Tolerance Limits

An organism survives best within a particular range of conditions. Four internal factors must be held within narrow limits: body temperature, water availability, blood glucose and blood carbon dioxide. If any of these factors moves outside the range the organism can tolerate, the organism is placed under stress. If the factor moves further beyond that range, survival is compromised.

TOLERANCE LIMITS AND TOLERANCE RANGES

• Tolerance limit - the minimum and maximum amount of a given factor that an individual organism can tolerate before survival is compromised.
• Tolerance range - the difference between the minimum and maximum amount of the factor tolerated across a population of a species.

The factors concerned are abiotic (non-living) factors, such as temperature, pH and carbon dioxide concentration. Different species living in the same physical environment have different tolerance ranges, and species are therefore distributed across ecosystems according to the conditions they can tolerate. A species with a wider tolerance range can survive in a greater variety of physical environments; a species with a narrow range is confined to its niche. In order to survive in a habitat, an organism must tolerate all of the environmental factors present there.

Activity peaks in the optimum range and falls away toward the tolerance limits (dashed lines) - the points beyond which survival is compromised.

THE FOUR FACTORS AND THEIR NORMAL RANGES

BODY TEMPERATURE

Mammals maintain their body temperature between 36 °C and 38 °C. This is because enzymes, the proteins which catalyse the reactions of cell metabolism, function most efficiently at 37 °C. Hyperthermia (an internal temperature above 40 °C) is life-threatening. Hypothermia (a core temperature below 35 °C) is equally life-threatening.

The narrow band of 36–38 °C keeps enzymes near their optimum of 37 °C; below 35 °C and above 40 °C the extremes are life-threatening.

WATER AVAILABILITY

Cells require both their internal and external environments to contain sufficient water and the correct solute concentration. If either is wrong, osmosis (the net movement of water across a membrane) causes cells to swell or shrivel. A solute concentration of 0.9% NaCl is isotonic to blood plasma - that is, it has the same solute concentration - and cells function most efficiently in isotonic environments. The body controls water availability through osmoregulation, which is covered with ADH in the endocrine system note.

BLOOD GLUCOSE AND BLOOD CARBON DIOXIDE

• Glucose - normally maintained at 75–95 mg/100 mL of blood. Deviation for extended periods causes health problems: diabetes is associated with high blood glucose, resulting from a lack of the hormone insulin or a poor tissue response to it.
• Carbon dioxide - non-toxic at normal levels (5–6%) and is the stimulus that drives both the rate and depth of breathing. Excess CO₂ lowers blood pH, a condition called acidosis, and levels above 10% are highly toxic, leading to unconsciousness and death.

COPING WITH CHANGE: UNICELLULAR AND MULTICELLULAR ORGANISMS

• Unicellular organisms - have only a limited ability to maintain their internal composition. Some bacteria form spores when water is unavailable and survive in this state until water returns.
• Multicellular organisms - maintain their internal composition far more effectively because their cells are bathed in tissue fluid, and passive diffusion and active transport control the solute and water balance of each cell.

EXCHANGE SURFACES

In order for cells to function efficiently and maintain homeostasis, nutrients must be taken up and waste products excreted. The sites where these exchanges occur are called exchange surfaces, and they are located throughout the body. Examples include capillaries in vascular beds, nephrons in the kidneys, alveoli in the lungs, and villi in the small intestine.

Three criteria must be met in order for an exchange surface to be efficient:

1. The exchange surface must be thin - to minimise the distance substances must travel.
2. The surface must be moist - to allow for diffusion of soluble substances.
3. The exchange surface needs a large surface area - to maximise the amount of substances that can be exchanged.

CAPILLARIES AND VASCULAR BEDS

The principal way of transporting nutrients and waste products between organ systems is through the blood. Blood vessels branch progressively and become smaller as they approach organs; the smallest vessels are the capillaries, which form networks called vascular beds. Capillary walls are only one cell thick (thin), they are surrounded by interstitial fluid (moist), and they are very numerous (large surface area) - so they satisfy all three criteria. Small gaps between the cells lining the capillary walls (fenestrae) allow fluid to be readily exchanged. The arteriole end of a capillary bed delivers oxygen and nutrients such as glucose to the cells; the venule end carries away carbon dioxide and other waste products such as urea.

A vascular bed: nutrients and oxygen leave the blood at the arteriole end; carbon dioxide and wastes enter at the venule end. Capillaries are the transport link between the body's organ systems and its cells.

HOMEOSTASIS: THREE MECHANISMS

Homeostasis is the maintenance of a steady internal state in response to changes in both the external and internal environments. Organisms achieve homeostasis through three main mechanisms (often described as adaptations):

1. Structural - particular physical features that assist in tolerating change (e.g. insulating fur).
2. Physiological - internal processes that detect and respond to changing conditions (e.g. sweating).
3. Behavioural - behaviours or actions that help the organism survive (e.g. moving into shade).

To hold conditions steady, all the organs of a system - and all the systems of the body - must communicate. This communication is carried out by nerves and hormones, which are the subject of the remaining notes in this topic.

Beyond the four factors above, other abiotic factors that affect an organism's survival include mineral composition, oxygen levels, light availability, wind, and the pH of the environment.