An average person is estimated to contain roughly 30 trillion human cells, according to recent research.
This is, of course, a rough approximation. Itís extraordinarily complicated to count human cells. Each of the 200 different types of cells in the human body has a different weight and size. Within the body, some cells are packed more densely, while others are more spread out. Many cells are constantly dying, and new ones being made simultaneously. Extra variances are due to age, height, weight, health, and environmental factors.
A recent study used a man between 20 and 30 years of age, weighing 70 kilograms (154 pounds) and measuring 170 centimeters (5 feet, 7 inches) in height, as a reference. In the study, researchers went through each cell type and used a variety of tedious methods to estimate the number of each type. They used the most up-to-date information available to make a detailed list of volumes and densities in every organ of the body. Once they arrived at an estimate of all the different cell types, they added them all together. The number they arrived at was 30 trillion, of which 25 trillion were red blood cells.
With respect to that person, about 5 million cells die every second. Red blood cells live for about four months and white blood cells live on average more than a year. Skin cells live about two or three weeks. And with all the wear and tear involved, colon cells die after about four days. After procreation, sperm cells have a life span of about three days.
With regard to the organs and bones, the body mostly replaces itself every seven to 15 years. However much of the cardiac muscle (about 2 to 3 billion cells) and central nervous system (about 86 billion neuron cells in the brain, about 1 billion in the spinal cord) can last an entire lifetime. This full number of neurons are present when we are born, similar to most of the cells in our heart muscle, and all of a female's ova.
You may have read that bacterial cells in the human body outnumber human cells 10 to 1. The primary source for that ratio dates back to the 1970s, when American microbiologists used a series of assumptions to calculate the number of bacteria inside the intestinal tract. The 10:1 ratio has since been disproven.
New data show that the number of bacterial cells inside a human body is around 38 trillion. This turns out to be much closer to the estimated 30 trillion human cells in the body. So, while there are likely more bacterial cells than human cells in your body at any given time, the difference isnít as great as previously thought. And they have a very tiny mass, perhaps 1 - 3% of our total body weight.
There are three types of blood cells: red blood cells, white blood cells, and platelets. Red blood cells (RBCs) are by far the most abundant type of cell in the human body, accounting for over 80 percent of all cells.
Adult humans have somewhere around 25 trillion RBCs in their body, on average. Women usually have fewer RBCs than men, while people living at higher altitudes will usually have more.
There are also about 147 million platelets and another 45 million lymphocytes (a type of white blood cell) in the body, based on recent calculations.
There are roughly 171 billion cells in the average male brain and spinal cord according to new research, including about 86 billion neurons. Neurons are cells that help transmit signals throughout the brain, along with motor neurons and sensory neurons throughout the central nervous system. There are also 85 billion other cells in the brain, called glial cells, that help support the neurons. All of the neurons are present when a child is born.
How do neurons die
Unlike normal cells, neurons do not reproduce, and they will die off like any other cell, though many may last a lifetime. Perhaps 85 thousand (one millionth of them) die daily without being replaced (that's about one per second). Neural function and an ever growing body of stored information (memory) is maintained because the neurons continue to grow over the course of the individual's life. Specifically, the cell generates new dendrites (structures on the neuron that receive electrical messages) and the existing ones get longer and branch out. This dendritic branching allows for an ever increasing number of connections to form. The result is an elaborately interconnected network.
When you bump your head and suffer a concussion, neurons die. When there is a glitch in the blood supply to the brain, also called a stroke, neurons die. Neurons also die when faced with changes in their own functions which happens in the so-called neurodegenerative diseases like Alzheimerís, Parkinsonís, Huntingtonís, and Ischaemia. The mode of cell death though is often unclear.
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