Scanning electron micrograph of blood cells. From left to right: human erythrocyte, thrombocyte (platelet), leukocyte.

White blood cells attacking a parasite.

Blood is a bodily fluid in some animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells.

Blood accounts for 7% of the human body weight. The average adult has a blood volume of roughly 5 liters (11 pints).

Blood is slightly saline - about the same concentration as seawater.


1   Substance

Blood consist of blood cells suspended in blood plasma. Blood plasma is 92% water by volume, and contains dissipated proteins, glucose, mineral ions, hormones, `carbon dioxide`_., and blood cells themselves.

The blood cells are mainly red blood cells (also called RBCs or erythrocytes), white bloods cells (also called WBCs or leukocytes) and platelylets. By volume, the red blood cells constitute about 45% of whole blood, the plasma about 54.3%, and white cells about 0.7%. These contain hemoglobin_, an iron-containing protein, which facilitates oxygen transport.

2   Properties

2.1   Oxygen level

Vertebrate blood is bright red when its hemoglobin is oxygenated and dark red when it is deoxygenated.

Oxygenated blood absorbs light preferentially at 905 nm (near infra-red light), where deoxygenated blood blood absorbs light preferentially at 660 nm (red light).

A pulse oximeter works by passing a beam of red and infra-red light through a pulsating capillary bed and then measures the amount of red and infra-red light emerging from the tissues via a sensor.

The relative absorption of light by oxyhemoglobin (hbO) and deoxyhemoglobin is then processed according to the `Beer-Lambert's law`_ and the oxygen saturation level (SpO2) determined.

SpO2 stands for periphereal capillary oxygen saturation. Oxygen saturation is defined as the ratio of oxyhemoglobin (HbO2) to the total concentration of hemoglo (i.e. oxyhemoglobin + reduced hemoglobin) present in the blood.

A pulse oximeter uses a light emitting diode and photo detector.

3   Cardiovascular system


Blood is circulated around the body through blood vessels by the pumping action of the heart. In animals with lungs, arterial blood carries oxygen from inhaled air to the tissues of the body, and venous blood carries carbon dioxide, a waste product of metabolism produced by cells, from the tissues to the lungs to be exhaled.

3.1   Pulse

Your pulse (= heart rate) is the number of times your heart beats per minute. The normal pulse for healthy adults range from 60 to 100 beats per minute.

3.2   Blood pressure



Blood pressure is pressure of circulating blood on the walls of blood vessels. Blood pressure is expressed in terms of the systolic pressure (maximum during one hear beat) over diastolic pressure (minimum in between two heart beats) and is measured in millimeters of mercury. Normal resting blood pressure in an adult is approximately 120/80 mmHg. (Why mercury? Mercury was used in the first accurate pressure gauges and is still used as the standard unit of measurement for pressure in medicine.)

In most people, systolic blood pressure rises steadily with age due to the increasing stiffness of large arteries and a long-term build up of plaque.

4   Blood Types

40 percent of Caucasians have type A blood, while only 27 percent of Asians do. [1]

5   History


Renaissance doctors mused about what would happen if they put blood into the veins of their patients. Some thought that it could be a treatment for all manner of ailments, even insanity. Finally, in the 1600s, a few doctors tested out the idea, with disastrous results. A French doctor injected calf’s blood into a madman, who promptly started to sweat and vomit and produce urine the colour of chimney soot. After another transfusion the man died.

Such calamities gave transfusions a bad reputation for 150 years. Even in the 19th century only a few doctors dared try out the procedure. One of them was a British physician named James Blundell. Like other physicians of his day, he watched many of his female patients die from bleeding during childbirth. After the death of one patient in 1817, he found he couldn’t resign himself to the way things were.

Human patients should only get human blood, Blundell decided. But no one had ever tried to perform such a transfusion. Blundell set about doing so by designing a system of funnels and syringes and tubes that could channel blood from a donor to an ailing patient. After testing the apparatus out on dogs, Blundell was summoned to the bed of a man who was bleeding to death.

Several donors provided Blundell with 14 ounces of blood, which he injected into the man’s arm. After the procedure the patient told Blundell that he felt better – “less fainty” – but two days later he died.

Still, the experience convinced Blundell that blood transfusion would be a huge benefit to mankind, and he continued to pour blood into desperate patients in the following years. All told, he performed ten blood transfusions. Only four patients survived.

The first clues as to why the transfusions of the early 19th century had failed were clumps of blood. When scientists in the late 1800s mixed blood from different people in test tubes, they noticed that sometimes the red blood cells stuck together. But because the blood generally came from sick patients, scientists dismissed the clumping as some sort of pathology not worth investigating. Nobody bothered to see if the blood of healthy people clumped, until Karl Landsteiner wondered what would happen. Immediately, he could see that mixtures of healthy blood sometimes clumped too.

Landsteiner set out to map the clumping pattern, collecting blood from members of his lab, including himself. He separated each sample into red blood cells and plasma, and then he combined plasma from one person with cells from another.

Landsteiner found that the clumping occurred only if he mixed certain people’s blood together. By working through all the combinations, he sorted his subjects into three groups. He gave them the entirely arbitrary names of A, B and C. (Later on C was renamed O, and a few years later other researchers discovered the AB group. By the middle of the 20th century the American researcher Philip Levine had discovered another way to categorise blood, based on whether it had the Rh blood factor. A plus or minus sign at the end of Landsteiner’s letters indicates whether a person has the factor or not.)

When Landsteiner mixed the blood from different people together, he discovered it followed certain rules. If he mixed the plasma from group A with red blood cells from someone else in group A, the plasma and cells remained a liquid. The same rule applied to the plasma and red blood cells from group B. But if Landsteiner mixed plasma from group A with red blood cells from B, the cells clumped (and vice versa).

The blood from people in group O was different. When Landsteiner mixed either A or B red blood cells with O plasma, the cells clumped. But he could add A or B plasma to O red blood cells without any clumping.

It’s this clumping that makes blood transfusions so potentially dangerous. If a doctor accidentally injected type B blood into my arm, my body would become loaded with tiny clots. They would disrupt my circulation and cause me to start bleeding massively, struggle for breath and potentially die. But if I received either type A or type O blood, I would be fine.

Landsteiner didn’t know what precisely distinguished one blood type from another. Later generations of scientists discovered that the red blood cells in each type are decorated with different molecules on their surface. In my type A blood, for example, the cells build these molecules in two stages, like two floors of a house. The first floor is called an H antigen. On top of the first floor the cells build a second, called the A antigen.

People with type B blood, on the other hand, build the second floor of the house in a different shape. And people with type O build a single-storey ranch house: they only build the H antigen and go no further.

Each person’s immune system becomes familiar with his or her own blood type. If people receive a transfusion of the wrong type of blood, however, their immune system responds with a furious attack, as if the blood were an invader. The exception to this rule is type O blood. It only has H antigens, which are present in the other blood types too. To a person with type A or type B, it seems familiar. That familiarity makes people with type O blood universal donors, and their blood especially valuable to blood centres.

Landsteiner reported his experiment in a short, terse paper in 1900. “It might be mentioned that the reported observations may assist in the explanation of various consequences of therapeutic blood transfusions,” he concluded with exquisite understatement. Landsteiner’s discovery opened the way to safe, large-scale blood transfusions, and even today blood banks use his basic method of clumping blood cells as a quick, reliable test for blood types.

In 1900 the Austrian physician Karl Landsteiner first discovered blood types, winning the Nobel Prize in Physiology or Medicine for his research in 1930.


The uncertainty slowly began to dissolve, starting in the 1990s with scientists deciphering the molecular biology of blood types. They found that a single gene, called ABO, is responsible for building the second floor of the blood type house. The A version of the gene differs by a few key mutations from B. People with type O blood have mutations in the ABO gene that prevent them from making the enzyme that builds either the A or B antigen.

Scientists could then begin comparing the ABO gene from humans to other species. Laure Ségurel and her colleagues at the National Center for Scientific Research in Paris have led the most ambitious survey of ABO genes in primates to date. And they’ve found that our blood types are profoundly old. Gibbons and humans both have variants for both A and B blood types, and those variants come from a common ancestor that lived 20 million years ago.


The most striking demonstration of our ignorance about the benefit of blood types came to light in Bombay in 1952. Doctors discovered that a handful of patients had no ABO blood type at all – not A, not B, not AB, not O. If A and B are two-storey buildings, and O is a one-storey ranch house, then these Bombay patients had only an empty lot.

Since its discovery this condition – called the Bombay phenotype – has turned up in other people, although it remains exceedingly rare. And as far as scientists can tell, there’s no harm that comes from it. The only known medical risk it presents comes when it’s time for a blood transfusion. Those with the Bombay phenotype can only accept blood from other people with the same condition.


Doctors first began to notice a link between blood types and different diseases in the middle of the 20th century, and the list has continued to grow. “There are still many associations being found between blood groups and infections, cancers and a range of diseases,” Pamela Greenwell of the University of Westminster tells me.

From Greenwell I learn to my displeasure that blood type A puts me at a higher risk of several types of cancer, such as some forms of pancreatic cancer and leukaemia. I’m also more prone to smallpox infections, heart disease and severe malaria. On the other hand, people with other blood types have to face increased risks of other disorders. People with type O, for example, are more likely to get ulcers and ruptured Achilles tendons.

These links between blood types and diseases have a mysterious arbitrariness about them, and scientists have only begun to work out the reasons behind some of them. For example, Kevin Kain of the University of Toronto and his colleagues have been investigating why people with type O are better protected against severe malaria than people with other blood types. His studies indicate that immune cells have an easier job of recognising infected blood cells if they’re type O rather than other blood types.

It may also be a clue as to why a variety of blood types have endured for millions of years. Our primate ancestors were locked in a never-ending cage match with countless pathogens, including viruses, bacteria and other enemies. Some of those pathogens may have adapted to exploit different kinds of blood type antigens. The pathogens that were best suited to the most common blood type would have fared best, because they had the most hosts to infect. But, gradually, they may have destroyed that advantage by killing off their hosts. Meanwhile, primates with rarer blood types would have thrived, thanks to their protection against some of their enemies.

6   Further reading

7   References

[1](1, 2) Karl Zimmer. July 15, 2014. Why do we have blood types?

Putting something in your mouth and then swallowing it is ALWAYS safer than poking it into your bloodstream. Just like humans, many predators have a whole line of "digesters" that can make things safer to eat. First, there is spit - chewing and mixing with spit is important for digestion because it helps make big chunks smaller, it helps smaller chunks slide down into the stomach, and the spit helps some things start to dissolve. If you are chewing a venomous animal, there is a good chance that the chewing will burst the tissues where the venom is stored - that makes it less likely that the fangs or stingers can actually inject the venom. Once the venomous parts get into the stomach, the acids within the stomach are often strong enough to "de-nature" many venom proteins. "De-nature" basically means it changes shape to that it doesn't react the same way that it would have. Then other enzymes begin to break everything down into even smaller chemical bits. Those small chemical bits are mostly turned into new proteins that you need to rebuild your own cells, or energy that helps you move.

In order for venom to be dangerous, it must enter the bloodstream. Many venomous toxins specifically target red blood cells themselves and break down those cells which results in organs being unable to receive oxygen. This leads to inflammation and necrosis of the cell tissues. Organ failure then leads to death. (Not all venom works this way. Sometimes venom attacks the nervous system instead.)

When consuming anything, animals have several defense mechanisms in the digestive system. The first defense is the mouth itself. If the lips start burning, the animal might not swallow. Then, the taste buds help the predator know whether this should be swallowed if it tastes bad. Next, the saliva in the mouth and chewing action helps break down molecules and foods into smaller pieces. Saliva is also full of white blood cells (leukocytes) that attack harmful items (such as bacteria) that might enter your mouth. After the mouth, the item enters the stomach. The acids, heat, and enzymes in the stomach are capable of denaturing proteins. For proper protein function, the proteins must consist of specific shapes. By denaturing potentially harmful proteins, they are rendered harmless in many cases. They are also broken apart. Gastric acid in the stomach is normally between 1.5 and 3.5 PH. (Stomach acid can be incredibly acidic!) So, if it reaches this point, the venom is most likely going to be broken down in the stomach.

However, if the venom happens to make it to the digestive tract intact, then it could potentially leech into your system. But, if it is possible for such a substance to reach this point, it is not actually a venom. It is a poison!

Bloodstream is a simplified term and circulatory system would probably be a better term to use. Venom initially enters a different system designed to transport white blood cells and remove toxins from the body (lymphatic system).

The heart does not need to beat; blood can be moved continuously through instead of in pumps. A device called an LVAD (Left Ventricular Assist Device) does this for .