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Your lungs hold about 480 million alveoli, tiny air sacs where oxygen passes into the blood and carbon dioxide passes out. Opened out flat, their combined surface would cover 70 to 80 square metres.

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Each alveolus is a cup-shaped pocket 0.2 to 0.5 millimetres across, wrapped in a mesh of capillaries, the body's finest blood vessels. The barrier between air and blood is usually 0.6 to 2 micrometres thick (a micrometre is a thousandth of a millimetre). Gases cross it by diffusion, simply moving from where they are more concentrated to where they are less, so no pump is needed. Special cells coat the inside of the sacs with surfactant, a detergent-like liquid that stops them collapsing when you breathe out. Babies born very early may not yet make enough of it, which is one reason they can struggle to breathe. The figure of about 480 million comes from a 2004 microscope study of adult lungs, and new alveoli keep forming until about the age of eight.

Türkçe oku
Akciğerlerinizde alveol denen yaklaşık 480 milyon minik hava keseciği bulunur; oksijen bu keseciklerden kana geçer, karbondioksit de kandan keseciklere. Hepsi açılıp düz bir yüzeye serilse 70-80 metrekarelik bir alanı kaplardı.

Her alveol fincan biçiminde küçük bir ceptir; çapı 0,2 ile 0,5 milimetre arasındadır. Çevresini, vücudun en ince damarları olan kılcal damarlar bir ağ gibi sarar. Hava ile kan arasındaki zar genellikle yalnızca 0,6-2 mikrometre kalınlığındadır; bir mikrometre, milimetrenin binde biridir. Gazlar bu zardan difüzyonla, yani çok oldukları yerden az oldukları yere kendiliğinden geçerek taşınır; bunun için bir pompaya gerek yoktur. Özel hücreler keseciklerin içini sürfaktan denen, deterjana benzer bir sıvıyla kaplar; bu sıvı nefes verirken keseciklerin sönüp yapışmasını önler. Çok erken doğan bebekler bu sıvıyı henüz yeterince üretemeyebilir; nefes almakta zorlanmalarının nedenlerinden biri budur. Yaklaşık 480 milyonluk sayı, 2004'te yetişkin akciğerleri üzerinde yapılan bir mikroskop çalışmasından gelir. Yeni alveoller yaklaşık sekiz yaşına kadar oluşmaya devam eder.

Sources and further reading

Background reading: Pulmonary alveolus
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