Lecture № 5Serous Sacs, Features and Functions

There are three serous sacs in the living man. There are: Pericardial, Pleural, and Peritoneal.

They all are developed from intra embrionic coelome.

Development of the Serous Sacs.

At the stage of the 3-4 week of embrionic development each of mesoderms are divided into

two layers and the space appered between them. This space is called coelome. Right and left

parts of the body have their own coelome.

There are two layers of coelome:

1. Somato-pleurae layer – layer which lines the amniotic cavity (outer aspect). Parietal

layer of Serous sac is developed from this layer.

2. Splanchno-pleurae layer – Layer which lines the yorc sac and developing primitive di-

gestive tube. Visceral layer is developed from this layer.

Two visceral layers connect each other in saggital plane. Thus dorsal and ventral mesen-

terium are perfomed. At the period of 7-8 embrionic week diaphragma begins to develop.

Diaphragma devides each coelom into three absolutely isolated cavities: unpaired thoracic

cavity situated cranially and paired abdominal cavity situated caudally. From thoracic cavity

three serous sacs are developed: two pleural sacs and one pericardial sac. Caudal part of the

coelom became unpaired, because ventral mesenterium disappeared and digestive tube is

connected with the body wall only by dorsal mesenterium.

Three absolutely isolated serous sacs are present in the thorasic cavity, one for each lung

and one, middle, sac for the heart. The serous covering of the lung is called pleura. It has

two layers: the visceral pleura (pleura visceralis) and the parietal pleura (pleura pari-

etalis).

The visceral, or pulmonary pleura, covers the lungs and fused with the pulmonary

substunce so closely that it cannot be removed without injury to the tissue. The visceral

pleura invests the lung completely and is continuous with the patietal pleura at the root of

the lung.

The potential space between two layers is called pleural cavity. In a healthy subject

the pleural cavity is not visible macroscopically. Under normal conditions it contains 1 or 2

ml of fluid. This cavity becomes patent when air (Pneumothorax), fluid (Pleurisy with effu-

sion), blood (Haemothorax) or pus (empyema) are in it. Parietal pleura is subdivided accord-

ing to regions it lines into three parts: the costal pleura, the diaphragmatic pleura and medi-

astinal pleura.

The costal pleura (pleura costalis), the most extensive part of the parietal pleura,

lines the inner surfaces of the ribs and intercostal spaces.

The diaphragmetic pleura (pleura diaphragmatica) covers the superior surface of

the diaphragm except for the middle part where the pericardium is in direct contact with the

diaphragm.

The mediastinal pleura (p. mediastinalis) lines the medial surface of lung. In front it

is continuous with the costal pleura at costomediastinal line of pleural reflection below it is

continuous with diaphragmatic pleura at costodiaphragmetic line of pleural reflection and

posteriorly with the costal pleura at costovertebral line of pleural reflection. The mediasti-

nal pleura bounds laterally the mediastinal organs. The complex of organs occupying the

space between the mediastinal pleura is called mediastinum. This complex forms as if a

septum between the two pleural sacs. An anterior and posterior part are distinguished in the

mediastinum. It is bounded by frontal plane drawn through the posterior part of both pul-

monary roots.

The anterior mediastinum (mediastinum anterius) contains the heat with the peri-

cardium, the thymus, the vena cava superior, the ascending aorta and the aortic arch with its

branches, the pulmonary viens, the trachea and bronchi, the phrenic nerves, the bronchial ar-

teries and veins, and the lymph nodes. The posterior mediastinum (mediastinum pos-

2

terius) is ocuppies by the oesophagus the thoracic aorta, the thoracic duct, the lymph node,

the vena cava inferior, the azygos and hemiazygos veins, the splanchnic nerves and the va-

gus nerves streching on the oesophageal walls.

The boundaries of the lungs coincide not in all places with those of the pleural sacs.

Reserve spaces are formed by two parietal layers of the pleura where the pulmonary borders

do not coincide with the pleural bounderies; these are the pleural recesses, or sinuses (re-

cessus pleuralis).The lung enters them only during very deep inspiration. The largest sinus

the costodiapfragmatic recess (recessus costodiaphragmaticus) is situated on the right and

left sides along the inferior boundary of the pleura, between the diaphragm and the thoracic

cage; the inferior pulmonary borders do not reach the pleural boundaries here. Another,

smaller, reserve space is at the anterior border of the left lung. It extends for the distance of

the cardiac notch between the costal and mediastinal pleurae and is called the costomedi-

astinal recess (recessus costomediastinalis).Fluid (inflammatory exudate) produced in in-

flammation of the pleura accumulates first of all in the pleural sinuses. The pleural sinuses

are the part of the pleural cavity, but they are nevertheless differ from it. The pleural cavity

is the space between the visceral and parietal pleurae; pleural sinuses are reserve spa-

ces of the pleural cavity between two layers of the parietal pleura.

The pericardial sac (pericardium) is a closed serous sac, in which two layers are dis-

tinguished: an outer fibrous layer, the pericardium fibrosum, and an inner serous layer ,the

pericardium serosum. The serous pericardium is divided into two layers: a visceral layer or

the epicardium mentioned above, and a parietal layer, which fuses with the inner surface of

the fibrous pericardium and lines it. The space between parietal and visceral layers is called

serous cavity (cavum pericardii). It contains a small amount of serous fluid (liquor peri-

cardii). On the trunks of large vessels close to the heart, the visceral and parietal layers are

continuous. The pericardium is directly attached to the mediastinal pleura on both sides.

The posterior surface of the pericardial sac adjoins the oesophagus and the descending aorta.

The passage behind the aorta and pulmonary trunk is called the transverse sinus of the peri-

cardium (sinus transversus pericardii). The space bounded below and to the right by the in-

ferior vena cava and above and to the left by the left pulmonary veins is the oblique sinus of

the pericardium (sinus obliquus pericardii).

3

The peritoneum is a serous membrane which is located into abdominal cavity. It

consists of two layers, parietal (peritoneum parietale) and visceral (peritoneum viscerale).

The parietal layer lines the abdominal wall, while the visceral layer invests the internal or-

gans. The space between parietal and visceral layers is called peritoneal cavity (cavum

peritonei).This space contains a small amount of serous fluid; this fluid moistens the surface

of the organs and so makes easier their movement against one another. In male the peri-

toneal cavity is closed serous sac, but in female it is an open cavity because the fimbriated

end of uterine tube opens into the peritoneal cavity. So the peritoneal cavity through uterine

tube, uterus and vaginal canal communicates with the outside. When air enters the cavity

during operation or postmortem examination or when pathological fluids accumulate in it,

this cavity is become large.

The parietal peritoneum forms а continuous lining on the anterior and lateral walls of

the abdomen. Then it passes on to the diaphragm and the posterior abdominal wall. Неrе it

is reflected on the viscera and is directly continuous with the visceral peritoneum investing

them.

In the lower part of the anterior abdominal wall the peritoneum forms five folds

converging on the umbilicus: one unpaired medial umbilical fold (plica umbilicalis mediana)

and two paired medial and lateral umbilical folds (plicae umbilicales mediales and plicae

umbilicales laterales). These folds bound on each side two inguinal fossae (fossae inguinales)

above the inguinal ligament, which аrе related to the inguinal canal.

It is very difficult to have clear conception of the disposition of peritoneum. So we

shall study peritoneum in two ways or dispositions.

1.Vertical disposition.

For easier understanding of the complex relations, the whole peritoneal cavity can be

separated into three regions, or storeys: (1) an upper storey bounded superiorly by the

diaphragm, and inferiorly by the mesocolon transversum; (2) а middle storey extending

downward from the mesocolon transversum to the entry into the true pelvis; 3) the lower

storey begins at the line of entry into the true pelvis and corresponds to the cavity of the true

pelvis which is the lowest part of the abdominal cavity.

4

1. From the inferior surface of diaphragm the peritoneum will be reflected to upper surface of

the liver. It is form ligaments of liver: the coronary ligament of the liver (lig. coronarium

hepatis), This ligament then perfom the right and left triangular ligaments of the liver (lig.

triangulare dextrum and sinistrum); the falciform ligament of the liver.(lig. falciforum).

This ligament divides the liver into right and left lobes. From the hepatic porta to the lesser

curvature of the stomach peritoneum is fomed the hepatogastric ligament (lig. hepatogas-

trica),and to the part of duodenum nearest to the stomach it forms the hepatoduodenal

ligament (lig. hepatoduodenale). The hepatogastric and hepatoduodenal ligaments are

duplications of the peritoneum because two peritoneal layers are encountered in the region of

the porta hepatis. One layer passing tо the porta from the anterior раrt of the visceral surface

of the liver and the other from the posterior раrt. The hepatoduodenal and hepatogastric

ligaments are а continuation of one another and form together the lesser omentum (omentum

minus). Between the layers of the hepatoduodenal ligament pass the сотптлоп bile duct (on

the right), the common hepatic artery (on the left) and the portal vein (posteriorly and

between these structures), as well as lymphatic vessels, nodes and nerves.

On the lesser curvature of the stomach both layers of the lesser omentum separate, one

to cover the anterior and the other the posterior surface of the stomach. On the greater

curvature they again join and descend in front of the transverse colon and the loops of the

small intestine to form the anterior lamina of the greater omentum (omentum majus). On

some level both layers fold over to ascend and form its posterior lamina. The great omentum

consists therefore of 4 layers.

The upper storey of the peritoneal cavity separates into three sacs: hepatic bursa (bursa

hepatica), pregastric bursa (bursa pregastrica), and omental bursa (bursa omentalis). The

hepatic bursa is related to the right lobe of the liver and is separated from the pregastric bursa

by the falciform 1igament of the liver; it is bounded posteriorly by the coronary ligament of

the liver. The pregastric bursa is related to the left lobe of the liver, anterior surface of the

stomach, and the spleen;

The omental bursa, lesser sac of the peritoneum (bursa omentalis), is part of the general

peritoneal cavity lying behind the stomach and the lesser omentum.

5

The cavity of the omental bursa communicates with the general peritoneal cavity only by

means of а relatively narrow epiploic foramen, opening into the lesser sас (foramen

epiploicum). The foramen is bounded above by the caudate lobe of the liver, in front by the

free margin of the hepatoduodenal ligament, below by the superior part of the duodenum, The

parietal peritoneum forming here the posterior wall of the omental bursa covers the

abdominal aorta, vena cava inferior, and the pancreas.

2. The middle storey of the peritoneal cavity can be visualized when the greater omentum and

transverse colon are raised. Using as the boundaries the ascending and descending parts of the

colon on the sides аnd the . mesentery of the small intestine in the middle, this storey can be

subdivided into four compartments; between the lateral abdominal walls and the colon

ascendens and colon descendens are the right and left lateral canals (canales laterales

dexter and sinister); the space bounded by the colon is divided by the mesentery of the small

intestine, descending obliquely from left to right, into the right and left mesenteric sinuses

(sinus mesentericus dexter and sinus mesentericus sinister).

The mesentery (mesenterium) is а fold of two peritoneal layers by means of which the small

intestine is attached tо the posterior abdominal wall. The posterior margin of the mesentery

attached to the abdominal wall is the root of the mesentery (radix mesenterii). It is relatively

short (f5-17 cm), whereas the opposite free end related to the mesenteric part of the small

intestine (jejunum and ileum) is the length of these two segments. The line of attachment of

the root passes obliquely from the left side of the second lumbar Blood vessels, nerves,

lymphatic vessels and lymph nodes pass in the thickness of the mesentery between the two

serous layers containing more or less fatty tissue.

At the junction of the small intestine and the colon are two recesses, the inferior and superior

ileocaecal recesses (recessus ileocecalis inferior and superior), situated below and above the

ileocaecal fold passing from the ileum to the medial surface of the caecum..

А recess in the parietal peritoneum lodging the caecum is called the fossa of caecum

3.The lower storey. Further tracing of peritoneum differs in male and female. On pass-

ing from the anterior surface of the recrum to the posterior surface of the urinary bladder in

males the peritoneum forms a pouch between its layers behind the bladder this is retrovesical

pouch (excavatio vesicorectalis). In females, therelations of the peritoneum in pelvis are dif-

6

ferent because between the urinary bladder and the rectum is the uterus, which is also covered

by the peritoneum. There are two peritoneal pouches in the female pelvis, namely the recto-

uterine pouch (excavatio recto-uterina) between the rectum and uterus and the uterovesical

pouch (excavatio vesico-uterina) between the uterus and the urinary bladder.

The tunica vaginalis testes is the serous sac too. It has two layers: parietal and visceral

(tunica albuginea).

7