Metabolic Pathway of Oxygen

The journey of oxygen through the human body is one of perpetual nature, with new oxygen entering the body through inhalation and utilized oxygen leaving as carbon dioxide and water.


Oxygen begins its journey through the human body as air is inhaled through the nose or the mouth. From there, the inhaled air travels through the larynx, down the trachea, and finally into the lungs. Specifically in the lungs, the oxygenated air bifurcates in its path into the left and right lungs via the bronchial tubes. This air further travels into smaller and smaller pathways that eventually end in alveoli cells. Any molecule of oxygen travels through an alveolus cell and into a capillary wall cell.

Through the Bloodstream

From the, alveolus, said molecule will travel out of the capillary wall into the blood plasma, where it will finally enter a red blood cell. This constitutes five membranes oxygen passes through when entering the blood stream: the two membranes of the alveolus, two of the capillary wall cell, and one of the red blood cell plasma membrane.

Oxygen, now bound to hemoglobin in red blood cells in the blood stream, is pumped throughout the body by the heart to all of the somatic cells. Oxygen diffuses across the somatic cells’ cellular membranes and into the cytoplasm.

Cellular Level

As per rate of cell respiration, a cell will utilize oxygen accordingly. If a cell needs to perform more cell respiration, it will require more oxygen. The citric acid cycle (also known as the Krebs cycle) is a major process of cell respiration that takes place in the mitochondrial matrix, where NAD+ is reduced into NADH, FAD to FADH2. This provides stored chemical energy which is later turned to ATP by the electron transport chain.

It is within the process of the electron transport chain at the boundary of the inner and outer mitochondrial matrices where oxygen is required. As electrons pass along the ETC from NADH and FADH2 to oxygen, there are proton pumps that move free floating hydrogen ions in the matrix to the intermembrane space. As the protons are pumped, the electrons given to the ETC by the donor must have a corresponding acceptor, in this case, oxygen. As a result, the oxygen within the matrix combines with two hydrogen atoms to form water as a byproduct of the ETC (the hydrogen ions are attracted by the electron that is accepted by the oxygen). This excess water eventually diffuses out of the mitochondria and the somatic cell in order to maintain osmotic equilibrium. This water, a combination of inhaled oxygen and protons of the mitochondrial matrices, now osmoses through the cell walls into the blood stream, where it travels through to the kidneys.

Through the Gastrointestinal System

The blood (with waste and more water) passes through the renal artery, where it enters any one of the 500000 nephrons in either kidney. At the level of the nephron, blood will flow into the glomerulus of Bowman’s capsule. The solutes present in the unfiltered blood pass through the walls of 50 or so capillaries in each glomerulus and follow a course through the proximal tubule, loops of Henle, distal tubule, and the collecting tubule. These tubules surround the peritubular capillaries, though which nutrients needed by the body are reabsorbed into (nutrients generally have a higher membrane permeability than wastes do). Also, the loop of Henle establishes a balance in osmolarity between the filtrate and the filtered blood. From here, the previously filtered blood circulates back out of the renal vein to the heart.

Liquid Excretion

The remaining aqueous fluid and solutes (now waste products) are collected as urine and are sent to the urinary bladder via the ureters. The function of the bladder is to store urine for later secretion out of the body (urination). We now see some of the oxygen that was inhaled in the water that constitutes urine, which finally exits the body as the bladder muscles contracts and two sphincters open, allowing urination.

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