When you climb Kilimanjaro, you expose your body to increasing levels of altitude.

At the summit of Kilimanjaro (19,341 feet/5,895 meters), there is about 50% less oxygen than there is at sea level. This low oxygen environment significantly impacts your health and physical performance. In this article, we’ll discuss what happens to the human body on Mount Kilimanjaro.

Increased Breathing and Heart Rate

Our bodies are primarily adapted for living at low altitudes. The ideal oxygen level for human life is found at sea level, where the atmosphere contains around 21% oxygen. At this level, the partial pressure of oxygen is sufficient for our lungs to effectively transfer oxygen into the bloodstream. This enables all bodily systems to function optimally.

Higher altitudes can challenge the body’s ability to function effectively. As oxygen levels in the environment drop, so does the oxygen level in your body. This is known as hypoxia.

In order to compensate, the body must work harder to increase oxygen intake and delivery. So your breathing rate increases to pull in more air. At the same time, your heart pumps faster to circulate more blood throughout the body.

But even with these adjustments, your muscles and organs still aren’t receiving their usual oxygen supply. You can expect to feel shortness of breath, fatigue, and perhaps early exhaustion. Activities that require strength and endurance feel more strenuous. Simple tasks like walking, eating, and even thinking can become difficult. 

As you spend more time at these elevated heights, your body begins to adjust to the new conditions. Gradually, these symptoms often diminish as your physiological systems adapt to the lower oxygen environment.

Red Blood Cell Production

One of the key adaptations to high altitude involves the production of red blood cells. The kidneys sense the decreased oxygen levels and release a hormone called erythropoietin (EPO). This hormone stimulates the body to produce more red blood cells. This is a crucial facet of acclimatization. By increasing the number of red blood cells, the body enhances its capacity to transport oxygen to vital organs and muscles.

However, this adaptation comes with its own set of challenges. The influx of red blood cells thickens the blood, increasing its viscosity. This means the heart must work harder to pump blood through the circulatory system. The body is essentially working overtime, which can lead to sluggishness or fatigue. Any strenuous activity may leave you feeling unusually winded, and even at rest, your heart may pound more vigorously than normal.

But, as your body continues to adjust, these sensations often lessen. The heart becomes more efficient at handling the thicker blood, and the enhanced oxygen delivery can lead to improved stamina and endurance. Staying well-hydrated can also help mitigate these effects, as fluid intake aids in maintaining optimal blood viscosity and eases the strain on your heart.

Inflammation and Swelling

At high altitudes, fluid movement within the body changes. Hypoxia triggers inflammation and disrupts the lining of blood vessels. These vessels become more permeable, allowing fluid to leak into the surrounding tissues. At the same time, your lymphatic system—which normally helps drain excess fluid from tissues—becomes less efficient at higher elevations. As a result, the body struggles to clear the fluid buildup. You may notice swelling in your hands, face, and feet while climbing Kilimanjaro.

Some climbers experience changes in vision at high altitudes. Reduced oxygen levels can cause swelling in the cornea, leading to blurred vision or difficulty focusing. In rare instances, high-altitude retinal hemorrhages can occur, where tiny blood vessels in the eyes leak, potentially affecting vision.

The magnitude of the inflammation and swelling in your body typically decreases as you adjust to the lower oxygen levels. 

Sleep Difficulty

As you ascend up Mount Kilimanjaro, you may notice that sleeping becomes increasingly difficult. Sleep disturbances arise from several physiological changes that occur as the body adapts to lower oxygen levels.

One of the primary reasons for disrupted sleep is a phenomenon known as periodic breathing, specifically Cheyne-Stokes respiration. At high altitudes, the decreased oxygen stimulates the respiratory centers in the brain to increase both the rate and depth of breathing. During sleep, this heightened respiratory drive can cause a cyclical pattern of rapid breathing followed by brief periods where breathing nearly stops, called apneas.

Here’s what happens: the body, in an effort to compensate for low oxygen levels, begins to hyperventilate, which expels more carbon dioxide than usual. A drop in carbon dioxide levels reduces the stimulus to breathe, leading to a temporary cessation of breathing. As carbon dioxide builds up again, the brain responds by restarting rapid breathing, and the cycle repeats.

These fluctuations disrupt the normal sleep cycle. Instead of progressing smoothly through the stages of sleep, including deep sleep and REM sleep, you may find yourself frequently shifting between lighter stages of sleep or waking up altogether. This fragmented sleep can leave you feeling unrefreshed and more fatigued during the day. Medications like acetazolamide (Diamox), which are used to aid acclimatization, can help reduce episodes of sleep apnea by stimulating breathing.

Hypoxia also affects the production of neurotransmitters and hormones that regulate sleep, such as melatonin and serotonin. The imbalance of these chemicals can alter your circadian rhythms. These biological factors manifest as difficulty falling asleep, frequent awakenings, vivid or unsettling dreams, and a general sense of restlessness.

Fortunately for most people, sleep improves after spending a few nights at higher altitudes. So if you have trouble sleeping in the beginning, know that it probably will get better as the days go by.

Gastrointestinal Disturbances

The digestive system doesn’t escape the effects of altitude. At high elevations, the body prioritizes the distribution of oxygen to essential organs like the brain and heart. Blood flow and oxygen delivery to the stomach and intestines is reduced, impairing gastrointestinal function. Digestive processes slow down, and the production of enzymes necessary for breaking down food diminishes. This can result in delayed gastric emptying, where food remains in the stomach longer than usual, causing sensations of fullness or bloating. You might experience a loss of appetite, finding it difficult to consume the food you know you need for energy. Nausea or a general sense of queasiness is also quite common.

Moreover, the reduced oxygen levels affect the smooth muscle activity in the walls of the GI tract. The coordinated contractions that move food along—known as peristalsis—become irregular or weakened. This disruption can lead to symptoms like nausea, constipation, or diarrhea if the intestinal movements become erratic.

As your body acclimatizes, your appetite and digestive comfort are likely to improve.

Altitude Sickness (Acute Mountain Sickness)

Altitude affects every climber differently, but everyone feels its effects on Kilimanjaro.

As you can see, altitude has many physiological effects on the body. Many of the things mentioned above are actually mild symptoms of Acute Mountain Sickness (AMS). It is very likely that you will experience one or more of these symptoms while climbing Kilimanjaro, typically at elevations above 8,000 feet (2,500 meters):

• Headaches
• Shortness of breath
• Nausea
• Dizziness
• Loss of appetite
• Fatigue
• Disturbed sleep
• General feeling of malaise

These are warning signs that your body is not acclimatized to the current altitude. The symptoms often appear within 6 to 24 hours of reaching a high altitude. With time, many of the adverse symptoms lessen or disappear entirely. 

However, continuing to ascend while experiencing symptoms of AMS can be deadly. The worst case scenario involves fluid accumulation in critical organs such as the lungs and the brain, leading to conditions known as High-Altitude Pulmonary Edema (HAPE) and High-Altitude Cerebral Edema (HACE). Both of these can be life-threatening.

HAPE occurs when the blood vessels in your lungs constrict—a response called pulmonary vasoconstriction. This increases the pressure within the pulmonary arteries. The elevated pressure forces fluid from the capillaries into the air sacs in your lungs where oxygen exchange occurs. The presence of excess fluid in the lungs hampers breathing, causing shortness of breath and reducing the amount of oxygen that reaches your bloodstream.

Similarly, HACE is caused by increased pressure in the capillaries of the brain, leading to fluid leakage within the skull. This fluid buildup causes the brain to swell, increasing intracranial pressure. The swelling can quickly become dangerous, resulting in symptoms such as confusion, loss of coordination and unconsciousness.

HAPE and HACE are extremely serious and require immediate evacuation. By conducting health checks and through visual observation, our guides are constantly on the lookout for indications that one might be at risk for developing these severe forms of altitude sickness. If our team suspects that you have symptoms that could turn into HAPE or HACE, you will be turned around on the mountain and brought back to town at a minimum or taken to the hospital for treatment. 

While mild symptoms like headaches, fatigue, or nausea can feel daunting, they are usually manageable. By taking proactive measures, you can greatly reduce the risks. The best way to prevent altitude sickness is by ascending gradually and allowing your body time to acclimatize. That’s why we always recommend longer routes—they’re safer, less strenuous, and lead to a more enjoyable experience.