**6.2 Transport**

When contraction traverses at 50% occlusion, there is a higher tendency for the particles to undergo circulation; favoring mixing [56]. However, at 80% occlusion, the particles tend to under more of axial displacement and less of a radial displacement with no circulation; favoring transport. Particles positioned near the center were found to travel a longer distance in comparison to those near the wall. Such behavior reminds us the parabolic velocity profile in case of pressure driven flows in pipe. Previous studies corroborates with the understanding that the flows in occlusion regions tend to show a parabolic profile [44]. Rheological effects of the particle displacement suggest that the eccentricity of the particle trajectory for Newtonian fluid is more and undergoes a near complete circulation. Particle trajectory for dilatants showed formation of a complete circulation. For fluid having flow behavior index less than 1.0, following observations were made (1) particles tend to travel with higher velocity over longer distance and (2) particles showed more of a radial predominance. There were no significant changes in the flow developed by introducing the LLS; however, due to additional momentum along the axial direction they tend to suppress the radial displacement of the tracer leading to a more translocation. The transport has been linked to malabsorption of the nutrients and electrolyte concentration. Alternations in the intestinal transit can disturb the equilibrium of osmolality and intestinal absorption leading to diarrhea or constipation [57]. Knowledge of the intestinal transit of bolus is essential when design the drug. Orally administered drugs have to be tuned to the environmental conditions of the small intestine so that drug bioavailability can be maximized. Since the physical properties of the meal, such as viscosity can greatly influence the transport behavior, clinical preparation of the food can be administered to help manage the patient suffering from motility disorders.

#### **6.3 Frictional advantage**

Frictional effects of the intestinal wall have been attributed to a disadvantage when considering transport. By estimating the flow resistance, the author was able to assess the importance of the slowing down of the fluid flow and increase in the retention time of the fluid near the mucosa; providing more time to undergo chemical digestion and absorption [56]. The extent of friction offered by the intestine to fluid of different flow behavior index (n = 0.6, 1.0, and 1.4) suggests that the friction is highest for pseudo-plastics and decreases with increase in flow behavior index [56]. In addition to this, friction is found to be dependent on pressure gradient; showing increasing trend with increase in pressure. They are linearly related to the Reynolds number; higher the *Re* higher is the resistance. Since the friction is analyzed for a channel with smooth inner surface, we presume that the contribution resulting from plicae circulares would be much higher.

Friction is more at the occlusion center and drops significantly as one recedes away from occlusion center to the wave end. The friction due to mucosal layer of the intestine is a subject matter of interest to intestinal digestion. We may consider the problem similar to the flooding of the terrain occupied by numerous trees. At the flood end, where the fluid velocity is very high, the fluid particles tend to slow down upon interacting with the tree. Since the surface area of the tree is more, the effectiveness to slow the fluid particle is much higher. In physiology, such resistance to flow is provided by the intestinal folds of mucosa known as the plicae circulares or the valves of kerckring. The author speculates that these structures help in reducing the luminal transport and increase the time of retention of the fluid near the mucosa so as to allow for increased absorption of the nutrients. Depending on the flow regime, the flow may be highly agitated to flush the contents and allow for replenishment of the nutrient-rich contents. Such a behavior prevents the formation of trapped fluids and cause continuous flushing of the mucosa without stagnation. Such understanding is necessary to know the dynamics of nutrient transport near the intestinal mucosa and equilibration. While, stagnation of the acidic contents near the duodenum can have drastic impact on the mucosal layer leading to duodena ulceration.

#### **6.4 Power demands of peristalsis**

Contraction leading to flow is majorly determined by the muscular contractions of the circular and longitudinal muscle layers of the intestine. Although extramural pressure forces may contribute in the modulating the flow patterns, much of the mechanics is initiated and driven by the muscles. Efficiency to pump is defined as the ratio of energy due to pressure force to the energy spent by intestine through muscular contraction. The circular contractions are majorly known to cause the positive displacement of the fluid, and hence primarily responsible to transport. However, the LLS results in the developed for axial forces that are small in comparison to circular contraction and have minor contribution to efficiency at lower occlusions. LLS is advantageous at higher occlusion, where they primarily help to forcefully shrink the intestinal wall along the axial direction to concentrate more circular fibers. The energy spent on contraction can be reduce dramatically from 26.5 (CC along) to 22.5 units (CC with 0.65% LLS) approximately; a 15% reduction in energy spent by the intestinal motility to drive shear thickening fluid. However, in contrast to the above, we also identify that power advantage of LLS negatively correlates for shear-thinning fluid driven by CC with 0.65% LLS. Suggesting that rheology of the luminal contents shares some relation with the nature of LLS. This emphasizes an important observation as to whether such a correlation exists, and

**63**

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

energy it spends to perform the peristalsis.

**6.5 What is the optimal parameter space for contractility?**

if so, how does the intestine senses the fluid rheology? Although there are no direct sensors to detect the rheology or viscosity of the contents, we speculate that the gut may use an indirect mechanism to serve the purpose of assessing the rheology through stretch sensors. Since this sensor respond to distension, we also speculate that the difficulty to pump highly viscous fluid are reflected in the form of stretch. The concept of mechanical sensing of the stretch in the intestinal wall was observed by infusing a larger bolus of isotonic saline directly into the intestinal lumen [58]. The study reported that a controlled distension of the intestine activates a subset of vagal sensory neurons. Perhaps, the sensors data are relayed to the higher centers of the brain or through the local reflex to trigger certain feedback controls. Somehow, the intestine is aware of the trade-offs between the power demands of peristalsis at a certain occlusion against the percentage LLS. It may not prefer to contract at higher LLS for circular contraction of lower occlusion; since it would be non-economical. However, on the contrary, it is economical to contract at higher LLS for circular contraction of higher occlusion; an optimal strategy in conserving the amount of

The intestine has its own ability to perform muscular contraction to an extent that can be mapped onto a phase space (multidimensional space in which each state variable represented by an axis is constructed to specify the state of a physical system at a given point of time). To derive such plots for intestine, we resorted to literature reports related to the clinical observations of the intestinal motility during fed and fasted state, and, normal and pathology condition [22]. The parameters of interest are: incidence of propagatory versus stationary contractions per min, percentage incidence of antegrade and retrograde propagating waves, frequency of the wave, wave velocity in mm/s, and duration of MMC (interdigestive contractions) cycle. The ability of the intestine to perform digestion optimally depends on how well it coordinates with neurohormonal system. Eliciting segmental contraction on duodenal infusion of fat or hydrochloric acid requires that the contents are mixed well with the biliopancreatic secretions to cause buffering and emulsification. Such motility patterns are known to transform the fat into droplets which help providing more surface area for lipase binding to take place and perform the digestion [59]. Previous study by the author shows intestinal preference to digestion especially extent of mixing, and volume of mixing, and to-and-fro motion of contents [44]. Since the peristalsis provides sufficient shearing forces to help cause the droplet formation, we learn that some correlation exist between the motility and emulsification. Similarly, transport of the contents requires forceful expulsion of the contents by through muscular contraction of the intestinal wall; which demands generation of sufficient forces or right motility patterns. Studies indicate that the intestine utilizes the LLS at its advantage to perform forceful contractions; with peak LLS not exceeding 65% [18, 52]. The optimal choice of wavelength at which the shearing attains its maximum value is equal to the intestinal diameter (1 unit); higher wavelength (1.5 units) is inefficient [56]. Similarly occlusive contractions show two functions—mixing at lower occlusion and transport at higher occlusion. The choice of occlusion is dependent on whether the meal needs further processing or not.

**7. From wall shear and strain to influencing cell biology of the intestine**

As a result of the mechanical forces arising from muscular contraction (CC, LLS, due to muscularis mucosa) or due to luminal contents (distension during

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

*Digestive System - Recent Advances*

Frictional effects of the intestinal wall have been attributed to a disadvantage when considering transport. By estimating the flow resistance, the author was able to assess the importance of the slowing down of the fluid flow and increase in the retention time of the fluid near the mucosa; providing more time to undergo chemical digestion and absorption [56]. The extent of friction offered by the intestine to fluid of different flow behavior index (n = 0.6, 1.0, and 1.4) suggests that the friction is highest for pseudo-plastics and decreases with increase in flow behavior index [56]. In addition to this, friction is found to be dependent on pressure gradient; showing increasing trend with increase in pressure. They are linearly related to the Reynolds number; higher the *Re* higher is the resistance. Since the friction is analyzed for a channel with smooth inner surface, we presume that the contribution

Friction is more at the occlusion center and drops significantly as one recedes away from occlusion center to the wave end. The friction due to mucosal layer of the intestine is a subject matter of interest to intestinal digestion. We may consider the problem similar to the flooding of the terrain occupied by numerous trees. At the flood end, where the fluid velocity is very high, the fluid particles tend to slow down upon interacting with the tree. Since the surface area of the tree is more, the effectiveness to slow the fluid particle is much higher. In physiology, such resistance to flow is provided by the intestinal folds of mucosa known as the plicae circulares or the valves of kerckring. The author speculates that these structures help in reducing the luminal transport and increase the time of retention of the fluid near the mucosa so as to allow for increased absorption of the nutrients. Depending on the flow regime, the flow may be highly agitated to flush the contents and allow for replenishment of the nutrient-rich contents. Such a behavior prevents the formation of trapped fluids and cause continuous flushing of the mucosa without stagnation. Such understanding is necessary to know the dynamics of nutrient transport near the intestinal mucosa and equilibration. While, stagnation of the acidic contents near the duodenum can have drastic

Contraction leading to flow is majorly determined by the muscular contractions of the circular and longitudinal muscle layers of the intestine. Although extramural pressure forces may contribute in the modulating the flow patterns, much of the mechanics is initiated and driven by the muscles. Efficiency to pump is defined as the ratio of energy due to pressure force to the energy spent by intestine through muscular contraction. The circular contractions are majorly known to cause the positive displacement of the fluid, and hence primarily responsible to transport. However, the LLS results in the developed for axial forces that are small in comparison to circular contraction and have minor contribution to efficiency at lower occlusions. LLS is advantageous at higher occlusion, where they primarily help to forcefully shrink the intestinal wall along the axial direction to concentrate more circular fibers. The energy spent on contraction can be reduce dramatically from 26.5 (CC along) to 22.5 units (CC with 0.65% LLS) approximately; a 15% reduction in energy spent by the intestinal motility to drive shear thickening fluid. However, in contrast to the above, we also identify that power advantage of LLS negatively correlates for shear-thinning fluid driven by CC with 0.65% LLS. Suggesting that rheology of the luminal contents shares some relation with the nature of LLS. This emphasizes an important observation as to whether such a correlation exists, and

resulting from plicae circulares would be much higher.

impact on the mucosal layer leading to duodena ulceration.

**6.4 Power demands of peristalsis**

**6.3 Frictional advantage**

**62**

if so, how does the intestine senses the fluid rheology? Although there are no direct sensors to detect the rheology or viscosity of the contents, we speculate that the gut may use an indirect mechanism to serve the purpose of assessing the rheology through stretch sensors. Since this sensor respond to distension, we also speculate that the difficulty to pump highly viscous fluid are reflected in the form of stretch. The concept of mechanical sensing of the stretch in the intestinal wall was observed by infusing a larger bolus of isotonic saline directly into the intestinal lumen [58]. The study reported that a controlled distension of the intestine activates a subset of vagal sensory neurons. Perhaps, the sensors data are relayed to the higher centers of the brain or through the local reflex to trigger certain feedback controls. Somehow, the intestine is aware of the trade-offs between the power demands of peristalsis at a certain occlusion against the percentage LLS. It may not prefer to contract at higher LLS for circular contraction of lower occlusion; since it would be non-economical. However, on the contrary, it is economical to contract at higher LLS for circular contraction of higher occlusion; an optimal strategy in conserving the amount of energy it spends to perform the peristalsis.

#### **6.5 What is the optimal parameter space for contractility?**

The intestine has its own ability to perform muscular contraction to an extent that can be mapped onto a phase space (multidimensional space in which each state variable represented by an axis is constructed to specify the state of a physical system at a given point of time). To derive such plots for intestine, we resorted to literature reports related to the clinical observations of the intestinal motility during fed and fasted state, and, normal and pathology condition [22]. The parameters of interest are: incidence of propagatory versus stationary contractions per min, percentage incidence of antegrade and retrograde propagating waves, frequency of the wave, wave velocity in mm/s, and duration of MMC (interdigestive contractions) cycle. The ability of the intestine to perform digestion optimally depends on how well it coordinates with neurohormonal system. Eliciting segmental contraction on duodenal infusion of fat or hydrochloric acid requires that the contents are mixed well with the biliopancreatic secretions to cause buffering and emulsification. Such motility patterns are known to transform the fat into droplets which help providing more surface area for lipase binding to take place and perform the digestion [59]. Previous study by the author shows intestinal preference to digestion especially extent of mixing, and volume of mixing, and to-and-fro motion of contents [44]. Since the peristalsis provides sufficient shearing forces to help cause the droplet formation, we learn that some correlation exist between the motility and emulsification. Similarly, transport of the contents requires forceful expulsion of the contents by through muscular contraction of the intestinal wall; which demands generation of sufficient forces or right motility patterns. Studies indicate that the intestine utilizes the LLS at its advantage to perform forceful contractions; with peak LLS not exceeding 65% [18, 52]. The optimal choice of wavelength at which the shearing attains its maximum value is equal to the intestinal diameter (1 unit); higher wavelength (1.5 units) is inefficient [56]. Similarly occlusive contractions show two functions—mixing at lower occlusion and transport at higher occlusion. The choice of occlusion is dependent on whether the meal needs further processing or not.

### **7. From wall shear and strain to influencing cell biology of the intestine**

As a result of the mechanical forces arising from muscular contraction (CC, LLS, due to muscularis mucosa) or due to luminal contents (distension during

gasification), the intestinal tissues are remodeled in accordance to the nature of forces. The epithelial and non-epithelial cells undergo various types of mechanical forces during the physiology function. Contractions of the circular muscle leads to generation of a tangential force along the periphery (shear) and contraction of longitudinal muscle layer leads to axial force (shear). In reality, such contractions are highly irregular and occur in conjunction that varies in wave geometry and kinetics (velocity). Shear forces at the mucosal layer affect the villi structure which modulates the adsorptive function of the organ and strain in the intramural structure affect the tissue (intestinal wall) and its compliance. The responsive nature of the intestine comes from the fact that the intestinal walls have several mechanosensitive cell types that respond to various types of mechanical stimuli such as—epithelial enterochromaffin cells (ECL), enteric neuronal cells (intrinsic and extrinsic), smooth muscle cells, and interstitial cells of Cajal (ICC). These cells contain ion channels (stretch-activated ion channel) that respond to mechanical forces and in response to stimuli they generate ionic currents in the channel thereby affecting mechanotransduction process. In mechanotransduction, the mechanical forces such as shear, stretch, and pressure trigger a biochemical pathway (through conformation change) initiating the chain reaction (involving second messengers) to affect the gene expression, and protein synthesis. *In vitro* experiment involving the seeding of scaffolds with human umbilical vein endothelial cells (HUVECs) demonstrated that the mechanical stimuli provided in the form of a pulsatile shear stress (12 ± 4 dyne cm<sup>−</sup><sup>2</sup> ) leads to changes in the expression of the mechanosensitive genes (Pecam1, Enos) [60].

#### **8. Conclusion**

The digestive process of the intestine is complex and depends on multiple parameters such as rheology of food, chemical composition, motility pattern, and neurohormonal signaling. In this chapter, we have addressed the question as to how the mechanics play a key role in performing the disintegration of the partially digested food through shearing action of the peristalsis. Both circular and longitudinal contraction participate in the process in a way to optimally perform the digestion at ease; which otherwise would be uneconomical. LLS is advantageous when driving contents having shear thickening behavior, where the longitudinal shortening brings the circular muscles closer to reduce the tension in the individual fibers during peristalsis. LLS have no significant contribution in the development of the flows. In conclusion, biomechanical studies indicate that the flow is highly sensitive to the motility patterns (geometry and wave parameters), and in order to perform the digestion, the intestine elicits the right kinds of contraction to perform the physiological functions (such as preventing duodenal ulceration through segmental contraction, buffering of chyme in the duodenum, preventing duodenogastric reflux, and digestion of meal).

#### **9. Future scope**

Previous study involving the 3D computer simulations of the flow provided details of relevance to physiology. Contraction types analyzed so far include: (1) stationary contractions (contractions that close and open at a given location) (a) closure type, (b) Opening type\*, (c) multiple contractions, (d) cluster/repetitive contractions; (2) propulsive contractions (contractions moving in either direction) (a) antegrade type, (b) retrograde type, (c) multiple contractions, (d) short distance traveling contractions\*, (e) long distance traveling contractions\*; and (3)

**65**

**Conflict of interest**

**Author details**

Ravi Kant Avvari

India

*bulb, and (4) distal duodenum.*

**Figure 8.**

There are no conflicts of interest.

provided the original work is properly cited.

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

mixed (mixture of both stationary and propulsive contractions)\*. The contractions marked with \* could not be analyzed due to computational limitations. This gives us a huge opportunity to the biomechanical engineers to explore the mechanism as to how the motility leads to digestion. Literature suggests a compartmental model to describe the physiological relevance of antrum, pylorus and the duodenum (**Figure 8**). The jejunal and ileal segments still remain a mystery as to how they

Sasi Institute of Technology and Engineering, Tadepalligudem, Andhra Pradesh,

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium,

*Compartment model of the APD segment showing various segments (1) stomach, (2) pylorus, (3) duodenal* 

coordinate with each other and how they contribute to digestion.

\*Address all correspondence to: ravikant.iitk@gmail.com

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

#### **Figure 8.**

*Digestive System - Recent Advances*

stress (12 ± 4 dyne cm<sup>−</sup><sup>2</sup>

**8. Conclusion**

**9. Future scope**

genes (Pecam1, Enos) [60].

gasification), the intestinal tissues are remodeled in accordance to the nature of forces. The epithelial and non-epithelial cells undergo various types of mechanical forces during the physiology function. Contractions of the circular muscle leads to generation of a tangential force along the periphery (shear) and contraction of longitudinal muscle layer leads to axial force (shear). In reality, such contractions are highly irregular and occur in conjunction that varies in wave geometry and kinetics (velocity). Shear forces at the mucosal layer affect the villi structure which modulates the adsorptive function of the organ and strain in the intramural structure affect the tissue (intestinal wall) and its compliance. The responsive nature of the intestine comes from the fact that the intestinal walls have several mechanosensitive cell types that respond to various types of mechanical stimuli such as—epithelial enterochromaffin cells (ECL), enteric neuronal cells (intrinsic and extrinsic), smooth muscle cells, and interstitial cells of Cajal (ICC). These cells contain ion channels (stretch-activated ion channel) that respond to mechanical forces and in response to stimuli they generate ionic currents in the channel thereby affecting mechanotransduction process. In mechanotransduction, the mechanical forces such as shear, stretch, and pressure trigger a biochemical pathway (through conformation change) initiating the chain reaction (involving second messengers) to affect the gene expression, and protein synthesis. *In vitro* experiment involving the seeding of scaffolds with human umbilical vein endothelial cells (HUVECs) demonstrated that the mechanical stimuli provided in the form of a pulsatile shear

) leads to changes in the expression of the mechanosensitive

The digestive process of the intestine is complex and depends on multiple parameters such as rheology of food, chemical composition, motility pattern, and neurohormonal signaling. In this chapter, we have addressed the question as to how the mechanics play a key role in performing the disintegration of the partially digested food through shearing action of the peristalsis. Both circular and longitudinal contraction participate in the process in a way to optimally perform the digestion at ease; which otherwise would be uneconomical. LLS is advantageous when driving contents having shear thickening behavior, where the longitudinal shortening brings the circular muscles closer to reduce the tension in the individual fibers during peristalsis. LLS have no significant contribution in the development of the flows. In conclusion, biomechanical studies indicate that the flow is highly sensitive to the motility patterns (geometry and wave parameters), and in order to perform the digestion, the intestine elicits the right kinds of contraction to perform the physiological functions (such as preventing duodenal ulceration through segmental contraction, buffering of chyme in the duodenum, preventing duodenogastric reflux, and digestion of meal).

Previous study involving the 3D computer simulations of the flow provided details of relevance to physiology. Contraction types analyzed so far include: (1) stationary contractions (contractions that close and open at a given location) (a) closure type, (b) Opening type\*, (c) multiple contractions, (d) cluster/repetitive contractions; (2) propulsive contractions (contractions moving in either direction) (a) antegrade type, (b) retrograde type, (c) multiple contractions, (d) short distance traveling contractions\*, (e) long distance traveling contractions\*; and (3)

**64**

*Compartment model of the APD segment showing various segments (1) stomach, (2) pylorus, (3) duodenal bulb, and (4) distal duodenum.*

mixed (mixture of both stationary and propulsive contractions)\*. The contractions marked with \* could not be analyzed due to computational limitations. This gives us a huge opportunity to the biomechanical engineers to explore the mechanism as to how the motility leads to digestion. Literature suggests a compartmental model to describe the physiological relevance of antrum, pylorus and the duodenum (**Figure 8**). The jejunal and ileal segments still remain a mystery as to how they coordinate with each other and how they contribute to digestion.

### **Conflict of interest**

There are no conflicts of interest.

#### **Author details**

Ravi Kant Avvari Sasi Institute of Technology and Engineering, Tadepalligudem, Andhra Pradesh, India

\*Address all correspondence to: ravikant.iitk@gmail.com

© 2019 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
