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腹膜（peritoneum）由连续的两层组成，即壁层腹膜（the parietal peritoneum，源于somatic mesoderm，体壁中胚层）和内脏腹膜（visceral peritoneum，源于splanchnic mesoderm，脏层中胚层），都由单纯性鳞状上皮细胞（simple squamous epithelial cells）组成，称为腹膜间皮（mesothelium）。The peritoneum consists of two layers that are continuous with each other: the parietal peritoneum and the visceral peritoneum. Both types are made up of simple squamous epithelial cells called mesothelium. 
腹膜腔（The peritoneal cavity）和腹腔是两个不同的解剖学名词，腹膜腔是壁层覆膜和脏层腹膜之间的潜在空间，内含少量的液体。腹膜的炎症等病变，可能会出现腹膜粘连（Peritoneal Adhesions），表现为脏层腹膜和壁层覆膜间纤维组织形成（formation of fibrous scar tissue），这种粘连可能引发腹痛、肠扭转（volvulus）、肠梗阻（bowel obstruction）等症状。The peritoneal cavity is a potential space between the parietal and visceral peritoneum. It normally contains only a small amount of lubricating fluid.根据发育过程中腹部器官被腹膜包裹的深度，这些器官被分为腹膜内器官（Intraperitoneal organs）和腹膜后器官(Retroperitoneal organs，又进一步被分为原发性和继发性腹膜后器官)。Depending on how deep the abdominal organs dive into the peritoneum during development, they can be classified as: The abdominal viscera can be divided anatomically by their relationship to the peritoneum. There are two main groups: intraperitoneal and retroperitoneal organs.Intraperitoneal organs are enveloped by visceral peritoneum, which covers the organ both anteriorly and posteriorly. Examples include the stomach, liver and spleen.腹膜内器官（Intraperitoneal organs）是完全被脏层腹膜（visceral peritoneum）包裹，这些器官包括腹腔的大部分：肝、脾、胃、十二指肠上部、空肠（jejunum）、回肠（ileum）、横结肠（transverse colon）、乙状结肠（sigmoid colon）和直肠上部。Note: this is a somewhat confusing term, as the organ does NOT lie inside the peritoneal cavity, but that’s how it’s been called for centuries, so we’ll stick to it. 腹膜内器官其实不在腹膜腔内。Intraperitoneal organs are completely wrapped by visceral peritoneum. These organs are the liver, spleen, stomach, superior part of the duodenum, jejunum, ileum, transverse colon, sigmoid colon and superior part of the rectum. 腹膜后器官（Retroperitoneal organs）是位于腹膜后（腹膜后区，腹膜后腔，腹膜外，retroperitoneal space），只有前壁被壁层覆膜（parietal peritoneum）覆盖。这些器官如果在发育过程中一直位于腹膜外，就是原发性腹膜后器官（primarily retroperitoneal organs），包括: 肾脏、肾上腺和输尿管；如果有些器官起初在腹膜腔内，但随着发育续逐渐向下移动，进入了腹膜后（腹膜下），就是继发性腹膜后器官（secondarily retroperitoneal organs），包括： 胰腺、远端十二指肠、升结肠（ascending colons）和降结肠（descending colons）。Retroperitoneal organs are found posterior to the peritoneum in the retroperitoneal space with only their anterior wall covered by the parietal peritoneum. If they develop and remain outside the peritoneum, they are primarily retroperitoneal organs: kidney, adrenal glands and ureter. Other retroperitoneal organs develop inside the peritoneum, but then move beneath it: pancreas, distal duodenum, ascending and descending colons.Retroperitoneal organs are not associated with visceral peritoneum; they are only covered in parietal peritoneum, and that peritoneum only covers their anterior surface.
They can be further subdivided into two groups based on their embryological development:- Primarily retroperitoneal organs developed and remain outside of the parietal peritoneum. The oesophagus（食管）, rectum（直肠） and kidneys （肾脏）are all primarily retroperitoneal.
- Secondarily retroperitoneal organs were initially intraperitoneal, suspended by mesentery. Through the course of embryogenesis, they became retroperitoneal as their mesentery fused with the posterior abdominal wall （随着肠系膜与腹壁后壁的融合，移动到了腹膜后）. Thus, in adults, only their anterior surface is covered with peritoneum. Examples of secondarily retroperitoneal organs include the ascending and descending colon.
A useful mnemonic to help in recalling which abdominal viscera are retroperitoneal is SAD PUCKER:S = Suprarenal (adrenal) Glands, SRO A = Aorta/IVC, PRO D =Duodenum (except the proximal 2cm, the duodenal cap), SRO P = Pancreas (except the tail), SRO U = Ureters, PRO C = Colon (ascending and descending parts), SRO K = Kidneys, PRO E = (O)esophagus, PRO R = Rectum, PRO
继发性腹膜后器官（secondarily retroperitoneal organs）由脏层腹膜完全包裹，属于腹膜内器官，但位置在腹膜后腔。比如，肾上腺，位于腹膜后腔。Extraperitoneal，腹膜外，腹膜腔外，Subperitoneal，腹膜下，腹膜腔下，这些器官与属于原发性腹膜后器官（primarily retroperitoneal organs）。
以上，腹腔内的脏器被分为腹膜内器官（Intraperitoneal organs），原发性腹膜外器官（Primarily retroperitoneal organs）和继发性腹膜外器官（Secondary extraperitoneal organs）。但我们平常听到是腹膜内位器官，腹膜外位器官和腹膜间位器官的称呼，基本的定义是基于腹膜覆盖脏器的多少，“腹膜内位器官，腹膜间位器官，腹膜外位器官”很少出现在医学文献中，也就是说不是正式的医学名词。 腹膜内位器官几乎全部包被腹膜，活动度较大，主要的器官有：胃、十二指肠降部和水平部、空肠、回肠、阑尾、横结肠、乙状结肠、脾、卵巢、输卵管等。就是腹膜内器官。Intraperitoneal organs. 腹膜间位器官多面包被腹膜，活动度较小，主要的器官有：升结肠、降结肠、肝、（胆囊），膀胱、子宫，直肠上部等。大部分属于继发性腹膜外器官。Interperitoneal organs. 腹膜外位器官只有一面包被腹膜，几乎不能活动，主要的器官有：胰、肾、输尿管、肾上腺等。大部分属于原发性腹膜外器官。Extraperitoneal organs.
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