicle to a vital bodily organ, energy derived from ATP is the common denominator for cellular function.

Cells can produce ATP aerobically or anaerobically, that is with oxygen or without oxygen. In the presence of oxygen, glucose is broken down to carbon dioxide and water and without oxygen, fermentation occurs and lactic acid is instead produced. In the presence of oxygen, far more energy in the form of ATP is produced and and in the absence of oxygen very little ATP is produced.

During exercise, there is greater consumption of oxygen as the cells need to produce more ATP derived energy to fuel muscle contraction. With vigorous physical activity, if oxygen demand outstrips supply, then the cells start accumulating lactic acid. The cells are said to be in oxygen debt. This oxygen debt is repaid by hard breathing that continues after finishing exercise. Lactic acid is slowly washed out. Till lactic acid is completely washed out, muscles remain tired and achy. Anyone who does regular physical activity would have experienced this.
Since every living cell in the human body produces ATP, the raw materials for ATP production (glucose and oxygen) needs to be transported to every cell. Cells cannot store these raw materials, therefore there needs to be a continuous supply. This is achieved through the blood stream. Food that we consume is broken down into basic building blocks such as glucose or sugar. The air that we breathe in is a mixture of gases such as nitrogen and oxygen. Oxygen is extracted from air in the lungs. Circulating blood picks up glucose from the gastrointestinal tract and oxygen from the lungs. These are then delivered to various parts of the body.

The human heart pumps about 5 litres of blood every minute. The main function of the heart is exporting blood. One of the largest consumers of blood exported by the heart is the human brain. Despite representing only 2% of the body weight, the brain accounts for 20-25% of the total body utilization of oxygen and glucose. Since the brain consumes so much oxygen and glucose, it must produce a lot of energy. This energy is utilized for the functioning of the neurons which are the basic building blocks of the brain.

Just as the heart is connected to every cell of the body through blood vessels, the brain is connected to every cell of the body through the nervous system. The body is in fact a network of red (blood) and white (nerve) highways. How does the heart know how much blood to pump to various parts of the body? The driving force is oxygen demand. The brain and the heart work in tandem to meet this oxygen demand. The brain “talks” to the heart and makes it work harder by sending impulses through a set of nerves called the sympathetic nervous system. Conversely, the brain also “tells” the heart to slow down and rest through the activity of certain nerves called the parasympathetic nervous system.

The human brain also “sees” the world outside. Our eyes are in fact an extension of the brain. A thin wire called the optic nerve connects the eyes to a small portion of the brain near the back of the head. From here, the brain sees the outside world. Through the network of nerves, the brain sees everything that is going on inside the body. The brain also sees what is going on in our minds though part of the brain called the limbic system.

Everything that we see with our eyes and in our minds affects our emotional state. This in turn influences brain activity and in turn the functioning of the heart. Just as vigorous physical activity increases the workload of the heart, heightened emotional states also increase the workload of the heart and the brain.

As the brain and heart work harder, more energy is produced in the form of ATP. As mentioned previously the byproducts of ATP production from glucose (utilizing oxygen) is carbon dioxide and water. Carbon dioxide and water excreted out of the body through the lungs and the kidneys. Apart from the production of harmless water as a byproduct, oxygen can also accept electrons and form potentially harmful byproducts called free radicals. One example of these  highly harmful byproducts of ATP generation hydrogen peroxide. Free radicals such as hydrogen peroxide are highly toxic to cells. You may have seen hydrogen peroxide used on cuts and bruises to clean wounds. Hydrogen peroxide acts as an antiseptic killing potential harmful bacteria on wounds.

Free radical production inside cells can damage cellular mechanisms and lead to cell death. One proposed theory of aging is free radical damage to cellular function. Luckily, the body produces natural antioxidants that neutralize this process. Moreover in a healthy state, very little free radicals are produced which are quickly neutralized by the body’s defense mechanisms. One example of an antioxidant that we frequently use as a supplement is Vitamin C.

Free radicals are also produced in the blood vessels. A very important regulator of blood vessel function and health is a molecule called nitric oxide. It is produced in the lining of the blood vessel and keeps the blood vessels supple and healthy. Nitric oxide relaxes blood vessels decreasing blood pressure among other beneficial effects. When free radicals combine with nitric oxide, they not only deplete the stores of beneficial nitric oxide, but also generate a new compound called peroxynitrite that is highly toxic to the blood vessel wall. The damaged blood vessel wall is a easy target for the formation of cholesterol plaques that may ultimately lead to strokes and heart attacks.

The brain receives about of the quarter of the blood ejected from the heart with every heart blood. This blood travels from the heart to the brain through four major blood vessels. These four blood vessels, two in the front of the neck and two in the back of the neck are like four pillars that support a healthy brain. If damage occurs to any of these blood vessels the brain is put in great jeopardy. Blockages frequently form in the carotid arteries, which are the two major blood vessels supplying the brain.

Besides these relatively large blood vessels that supply blood to the brain, there is a vast network of very fine blood vessels within the brain itself. These delicate blood vessels are easily susceptible to damage resulting from hypertension. Hypertension increases outward force on these delicate blood vessels.

Compromise to blood flow to the brain can occur due to problems in the heart. In some cases, blood clots can form in the heart and travel up to the brain resulting in a stroke. Majority of the cases of stroke are due to lack of blood flow to the cells of the brain from one or more of these causes. A smaller number of strokes are caused by damaged blood vessels in the brain that break and leak blood into the brain.

One of the keys to staying young and healthy is reducing the workload of the cells. There is a greater metabolic turnover of cells with mental stress, overeating and sedentary lifestyle. Greater the metabolic turnover of cells, greater is the chance for cell damage and death. Metabolic activity of cells can be reduced by keeping the mind calm, eating fresh and easily digestible foods, and getting regular aerobic activity.