Human Anatomy — A Course of Lectures - Kostylenko, Yu.P. 2015

General Splanchnology. General Anatomy of the Digestive System

Lecture Outline:

4.1 Doctrine of Internal Organs.

4.2 General Principles of The Structure of internal organs.

2.3 Topography and Variability of internal organs.

4.4 Outline for describing internal organs.

4.5 General characteristics of the STRUCTURE OF THE Digestive System.

4.6 Main Selection/3.html">Stages of development of the digestive system.

4.1 Doctrine of internal organs

Splanchnology is the branch of anatomy that studies the internal organs located within the Body Cavities (the oral, cervical, thoracic, abdominal, and pelvic cavities) which mediate metabolic processes between the Organism and the external environment. To better understand the material presented in this section, we must first define several general morphological concepts that are studied in greater detail in the Histology course.

A tissue is a phylogenetically evolved system of Cells and their derivatives, organized to perform specific Functions and capable of self-maintenance.

Each tissue consists of the following basic elements: 1 - cells; 2 - a fibrous component; 3 - an amorphous substance.

Tissues serve as the building blocks from which organs are formed. The Human Body comprises 4 MAIN TYPES OF tissues: epithelial, Internal Environment Tissues (Connective Tissues), muscular, and nervous tissues.

Epithelial Tissues lie on the free surfaces of the body and line the mucous membranes of hollow organs, forming a selective barrier between the organism and the external environment, as well as constituting glands. Morphologically, Epithelial Tissue is characterized by cells united into sheets consisting of one or more layers.

Internal environment tissues (connective tissues) occupy the entire internal space of the body, surrounding and supporting all other tissue types; they are highly diverse in Structure and function. They are characterized by cells separated by an abundant Extracellular matrix, which consists of a fibrous complex (Collagen and elastic fibers) and a ground (amorphous) substance.

Connective tissues are subdivided into proper connective (fibrous) tissue, Connective Tissues with special properties (adipose, reticular, etc.), and Skeletal Tissues—Cartilage and bone. Fibrous tissue can be loose or dense. Loose Connective Tissue accompanies Blood Vessels and nerves and is a component of various organs. Cytology/practical/45.html">Dense connective tissue can be unorganized or organized; the latter contains regularly oriented bundles of fibers and participates in The formation of ligaments, membranes, and tendons.

Muscle Tissues provide movement for the body or its individual parts, as they are formed by contractile cells containing specialized contractile Proteins in their Cytoplasm—Actin and Myosin—which form myofibrils. The following types of Muscle tissue are distinguished: smooth (non-striated), which is found in the walls of most hollow internal organs, skeletal striated, and cardiac striated muscle.

Nervous Tissue receives information from the external and internal environments of the organism. It consists of specialized cells—Neurons—which have one or more processes and are interconnected via specialized intercellular contacts known as synapses. Neurons are capable of generating, transmitting, and integrating electrical impulses.

As stated above, tissues combine to form organs within the body. An organ is composed of various tissues (often representing all four basic groups), with one or more predominating to determine its specific structure and function.

As part of the organism, an organ is characterized by the following features: a specific position in the body, a characteristic shape, structure, and spatial relationships with other organs. Each organ performs its own specific function in the body; however, most organs fulfill multiple functions, meaning they are multifunctional.

Within the intact organism, organs are combined into complexes known as anatomical and functional systems. An organ system is defined as a set of organs interconnected anatomically, topographically, and functionally, sharing a common origin and similar structural features. The constituent organs of a system perform a part of the general function or the entire primary function of that system. In addition to METABOLISM/2.html">THE CONCEPT OF an "organ system," some authors use the term "apparatus," such as the musculoskeletal apparatus, vocal apparatus, and so on. An apparatus is a functional combination of organs that may be homogeneous or heterogeneous in origin and structure. For example, bones and Muscles make up the musculoskeletal apparatus.

In the human body, as in all mammals, the following Organ Systems and apparatuses are distinguished.

1. Organs that carry out metabolism with the external environment. This process represents The Unity of opposing phenomena: assimilation (anabolism) and dissimilation (Catabolism). The assimilation of nutrients and oxygen is provided by the digestive and respiratory systems. The excretion of metabolic waste is performed by the Urinary System. Metabolic products are also excreted by the digestive and respiratory systems.

2. Organs that serve for species preservation—the Reproductive System or sex organs. The URINARY AND REPRODUCTIVE organs are closely linked in Development and Structure, which is why they are grouped together into the Urogenital apparatus.

3. Organs through which the material received by the mother's digestive and respiratory systems is distributed throughout the body, while substances destined for elimination are transported to the excretory system. These organs constitute The Cardiovascular system.

4. Organs that effect chemical communication and regulation of all body processes—ductless glands (Endocrine glands).

5. The organs responsible for spatial locomotion form The Musculoskeletal System, which consists of bones (Skeletal System), their joints (joints and ligaments), and muscles that drive them (muscular system).

1. The organs that perceive stimuli from the external environment constitute the sensory system.

2. The organs that establish neural connections and integrate the functions of all organs into a single whole make up The Nervous System, which is associated with Higher Nervous Activity in humans.

The digestive, respiratory, urinary, and reproductive organs, Blood Vessels, and endocrine glands are collectively referred to as vegetative (or plant-like) organs, since plants possess analogous functions.

The musculoskeletal system, Sensory Organs, and nervous system are grouped under the term animal system, because the functions of active locomotion and neural activity are characteristic exclusively of animals and are practically absent in plants.

The musculoskeletal system, covered by the Skin, forms the body proper—the "soma"—inside of which lie cavities: the thoracic, abdominal, and pelvic cavities. Thus, the soma forms the walls of these cavities. The contents of these cavities are referred to as the "viscera." These include the digestive, respiratory, urinary, and reproductive organs, along with their associated endocrine glands. Running into the viscera and soma are fluid-conducting pathways, namely Blood and Lymphatic vessels that form The Vascular System, as well as conducting pathways—nerves—which, together with the Spinal Cord and brain, constitute the nervous system.

The pathways conducting stimuli and fluids form the anatomical basis for integrating the organism through neurohumoral regulation. Therefore, the viscera and soma are parts of a single, unified organism, and their Separation is largely conventional. Currently, the viscera comprise three organ systems: the digestive, respiratory, and urinary systems.

When studying internal organs, attention is focused on their external and internal structure, as well as their topography.

Based on their structural characteristics, most internal organs can be classified as hollow or tubular organs (Esophagus, Stomach, intestines, Trachea) and parenchymal organs (Liver, Pancreas).

There are also muscular organs (Tongue), organs composed of hard tissues (Teeth), and organs of mixed (complex) structure.

4.2 General Principles of the Structure of Internal Organs

Hollow (tubular) organs feature an internal cavity enclosed by a multilayered wall. The wall comprises three layers: an inner mucous membrane, a middle muscular layer, and an outer layer.

The mucous membrane, or tunica mucosa, lines the entire internal surface of the hollow organs belonging to the digestive, urogenital, and respiratory systems. The external body covering transitions into the mucous membrane at the openings of the Mouth, Nose, anus, Urethra, and Vagina.

The mucous membrane is covered with epithelium, beneath which lie connective tissue and muscular plates. The transport of luminal contents is facilitated by mucus secreted by glands located within the mucous membrane. The mucous membrane provides mechanical and chemical Protection of the organs against damage. It also plays a major role in the biological defense of the organism. The mucous membrane contains accumulations of lymphoid tissue in the form of lymphatic follicles and more complex structures, such as Tonsils. These formations are part of the body's immune (protective) system.

An essential function of the mucous membrane is the absorption of nutrients and fluids. In organs where absorption is most intensive, the surface area of the mucous membrane is increased through folds and villi. For instance, due to these structures, the internal surface area of the Small Intestine reaches 4 m², compared to a total body surface area of about 1.5 m².

In addition, the mucous membrane secretes glandular products and certain Metabolic waste products, particularly under conditions of impaired excretory system function (renal failure).

The mucous membrane of most hollow organs rests on a submucosal layer, or tela submucosa, which consists of loose connective tissue and allows the mucous membrane to shift. The submucosa houses branches of blood vessels that supply the walls of the hollow organ, as well as Lymphatic vessels and nerve plexuses.

The muscular coat, or tunica muscularis, forms the Middle layer of the wall of a hollow organ. In most viscera, with the exception of the initial sections of the digestive and respiratory systems, it is represented by Smooth muscle tissue.

The muscular coat typically features an inner circular layer and an outer longitudinal layer. It has been established that both circular and longitudinal bundles of muscle fibers follow a spiral course. Unlike the longitudinal layer, the spirals in the circular layer are steeper. When the inner circular layer of the digestive tube contracts, it narrows and slightly lengthens at that site, whereas where the longitudinal musculature contracts, it slightly shortens and widens. The coordinated contractions of these layers ensure the propulsion of contents through a given tubular system. At certain points, circular muscle cells concentrate to form sphincters capable of closing the organ's lumen. Sphincters play a role in regulating the movement of contents from one organ to another (e.g., the pyloric sphincter of The Stomach) or expelling them outward (sphincters of the anus and urethra).

The outer layer of hollow organs has a dual structure. In some, it consists of loose connective tissue known as the adventitial layer, or tunica adventitia, while in others it is represented by the visceral layer of one of the body's serous membranes (Pleura, Peritoneum, Pericardium) and is called the serous layer, or tunica serosa (Fig. 4.1).

Parenchymal organs are predominantly large glands. The composition of such organs includes parenchyma and stroma. The parenchyma refers to the specific functional tissue of the organs, typically represented by epithelium, whose morphofunctional features determine the specific functions performed by the given organ.

The cellular complexes of the parenchyma are embedded within a connective tissue framework known as the stroma. The stroma in these organs lacks fundamental differences and consists of loose Fibrous connective tissue through which nerves and blood vessels pass; thus, the stroma performs supporting, trophic, and shape-forming functions for the organs.

Class="center">

Fig. 4.1 Diagram of the wall structure of a hollow organ (after R. D. Sinelnikov):

1 — mucous membrane (inner); 2 — submucosa; 3 — muscular coat (middle), A — circular layer, B — longitudinal layer; 4 — outer layer (serous or adventitial)

Parenchymatous organs consist of anatomical structural units, namely lobes and lobules. In certain organs, anatomical-surgical units—segments—are distinguished, which are of significant importance in organ-sparing surgical interventions.

The smallest structural part of an organ that performs all the functions inherent to that organ is called a Structural and functional unit (e.g., the nephron in the Kidneys, the hepatic lobule in the liver, and the acinus in the Lungs).

4.3 Topography and Variability of Internal Organs

Knowledge of visceral topography is of paramount practical importance. When describing THE POSITION OF an organ within the body, The concepts of holotopy, skeletotopy, and syntopy are used.

Holotopy refers to the topographical relationship between organs and specific body regions. When studying splanchnology, it is essential to know the Regions of the HEAD, neck, chest, abdomen, back, and Perineum, as well as how to project organs onto the body surface.

Skeletotopy is The Relationship of organs to PARTS OF THE Skeleton and bony landmarks that can be palpated in a living person or identified on a radiograph.

Syntopy describes the spatial relationship of the organ under study to adjacent organs and anatomical structures (such as blood vessels and nerves).

Concepts of developmental variants and anomalies of internal organs. In clinical practice, one encounters not only anatomical variants but also anomalies resulting from impaired organ development during the embryonic and postnatal periods. These are frequently the cause of functional insufficiency and pathological disorders.

All developmental defects of internal organs can be divided into 4 groups:

I. Anomalies of number:

A) Absence of an organ associated with agenesis or aplasia.

1) Agenesis — the failure of an organ to develop due to the absence of its primordium in the embryo.

2) Aplasia — the failure of the embryonic primordium of an organ to develop further.

B) Duplication of an organ or the formation of additional organs — caused by multiple embryonic primordia or the splitting of a single organ primordium.

C) Fusion (non-separation) of organs.

II. Anomalies of position:

A) Heterotopia — The Development of an organ from a primordium formed in an unusual site, where its subsequent growth takes place.

B) Dystopia — the displacement of an organ to an abnormal Location during the Embryonic period.

C) Inversion — the reversal of an organ's position relative to its own axis or the median plane of the body As a result of disrupted embryonic rotation.

III. Anomalies of shape and size:

A) Hypoplasia — underdevelopment of an organ due to arrest at a certain stage of Embryogenesis. A hypoplastic organ is reduced in size, and its function is markedly impaired or entirely absent.

B) Hyperplasia (hypertrophy) — an increase in the relative mass or size of an organ resulting from an increase in Cell number (hyperplasia) or cell volume (hypertrophy).

C) Fusion of paired organs — resulting from the merging of their primordia during the embryonic period.

IV. Anomalies of structure:

A) Atresia — the congenital absence or closure of a tubular structure or natural body orifice.

B) Stenosis - narrowing of a canal or opening.

C) Diverticula - abnormal outpouchings of hollow organs.

D) Heteroplasia - impaired differentiation of specific tissue types.

E) Dysplasia - abnormal Formation of the constituent tissue elements of an organ.

F) Hamartia - abnormal proportion of tissues within anatomical structures or the presence of persistent embryonic remnants in a mature organism.

4.4 Outline for Describing Internal Organs

During Practical Classes, when describing internal organs, it is advisable to follow this outline:

1. Name of the organ: in Ukrainian, Latin (Greek);

2. Topography of the organ:

a) holotopy

b) skeletotopy

c) syntopy.

3. Dimensions and mass of the organ.

4. CHARACTERISTICS OF THE External structure of the organ.

5. Characteristics of the Internal Structure of the organ (for hollow organs, the structure of the wall).

6. Functions of the organ.

7. Variants and developmental Anomalies of the organ.

4.5 General Characteristics of the Digestive System Structure

The digestive system can be viewed as a set of sequentially connected hollow organs (the digestive tract) beginning cranially with the oral fissure and ending caudally with the anus, along with large digestive glands located outside the digestive tract and connected to it by excretory ducts.

The digestive system includes the Oral Cavity with its contents, Pharynx, esophagus, stomach, small intestine consisting of the duodenum, jejunum, and ileum, Large Intestine consisting of the cecum, ascending colon, transverse colon, descending colon, sigmoid colon, and rectum, as well as the Salivary Glands, liver, and pancreas (Fig. 4.2).

The Organs of the digestive system perform food ingestion, taste assessment, mechanical and chemical Processing, nutrient absorption, and elimination of indigestible residues.

The oral cavity is the beginning of the digestive system. Here, food is chewed by the teeth, its taste properties are assessed by the tongue, and the food bolus is mixed and combined with saliva entering the oral cavity from the salivary glands. From the oral cavity, food passes into the pharynx.

Fig. 4.2 General layout of the digestive system:

1 - oral cavity;

2 - salivary glands;

3 - pharynx;

4 - esophagus;

5 - stomach;

6 - duodenum;

7 - jejunum;

8 - ileum;

9 - cecum;

10 - ascending colon;

11 - transverse colon;

12 - descending colon;

13 - sigmoid colon;

14 - rectum;

15 - liver;

16 - pancreas.

The pharynx is a funnel-shaped, slightly flattened, non-collapsing tube. The upper wall of the pharynx is fused with the Base of the Skull; at the border between the VI and VII cervical vertebrae, the pharynx narrows and transitions into the esophagus. The functions of the pharynx are diverse. It participates in Respiration: inhaled air passes from the Nasal cavity into the pharyngeal cavity and then into the trachea; food passes from the oral cavity through the pharynx into the esophagus, and its muscular coat actively takes part in the act of swallowing. Furthermore, acting as a resonator, the pharynx is of great importance for articulated speech, especially singing. Air enters the Middle ear cavity from the pharynx via the auditory tube, thereby maintaining equal pressure between the tympanic cavity and the external environment. The esophagus is a cylindrical muscular tube, slightly flattened anteroposteriorly, located between the pharynx and the stomach, with a length of 22-30 cm and a width of about 3 cm. The esophagus is lined with a mucous membrane, in the submucosa of which numerous proper glands are located; their secretion moistens the food bolus as it passes through the esophagus into the stomach. The movement of the food bolus along the esophagus is driven by wave-like contractions of its wall, where contractions of individual segments alternate with their relaxation. From the esophagus, food enters the stomach.

The stomach is a pouch-like dilation of the digestive tract, resembling a retort in shape. The mucous membrane of the stomach contains glands that produce mucus, Enzymes, and Hydrochloric acid. The stomach serves as a reservoir for ingested food, where it is mixed and partially digested under The Influence of gastric juice.

Under the action of enzymes, the food bolus in the stomach is converted into a partially digested semi-liquid mass (chyme), which then enters the duodenum. The mixing of chyme with gastric juice and its subsequent propulsion into the small intestine are carried out by the contraction of the Muscles of the stomach wall.

The small intestine occupies a large part of the Abdominal cavity AND lies in loops. It is the longest section of the digestive tract, reaching a length of 6-7 meters in a cadaver. The small intestine, in turn, is divided into the duodenum, jejunum, and ileum. It is here that the majority of food Digestion and the absorption of its contents take place.

The surface area of the inner lining of the small intestine is increased due to the presence of villi, circular folds, and microvilli on the apical surface of the mucosal epithelial cells (enterocytes).

Next, the undigested food mass enters the large intestine, where Water absorption, formation of fecal masses, and their elimination from the body take place. The large intestine is 1.2-1.5 m long and consists of the cecum with the vermiform Appendix, the colon (the latter includes the ascending, transverse, descending, and sigmoid colons), and the rectum.

General characteristics of the peritoneum. The peritoneum is a serous membrane that lines the abdominal cavity and completely or partially covers the internal organs located within this cavity. The peritoneum is formed by connective tissue covered with a single-layer squamous epithelium—mesothelium.

The peritoneum plays a multifaceted role in the body. First of all, its lubricated, slippery surface facilitates the constant physiological movement of mobile organs in the abdominal cavity (loops of the small intestine, transverse and sigmoid colons). In response to various mechanical, chemical, and thermal stimuli, the peritoneum secretes a sticky fibrinous exudate that ensures the adhesion (gluing) of damaged areas; through the formation of adhesions, it is often possible to localize the focus of an inflammatory process. The peritoneum of the greater omentum exhibits particularly high plasticity, as it consistently adheres to sites of peritoneal injury. Surgeons refer to the greater omentum as the "watchman of the abdominal cavity." Through peritoneal folds (mesenteries, folds, and certain ligaments), blood vessels and nerves approach the abdominal organs. Finally, the peritoneum and its derivatives play a significant role in fixing internal organs and participate in the production and utilization of serous fluid, as will be discussed in more detail below.

The peritoneum lining the walls of the abdominal cavity is called the parietal peritoneum. The peritoneum covering the organs is called the visceral peritoneum. The total surface area of the peritoneum in an adult averages 1.71 m². The visceral peritoneum primarily performs the function of transudation (production of serous fluid), as it is dominated by blood capillary plexuses. The resorption function (absorption of excess serous fluid) is predominantly performed by the parietal peritoneum in the region of the thoracoabdominal partition and the pelvic Diaphragm. In these areas, lymphatic capillary plexuses are located, into which absorption takes place. In other areas, the peritoneum performs both transudative and resorptive functions. The normal total volume of serous fluid in the peritoneal cavity is 20-25 ml.

When the equilibrium between transudation and exudation processes in the peritoneal cavity is disrupted, a significant amount of fluid accumulates (ascites), which occurs in chronic Diseases of the cardiovascular system and liver disorders.

The peritoneal cavity is a labyrinth formed by a set of slit-like spaces between the visceral and parietal leaves of the peritoneum. Thus, the concepts of the "abdominal cavity" and the "peritoneal cavity" are not synonymous, with the latter making up only a part of the former. In males, the peritoneal cavity is closed; in females, it communicates with the external environment through the abdominal openings of the uterine tubes, the uterine cavity, and the vagina.

Topographically and anatomically, the peritoneal cavity is divided into three tiers: upper, middle, and lower. The upper tier is bounded by the diaphragm superiorly and the transverse mesocolon inferiorly. The upper tier of the peritoneal cavity is subdivided into three somewhat isolated spaces: the hepatic, pre-gastric, and omental bursae.

The middle tier of the peritoneal cavity lies between the transverse mesocolon and the pelvic inlet. The middle tier comprises the right and left lateral channels, mesenteric sinuses, as well as recesses and fossae.

The lower tier corresponds to the pelvic cavity. As the peritoneum transitions from organ to organ in the lower tier, recesses are formed: the rectovesical pouch in males and two pouches in females—the rectouterine and vesicouterine pouches.

The relationship of the peritoneum to internal organs varies. Some organs are covered by the peritoneum on only one side, meaning they lie outside the peritoneum, retro- or extraperitoneally (duodenum, pancreas). Other organs are covered by the peritoneum on only 3 sides and are referred to as mesoperitoneal organs (liver, ascending and descending colons, middle third of the rectum).

The third group of organs is covered by the peritoneum on all sides and occupies an intraperitoneal position (stomach, Spleen, jejunum, ileum, cecum, transverse and sigmoid colons, upper third of the rectum, appendix).

During the transition of the peritoneum from the abdominal wall to an organ, or from organ to organ, peritoneal derivatives are formed, which include: 1) peritoneal ligaments, 2) mesenteries, 3) omenta, 4) folds.

Peritoneal ligaments are areas of the peritoneum at the sites where the parietal peritoneum transitions into the visceral peritoneum, from the abdominal wall to an organ, or where the visceral peritoneum transitions from one organ to another. Structurally, they are classified as single-layered or double-layered ligaments.

Vessels, nerves, glandular ducts, or adipose tissue may run between the layers of the visceral peritoneum (falciform ligament of the liver, triangular ligaments of the liver, hepatogastric ligament, hepatoduodenal ligament, gastrosplenic ligament, gastrophrenic ligament, gastrocolic ligament, broad ligament of the Uterus).

Mesenteries are double-layered ligaments formed during the transition of the peritoneum from the abdominal wall to an organ. Within the mesentery, between the peritoneal layers, there is connective tissue, adipose tissue, blood vessels, nerves, and Lymph Nodes. An organ with a mesentery is always positioned intraperitoneally relative to the peritoneum (with the exception of the Ovary, which has a mesentery but is not covered by the peritoneum) and is more or less mobile. Furthermore, the longer the mesentery, the more mobile the organ.

Omenta—the greater and lesser—are ligaments containing a significant amount of adipose tissue.

Large Glands of the Digestive System. The human liver is the largest gland, with an adult mass of 1.5–2.0 kg (2% of body weight). The liver is located in the abdominal cavity beneath the diaphragm, with the greater part lying on the right. To some extent, the shape of the liver resembles a mushroom cap; it features two surfaces (diaphragmatic and visceral) and two main lobes—right and left. On the visceral surface, furrows divide the liver into secondary lobes: the caudate and quadrate lobes. The transverse furrow, known as the porta hepatis (hepatic hilum), is also located on this surface. Entering the liver through the porta hepatis are the proper hepatic artery, nerves, and portal vein, while lymphatic vessels and the common hepatic duct (through which Bile flows out of the liver) exit from it.

As I.P. Pavlov aptly put it, the liver is the "main laboratory of the organism," and it is rightfully considered a multifunctional organ, performing the following vital functions in the human body:

1. Exocrine function. The liver produces bile, which then flows through a system of ducts into the duodenum, where it participates in digestion. Bile consists of Cholesterol, bilirubin, primary bile acids, Phospholipids, and electrolytes. The volume of bile and the concentration of bile acids in it increase when consuming fat-rich foods.

2. Detoxification function. Blood from the stomach, small intestine, and large intestine flows into the liver via the PORTAL VEIN SYSTEM. Along with nutrients and substances beneficial to the body, this blood contains toxic substances that have entered the body alimentarily or been produced during digestion. The neutralization of these substances takes place precisely in the liver, after which only purified blood enters the systemic Circulation.

3. During the prenatal period, Blood Cells are formed in the liver (hematopoietic function).

4. The liver participates in protein, lipid, and Carbohydrate Metabolism (it carries out the Synthesis and Breakdown of Glycogen, the synthesis of Blood Plasma Proteins, and so on).

The internal structure of the liver is characterized by a regular distribution of parenchyma, bile ducts, and blood vessels within it.

The structural and functional unit of the liver is the hepatic lobule; the earliest concepts of its structure emerged in the late 19th century.

The human liver consists of approximately 500,000 hepatic lobules. According to classical concepts, a hepatic lobule has the shape of a hexagonal prism, 1–1.5 mm in diameter and 1.5–2 mm in height. The lobule consists of hepatic cords radiating outward from the center, each formed by a double row of liver cells—hepatocytes. A central vein is located at the center of the lobule. From the periphery (at the angles of the hexagon), blood capillaries penetrate the hepatic lobule; these are continuations of interlobular Veins (from the portal vein system) and interlobular Arteries (from the proper hepatic artery system) running within the interlobular connective tissue septa.

Inside the lobule, the venous and arterial capillary networks continue into intralobular capillaries (sinusoids) located between the cords of liver cells. The intralobular capillaries of the liver differ from capillaries in other organs by their large diameter, and their walls closely adhere to The surface of the hepatocytes.

During Blood Circulation through the intralobular capillaries, toxic substances contained within the blood (which entered the liver via the portal vein system) are inactivated by specialized cells—stellate macrophages (Kupffer cells).

Vessels emerging from the capillary network empty into the central vein of the lobule, through which blood flows into the interlobular collecting veins. The latter subsequently form the hepatic veins, which empty into the INFERIOR VENA CAVA.

Bile canaliculi are located between the rows of hepatocytes forming the cords. These canaliculi lack walls of their own. Their walls are formed by the contacting surfaces of hepatocytes, which feature small depressions matching each other and together creating the lumen of the bile canaliculus (Fig. 4.3).

Bile canaliculi end blindly in the central regions of the lobule, while at the periphery they form interlobular bile ducts. Connecting successively, the latter transform into segmental, sectoral, lobar (right and left hepatic) ducts, and finally into the common hepatic duct.

Fig. 4.3 SCHEMATIC STRUCTURE OF a hepatic lobule:

1 — sinusoid; 2 — bile canaliculi; 3 — hepatocytes; 4 — Kupffer cells; 5 — central vein; 6 — interlobular septa; 7 — interlobular hepatic triads; 8 — interlobular artery; 9 — interlobular vein; 10 — interlobular bile duct.

Interlobular arteries, veins, and bile ducts run parallel to each other within the layers of interlobular connective tissue, forming the portal triads of the liver.

Due to advancements in liver surgery, larger structural units—hepatic segments—are distinguished in its architecture. The segmentation of the liver is based on the branching pattern of the portal vein. At the porta hepatis, the portal vein divides into right and left branches, which in turn give rise to second-order branches. The regions of the liver supplied by second-order veins, along with their corresponding Branches of the hepatic artery and bile ducts, are regarded as hepatic segments.

The most widely accepted division of the liver is into 8 segments: the left lobe of the liver, including the quadrate and caudate lobes, is subdivided into 4 segments, while the right lobe also comprises 4 segments. These segments are separated by relatively avascular planes, which serve as safe surgical pathways for partial liver resection in the presence of focal pathological processes.

Gallbladder and Extrahepatic bile ducts. In adults, the gallbladder is an elongated, pear-shaped reservoir comprising a fundus, body, and neck that gradually transitions into the cystic duct. The gallbladder functions as a bile storage reservoir. In a living individual, the gallbladder contacts the abdominal wall at the point defined by the angle formed between the costal margin and the lateral border of the rectus abdominis muscle.

Bile is secreted continuously by the liver, but encountering a sphincter on its path to the duodenum, it is diverted into the gallbladder. Here, the mucous membrane absorbs water, concentrating the bile approximately 5-fold.

The liver produces between 3 and 4.5 liters of preliminary bile per day, which is then concentrated in the gallbladder. When about 40 cm³ of bile accumulates in the gallbladder, its neural elements are stimulated, reflexively contracting the muscular layer and relaxing the spiral fold, thereby releasing a portion of bile into the duodenum. Bile is likewise expelled reflexively upon The entry of food into the duodenum.

The right and left hepatic ducts join at the porta hepatis to form the common hepatic duct. The latter unites with the cystic duct to form the common bile duct. In half of all cases, the common bile duct joins the pancreatic duct to form the hepatopancreatic ampulla, which is located within the major duodenal papilla. A specialized apparatus regulating the flow of BILE AND PANCREATIC juice into the small intestine is formed here—the sphincter of the hepatopancreatic ampulla, which is continuous with the sphincter of the common bile duct and the pancreatic sphincter.

Pancreas. Situated posterior to the stomach, adjacent to the duodenum, is the pancreas, the second-largest gland of the digestive system. It is a mixed-secretion gland (excreting its products both into the bloodstream and into the lumen of the digestive tract).

The pancreas lies horizontally in the retroperitoneal space behind the stomach, at the level of the XI–XII thoracic and I–II lumbar vertebrae. Three parts are distinguished in the pancreas: the head, body, and tail. In cross-section, the head and body of the pancreas are most frequently prismatic in shape, whereas the tail is oval. The length of the gland ranges from 12 to 22 cm, its width (height) from 3 to 9 cm, and its thickness is 2–3 cm. The weight of the gland is approximately 70–90 grams. It reaches its maximum weight during mature adulthood (25–40 years), after which it gradually decreases, dropping to 50–60 grams in old age.

The pancreas is surrounded by adipose tissue, The amount of which varies considerably. Most commonly, the fatty tissue is located only posteriorly and along the margins, while in overweight individuals it sometimes completely surrounds the gland. The splenic artery and vein run parallel to the gland within this adipose tissue.

The main pancreatic duct (duct of Wirsung) runs the entire length of the pancreas from the tail to the head and opens into the same orifice as the common bile duct on the major duodenal papilla in the descending part of the duodenum. The diameter of the pancreatic duct is 3–4.8 mm in the head, 2–3.5 mm in the body, and 0.9–2.4 mm in the tail. Some individuals possess an accessory pancreatic duct that opens onto the minor duodenal papilla.

The exocrine portion of the pancreas is a complex compound alveolar-tubular gland divided into lobules by very thin interlobular connective tissue septa extending from the capsule. The lobular structure of the gland is visible to the naked eye, with each lobule measuring approximately 5 mm. Closely packed bubble-like structures with a diameter of 100–150 µm—acini—lie within the lobules; their walls are formed by a single layer of secretory epithelial cells, the acinar cells.

An acinus together with its intercalated duct constitutes the structural and functional unit of the Exocrine Pancreas. Pancreatic secretions collect in the intercalated ducts, then flow into the interlobular ducts, and finally into the main pancreatic duct. Per day, the pancreas produces about 1500–2000 ml of secretion, which plays a major role in digestion.

The Endocrine portion of the pancreas is represented by isolated islets (pancreatic islets, islets of Langerhans). Each islet is a cluster of cellular elements lying between the exocrine acini as small, predominantly spherical formations ranging from 0.1 to 0.3 mm in diameter (rarely up to 1 mm), with no connection to the excretory ducts of the gland.

The number of pancreatic islets ranges from 200,000 to 2.5 million, and their total mass is 0.6 to 1.5 g, which accounts for approximately 3% of the total pancreatic mass. Although the islets are diffusely scattered throughout the gland, the vast majority are concentrated in the tail region.

Modern histochemical techniques allow the identification of 5 main cell types among the islet cells (B-cells, A-cells, D-cells, D₁-cells, and PP-cells).

B-cells constitute the bulk of the islet cells (about 70–75%), with the majority located in the center of the islets. These endocrinocytes synthesize the hormone Insulin. The primary physiological function of insulin is its ability to lower blood glucose levels. Simultaneously, it enhances protein and lipid synthesis while inhibiting The conversion of proteins and Lipids into CARBOHYDRATES, and it increases the permeability of cell membranes to glucose. Insulin deficiency leads to a drop in tissue glucose levels alongside a sharp increase in blood glucose, resulting in the development of Diabetes Mellitus.

A-cells produce Glucagon, which performs functions antagonistic to those of insulin.

D-cells synthesize Somatostatin. The latter inhibits the pituitary secretion of Growth Hormone and the synthesis of enzymes by acinar cells, as well as suppressing the release of insulin and glucagon.

PP-cells, located at the periphery of the islets, synthesize a polypeptide that stimulates the secretion of gastric juice and the exocrine pancreatic secretion.

MAIN STAGES OF the Development of the digestive system. The primitive form of the digestive system in vertebrates is a gut tube with openings at the anterior and posterior ends of the body.

During the 3rd to 4th weeks of Human embryonic development, the embryonic entoderm gives rise to the primary gut, which extends from the cranial to the caudal end of the body ventral to the notochord. Subsequently, the entoderm gives rise to the epithelium of the digestive tract (except for parts of the Oral Cavity and the anal region) as well as both small and large digestive glands.

The outer layers of the digestive tube develop from the splanchnopleure (visceropleure), which is the inner layer of the unsegmented portion of the mesoderm adjacent to the primitive gut.

At Early stages of embryonic development, both the cranial and caudal ends of the gut end blindly. By the 4th to 5th week of development, two depressions appear On the surface of the embryo's body in the head and caudal regions—the oral and anal pits. These gradually deepen until they meet the blind ends of the primitive gut and subsequently rupture, forming the oral and cloacal openings. Thus, the alimentary canal becomes continuous.

The primitive gut is subdivided into the head gut and the trunk gut. The head gut, in turn, is divided into the oral and pharyngeal parts. The trunk gut comprises three sections: the foregut, midgut, and hindgut.

The oral pit is lined with ectoderm-derived epithelium, which gives rise to a portion of the oral cavity. The pharyngeal gut, lined with endoderm-derived epithelium, develops into the deeper regions of the oral cavity and the pharynx.

The primitive mouth, or stomodeum, is a site of complex morphogenetic processes during which the Lips, Cheeks, tongue, Gums, teeth, and salivary glands are formed. The primitive oral cavity, appearing as a narrow slit, is bounded by five prominences: dorsally by the unpaired frontonasal prominence, laterally by the paired maxillary prominences, and ventrally by the paired mandibular prominences. These prominences not only bound the oral fissure but also form the walls of the oral cavity. The frontonasal prominence divides into three parts: an unpaired medial part and paired lateral (nasal) parts. The medial part of the frontonasal prominence gives rise to the central section of the upper lip and the primary palate. The lateral (nasal) parts form the nasal pits and, upon fusing with the maxillary prominences, close off the nasolacrimal duct. The maxillary prominences form the upper jaw, palate, cheeks, and lateral portions of the upper lip. The mandibular prominences fuse to form the lower jaw, chin, and lower lip. By the 7th week of development, the labiogingival laminae form the oral vestibule. The primitive oral fissure is initially very wide, reaching the external acoustic meatus laterally. As the embryo develops, the outer margins of the oral fissure fuse, forming the cheeks and narrowing the oral orifice.

The formation of the palate results in the separation of the nasal cavity. As a result, the initial segments of the digestive and respiratory tracts become segregated.

In the walls of the primitive pharynx, pharyngeal arches appear and pharyngeal (branchial) pouches are formed. Both structures contribute to the formation of many cranial bones, branchiomeric muscles, the greater part of the tongue, pharyngeal muscles, as well as the cartilages and muscles of the Larynx. The pharyngeal pouches also give rise to the primordia of The Thyroid Gland, Parathyroid glands, and Thymus. The respiratory primordium buds off from the primitive pharynx, eventually forming the trachea, Bronchi, and lungs.

The salivary glands develop from epithelial outgrowths of the ectoderm of the primitive oral cavity. The minor salivary glands begin to form during the 6th week. By the 7th and 8th weeks, the epithelium proliferates toward the ear region to establish the glandular tissue of the parotid gland; subsequently, epithelial cords grow toward the floor of the oral cavity, and through the fusion of smaller glands, the submandibular and sublingual salivary glands are formed.

The foregut gives rise to the esophagus, stomach, and part of the duodenum. The stomach primordium appears during the 4th week of intrauterine development as a dilation of the foregut.

On the ventral wall of the developing duodenum, two endodermal outpouchings appear between the layers of the ventral mesentery. These represent the primordia of The Liver and gallbladder. During the development of the liver, the anterior section of the mesentery transforms into the falciform ligament of the liver, while the posterior section becomes the lesser omentum.

The pancreas develops from two endodermal outpouchings of the primitive gut wall—a dorsal and a ventral one. The dorsal bud forms during the 4th week of embryonic development and gives rise to the body and tail of the gland. The ventral bud appears in the 5th week and forms the head of the gland. By the 7th week, both buds fuse. The pancreatic primordium subsequently invaginates between the layers of the dorsal mesentery.

The midgut constitutes the segment of the intestinal tube that communicates with the yolk sac in early embryogenesis. It gives rise to a portion of the duodenum, the jejunum, the ileum, and the initial segments of the large intestine—the cecum, the ascending colon, and part of the transverse colon.

The hindgut forms part of the transverse colon, the descending and sigmoid colons, and the rectum. Initially, the rectum opens into the cloaca in the embryo. As the cloaca is partitioned by the urorectal septum, the rectum becomes separated from the urogenital sinus. Concurrently with the regional division of the primitive gut, the walls of the digestive tube undergo differentiation, and the digestive glands are formed.

The digestive system begins to function during the prenatal period. By the 4th month, a greenish content known as meconium appears in the fetus's intestine. It consists of desquamated epithelial cells, mucus, bile, and substances contained in the Amniotic Fluid swallowed by the fetus. From this time onward, digestive enzymes are also present in the gut. By the end of the intrauterine period, the digestive system reaches a level of maturity capable of supporting the vital functions of the newborn. The structures responsible for breastfeeding—most notably the sucking reflex—are the most highly differentiated. This determines the specific Structural Features of the digestive organs in newborns.