In mammals the olfactory mucous membrane is located in the upper part of the nose, called the nasal roof. In this area the inhaled air passes by free nerve endings that are able to sense various molecules in the air, thus functions our sense of smell. Randomly molecules and particles may have contact with these nerves, and by this means directly reach the olfactory bulb, which is connected to our forebrain. This pathway guides into the brain without the necessity of crossing the blood-brain-barrier.


Gastro-Intestinal Tract  (c) yodijim /

Our gastro-intestinal tract is highly specialised for the uptake of different materials (food), their digestion, and the delivery of the nutrients via the blood to the organs. There is also the possibility for nanomaterials to cross the gastro-intestinal tract barrier. However, this only relates to a very small amount of nanomaterials that are considered to be unproblematic.


Lung © Nerthuz /

The air-blood barrier is a structure present in the lungs that controls gas exchange in the lungs by means of pressure and concentration gradients. However, all other foreign material in our breathing air will be inhaled too if it is small enough, such as bacteria, viruses including nanomaterials. With this being a very thin barrier, the chance for nanoparticles to cross and enter the interior of our body is relatively high.


Immune defenses © ag visuell /

Nanoparticles that are present in the body (i.e. after injection) are largely taken up and eliminated by the reticulohistocytic system (RHS). This system represents a network of cells that are distributed throughout the body within its organs. The function of the RHS is the inactivation and elimination of dead cells, bacteria, viruses, and infiltrated small particulates. Nanoparticles also belong to these "infiltrates".


[© Gray38.png: User Magnus Manske on en.wikipedia derivative work: Amada44

The placenta is an important tissue barrier that separates the unborn child from the mother. The foetus is totally dependent on proper functioning of the placenta during its development as it is fed and protected by the unique physiology and structural complexity of this organ. As nutrients and oxygen have to penetrate to the foetus, waste and carbon dioxide have to be transported in the other direction. Nanoparticles may cross this barrier and affect the unborn child.


Cross-section through the skin © 7activestudio /

The skin is the body's largest organ and protects the body against diseases caused by organisms, toxic chemicals, and mechanical damage. Nanoparticles are neither able to easily penetrate intact skin nor through superficial injuries within the skin. Low level mechanical stresses applied to the skin are buffered by a layer called the stratum corneum. Secretion of ichor and other blood components protect deeper abraded skin sites from harmful impacts. Nanoparticles, however, can be deposited in the hair follicles, and this route is used in medicine for the release of nanoparticle-bound drugs or vaccines, which subsequently are and delivered to the body. Normally, nanoparticles applied to the skin can be removed by washing.

Immune system © ag visuell / fotolia

The immune system recognizes both self and non-self-entities. When non-self agents are recognized, like nanoparticles, immune cells decide whether they constitute a danger or not. If the body recognizes non-self and considers it dangerous, the immune system responds with inflammation. In the case of pathogens which induce disease, they are confined to a local site, are destroyed, and if an injury has occurred, wound healing is induced. Some technically produced nanomaterials are suspected to promote inflammation. However, an ability to influence immune responses can be medically useful. Research about the safety of nanomaterials can thus also contribute to the development of novel medical therapies.


Schematic sketch showing the blood-brain barrier © von Kuebi = Armin Kübelbeck, and for the brain: Patrick J. Lynch [CC-BY-3.0 (], via Wikimedia Commons

The blood-brain barrier separates the vascular system from the brain and is formed by endothelial cells which are enclosing the brain capillary blood vessels. Under normal circumstances this barrier is impermeable to nanoparticles.


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