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Problems in Oxygen Transport

When a person travels quickly from sea level to elevations above 8000ft, where the atmospheric pressure and partial pressure of oxygen are lowered, the individual responds with symptoms of acute mountain sickness (AMS) – headache, shortness of breath, nausea and dizziness due to poor binding of O2 with haemoglobin.

When the person moves on a long-term basis to mountains from sea level is the body begins to make respiratory and haematopoietic adjustments. To overcome this situation kidneys accelerate production of the hormone erythropoietin, which stimulates the bone marrow to produce more RBCs.

When a person descends deep into the sea, the pressure in the surrounding water increases which causes the lungs to decrease in volume. This decrease in volume increases the partial pressure of the gases within the lungs. This effect can be beneficial, because it tends to drive additional oxygen into the circulation, but this benefit also has a risk, the increased pressure can also drive nitrogen gas into the circulation.

This increase in blood nitrogen content can lead to a condition called nitrogen narcosis. When the diver ascends to the surface too quickly a condition called ‘bends’ or decompression sickness occurs and nitrogen comes out of solution while still in the blood forming bubbles.

Small bubbles in the blood are not harmful, but large bubbles can lodge in small capillaries, blocking blood flow or can press on nerve endings. Decompression sickness is associated with pain in joints and muscles and neurological problems including stroke. The risk of nitrogen narcosis and bends is common in scuba divers.

During carbon-dioxide poisoning, the demand for oxygen increases. As the O2 level in the blood decreases it leads to suffocation and the skin turns bluish black.

In terms of O2 transport, decreased arterial blood oxygenation (hypoxemia) is the primary limitation, and thus, the problem resides with the respiratory system. First, additional capillaries open to reduce diffusion distances and increase the surface area for oxygen exchange; oxygen extraction subsequently increases.

Carbon dioxide levels, blood pH, body temperature, environmental factors, and diseases can all affect oxygen’s carrying capacity and delivery. A decrease in the oxygen-carrying ability of hemoglobin is observed with an increase in carbon dioxide and temperature, as well as a decrease in pH within the body.

Oxygen is transported in the blood in two ways: A small amount of O2 (1.5 percent) is carried in the plasma as a dissolved gas. Most oxygen (98.5 percent) carried in the blood is bound to the protein hemoglobin in red blood cells. A fully saturated oxyhemoglobin (HbO2) has four O2 molecules attached.

The blood hemoglobin concentration is determinant of oxygen delivery. In anemic patients, oxygen delivery decreases and oxygen extraction is increased. This leads to decreased venous hemoglobin saturation and a lower tissue oxygen saturation.

Since carbon dioxide reacts with water to form carbonic acid, an increase in CO2 results in a decrease in blood pH, resulting in hemoglobin proteins releasing their load of oxygen. Conversely, a decrease in carbon dioxide provokes an increase in pH, which results in hemoglobin picking up more oxygen.

Oxygen delivery (DO2) represents the amount of oxygen transported to tissues and is defined as the product of cardiac output (CO) and oxygen content. Normal value is 520 to 570 mL/min/m2. Oxygen delivery can be improved by increasing cardiac output, oxygen saturation, or hemoglobin.

Carbon dioxide is transported in the blood from the tissue to the lungs in three ways:

  • Dissolved in solution
  • Buffered with water as carbonic acid
  • Bound to proteins, particularly haemoglobin.

Approximately 75% of carbon dioxide is transport in the red blood cell and 25% in the plasma. Tissue oxygenation may be impaired either by a low arterial oxygen saturation, as in cyanotic congenital heart disease, or by a reduced blood flow, as occurs with myocardial failure or with obstruction to the left heart (aortic atresia). Anaerobic metabolism results in the accumulation of lactic acid in the tissues.