Physiology of Giraffes

The brown spots and yellowish coat, the long legs, the small “horns” on the head, the tall, intimidating stature, and especially the long neck, are what make giraffes, of genus Giraffa, stand out as a fascinating creature, striking the curiosity of many scientists. According to the World Animal Foundation, giraffes are one of the world’s tallest mammals and a vulnerable species, unique in many ways. Male giraffes outnumber the females in size, as they weigh approximately 2,400 to 3,000 pounds, while the females can weigh around a thousand or more pounds less, with a maximum weight around 2,600 pounds. The male giraffes are much bigger than the female giraffes for they can grow to be approximately 19 feet tall, and the females to about 16 feet tall, with most of their height coming from just their necks. Giraffes can be found in areas throughout Central, Eastern, and Southern Africa. They travel mainly through the savannas of Africa, staying near tall trees to feed on. Giraffes feed off leaves on tall trees, preferably acacia trees, with the help of their long necks and extremely long tongue, which is about 18 inches in length. The tongue is quite tough for it must be immune to the effects of the vicious thorns of the acacia trees they feed on. If necessary, giraffes can go several days without water, simply staying hydrated by the moisture from leaves. Giraffes are non-territorial, social animals, traveling in large herds, which are not organized in any way consisting of a combination of sexes or ages.

Along with the giraffe’s interesting physical appearance and behaviors, what is quite fascinating is their long necks and how it affects their physiological systems, especially their cardiovascular system. The question is how do giraffes manage to get blood all the way up to their brain? Their long necks not only affect their cardiovascular systems, but also their respiratory, nervous, and musculoskeletal system. Does having such a long neck affect their breathing or does it affect their musculoskeletal system in attempting to balance such a neck? This and many more questions will be answered to allow us to gain further insight to the unique mammal, the giraffe.

The giraffe not only has a distinct and unique physical appearance, but it also has a unique and curious circulatory system. The giraffe’s circulatory system is modified in many ways to pump blood vertically approximately 2?m from its heart to the brain. Giraffes have evolved a strong heart and two times greater blood pressure than in other mammals. The main goal of every circulatory system is to generate and regulate blood pressure to provide flow to the tissues. Blood flows throughout the giraffe’s body, nourishing tissues by supplying oxygen and other nutrients, and removing carbon dioxide and other waste products.

The most striking aspect of the giraffe’s cardiovascular system, is its extremely high blood pressure compared to the blood pressure in humans. The giraffe’s head is “2,500-3,000 millimeters above its heart, therefore, the giraffe’s heart, which weighs up to 12 kg, must pump powerfully to overcome the large hydrostatic pressure generated by column of blood pooling in its right and left carotid arteries in the neck” (Zhang, 2006). Giraffes need to overcome this hydrostatic pressure to maintain blood flow and perfusion to the brain, supplying it with sufficient nutrients and oxygen, to simply survive. Due to this, giraffes have evolved a remarkably high blood pressure in relation to other mammals (Zhang, 2006). Examination of the giraffe heart in Zhang’s study, found that the heart of the giraffe has a heavier weight, an enlarged left ventricular chamber, a thicker left ventricular wall, as well as thicker arterial walls. These characteristics of the heart, allow the heart and vessels the ability to withstand such high pressure and increase cardiac output. In another study by Smerup, et al. (2015), similar results were found showing that arterial pressures in the giraffe may exceed 300 mmHg, attributed to an exceptionally large heart, but the mass of giraffe heart is like that of other mammals, when it is expressed in relation to body mass. The giraffe hearts were seen to have a “small intraventricular cavity and a relatively thick ventricular wall” (Smerup, et al. 2015). Despite such a high blood pressure in giraffes, in the study performed by Zhang, he saw that in giraffes, “high blood pressure was not found to result in severe vascular lesions, nor did it lead to heart and kidney failure, while in humans, the same blood pressure is extremely dangerous” (Zhang, 2006). This can infer that there must be some mechanisms in place in giraffes, to prevent such damage.

The main reason for the giraffe’s high blood pressure, is simply its adaptation to flow against gravity’s forces. According to Hargens, Petterson, & Millard (2006), giraffes have evolved mechanisms to provide good blood flow and nutrition to their brains, while also restricting blood flow and tissue swelling in their legs. As discussed earlier, arterial pressure near the giraffe’s heart is about twice as large as in humans, to provide more normal blood pressure and perfusion to the brain, but high blood pressures in arteries of giraffe feet can cause severe dependent edema. Due to such pressure, the giraffe circulatory system exhibits a significant increase in vascular wall muscle thickness, capillary basement membrane thickness, and other vascular control systems in blood vessels that participate in capillary fluid dynamics and edema prevention. Tight skin and a fascial “antigravity suit,” moves venous blood and tissue fluid upward against gravity, which then prevents the pooling of blood and edema in dependent tissues. The vessels exposed to high blood pressure have also, developed smooth-muscle hypertrophy and narrow lumens to accommodate the extraordinarily high blood pressures (Hargens, Petterson, & Millard, 2006). It was also found that an extremely high blood pressure is not only confined to the systemic circulation of the giraffe but also to the pulmonary circulation. In a 775 kg giraffe, pulmonary blood pressure was found to be 38 mm Hg compared to 15 mm Hg in other mammals (Mitchell and Skinner, 2009). Lastly, there are two other ways that blood can travel up such a large neck and create such a high blood pressure. There are a series of non-return values in the blood vessels travelling up the neck that stop the blood from flowing back towards the heart, and the skin and muscle around the legs fit tightly, increasing the blood pressure in the lower body, stopping blood from draining down and pooling (Giraffe-Circulatory System, 2016).

Even with all these mechanisms to help blood get to the head, giraffes still must deal with the effects of sudden head lowering and raising to drink water. Blood flow and blood pressure to the giraffe's brain are regulated when drinking, but there is still a lot of confusion on how this regulation is done. In a study by Brøndum, et al. (2009), simultaneous blood flow and pressure in the carotid artery and jugular vein of anesthetized and spontaneously breathing giraffes were measured. In the upright position, it was found that, “mean arterial pressure (MAP) of the giraffe was 193 ± 11 mmHg and when the head was lowered, mean arterial pressure decreased to 131 ± 13 mmHg” (Brøndum 2009). When a giraffe lowers its head to drink, it reduces the force of gravity and ends up reversing it. This means that blood is forced towards the brain at very high pressure, possibly leading to major brain damage, but there is an organ, known as the ‘Rete mirabile’ specifically designed to reduce the pressure before the blood enters the brain. The rete mirabile ‘pools’ the blood at the base of the skull and regulates the amount of blood released into the brain. This works in reverse as the giraffe quickly lifts its head, stopping blood from draining out the brain too quickly, preventing the giraffe from fainting (Giraffe-Circulatory System, 2016). Sympathetic nerves were also found in jugular veins. During upright posture, the jugular veins do not need active regulation, but during head-down drinking, when the pressure gradient is reversed, and jugular pressure increases, the nerves are useful. Along with valves present in the jugular veins, the sympathetic nerves contribute to prevent blood flow in the retrograde direction and pooling of blood in the head during those short periods when giraffes lower their head below heart level to drink (Hargens, Petterson, & Millard, 2006).

The giraffe can act as an important animal model. According to the August Krogh Principle, 'For many problems there is an animal on which it can be most conveniently studied' (Krebs, 1975). As seen in the study by Hargens, Millard, Petterson, & Johansen (1987), despite such high blood pressure in giraffes, there is no damage to the heart. This aspect of giraffe physiology and “their protective mechanisms in the cardiovascular system may provide a framework for further investigations into mechanisms that can assist in prevention and treatment of human hypertension and cardiovascular disease” (Hargens, et al. 1987). Also, the giraffe’s tall build can provide scientists with an important model for studying “adaptive mechanisms to orthostatic (gravitational) pressure changes” (Hargens, Petterson, & Millard, 2006). Humans are relatively tall compared to other species of animals, so it can be inferred that they too have developed some sort of mechanism to maintain cerebral perfusion against gravity and prevent lower extremity edema while in standing in an upright position. Understanding of gravitational mechanisms in giraffes may assist us in understanding those mechanisms in humans.

The long necks of giraffe’s not only strike the curiosity of scientists about how blood gets all the way up to the head against gravity, but also how respiration is affected. There were some theories that were generated regarding giraffe’s respiration. It was believed that giraffes have an abnormally large respiratory dead space, which has been unsupported by a study done by Langman, Bamford, and Maloiy in 1982. The theory was that the large dead space in giraffes are overcome by slow deep respirations and a large lung capacity. Another theory was that the relatively small diameter of the giraffe trachea helps to minimize water loss and the giraffe’s respiratory dead space is within normal values. The study performed by Langman, Bamford, and Maloiy (1982) looked to answer the questions, do giraffes really have a large dead space and if so does this affect pulmonary ventilation? In the study, three giraffes with a mean weight of just under 600kg were used. Respiratory data was collected by using a light close-fitting mask made from a 5-L polythene bottle fitted over the head of the giraffe extending from just forward of the eyes to beyond the muzzle with a Douglas bag attached. Three-way valves in the mask allowed exhaled air to be collected or vented to the atmosphere and sealed the desiccant from the atmosphere until required. Twenty-five breaths were collected for analysis, looking at air volume and water evaporation.

Overall, through the course of the study, anatomical evidence showed that the respiratory dead space was not large, so there was no need for a special behavior to compensate for it. It was noticed though that the trachea is abnormally long and narrow. With such a structure of the trachea, it would be expected to force an increase to the work of breathing, but the effect was not large enough to increase oxygen consumption as predicted. It was possible that the resistance of the trachea would become obvious during exercise, but this theory was not supported entirely (Langman, 1982). It was also found that a strong labile body temperature of approximately 37.2 degrees Celsius has important ecological consequences for food and water requirements in giraffes. The ability to survive a high body temperature depends on maintaining brain-body temperature difference, usually with the aid of the carotid rete-cavernous sinus system, which uses cool venous blood from the nasal mucosa. A carotid rete has been reported to be present in giraffes. According to Langman et al. (1982) it has recently been shown that cooling of exhaled air by heat exchange in the nasal mucosa conserves about half the water that would be lost if air were exhaled at body temperature in giraffes. Therefore, giraffes use water very economically, which was also seen in Langman’s results showing low values for evaporative water loss through ventilation. This aspect of giraffe physiology is quite important, especially for giraffes that are most vulnerable when they are drinking water and living in arid savannahs. These results overall, showed that such an abnormally long and narrow trachea has no obvious effect on giraffe respiration, with respiratory dead space normal in comparison to other mammals.

When looking at the extreme size of giraffes, there is a question of whether it affects the control of the nervous system in giraffes and how such a long neck is supported by the giraffe’s musculoskeletal system. The ability of an animal to “sense and respond to changes in the environment is just one aspect crucial to the survival of animals” (More et al., 2013). In a study by More, et. al. (2013), the sensorimotor control of giraffes was investigated. There are two elements of sensorimotor control; “the time taken to respond to a stimulus and the precision of stimulus detection and production of a response” (More et al., 2013). These elements of sensorimotor control are restricted due to a struggle for space within peripheral nerves and muscles, that become more severe as animal size increases. While giraffes have some advantages to being tall, such as being able to reach high food sources, there are some disadvantages as well. Both giraffes and other large animals “must deal with greater sensorimotor delays and lower innervation density in comparison to smaller animals” (More et. al., 2013). According to More’s study, giraffes are less able to precisely and accurately sense and respond to stimuli, especially when moving quickly. Also, as a result of their very large leg length, “giraffes may experience even longer delays compared to other animals of the same mass when sensing distal stimuli” (More et al., 2013). These results suggested that giraffes may require additional mechanisms for more effective sensorimotor control to compensate for the delay. Also, overall the corticospinal tract in giraffes was seen to exhibit no unique anatomical features related to its very large length compared to other ungulates (Badlangana, Bhagwandin, Fuxe, & Manger, 2007).

It is incredible to see how such a difference in size of giraffes compared to other mammals, does not result in a drastic effect on the nervous system of giraffes, but when looking at a giraffe’s musculoskeletal system, the need to support such a large vertically elongated body mass in giraffes affects the spine, as seen in a study by Endo, et. al. (1997). Endo found that as the neck grows longer in giraffes, the cervical vertebrae and its spinous processes become more enlarged than in other mammals. The modified and enlarged spinous processes in giraffes provide a large attachment surface for a very strong nuchal ligament. This nuchal ligament, which runs down the dorsal surface of the cervical vertebrae and attaches to the anterior thoracic vertebrae, sustains the weight of the long neck and head, unlike that in other animals.

The giraffe’s physiology and anatomy are quite unique, but it is also quite similar to that of other animals. From what was learned in class and dissections, the giraffe’s unique anatomy can be compared to animals such as the shark, rat, pigeon, and perch. Giraffes and sharks are very different from each other. The shark, unlike giraffes, has gills involved in its respiratory system, through which water passes through and oxygen is picked up, unlike in giraffes, where air is inhaled directly through nasal cavities. Also, the giraffe has a bony skeleton like most higher vertebrate, but the shark in contrast has a skeleton composed of cartilage. The circulatory system is also quite different. Sharks have a sinus venous which is a non-muscular sac that collects venous blood. A shark heart only contains one atrium and one ventricle. The giraffe can also be compared to a rat, as they have quite a similar anatomy. Both rats and giraffes have a closed circulatory system with a four-chambered heart, but the rat is simply smaller overall, with a smaller blood pressure and heart. A perch overall, is very different from giraffes, but quite like sharks. A perch lives in the water, with gills serving to help in its respiratory system and water flowing through multiple pumps and a gas bladder to assist in buoyancy. A giraffe simply has a closed circulatory system with a four-chamber heart, while a perch, has a branchial heart contained in a tough pericardial sac. The branchial heart supplements the action of the main, systemic heart, pumping blood to the gills. Lastly, a pigeon, unlike giraffes, can fly. A pigeon’s skeletal system is modified to facilitate flight therefore, it has pneumatic cavities filled with water, to do so. In similarity with giraffes, pigeons have a closed circulatory system, with a four-chamber heart, but a pigeon’s heart is much smaller, as is blood pressure. It is amazing to see how similar animals are to each other, despite looking so different.

The dynamics of the giraffe’s physiological systems are quite interesting and challenging. Every physiological system in the giraffe is unique, yet so alike other animals around it. The giraffe’s imposing stature and long neck serve as the basis for its wildly diverse physiological systems. With the giraffe’s cardiovascular system functioning so that blood can reach the brain by increased blood pressure up its very long neck, the respiratory system functioning normally despite its abnormally long trachea, a strong musculoskeletal system, and a delayed sensorimotor control in the nervous system due to the large size of the giraffe, the giraffe shows how over time it has evolved to face such problems, to continue thriving.  

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