1.8.2: Survival Needs

To sustain human life, certain physiological needs include air, water, food, shelter, sanitation, touch, sleep and personal space.

Master this section and you'll be able to:

• Explain the five essential requirements for human survival.
• Describe how the absence or deficiency of each affects physiological function.
• Analyze the physiological consequences of environmental changes, such as dehydration, hyper- or hypothermia, and high altitude or deep-sea conditions.

Survival Needs

While the development of tools, plumbing systems and food preservation have enabled modern man to live life with many creature comforts, the truth remains that the requirements for human survival are quite basic.  

1) Oxygen Availability

Oxygen, within a particular range of pressure, concentration, and purity, is essential for aerobic cellular respiration, the process by which cells generate ATP, the primary energy currency of the body. Insufficient oxygen delivery (hypoxia) rapidly impairs organ function, particularly in the brain. Without oxygen, brain damage begins in minutes and death quickly follows.
 

2) Food Availability

Our food must be adequate in calorie and nutritional standards in order to sustain life. A continuous supply of nutrients is necessary and can be categorized into:

• Macronutrients,  specifically carbohydrates, lipids, and proteins. These macronutrients provide the energy necessary for cellular processes and the structural components for growth and repair.
• ​​​​Micronutrients, such as vitamins and minerals are critical for enzymatic activity, nerve conduction, immune function, and metabolic regulation.
   

3) Water Availability

Access to a safe, clean, and adequate water supply is absolutely necessary for human survival. Indeed, water is as critical as oxygen. It serves as the solvent for biochemical reactions, assists in thermoregulation, and facilitates nutrient transport and waste elimination. Even mild dehydration can lead to headaches, dizziness, fatigue, and impaired cognitive function. Severe dehydration affects blood volume, reduces blood pressure, and can lead to organ failure and death.

⚠️ Fun-but-Scary Fact: You can survive weeks without food, but only a few days without water. Water is not optional! It is a top priority for survival.

The typical survival time for a human without water is about three days (72 hours). However, this can vary significantly depending on several factors. In some cases, individuals may survive anywhere from two to seven days without water. Environmental conditions play a major role — extreme heat and dry climates accelerate water loss through sweat and evaporation, while cooler, more humid environments may slow dehydration. Other influencing factors include a person’s level of physical activity, age, overall health, body size and fat reserves, and their hydration status before water deprivation begins. These variables determine how quickly dehydration sets in and how long the body can continue to function without adequate fluid intake.
 

4) Stable Body Temperature

Enzymatic reactions and physiological processes in our body are optimized around a core temperature of approximately 37°C (98.6°F).

• Hyperthermia can denature proteins and disrupt cellular integrity.  It begins when core body temperature rises above 38.3°C (101°F) and becomes dangerous above 40°C (104°F).
  • Around ~38–40°C (100.4–104°F) heat exhaustion often kicks in, leading to heavy sweating, weakness, and dizziness.
  • Above 40°C (104°F) heat strokes start build up and cause organ failure, altered mental status, and is a medical emergency.
     
• Hypothermia on the other hand impairs organ systems by slowing down metabolic activity. It occurs when core body temperature drops below 35°C (95°F).  
  • Mild hypothermia:  32–35°C (89.6–95°F) causes vasoconstriction, shivering, and confusion.
  • Moderate hypothermia: 28–32°C (82.4–89.6°F) slows the metabolism and decreased consciousness.
  • Severe hypothermia: <28°C (<82.4°F) will ultimately lead to cardiac arrest and death.

 

5) Appropriate Atmospheric Pressure

Atmospheric pressure is the force of the air around us — and, believe it or not, your ability to breathe depends on it. This pressure helps create the differences in air pressure that your body needs to move oxygen into your blood and get rid of carbon dioxide. Atmospheric pressure enables adequate ventilation and gas exchange in the lungs. A sufficient pressure gradient is required for oxygen to diffuse into the blood and for carbon dioxide to be expelled. Changes in barometric pressure, such as at high altitude, can hinder respiratory efficiency.

Here is the short version of how it works:

• Oxygen moves from areas of higher pressure (the air in your lungs) to areas of lower pressure (your blood).

• Carbon dioxide does the opposite. It moves from your blood (where it is higher) into your lungs to be exhaled.
  This only works if the pressure outside your body is high enough to keep this gas exchange going.  (Don't worry, if you do not totally understand this now, we will cover it in more detail when we study the respiratory system).

Even though the percentage of oxygen in the air (about 21%) stays the same no matter where you are, the air pressure changes with altitude. As you go higher — like climbing a mountain — the atmospheric pressure drops. This causes the partial pressure of oxygen (the amount of pressure oxygen alone exerts) to drop too. When that happens, it becomes harder for oxygen to move from your lungs into your blood because the pressure difference that drives gas exchange is too small. Less oxygen gets into your bloodstream, and that affects your whole body, right down to your mitochondria, which are the tiny structures in cells that use oxygen to make energy.

For example, at around 5500 meters (18,000 feet), approximately the hight of the Mt. Everest North Base Camp, the oxygen pressure is only about half of what it is at sea level. At the top of Mount Everest (about 8900 meters or 29,000 feet), it is only about 30% of sea level. This drop in oxygen availability can cause symptoms all the symptoms of hypoxia. And if your body cannot adapt or you do not use supplemental oxygen, it can become life-threatening.

On the flip side, in situations with too much pressure (like deep-sea diving), other problems can occur, such as too much oxygen or nitrogen entering the blood, which also is dangerous. 

Bottom line: atmospheric pressure is not just about the weather! Your life depends on it, because without the right pressure, your body cannot get the oxygen it needs to survive.

Figure \(\PageIndex{1}\): Mount Everest North Face as seen from the path to the base camp.

 

Beyond the five key requirements for maintaining homeostasis and supporting basic survival, humans also depend on other factors to thrive.

• Shelter is critical for protection from extreme heat, cold, sun, and precipitation, conditions that could otherwise lead to hypothermia or hyperthermia.
• Sanitation helps prevent disease by safely removing waste and reducing exposure to harmful toxins and pathogens.
• Sleep is vital for physical and cognitive restoration. Ideally seven to eight uninterrupted hours per night are needed.
• Adequate space reduces overcrowding by promoting mental well-being and maintenance of healthy social structures.
• Finally, touch is also a fundamental human need. As humans have evolved to interact in community settings, both hunting and gathering in groups, touch (as in a caring caress) is often considered a basic human survival need. In fact, studies have shown that physical contact fosters bonding, reduces stress, and even supports immune function, especially in infants and young children and is therefore essential to the early growth and development of healthy humans. 

Figure \(\PageIndex{2}\): Sleeping Child. Adequate sleep is necessary to sustain life.