**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

#### **Figure 2.**

*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.*

segments), which help in regulation of motor patterns mediated by some kind of sensor mechanism. To support the relevance of sensory-motor integrity, let us consider the report by Peter Holzer who suggested that prevention of acid damage to the mucosal tissues are carried out by an *elaborate network of acid-governed mechanisms* that help in protecting the tissue from acidosis and maintaining homeostasis [12]. The pH distribution of the gut lumen follows a particular trend, having lowest at the stomach (pH 1–3) and then goes on increasing from duodenum (pH 1.7–5 at proximal duodenum and pH 5–6 at distal duodenum) to terminal ileum (pH 7–9) [13, 14]. On exposure of the duodenum to acidic contents they stimulate various defense mechanisms which include increase in mucosal secretions, bicarbonate secretions, and blood flow. Together with this, hormones also play a major role in acid secretion at the stomach which in turn may contribute to the overall homeostasis. The pylorus also plays its role in regulatory mechanisms which run across the terminal stomach to proximal duodenum and shares the neural tracts and the circular muscle layer with antrum and duodenum. Besides being a muscular tissue, it also has sensors embedded within its mucosal layers, which are involved in some control related activities that are relayed through enteric nervous system or local mediated reflex pathway. Digestive processes are driven by the peristalsis motion that grinds the food rigorously in the antrum, so as to grind into small pieces less than 3 mm, so that they can escape the pyloric channel. The remaining part of the digestion is driven by the intestinal peristalsis which together with chemical secretions facilitates the process of mechanical and chemical degradation of the chyme into macromolecules and further down to simpler molecules so they can be absorbed (**Figure 2**)—steps 3–5.

#### **2.4 Coordination among the small intestinal segments**

The APD contractions are very much time synchronized and work in coordination. These neurally activated contractions push the luminal contents by transferring their momentum, which helps to facilitate mixing, grinding, and transporting of the food. Two kinds of pumping action take place here, one at the antral side and the other in duodenum which tries to push their contents to the other side. It is not well understood on how these two motor actions play their part in causing the transport, i.e., either gastric emptying or reflux of duodenal content back into the stomach. However, from a mechanics point of view, we know that the flow would result in emptying when the pressure at the antral side is higher than duodenal side (**Figure 3**). However, a reverse situation can exist, i.e., reflux when duodenal pressure becomes higher than antral pressure leading to a disease condition known as the duodenogastric reflux (DGR). The mechanism by which the transport across the pylorus occurs is not clear; however, it is known that the transport occurs by developing two kinds of pressure waves via pressure pump (common cavity pressure wave) and peristaltic activity [15]. Multiple studies have been performed for estimating gastric emptying in relation to the generation of intragastric pressures using manometric studies. Though little is known about the relationship between the pump mechanism (gastric pumping) and the coordinative muscle contractions, few researchers have reported that the process of gastric emptying is observed only during those occasions when the antral pressure (Pa) is higher than the duodenal pressure (Pd). Study also indicates that the base line pressure or the common cavity pressure is the major determinant of gastric emptying (GE) rather than the antral contraction-induced emptying [15]. This idea is supported by literature which demonstrates that alternations in pressure inside the proximal stomach correlate well with the varying rates of gastric emptying of different liquid meals [16].

**51**

eventually developing the flow.

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

contraction patterns.

**Figure 3.**

ing pyloric channel.

The local generation of high pressure and appearance of anterograde and retrograde flow patterns suggest that the geometry is closely linked to the way emptying proceeds. In addition to the complexity present in gastric emptying predictions, the gut works by intelligently sensing the food content and accordingly modulating the

*A cartoon diagram depicting the antrum, pylorus, and the duodenal (APD) segments, respectively. The dashed line represents the APD axis, while the blue colored plot shows pressure profile along the axial direction* 

*indicating higher antral pressure (Pa) in comparison to lower duodenal pressure (Pd).*

The coordination is established through neural feedback means; e.g., enterogastric, ileogastric, intestino-intestinal reflex, and vago-vagal reflexes. One of the well-known reflexes is the ileal brake. The ileal brake refers to the ability of the ileal segments to modulate the motility patterns upon exposure to the nutrients such as lipid through enteric reflex [17]. The intestinal segments communicate with each other through such reflex in process to regulate the digestive process such as regulating the flow at which the gastric contents enter into the duodenum by suppress-

A fluid is a substance which continually deforms under the application of a shear force. When an external force is applied to a solid object it undergoes whole body translation; whereas, fluid undergoes both translation and deformation. Transport of fluid can be better appreciated by considering an example of the flow through a cylindrical pipe, also referred to as the Hagen-Poiseuille flow (flow in a cylindrical pipe). Applying a relatively higher pressure force at left end of the tube, in comparison to right end, sets up a pressure gradient along the length of the tube. As a result of this, the fluid tends to move down the pressure gradient only if it has overcome the viscous resistance. In the case of viscous flow the fluid eventually gains inertia and reaches a steady state when the axial velocity profile becomes parabolic. In a similar fashion, we can draw some parallels between the Hagen-Poiseuille flows to those of intestinal flows. In physiological scenario, as the contraction (i.e., the circular constriction that appear around the periphery) propagate thorough the small intestinal segment, it imparts a part of the momentum to the fluid underneath, which as a consequence of having gained the momentum can now hit the neighboring fluid particle and transmits a part of its momentum;

**3. From small intestinal motility to flows to the digestion**

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

**Figure 3.**

*Digestive System - Recent Advances*

absorbed (**Figure 2**)—steps 3–5.

**2.4 Coordination among the small intestinal segments**

The APD contractions are very much time synchronized and work in coordination. These neurally activated contractions push the luminal contents by transferring their momentum, which helps to facilitate mixing, grinding, and transporting of the food. Two kinds of pumping action take place here, one at the antral side and the other in duodenum which tries to push their contents to the other side. It is not well understood on how these two motor actions play their part in causing the transport, i.e., either gastric emptying or reflux of duodenal content back into the stomach. However, from a mechanics point of view, we know that the flow would result in emptying when the pressure at the antral side is higher than duodenal side (**Figure 3**). However, a reverse situation can exist, i.e., reflux when duodenal pressure becomes higher than antral pressure leading to a disease condition known as the duodenogastric reflux (DGR). The mechanism by which the transport across the pylorus occurs is not clear; however, it is known that the transport occurs by developing two kinds of pressure waves via pressure pump (common cavity pressure wave) and peristaltic activity [15]. Multiple studies have been performed for estimating gastric emptying in relation to the generation of intragastric pressures using manometric studies. Though little is known about the relationship between the pump mechanism (gastric pumping) and the coordinative muscle contractions, few researchers have reported that the process of gastric emptying is observed only during those occasions when the antral pressure (Pa) is higher than the duodenal pressure (Pd). Study also indicates that the base line pressure or the common cavity pressure is the major determinant of gastric emptying (GE) rather than the antral contraction-induced emptying [15]. This idea is supported by literature which demonstrates that alternations in pressure inside the proximal stomach correlate well with the varying rates of gastric emptying of different liquid meals [16].

segments), which help in regulation of motor patterns mediated by some kind of sensor mechanism. To support the relevance of sensory-motor integrity, let us consider the report by Peter Holzer who suggested that prevention of acid damage to the mucosal tissues are carried out by an *elaborate network of acid-governed mechanisms* that help in protecting the tissue from acidosis and maintaining homeostasis [12]. The pH distribution of the gut lumen follows a particular trend, having lowest at the stomach (pH 1–3) and then goes on increasing from duodenum (pH 1.7–5 at proximal duodenum and pH 5–6 at distal duodenum) to terminal ileum (pH 7–9) [13, 14]. On exposure of the duodenum to acidic contents they stimulate various defense mechanisms which include increase in mucosal secretions, bicarbonate secretions, and blood flow. Together with this, hormones also play a major role in acid secretion at the stomach which in turn may contribute to the overall homeostasis. The pylorus also plays its role in regulatory mechanisms which run across the terminal stomach to proximal duodenum and shares the neural tracts and the circular muscle layer with antrum and duodenum. Besides being a muscular tissue, it also has sensors embedded within its mucosal layers, which are involved in some control related activities that are relayed through enteric nervous system or local mediated reflex pathway. Digestive processes are driven by the peristalsis motion that grinds the food rigorously in the antrum, so as to grind into small pieces less than 3 mm, so that they can escape the pyloric channel. The remaining part of the digestion is driven by the intestinal peristalsis which together with chemical secretions facilitates the process of mechanical and chemical degradation of the chyme into macromolecules and further down to simpler molecules so they can be

**50**

*A cartoon diagram depicting the antrum, pylorus, and the duodenal (APD) segments, respectively. The dashed line represents the APD axis, while the blue colored plot shows pressure profile along the axial direction indicating higher antral pressure (Pa) in comparison to lower duodenal pressure (Pd).*

The local generation of high pressure and appearance of anterograde and retrograde flow patterns suggest that the geometry is closely linked to the way emptying proceeds. In addition to the complexity present in gastric emptying predictions, the gut works by intelligently sensing the food content and accordingly modulating the contraction patterns.

The coordination is established through neural feedback means; e.g., enterogastric, ileogastric, intestino-intestinal reflex, and vago-vagal reflexes. One of the well-known reflexes is the ileal brake. The ileal brake refers to the ability of the ileal segments to modulate the motility patterns upon exposure to the nutrients such as lipid through enteric reflex [17]. The intestinal segments communicate with each other through such reflex in process to regulate the digestive process such as regulating the flow at which the gastric contents enter into the duodenum by suppressing pyloric channel.
