Question

How does the concentration of dissolved oxygen and nitrogen in the bloodstream change as a scuba diver swims to lower depths of the ocean? Explain.

Short answer

As a scuba diver descends and the pressure increases, the concentrations of both oxygen and nitrogen dissolved in the bloodstream increase. Dissolved oxygen can lead to oxygen toxicity if it increases too much, so divers regulate their tanks to limit oxygen content. Excess nitrogen can cause decompression sickness if a diver ascends too quickly.

Step-by-step solution

Understanding Pressure And Its Impact On Gases

As the depth increases in ocean water, the pressure increases. With every 10 meters of water depth, the pressure increases by about 1 atmosphere. More pressure makes gases more dense, therefore, the same volume of air contains more molecules.

Impact on Oxygen Concentration

The scuba diver's tank contains a special gas mixture that is approximately 21% oxygen and 79% nitrogen. When a diver descends, the increasing pressure causes more oxygen to dissolve into the bloodstream. Too much oxygen, however, can lead to oxygen toxicity, causing seizures. To avoid this, scuba divers regulate their tanks to reduce the percentage of oxygen as they descend.

Impact on Nitrogen Concentration

Just like oxygen, the concentration of dissolved nitrogen also increases as the scuba diver descends. This is typically not a problem during the dive, but if the diver ascends too quickly, the drop in pressure can cause the excess nitrogen to come out of solution, forming bubbles in the bloodstream. This is known as decompression sickness or 'the bends'.

Key concepts

Partial Pressure and Gas Solubility

Understanding how gases behave under different pressures is crucial for scuba diving, as this directly affects gas solubility in the body. When a diver goes deeper into the ocean, the pressure from the water above increases. According to Henry's Law, the amount of a gas that can be dissolved in a liquid (such as blood) at a given temperature is directly proportional to the partial pressure of that gas. This means that with each additional 10 meters of depth, roughly equivalent to an increase of 1 atmosphere of pressure, the gases in the surrounding environment become denser, leading to a higher partial pressure. As a result, this increases the concentration of dissolved gases, like oxygen and nitrogen, in a diver's bloodstream. It's vital for divers to understand this relationship to manage their breathing gas mixtures correctly and prevent potential health risks such as oxygen toxicity and decompression sickness.

For instance, when a diver descends and the pressure increases, the partial pressure of oxygen in their tank also rises, resulting in more oxygen dissolving into their bloodstream. This is beneficial up to a point, as it ensures sufficient oxygen for metabolic needs. However, if the partial pressure of oxygen becomes too high, it may lead to oxygen toxicity. Therefore, divers often use gas mixtures with different oxygen concentrations or add helium to the mix to mitigate this risk as they dive deeper.

Oxygen Toxicity in Diving

Oxygen is essential for life, but in high concentrations under increased pressure, it can become toxic. Scuba divers must carefully monitor the partial pressures of gases they are breathing. Oxygen toxicity typically occurs when the partial pressure of oxygen exceeds safe limits, commonly above 1.4 atmospheres. The condition can cause symptoms ranging from nausea, muscle twitching, and dizziness to more severe forms like convulsions and loss of consciousness. These symptoms pose a significant threat underwater, where immediate assistance may not be available.

One way that divers manage the risk of oxygen toxicity is by reducing the fraction of oxygen in their breathing gas as they dive deeper. At greater depths, they might use gas mixtures like trimix (a mixture of oxygen, nitrogen, and helium) or heliox (a mixture of oxygen and helium) to keep the partial pressure of oxygen at a safer level. This approach also helps to prevent nitrogen narcosis, another condition affecting divers that is influenced by the effects of increased nitrogen solubility at depth.

Decompression Sickness

Decompression sickness (DCS), also known as 'the bends,' is a diving-related condition that occurs when a diver ascends too quickly to the surface. During the descent and while at depth, the increased pressure causes nitrogen from the breathing gas to dissolve into the diver's bloodstream. If the ascent is rapid, the reduced pressure doesn't allow enough time for the excess nitrogen to be expelled through the lungs. Instead, it forms bubbles in the blood and tissues.

These nitrogen bubbles can cause joint pain, dizziness, paralysis, and even death if not treated promptly. Symptoms can vary widely, and the severity depends on the amount of nitrogen accumulated and the rate of ascent. To avoid DCS, divers follow carefully planned decompression stops to allow the excess nitrogen to off-gas safely. The use of dive computers can assist divers in monitoring their ascent rate and decompression needs. Implementing 'safety stops,' even when not required, is a common practice where divers will remain at a shallow depth for a few minutes before surfacing to ensure additional off-gassing of nitrogen.