Climbing Mount Kilimanjaro exposes the body to increasing levels of altitude, which significantly impacts your physical performance and health. At the summit of Kilimanjaro (19,341 feet/5,895 meters), there is about 50% less oxygen than there is at sea level. This state of oxygen deprivation, known as hypoxia, affects the body in many ways.

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 drop, the body must work harder to compensate. The breathing rate increases to pull in more air and boost the oxygen intake. At the same time, the heart pumps faster to circulate more oxygen-rich blood throughout the body.

This heightened effort doesn’t come without a cost—fatigue, shortness of breath, and early exhaustion become common companions. Simple tasks like walking, eating, and even thinking demand more energy because your muscles and organs aren’t receiving their usual oxygen supply. Activities that require strength and endurance feel more strenuous. To ensure your brain gets enough oxygen, cerebral blood vessels expand, which can increase intracranial pressure and lead to headaches.

Yet, 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.

Blood Oxygen Saturation and 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.

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. This is a mild symptom of the body’s adjustment to altitude.

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.

However, in more severe cases, fluid can accumulate in critical organs such as the lungs and brain. This leads to serious conditions known as High-Altitude Pulmonary Edema (HAPE) and High-Altitude Cerebral Edema (HACE), both of which can be life-threatening.

HAPE occurs when the blood vessels in your lungs constrict—a response called pulmonary vasoconstriction. This constriction 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.

By ascending gradually and allowing time for acclimatization, you can significantly reduce the risk of these severe conditions.

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 throughout the night, vivid or unsettling dreams, and a general sense of restlessness.

Fortunately for most people, sleep improves after spending a few nights at higher altitudes.

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.

The body needs time to adapt to reduced oxygen at higher altitudes. Slow ascent allows your body time to adjust, reducing the risk of severe altitude sickness. If your body struggles to acclimatize, you may begin to experience the symptoms of Acute Mountain Sickness (AMS). These are warning signs that your body is not adjusting well to altitude. Note that mild AMS is common on Kilimanjaro, even for fit climbers, typically kicking in at elevations above 8,000 feet (2,500 meters).

Common symptoms include:

  • Headaches
  • Nausea
  • Dizziness
  • Loss of appetite
  • Fatigue

The symptoms often appear within 6 to 24 hours of reaching a high altitude and typically worsen as you ascend further without acclimatization. However, with proper acclimatization techniques—such as gradual ascent, staying hydrated, maintaining a balanced diet, and getting sufficient rest—many of the adverse symptoms lessen or disappear entirely. As your body adjusts to the altitude, you’ll likely experience improved energy levels, better sleep, and a greater sense of well-being.

By understanding your body’s responses and taking proactive steps to support its adaptation, you can enhance your safety and enjoyment throughout the climb.