Habits, routines, and jobs. If I play pickup soccer every few days when my friends and I can coordinate schedules, then there’s a certain average probability that I’ll be in the “soccer-playing” state at any given time. Our schedules, locations, and energy levels will determine whether we enter and exit the “soccer-playing state.” Our enjoyment of soccer vs. preference for other activities will determine how often we collectively play soccer—a sort of supply and demand curve (which is itself a dynamic equilibrium).
Cell turnover. Evolution and signaling are determinants of the rate at which cells reproduce, mature, and die. In a healthy organism, the function of the tissue to which the cell belongs determines, via natural selection, the rate of cell turnover. Blood cells, neutrophils, and gut cells account for 96% of total cell turnover on any given day. Although these cells dominate the turnover rate, it seems likely that differences in turnover rate in other cells are also important, and it would be interesting to take an evolutionary approach to understanding the differences in turnover rates at different rate magnitudes.
Highways and airports. The specific identity of the people driving or passing through these spaces changes, but the absolute number follows a pattern. Patterns of the workday and workweek, holidays, and other factors determine the flows in and out on one level; the airlines’ and road designers’ expectations about how many flights to schedule or lanes to build determine it on another.
I’d guess blood cells and neutrophils dominate turnover largely because there’s so many of them; IIRC blood cells turn over on a timescale of months, which isn’t especially fast. The stomach lining presumably turns over very quickly because it’s exposed to extreme chemical stress (mitigated by a mucus layer, but that can only do so much), so I’d guess that’s the dominant “gut cell” term.
That’s an interesting thing to know because it tells us what processes are likely to eat up bodily resources, aside from obvious things like moving muscles or firing neurons.
Just for erythrocytes, there are about 30 billion in the human body, which comprises a little under .1% of all cells. So they are enormously over represented in turnover relative to their abundance.
Huh. Now I am confused. Why is a cell which turns over on a timescale of months so over-represented in turnover? Skin cells, for instance, turn over at least that fast and should be at least as numerous.
The paper I linked above (“The distribution of cellular turnover in the human body,” lmk if you want me to send it to you) states that turnover is about 330 billion cells per day. It also states that erythrocytes account for 65% of that turnover, gastrointestinal epithelial cells account for 12% of turnover, while skin cells accounts for 1.1%. For skin cells, that would be 3.6 billion cells/day; for erythrocytes, 200 billion. That seems totally impossible given what I know about the turnover rate and absolute number of erythrocytes in the human body.
Another paper states that epidermal desquamation is about 500 million cells/day (Fig 1).
So yeah, both the proportions and the absolute number of cells being shed seem wildly divergent. The paper estimating cell turnover rates is in Nature Medicine. I’ll look closer at it and see if I can figure out the disconnect.
Oh shoot, I made a math mistake (wrong units). There’s actually almost 3 trillion erythrocytes in the human body, which is closer to 8% of the human body (~37.2 trillion cells). Their estimate of epidermal cell number and turnover is more than two orders of magnitude lower.
That still means that erythrocytes are heavily overrepresented in terms of cell turnover (of which they compose 65%), but not by as much as I’d originally thought.
Habits, routines, and jobs. If I play pickup soccer every few days when my friends and I can coordinate schedules, then there’s a certain average probability that I’ll be in the “soccer-playing” state at any given time. Our schedules, locations, and energy levels will determine whether we enter and exit the “soccer-playing state.” Our enjoyment of soccer vs. preference for other activities will determine how often we collectively play soccer—a sort of supply and demand curve (which is itself a dynamic equilibrium).
Cell turnover. Evolution and signaling are determinants of the rate at which cells reproduce, mature, and die. In a healthy organism, the function of the tissue to which the cell belongs determines, via natural selection, the rate of cell turnover. Blood cells, neutrophils, and gut cells account for 96% of total cell turnover on any given day. Although these cells dominate the turnover rate, it seems likely that differences in turnover rate in other cells are also important, and it would be interesting to take an evolutionary approach to understanding the differences in turnover rates at different rate magnitudes.
Highways and airports. The specific identity of the people driving or passing through these spaces changes, but the absolute number follows a pattern. Patterns of the workday and workweek, holidays, and other factors determine the flows in and out on one level; the airlines’ and road designers’ expectations about how many flights to schedule or lanes to build determine it on another.
I’d guess blood cells and neutrophils dominate turnover largely because there’s so many of them; IIRC blood cells turn over on a timescale of months, which isn’t especially fast. The stomach lining presumably turns over very quickly because it’s exposed to extreme chemical stress (mitigated by a mucus layer, but that can only do so much), so I’d guess that’s the dominant “gut cell” term.
That’s an interesting thing to know because it tells us what processes are likely to eat up bodily resources, aside from obvious things like moving muscles or firing neurons.
Just for erythrocytes, there are about 30 billion in the human body, which comprises a little under .1% of all cells. So they are enormously over represented in turnover relative to their abundance.
Huh. Now I am confused. Why is a cell which turns over on a timescale of months so over-represented in turnover? Skin cells, for instance, turn over at least that fast and should be at least as numerous.
The paper I linked above (“The distribution of cellular turnover in the human body,” lmk if you want me to send it to you) states that turnover is about 330 billion cells per day. It also states that erythrocytes account for 65% of that turnover, gastrointestinal epithelial cells account for 12% of turnover, while skin cells accounts for 1.1%. For skin cells, that would be 3.6 billion cells/day; for erythrocytes, 200 billion. That seems totally impossible given what I know about the turnover rate and absolute number of erythrocytes in the human body.
Another paper states that epidermal desquamation is about 500 million cells/day (Fig 1).
So yeah, both the proportions and the absolute number of cells being shed seem wildly divergent. The paper estimating cell turnover rates is in Nature Medicine. I’ll look closer at it and see if I can figure out the disconnect.
Oh shoot, I made a math mistake (wrong units). There’s actually almost 3 trillion erythrocytes in the human body, which is closer to 8% of the human body (~37.2 trillion cells). Their estimate of epidermal cell number and turnover is more than two orders of magnitude lower.
That still means that erythrocytes are heavily overrepresented in terms of cell turnover (of which they compose 65%), but not by as much as I’d originally thought.
Aha! This makes more sense now. Thanks for chasing that down, I feel much less confused.