Life is in the blood

by Andrew Hodge

Article from:
June 2011

“For the life of the flesh is in the blood”

life is in the blood

Photo: ©

So says Leviticus 17:11. Everyone knows that we must have enough blood flowing around our body or else our bodily functions deteriorate and we die. Yet for a long time the exact function of blood was little understood. In what ways has modern science shown Leviticus 17:11 to be true?

Blood is fundamental to the function of every cell of every component in our bodies. Cells need food to survive, grow, repair themselves and to fulfill their specific functions, and, to reproduce. Cellular food is transported in blood to provide energy for all the cells’ needs. As humans are multicellular organisms, having separate specialized organs with highly sophisticated functions, transport and communication between these structures is essential.


Do the cells of the body tell the blood how it should work? No. Does the blood carry around everything possible just in case? No. The cells and the blood work together to provide optimum conditions for correct functioning of all the cells—with their different requirements—in all the tissues and organs of the whole body, including the cells of the blood itself.

Blood provides this coordinated environment by regulating acidity/alkalinity (pH), providing oxygen (and removing carbon dioxide and other waste products), and carrying essential vitamins and minerals. Also, blood has to be in the right places at the right times, at the right temperature and pressure, and it carries regulatory messages between organs via blood ‘messengers’ called hormones. All this is organized within very specific limits—straying outside these (through injury, disease, toxins, etc.) rapidly reduces functionality.

Hormonal feedback

Words matter

FLESH (as used in many English translations of Leviticus 17:11): Hebrew בשר basar, the tissues that make up the body, and (by extension) also the body, the living creature.

TISSUE: a collection of cells (not necessarily the same type) grouped for a specific function. E.g. connective tissue, muscle tissue. Blood itself is, technically speaking, also a tissue.

ORGAN: several types of tissue functionally grouped together, e.g. liver, lung.

Hormones, those important chemical messengers in the blood, are involved in self-regulating feedback systems. These systems stimulate hormone production in times of lack, and suppress it in times of plenty. For example, when we eat, the sugars in the intestine are digested and absorbed into the local bloodstream. This blood then passes through the pancreas and its higher sugar level stimulates production of the hormone insulin. As insulin is distributed in the bloodstream, it reduces the blood sugar to normal levels again by increasing the amount of sugar that all cells take in. In fact the brain relies almost entirely on sugar (specifically glucose) for its energy supply; hence this feedback system is absolutely critical for proper brain activity. If the blood glucose ever drops too much, we lose consciousness.

The body’s systems tend to be wisely over-engineered, so that one might predict that there is also a system to cope with low sugar levels, for example when we exercise and use sugar up. This system uses the hormone glucagon (also from the pancreas) and it works by releasing glucose into the blood from stores located mostly in the liver.

There are about fifteen organs classed as hormone-producing (endocrine) glands,1 and their products, carried by the blood, affect either every cell in general or specifically target certain cells. Widely known examples are the male and female hormones testosterone and estrogen, adrenaline (epinephrine in the US), the thyroid hormone thyroxine, and many more.




Why is blood red?

The red colour of blood reflects the colour of the hemoglobin inside the red blood cells. This is because the hemoglobin contains iron. The ‘heme’ of the hemoglobin molecule in vertebrates (creatures with a backbone) is a porphyrin ring which surrounds ferrous iron atoms. It is the spatial relationship between heme, iron and globin which makes it possible to bind oxygen molecules reversibly—one to each iron—and which makes the system so efficient.

For example, thyroxine regulates the speed of metabolism in every cell, and having the correct amount (within narrow limits) allows normal cellular activity. Too much and we become ‘hyper’, too little and we are slow and lethargic.

Another example is gastrin. The target organ for gastrin is that part of the inner lining of the stomach which produces hydrochloric acid for digestion. Food in the last part of the stomach stimulates the production of gastrin, which is carried back by the blood to stimulate acid production. This is a positive feedback mechanism in which blood is the essential communicating link.


Blood also has a major role in body protection in that it is an integral part of the immune or infection-fighting system, involving antibodies and white blood cells. It also possesses a highly complex mechanism to prevent its own loss from the body (clotting) and to prevent clotting inside the body (thrombosis). The capacity to quickly initiate clotting outside and to limit—even reverse—clotting on the inside is provided by ‘cascades’—cumulative processes in which each step of the process is dependent on the one before it (see box). The cascades are of such complexity that new factors, cofactors and regulators are being constantly added to our body of knowledge. It is now known that there are more than a hundred factors or steps that make up the clotting cascade.2 Such details add to our appreciation of how finely balanced, effective and versatile the system is. But a greater marvel is that such a system, which is there in anticipation of blood loss, internal injury or disease, should be there at all.

Unique red blood cells

Red blood cells (RBCs or erythrocytes) form the majority of the cells in the blood—and a quarter of all cells in the human body. They are unique among all others—in mammals, they have no nucleus and none of the usual energy-producing structures in the cell outside the nucleus. This is a design feature of mammals (creatures which, like us, suckle their young). Normally, a cellular nucleus carries the DNA which instructs the cell on how to perform its functions, including repair and reproduction, at the appropriate times. RBCs cannot do this because instead they are especially designed to carry oxygen, and in humans, having a nucleus would hinder this essential function. So the nucleus is lost after formation, leaving them with their characteristic biconcave shape.

Blood bytes*

There are about 4–6 million red blood cells (RBCs) in every cubic millimetre of blood; 20–30 trillion of them in each person.

Every day about 1% of these are changed. New RBCs take about 7 days to form in the bone marrow, and are produced at the staggering rate of about 2 to 3 million every second.

Each RBC lasts about 120 days before its components are recycled to form new RBCs.

During its 4-month lifetime, each red cell travels some 500 km (300 miles) around the body, passing through the heart about 14,000 times per day.

Most of our blood vessels are the microscopic capillaries. If the blood vessels in one person were laid end to end, they would be about 150,000 km (100,000 miles) in length—enough to circle the earth at the equator about four times!

*All figures are for a healthy adult

Two reasons have been suggested for this. First, the relative size of RBCs (6–8 µm diameter and just 2 µm thick)3 and capillaries (tiny blood vessels) is such that red blood cells often have to deform in order to squeeze through. A nucleus (about 6 µm on average4) could prevent passage of the cell and make it get stuck, blocking the circulation.

Second, the shape and deformability of the red blood cell is optimized for the carrying and delivery of oxygen, and it maximizes the amount of hemoglobin that can be packed into the cell. Nevertheless birds, which have a very high oxygen requirement, do fine with nucleated RBCs, so there are other design features in birds that compensate for this.5

The system of the red blood cells giving oxygen to the cells of the tissues is reversed when the red blood cell reaches the lungs, where it gives up its carbon dioxide (though this is mostly carried by plasma6) and takes on a new load of oxygen. At rest, all the blood (5 litres in an adult) completes a circuit within a minute (spending 1 to 3 seconds in the capillaries). With exercise, circulation is as quick as every 10 seconds.7 Having a molecule such as hemoglobin which can handle oxygen so quickly and reversibly, when required, is amazing.


So is the life of the flesh in the blood? Although not confirmed by science until modern times, this statement from Leviticus 17:11 has always been true. Blood actively maintains life by providing a vital function for all cells, tissues and organs, and thus the life of the whole body. The more we find out about the astounding functional design and complexity of blood, the more marvellous it becomes to us, and the more honour and praise is due its Creator.

Irreducibly complex: The clotting cascade

The function of the blood clotting system is to prevent the escape of blood from a damaged vessel. To do this, the blood has a special and very complex repair procedure in place. Once initiated by a cut, the first component in the process is activated, which in turn activates the next component, and so on, in a series of cumulative, mutually-dependent steps. This physiological chain of production, or cascade, results in the formation of a solid obstruction (a clot) in order to seal over the damage.

Some of the main components of the clotting cascade are the proteins fibrinogen, prothrombin, Stuart (anti-hemophilic) factor and proaccelerin. None of these are used for any other purpose in the blood. The system is very finely tuned to result in a repair process that achieves just the repair needed at just the right place and time to stop bleeding and begin the process of healing. Importantly, the process is also self-limiting to ensure that coagulation (clotting) of the entire blood supply does not occur.

The Intelligent Design advocate Michael Behe, in his book Darwin’s Black Box, has noted that the clotting cascade is an example of irreducible complexity. The removal or degradation of just one, any one, of the components or steps would cause the cascade to fail. Obviously this would have dire consequences for the organism. It is exceedingly difficult to see how the clotting cascade could have evolved, as any postulated simplified or ‘primitive’ version of the process would result in failure.1

  1. See also Behe, M., In defense of the irreducibility of the blood clotting cascade: Response to R. Doolittle, K. Miller and K. Robison,, 2000.

References and notes

  1. Guyton, Arthur C., Textbook of Medical Physiology, Eds Arthur C. Guyton, John E. Hall, p. 838, W.B. Saunders Co., Philadelphia PA 19106, 10th Edition 2000. Return to text.
  2. E.g. there is an international scientific journal dedicated solely to Thrombosis and Haemostasis (Schattauer, ISSN 0340-6245, 12 issues/yr). Return to text.
  3. 1 µm (micrometre) is a millionth of a metre, or 1/25,400 inch. Return to text.
  4. ‘Cell Nucleus’, Encyclopaedia Brittannica: Ultimate Reference Suite 2005. Return to text.
  5. See e.g. Blown away by design; Return to text.
  6. Most of the oxygen in the bloodstream (98%) is carried on the hemoglobin in the RBCs; a little is dissolved in the plasma. Most of the carbon dioxide returning to the lungs is carried dissolved in the plasma, with a small amount in the RBCs. Return to text.
  7. Guyton, Ref.5, p. 3. Return to text.
Guyton, Arthur C., Textbook of Medical Physiology, Eds Arthur C. Guyton, John E. Hall, p. 838, W.B. Saunders Co., Philadelphia PA 19106, 10th Edition 2000.
E.g. there is an international scientific journal dedicated solely to Thrombosis and Haemostasis (Schattauer, ISSN 0340-6245, 12 issues/yr).
1 µm (micrometre) is a millionth of a metre, or 1/25,400 inch.
‘Cell Nucleus’, Encyclopaedia Brittannica: Ultimate Reference Suite 2005.
See e.g.
Most of the oxygen in the bloodstream (98%) is carried on the hemoglobin in the RBCs; a little is dissolved in the plasma. Most of the carbon dioxide returning to the lungs is carried dissolved in the plasma, with a small amount in the RBCs.
Guyton, Ref.5, p. 3.

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Readers’ comments
Terence T., South Africa, 24 September 2012
Thank you Andrew! There is so much debate about whether creationists should write as they do, but sometimes those who disapprove miss how you all lift up and point out the glory of our Creator. Terence
M. G., South Africa, 24 September 2012
If our bodies have 150,000 km of blood vessels, and the blood passes through the heart about 14000 times a day, how is it possible for a RBC to travel only 500 km in 4 months? Shouldn't it be a ***lot*** more?
Andrew Hodge responds
This query has obvious merit and clarification is required. I had thought that the figures were all sourced from Guyton’s textbook of Medical Physiology but I have not been able to readily confirm this. Nor have I found them with a quick review of the articles in Wikipedia but from memory they are correct, and appear reasonable, in line with other data in Guyton e.g. the volume and distribution of blood in the sequence of vessels from the heart and back again (p144). (1)If we use a total vascular distance of 500,000 km and 14,000 circuits per day we have a single circuit distance of 35.7 km. The time taken for each circuit would be 6.17 sec or 6243 km/hr! Clearly ridiculous. (2)If we use total distance travelled in 4 months as 500km, then the same calculations give a mean circuit distance of 0.298 m (not unreasonable for a combined distance out from the heart and back given that the majority of the cardiac output is to heart, kidneys and liver at rest) and a mean blood speed of 17.9 km/hr which is much closer to the physiological expectation. Guyton (p145) puts the average velocity of blood flow in the aorta (at rest) as 33cm/sec or 118 km/hr, and in the capillaries at 1/1000th of this at 0.3 cm/sec or only 0.12 km/hr. One can easily reconcile a mean velocity of 17.9 km/hr with these numbers. Why the difference between (1) and (2)? The calculation for (1) makes the assumption that all the blood passes through all the vessels on every circuit. It does not. If muscle tissue is taken as an obvious example, the volume of blood flowing at rest is 3 to 4 ml/min/100gm of muscle (Guyton p223). With extreme exercise in a well-trained athlete this volume can increase 15 to 25 times or 50 to 80 ml/min/100gm. The capacity of the capillary bed is huge, but the volume of blood flowing through it is regulated - according to muscular demand for oxygen - by the local arterioles and precapillary sphincters. At rest, not much (but the distribution remains appropriate to demand), and with exercise, a lot. If every muscle fibre is working at maximum capacity its capillary supply is maximally dilated (and at maximum pressure and volume). Does this then mean that at maximum muscular activity (and with all the capillaries open) the blood travels 35.7 km with every circuit and at 6,243 km/hr? No, because only those muscles that demand oxygen are served. All other tissues not directly involved reduce their blood flow e.g. the abdominal organs. This is why if we exercise too hard after a big meal we do not feel good because there is basically not enough blood (i.e. physiological compensatory mechanisms) to go round. The design of the large potential for variation in capillary flow is necessary for the wide demand in oxygen of all tissues according to their variable activities.
Don S., United States, 24 September 2012
Yes, there is life in the blood! That's why God requires (or required) blood sacrifices. Yet red blood cells are a beautiful picture of sacrifice. When the cells are created, they have a nucleus with DNA and have the potential to live and self-replicate. However, at about a week old, they extrude out their DNA yielding a sacrificed "dying" cell that is now able to carry the "breath of life" (O2) to every other cell in the body. Without their sacrifice, our life would not be possible. God designed it this way to declare His glory!
Erik W., United States, 25 September 2012
After doing most of the research into the origins debate and finding the Biblically based, technical answers and explanations to the tough questions I needed to resolve, I found myself sticking around AiG, ICR and CMI's sites. Why? I had already discovered what was true, so what purpose was there in continuing to read? The answer is that I love learning, and these sites continually put out FASCINATING in-depth articles that satisfy my intellect by teaching me new things. God's Word never ceases to teach us new things as we read it. I suggest that likewise, exposition of God's Word, and scientific research founded on God's Word are similarly capable to teach us new things the more we keep looking into it. Thank you for your technical and semi-technical articles, CMI! Articles like this one are as educational as a typical chapter in one of my university biology textbooks--and more so, because here I can see the *big picture* as well. I don't just come here for encouragement and answers to tough questions anymore, I come here to learn from the best! (I'm referring both to "thinking God's thoughts after Him" and to your staff) :) Sorry for the length of the comment, and may God continue to bless and mightily use your ministry.