In 1957, the eminent American snake researcher Karl Patterson Schmidt was bitten by a young booslang(Dispholidus typus), also known as the South African tree snake. At the time, boomslang bites were not clinically described, and dedicated to his field, Karl refused medical treatment as it would interfere with the symptoms of the venom, because he wanted to document the effect of the venom. Karl himself was convinced that the young boomslang could not produce enough venom to kill an adult human. Twenty-four hours after the little bite on his thumb, he died from internal bleeding in his lungs, kidneys, heart and brain. But what exactly in the snake’s venom caused Karl to bleed to death? To better understand the effects of venom on the human body, we start by looking at normally functioning body.

Blood circulation

The function of the body’s bloodstream is to transportO2,CO2, proteins, salts, hormones and many other necessary molecules around the body. It consists of a system of tubes (the blood vessels) and a pump (the heart). Blood vessels are divided into arteries, which carry blood from the heart to the body, and veins, which carry blood back to the heart.

The blood vessels are largest closest to the heart and smallest further away from the heart. Arteries slowly narrow and become capillaries, which gather into larger blood vessels that become veins. From the capillaries the blood’s life-giving substances (such as O2) are transported into the tissues and organs they are intended for. Capillaries are the smallest blood vessels in the body, with a diameter of less than 0.01 mm (thinner than a human hair!). There are about 100,000 kilometers of capillaries in the body. For transport of molecules to work optimally, blood needs to be fluid (so it doesn’t clog) and the blood vessels need to be free of holes to prevent blood from escaping.


Most people have probably cut themselves on a knife or a piece of paper. The quick (and wise) reaction is to grab a paper towel and apply pressure to the wound. Increasing pressure on tissue is one way to stop bleeding, but this is not anything. The blood vessels have their own ability to close by forming a clot that closes the wound. Stopping bleeding is called hemostasis which can be devided into primary and secondary hemostasis. In the following, secondary hemostasis will be described. It is recommended to follow Figures 11, 12 and 13 while reading through it.

Primary haemostasis described briefly

Primary hemostasis is activated when a wall of a blood vessel breaks. Through a series of processes, the so called platelets are activated. These platelets are used to form a blood clot, by coming together forming a “plug” that seals off the wound. Secondary hemostasis is essential to hold this created clot together, otherwise it will break just as quickly as it was formed.

Secondary hemostasis (also called the coagulation cascade)

Coagulation is the conversion of liquid fluid into a solidified mass. In other words, we need the liquid blood to coalesce into a solidified mass (clot) that can reseal our blood system where it has broken. There are two pathways to activate clotting in the body, called the internal pathway and the external pathway. Both end in a shared pathway. Coagulation is made up of many different factors (proteins) that exist in an activated and non-activated form. “Factor 9” is a non-activated form, while “Factor 9a” is an activated form.

The internal path:

  1. The internal pathway is activated when factor 12 comes into contact with an activated platelet, converting it into its active form, factor 12a
  2. Factor 12a then cleaves factor 11 to factor 11a, which in turn cleaves factor 9 to factor 9a
  3. Factor 9a, together with factor 8a, forms the tenase complex that activates factor 10 to factor 10a

Figure 11. The internal pathway in secondary hemostasis. The figure shows the activation of several different blood factors, ending with factor 10 being activated to factor 10a.

The external route:

  1. The external pathway is activated when blood comes into contact with material from damaged cell membranes
  2. Factor 7a comes into contact with factor 3 due to damage to the vessel wall and forms the tenase complex that activates factor 10 to factor 10a
  3. Factor 7a, together with calcium ions and tissue factor, forms the tenase complex that activates factor 10 to factor 10a


Figure 12. The external pathway in secondary hemostasis. The figure shows the activation of blood factors, which ends with factor 10 being activated to factor 10a.

The shared pathway:

  1. Factor 10a from either the internal and external pathway forms the prothrombinase complex together with factor 5a. This complex cleaves prothrombin into the active molecule thrombin, a key molecule in blood clotting
  2. Among other things, thrombin activates the conversion of fibrinogen to fibrin
  3. The long fibrin strands bind the platelets from the primary hemostasis together

Figure 13. An overview of the blood drying cascade, which consists of the external path, internal path and shared path. As described in the two previous figures (Figures 11 and 12), both the external and internal paths end with the activation of factor 10 to factor 10a. The two pathways then meet in the common pathway where factor 10a, through a series of effects, causes fibrinogen to cleave into fibrin, which causes a stable blood clot.

Snake toxins that affect blood clotting

Several snake venoms contain toxins that affect the blood clotting process. These are called hemotoxins. Hemotoxins can be derived from a wide range of toxin families, but Snake Venom Metalloproteases (SVMPs) and Snake Venom Serine Proteases (SVSPs) in particular are known to be extremely effective in their effect on the clotting process.

Let’s go back to Karl Patterson Schmidt, who you heard about in the beginning.

The venom from the African boomslang that killed Karl was once thought to cause blood clotting, but why did Karl die from internal bleeding? The explanation lies in the toxins contained in the venom and the body’s reaction to effects of these toxins. The venom of the African boomslang contains range of different toxins, but the most important toxin family is the Snake Venom Metalloproteases (SVMPs). These toxins attack the small blood vessels, making holes between the cells in the vessel wall. This makes the blood vessels filled with leaks and thus unstable (Figure 14). In addition, the venom also has a clotting effect on the blood. One of the SVMPs (dispholysin A) causes the blood to clot by activating the clotting factors prothrombin and factor 10 (whose effects are described earlier under secondary hemostasis), which starts the blood clotting process. If the venom is added to blood in a test tube, you can clearly see the blood forming a blood clot. However, the blood contains large ammounts of the enzyme plasmin, which breaks down these clots. This means that the activation of the coagulation factors prothrombin and factor 10 by SVMPs does not lead to stable blood clots in the body. Surprisingly the consequence is that all the coagulation factors are depleted – after which the blood can no longer clot. In short, the body’s entire defense against uncontrolled bleeding is consumed, which is called ‘Venom-Induced Consumption Coagulopathy’ (VIC). Together with the increased leakage of blood vessels, this means that it takes very little damage for a patient to suffer internal bleeding. In addition to internal bleeding, African boomslang bites also cause kidney failure. This is because broken down proteins in the blood are transported to the kidney, where they accumulate and destroy the kidney. Thus, a boomslang bite causes very severe symptoms that can lead to multi-organ failure due to bleeding.

Question: Karl, a fully-grown human, died from hemorrhaging from a snake venom, which is actually a toxin that triggers the blood clotting process. If the snake instead bites its intended prey, such as a small bird, what do you think would happen to the bird's body?

A: In a smaller prey animal, the coagulating effects of the venom will cause the animal to develop large blood clots and become unconscious or die, after which the snake can eat its meal.

Figure 14. Some SVMPs cause damage to the small blood vessels by cutting a hole between the cells in the blood vessel wall, allowing blood to drain out of the artery.

Another group of snake venoms are the Snake Venom Serine Proteases (SVSP). Some of these toxins act in the blood clotting cascade as enzymes that mimic the body’s natural enzyme thrombin (Figure 15). As previously mentioned, thrombin is an important part of the blood clotting cascade that leads to the formation of fibrin, a key component of blood clots. Under normal circumstances, fibrin formation would give rise to blood clots (e.g. to close an open wound). However, unlike thrombin, SVSPs do not sufficiently activate the blood clotting process. This means that the fibrin formed by SVSPs is unable to form stable blood clots. The unstable blood clots are quickly dissolved by the enzyme plasmin. This will mean that all of the body’s fibrin is used up, leading to the blood losing its ability to clot (VICC, as with SVMPs). This is one of many examples of how different toxins can cause the same symptoms with different biochemical mechanisms. Thus, it is important to know more about the toxins and not just the symptoms when developing or using antivenom.

Figure 15: Illustration of Snake Venom Serine Proteases (SVSP) function: SVSPs are a group of snake venoms that act as enzymes in the blood clotting cascade, mimicking the body’s natural enzyme thrombin. However, SVSPs do not activate the blood clotting process sufficiently, leading to the formation of unstable blood clots. These clots are rapidly dissolved by the enzyme plasmin, leading to depletion of the blood’s fibrin, rendering the blood unable to form clots.

Working questions

  1. Based on the information you have read so far, do you think an SVSP with a function like in figure 16 can dissolve blood clots in humans?
  2. If the body need to form these blood clots, why do you think the body also has plasmin, whose function is to dissolve blood clots?



1. No, the SVSP in the example is not part of clot dissolution, but simply causes an unstable clot to form, which can then be dissolved by the body’s clot-dissolving functions

2. The body has both activating and deactivating mechanisms for most processes. So, blood clotting is a function in the body that is “activated”, but in order for all our blood not to clot, this function must also be “deactivated” again. In other words, plasmin acts as a “brake” on the clotting cascade, ensuring that clotting doesn’t just continue until we die.