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Acid–base and blood gases are among the few blood constituents that exhibit substantial difference between arterial and venous values. [6] Still, pH, bicarbonate and base excess show a high level of inter-method reliability between arterial and venous tests, so arterial and venous values are roughly equivalent for these. [44]
The average red blood cell contains 250 million hemoglobin molecules. [7] Hemoglobin contains a globin protein unit with four prosthetic heme groups (hence the name heme-o-globin); each heme is capable of reversibly binding with one gaseous molecule (oxygen, carbon monoxide, cyanide, etc.), [8] therefore a typical red blood cell may carry up to one billion gas molecules.
Bicarbonate in the red blood cell (RBC) exchanging with chloride from plasma in the lungs. The underlying properties creating the chloride shift are the presence of carbonic anhydrase within the RBCs but not the plasma, and the permeability of the RBC membrane to carbon dioxide and bicarbonate ion but not to hydrogen ion.
Carbonic anhydrase catalyzes the conversion of carbon dioxide and water into carbonic acid. This molecule breaks down into bicarbonate and hydrogen ions. This break down process occurs in red blood cells. Ultimately, the concentration of bicarbonate ions in the bloodstream affects the formation of the protein carbaminohemoglobin in the body. [11]
The parameter standard bicarbonate concentration (SBC e) is the bicarbonate concentration in the blood at a P a CO 2 of 40 mmHg (5.33 kPa), full oxygen saturation and 36 °C. [13] Reference ranges for blood tests, comparing blood content of bicarbonate (shown in blue at right) with other constituents.
An arterial blood gas (ABG) test, or arterial blood gas analysis (ABGA) measures the amounts of arterial gases, such as oxygen and carbon dioxide. An ABG test requires that a small volume of blood be drawn from the radial artery with a syringe and a thin needle , [ 1 ] but sometimes the femoral artery in the groin or another site is used.
Red blood cells are much smaller than most other human cells. RBCs are formed in the red bone marrow from hematopoietic stem cells in a process known as erythropoiesis. In adults, about 2.4 million RBCs are produced each second. The normal RBCs count is 4.5 to 5 millions per cu.mm. RBCs have a lifespan of approximately 100-120 days.
At this stage, the hemoglobin is called free hemoglobin. [3] Free hemoglobin (also called naked hemoglobin) is the un-bound hemoglobin that is not enclosed in the red blood cell. The naked hemoglobin is devoid of its anti-oxidant sentries that are normally available within the RBC. [5] Free hemoglobin is thus vulnerable to be oxidized. [5]