Biomechanics of the Small Intestinal Contractions

*Ravi Kant Avvari*

#### **Abstract**

The small intestine is a part of the gastrointestinal segment comprising of the duodenum, jejunum, and ileum. They help to process the gastric contents for further digestion, which involves mixing with duodeno-biliary-pancreatic (DBP) secretions to facilitate the chemical digestion, and homogenization of the luminal contents through contractions of the circular and longitudinal smooth muscle fibers of the intestine. The contractions of these smooth muscle fibers develops the mechanical forces at the mucosal wall, which as a consequence, transfers its momentum to the underlying fluid to develop the fluid flows, suggesting relevance of mechanics in physiology. The resulting flows are what drive the digestion. Changes in contractility of wave shapes of circular and longitudinal smooth muscle contractions and fluid rheology are known to affect the digestive process through generation of various flow patterns that differ in luminal pressure, peak velocity, extent of shearing/ mixing, volume of mixing, and flow rate. Recent studies indicate that the digestive process can be very specific such as to cause lipid digestion through segmental contractions and transport by eliciting propagating contractions, suggesting that the intestine manages to digest a variety of food in an efficient manner by eliciting appropriate contractions.

**Keywords:** small intestine, small intestinal motility, peristalsis, circular contraction, local longitudinal shortening

### **1. Introduction**

The human small intestine is a part of the gastrointestinal tract which extends from end of the stomach to the inlet of large intestine. They form the visceral organ of our body which helps in processing the food at various levels such as mixing, digestion (mechanical grinding and chemical breakdown), and transport. They are arranged in a complex 3D manner, having numerous folds (convolutions) and flexures. The small intestine is functionally divided into duodenum, jejunum, and ileum; each of which has a specific physiology function to play in the digestion. They enable the digestion of meal in these compartments through coordinative effort. The small intestine elicits a complex series of motility patterns depending on the nature of meal to help (1) mixing with duodeno-biliary-pancreatic (DBP) secretions to facilitate the chemical digestion, (2) homogenization of the luminal contents of intestine, (3) regulation of pH in the duodenum, (4) mechanical disintegration, (5) absorption, and (6) transport. Since the generation of such motility patterns are highly variable and regulated by neurohormonal cues, the process of digestion has been a challenge, hitherto, to explore the mechanisms involved.

The mechanical relevance to digestion dates back to the classical study performed by Cannon on cat's intestine using X-ray [1]. The observations made by Cannon reports, *The constrictions causing the segmentation thoroughly mix the food and digestive juices, and bring the digested food into contact with the absorbing mechanisms* [1]. Even after a century has passed, the digestion still remains to be mystery; probably due to the multifaceted dimensions of the digestive process. In the recent past, there has been growing literature on the involvement of the mechanics in the digestion. Studies indicate that the mechanics of peristalsis is intertwined with physiological function of the intestine and still remains to be explored. The idea that the mechanics play a key role in the intestinal physiology is best described by Costa and Brookes which reads, *The discovery of the presence of multiple neurochemicals in the same nerve cells in specific combinations led to the concept of "chemical coding" and of "plurichemical transmission." The proposal that enteric reflexes are largely responsible for the propulsion of contents led to investigations of polarized reflex pathways and how these may be activated to generate the coordinated propulsive behavior of the intestine* [2]. We learn that the digestion system include highly complex organ which manages, *in house*, the enteric controls that are mediated through intramural reflexes (short and long range reflexes), and centralized control mediated through the central nervous system (involving higher nerves centers to process the information relating to gut sensing and relay through efferent nerves). While it has been a mystery for many decades as to how the digestion occurs in the gut, especially the mechanical breakdown, mixing and transiting over the long distance of the bowels, recent studies on mechanics are providing clues pertaining to the mechanisms that may contribute towards an understanding of the process involved at the level of mechanical digestion and their interaction with upstream and downstream players.

In this chapter, we present the current state of art in the area of intestinal biomechanics addressing various aspects of digestion through clinical, mathematical, and computational studies performed so far. This chapter is organized as follows: Section 2 describes the mechanics and physiology of the small intestine. Section 3 provides details as to how the small intestinal motility leads to the development of flows inside the lumen causing mixing and transport. The details of flow resulting from circular contraction are discussed in Section 4. In Section 5, the relevance of the local longitudinal shortening is explored followed by the physiological relevance of motility in Section 6. Since the nature of forces also affect the molecular biology of the cell, the basic principle behind the mechanotransduction is addressed in Section 7. The conclusions are drawn in Section 8 followed by the future scope of the work in Section 9.

### **2. Mechanophysiology of the small intestine and the small intestinal digestion**

#### **2.1 Anatomy**

The small intestine is the part of the gastrointestinal tract which connects to the stomach at one end through pylorus and the large intestine at the other end through ileocecal valve (**Figure 1**). The anatomy of the small intestine segments, that includes duodenum, jejunum, and ileum, are discussed in the following.

#### *2.1.1 Antrum*

The antrum is a distal part of the stomach which is highly muscular having a thickness of 5.1 ± 1.6 mm (depends on degree of distention of antrum [3]), which

**47**

*2.1.2 Pylorus*

**Figure 1.**

*2.1.3 Small intestine*

*2.1.3.1 Duodenum*

*Biomechanics of the Small Intestinal Contractions DOI: http://dx.doi.org/10.5772/intechopen.86539*

is higher than proximal stomach [4]. Its musculature helps the antral segment to undergo rigorous peristalsis to perform the grinding of the food. They also help

*Anatomical details of the stomach and duodenum showing three layers of muscles—oblique muscle layer* 

The pylorus (at L1 level or Lumber region 1) is a muscular tissue that connects the stomach at one end to the small intestine or more specifically the duodenum at the other end. Due to its musculature they contract radially to close or open the valve to cause the flow across the stomach and duodenum. It functions like a valve

The small intestine is a muscular and convoluted tube that extends from pyloric region to the ileocecal valve that connects to the large intestine. It is approximately 7 m long and 2–4 cm in diameters and divided into the duodenum, jejunum, and ileum.

Duodenum is the shortest segment of them all and is approximately 20–25 cm long and 2.5 cm in diameter. They are responsible to mix the chyme with DBP secretions, cause homogenization and pH transition from acidic to slightly alkaline. The process occurs inside the segment that is divided into four parts as follows: (1) the first part or par superior or duodenal bulb is about 5 cm long which begins its journey somewhere at the pylorus region and ends at the neck region of the gall bladder. Pars superior is the most movable region of the duodenum. (2) The second part or pars descendens is about 7–10 cm long and extends from the neck region of the gall bladder or L1 (lumbar region 1) to the upper border of L4 region. The common bile duct and the pancreatic duct together join and open at the major duodenal papilla into the medial side of this segment at approximately 7–10 cm distance from the pylorus. The minor duodenal papilla, if present, lies above the major duodenal papilla. (3) The third part or pars horizontalis is about 5–7.5 cm long and travels

regulating the gastric emptying and duodenogastric reflux (DGR).

*(OM), circular muscle layer (CM), and longitudinal muscle layer (LM).*

whereby it can regulate the flow of gastric content into the duodenum.

*Biomechanics of the Small Intestinal Contractions DOI: http://dx.doi.org/10.5772/intechopen.86539*

#### **Figure 1.**

*Digestive System - Recent Advances*

and downstream players.

**digestion**

**2.1 Anatomy**

*2.1.1 Antrum*

The mechanical relevance to digestion dates back to the classical study performed by Cannon on cat's intestine using X-ray [1]. The observations made by Cannon reports, *The constrictions causing the segmentation thoroughly mix the food and digestive juices, and bring the digested food into contact with the absorbing mechanisms* [1]. Even after a century has passed, the digestion still remains to be mystery; probably due to the multifaceted dimensions of the digestive process. In the recent past, there has been growing literature on the involvement of the mechanics in the digestion. Studies indicate that the mechanics of peristalsis is intertwined with physiological function of the intestine and still remains to be explored. The idea that the mechanics play a key role in the intestinal physiology is best described by Costa and Brookes which reads, *The discovery of the presence of multiple neurochemicals in the same nerve cells in specific combinations led to the concept of "chemical coding" and of "plurichemical transmission." The proposal that enteric reflexes are largely responsible for the propulsion of contents led to investigations of polarized reflex pathways and how these may be activated to generate the coordinated propulsive behavior of the intestine* [2]. We learn that the digestion system include highly complex organ which manages, *in house*, the enteric controls that are mediated through intramural reflexes (short and long range reflexes), and centralized control mediated through the central nervous system (involving higher nerves centers to process the information relating to gut sensing and relay through efferent nerves). While it has been a mystery for many decades as to how the digestion occurs in the gut, especially the mechanical breakdown, mixing and transiting over the long distance of the bowels, recent studies on mechanics are providing clues pertaining to the mechanisms that may contribute towards an understanding of the process involved at the level of mechanical digestion and their interaction with upstream

In this chapter, we present the current state of art in the area of intestinal biomechanics addressing various aspects of digestion through clinical, mathematical, and computational studies performed so far. This chapter is organized as follows: Section 2 describes the mechanics and physiology of the small intestine. Section 3 provides details as to how the small intestinal motility leads to the development of flows inside the lumen causing mixing and transport. The details of flow resulting from circular contraction are discussed in Section 4. In Section 5, the relevance of the local longitudinal shortening is explored followed by the physiological relevance of motility in Section 6. Since the nature of forces also affect the molecular biology of the cell, the basic principle behind the mechanotransduction is addressed in Section 7. The conclusions are drawn in Section 8 followed by the future scope of the work in Section 9.

**2. Mechanophysiology of the small intestine and the small intestinal** 

The small intestine is the part of the gastrointestinal tract which connects to the stomach at one end through pylorus and the large intestine at the other end through ileocecal valve (**Figure 1**). The anatomy of the small intestine segments, that includes duodenum, jejunum, and ileum, are discussed in the following.

The antrum is a distal part of the stomach which is highly muscular having a thickness of 5.1 ± 1.6 mm (depends on degree of distention of antrum [3]), which

**46**

*Anatomical details of the stomach and duodenum showing three layers of muscles—oblique muscle layer (OM), circular muscle layer (CM), and longitudinal muscle layer (LM).*

is higher than proximal stomach [4]. Its musculature helps the antral segment to undergo rigorous peristalsis to perform the grinding of the food. They also help regulating the gastric emptying and duodenogastric reflux (DGR).

#### *2.1.2 Pylorus*

The pylorus (at L1 level or Lumber region 1) is a muscular tissue that connects the stomach at one end to the small intestine or more specifically the duodenum at the other end. Due to its musculature they contract radially to close or open the valve to cause the flow across the stomach and duodenum. It functions like a valve whereby it can regulate the flow of gastric content into the duodenum.

#### *2.1.3 Small intestine*

The small intestine is a muscular and convoluted tube that extends from pyloric region to the ileocecal valve that connects to the large intestine. It is approximately 7 m long and 2–4 cm in diameters and divided into the duodenum, jejunum, and ileum.

#### *2.1.3.1 Duodenum*

Duodenum is the shortest segment of them all and is approximately 20–25 cm long and 2.5 cm in diameter. They are responsible to mix the chyme with DBP secretions, cause homogenization and pH transition from acidic to slightly alkaline. The process occurs inside the segment that is divided into four parts as follows: (1) the first part or par superior or duodenal bulb is about 5 cm long which begins its journey somewhere at the pylorus region and ends at the neck region of the gall bladder. Pars superior is the most movable region of the duodenum. (2) The second part or pars descendens is about 7–10 cm long and extends from the neck region of the gall bladder or L1 (lumbar region 1) to the upper border of L4 region. The common bile duct and the pancreatic duct together join and open at the major duodenal papilla into the medial side of this segment at approximately 7–10 cm distance from the pylorus. The minor duodenal papilla, if present, lies above the major duodenal papilla. (3) The third part or pars horizontalis is about 5–7.5 cm long and travels

#### *Digestive System - Recent Advances*

across the inferior vena cava and aorta above the upper border of the fourth lumbar region with the superior mesenteric vessels (the vein on the right and the artery on the left) on its front. (4) The fourth part or pars ascendens is about 2.5 cm long and continues to ascend toward the left side of the aorta. At its terminus, it abruptly transforms to a jejuna-like feature, where it forms the duodeno-jejunal flexure. The duodeno-jejunal flexure is connected to the superior mesenteric artery and celiac artery by suspensory muscles of the duodenum also known as the ligament of Treitz (a connective tissue), which marks the anatomical distinction between the duodenum and the jejunum.

#### *2.1.3.2 Jejunum*

It forms second part of the small intestine that is roughly 1.5–3.5 m (two-fifth of the small intestine) in length. They are attached to the posterior wall of the abdomen by the mesentery. The interior wall of the segment contains of numerous microscopic finger-like structures known as villi that help increase the surface area of absorption for the jejunum. Most of the nutrients are absorbed in this part of the small intestine. By the time the intestinal contents are emptied into the next segment (ileum), around 90% of all the available nutrients in the food has been absorbed. It also helps to shape the rheology of the digesta by absorbing about 90% of the secreted water, 6–8 l day<sup>−</sup><sup>1</sup> .

#### *2.1.3.3 Ileum*

It forms the last segment of the intestine that is roughly 2.5–3.5 m (three-fifth of the small intestine) in length and ends at the intraperitoneal pouch known as cecum (where undigested food settle down). The remaining parts of the nutrients that have passed through the jejunum are absorbed here (also absorbs vitamin B12 and bile acids). The segment contains numerous lymphoid follicles (forming Payer's patch; mainly function to survey and respond to pathogens). They are attached to the posterior wall of the abdomen by mesentery (giving flexibility to the bowels to adjust in the abdominal cavity during act of peristalsis and intestinal transit).

#### *2.1.4 Ileocecal valve (ileal ostium)*

The valve is a muscular tissue that separates the contents of the small intestine from those of the large intestine. They help in controlling the volume of flow occurring from the large intestine into the ileum and as a consequence of this, help in regulating the bacterial growth (involved in causing small intestinal bacterial overgrowth; SIBO) in the small intestine in conjunction with the small intestinal motility. It also helps in vitamin B12 absorption and collecting most of bile acid (terminal ileum) to replenish for the secreted bile for reuse (via entero hepatic circulation) [5]. They play a key role in preventing reflux of the bacteria-rich content from the large intestine into the small intestine; thus forming a barrier separating the two bowels.

#### **2.2 Generation of smooth muscle contractions: the precursor to luminal flows**

The intestinal musculature comprises of the smooth muscle fibers arranged in intertwined bundles; interconnected to the neighboring smooth muscle fibers through gap junctions. This enables two neighboring muscles to be electrical coupled. The gap junctions provide a way to propagate the electric potential (a wave of depolarization) from one fiber to the other, thereby spreading across adjacent segment of the intestine resulting in a muscular contraction (initiated as a consequence of

**49**

**Figure 2.**

reflex).

*Biomechanics of the Small Intestinal Contractions DOI: http://dx.doi.org/10.5772/intechopen.86539*

depolarization above threshold) to traverse the segment. In physiology, the membrane of the small intestinal smooth muscle (especially the myogenic cells) cell shows rhythmic changes in their electric potential which is referred to as the slow waves (resting membrane potential of −50 to −60 mV). Slow waves are the waves of partial depolarization of the membrane having the transmembrane potential of 5–15 mV. They help in nominal depolarization of the membrane, but do not initiate a muscle contraction. It is only during the condition when the membrane potential of smooth muscle cell cross the threshold level, an action potential is triggered causing contraction of the smooth muscle fiber. The event of spiking is known to occur at the crests of slow waves. To initiate the spike potential, it is necessary that smooth muscles of the segment are in the charged condition; having the neurotransmitters released in the vicinity by neurons. The neurotransmitters are released in response to a variety of stimuli such as neural signaling form higher center of the brain (mediated through vagus nerve), and distention-induced signaling (locally mediated through intramural

**2.3 Control of smooth muscle contractions through sensing**

Before we discuss the factors affecting APD motility, it is worth considering the sensory-motor integration of the intestinal segments (**Figure 2**). Generation of motility patterns is in some way hardwired to the sensors present and it is because of this reason that the APD segment can show a wide variation in its motility patterns. Little is known about the neurohormonal control, chemical control (pH [6], osmolarity [6, 7], lipid (also ileum) [8, 9], carbohydrates, and proteins), and other factors like size of bolus [10] and allergic responses through jejunal dysmotility [11]. They control muscles in the APD segment (also present in jejunal and ileal

*A cartoon representing mechanophysiology of the APD segment; indicating (1) the bolus undergoes disintegration due to grinding activity of the stomach, (2) smaller pieces of food, (3) food in its finely disintegrated form and yet to be mixed with DBP secretions, and (4 and 5) homogeneous mixture.*

#### *Biomechanics of the Small Intestinal Contractions DOI: http://dx.doi.org/10.5772/intechopen.86539*

*Digestive System - Recent Advances*

num and the jejunum.

of the secreted water, 6–8 l day<sup>−</sup><sup>1</sup>

*2.1.4 Ileocecal valve (ileal ostium)*

*2.1.3.2 Jejunum*

*2.1.3.3 Ileum*

across the inferior vena cava and aorta above the upper border of the fourth lumbar region with the superior mesenteric vessels (the vein on the right and the artery on the left) on its front. (4) The fourth part or pars ascendens is about 2.5 cm long and continues to ascend toward the left side of the aorta. At its terminus, it abruptly transforms to a jejuna-like feature, where it forms the duodeno-jejunal flexure. The duodeno-jejunal flexure is connected to the superior mesenteric artery and celiac artery by suspensory muscles of the duodenum also known as the ligament of Treitz (a connective tissue), which marks the anatomical distinction between the duode-

It forms second part of the small intestine that is roughly 1.5–3.5 m (two-fifth of the small intestine) in length. They are attached to the posterior wall of the abdomen by the mesentery. The interior wall of the segment contains of numerous microscopic finger-like structures known as villi that help increase the surface area of absorption for the jejunum. Most of the nutrients are absorbed in this part of the small intestine. By the time the intestinal contents are emptied into the next segment (ileum), around 90% of all the available nutrients in the food has been absorbed. It also helps to shape the rheology of the digesta by absorbing about 90%

It forms the last segment of the intestine that is roughly 2.5–3.5 m (three-fifth of the small intestine) in length and ends at the intraperitoneal pouch known as cecum (where undigested food settle down). The remaining parts of the nutrients that have passed through the jejunum are absorbed here (also absorbs vitamin B12 and bile acids). The segment contains numerous lymphoid follicles (forming Payer's patch; mainly function to survey and respond to pathogens). They are attached to the posterior wall of the abdomen by mesentery (giving flexibility to the bowels to adjust in the abdominal cavity during act of peristalsis and intestinal transit).

The valve is a muscular tissue that separates the contents of the small intestine from those of the large intestine. They help in controlling the volume of flow occurring from the large intestine into the ileum and as a consequence of this, help in regulating the bacterial growth (involved in causing small intestinal bacterial overgrowth; SIBO) in the small intestine in conjunction with the small intestinal motility. It also helps in vitamin B12 absorption and collecting most of bile acid (terminal ileum) to replenish for the secreted bile for reuse (via entero hepatic circulation) [5]. They play a key role in preventing reflux of the bacteria-rich content from the large intestine into the small intestine; thus forming a barrier separating the two bowels.

**2.2 Generation of smooth muscle contractions: the precursor to luminal flows**

The intestinal musculature comprises of the smooth muscle fibers arranged in intertwined bundles; interconnected to the neighboring smooth muscle fibers through gap junctions. This enables two neighboring muscles to be electrical coupled. The gap junctions provide a way to propagate the electric potential (a wave of depolarization) from one fiber to the other, thereby spreading across adjacent segment of the intestine resulting in a muscular contraction (initiated as a consequence of

.

**48**

depolarization above threshold) to traverse the segment. In physiology, the membrane of the small intestinal smooth muscle (especially the myogenic cells) cell shows rhythmic changes in their electric potential which is referred to as the slow waves (resting membrane potential of −50 to −60 mV). Slow waves are the waves of partial depolarization of the membrane having the transmembrane potential of 5–15 mV. They help in nominal depolarization of the membrane, but do not initiate a muscle contraction. It is only during the condition when the membrane potential of smooth muscle cell cross the threshold level, an action potential is triggered causing contraction of the smooth muscle fiber. The event of spiking is known to occur at the crests of slow waves. To initiate the spike potential, it is necessary that smooth muscles of the segment are in the charged condition; having the neurotransmitters released in the vicinity by neurons. The neurotransmitters are released in response to a variety of stimuli such as neural signaling form higher center of the brain (mediated through vagus nerve), and distention-induced signaling (locally mediated through intramural reflex).
