Chapter 9: Digestive and excretory system (C5695502)

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Getting started

Apart from the molecules already discussed, another important dietary requirement are dietary minerals, which are inorganic elements required by living organisms. Inorganic means compounds not containing carbon and hydrogen, and therefore nitrogen and oxygen usually bonded to either carbon and/or hydrogen. Minerals typically exist as ions in the body, and are used to create electrochemical gradients, for use as cofactors to permit a protein to function, and for use in bone matrix.

1 Digestive system

The digestive system from the mouth and through the alimentary tract (aka gut) to the anus, including, going from top to bottom–

Mouth, where food is first acquired into the mouth, known as ingestion. Food is then digested by mechanically chewing. And chemically by hydrolysis, which as stated , breaks down all macromolecules, including carbohydrates (disaccharide, polysaccharide), protein, nucleic acids and fats (mostly triglyceride), but require an enzyme for the reaction to be meaningful. Disaccharide is ingested as sucrose (from sugarcane or sugar beet, composed up of monosaccharides glucose and fructose) and lactose (from milk, composed of the monosaccharides glucose and galactose). About 80% of absorbed dietary sugar is glucose; galactose and fructose make up the difference. Polysaccharide is ingested as glycogen (from animals such as meat and fish), starch and cellulose (from plants), and chitin (from the cell wall of fungi, see for more). Humans cannot digest cellulose (as mentioned ) or chitin. Mouth contains saliva, which contains salivary amylase, which is an enzyme that breaks down carbohydrates into disaccharides and trisaccharides (but not monosaccharides). Mouth also contains mucus, which helps soften the food and form into a ball, known as a bolus. Swallowing not only involves the first voluntary channeling back of food, but involuntary closure of the anterior windpipe by the cartilaginous epiglottis, thereby redirecting food down the [posterior] esophagus

Esophagus, which pushes bolus down to the stomach via peristaltic contraction. Digestive activity in esophagus is entirely mechanical, and not chemical

Stomach, which stores and breaks down food, before slowly releasing the semifluid mass, known as chyme, into the intestine. Stomach churns and physically breaks down food. Stomach also has a low pH, denaturing proteins. Acidic environment is also conducive to pepsin, which is an enzyme that degrades proteins into peptides. Pepsin, is a endopeptidase, which are proteases which break peptide bonds in the middle of the peptide (cf. breaking peptide bonds from end pieces), thereby being unable to create monomer amino acids. Types of cells found in stomach include:

  • Mucous cell, which secrete viscous alkaline mucus to protect stomach epithelium from highly acidic stomach environment and physical abrasion
  • Parietal cell, which secrete gastric acid (containing hydrochloric acid)
  • G cell, which secrete gastrin into the bloodstream. Gastrin is a peptide hormone which stimulates parietal cells to secrete gastric acid
  • Chief cell, which secrete pepsinogen, which as indicated by the “-ogen” suffix is a zymogen, expressed as an inactive enzyme precursor. Pepsinogen is activated by hydrochloric acid ([latex]HCl[/latex]), the optimal pH of pepsin being 2

Small intestine, consisting of:

  • Duodenum, which is involved mainly with digestion. Duodenum produces various hormones in response to the presence of nutrients or increased acidity in the duodenum, including secretin, cholecystokinin, and gastric inhibitory peptide, which are peptide hormones which increase insulin in blood, remembering from  that insulin lowers blood glucose. Pancreas secretes bicarbonate ion and digestive enzymes through the pancreatic duct, and through the Ampulla of Vater, into the duodenum. Bicarbonate ion offsets the acidity of, and caused by the stomach. Digestive enzymes secreted by pancreas [into the duodenum] include:
    • Pancreatic amylase, which is analogous to salivary amylase
    • Protease, including trypsin and chymotrypsin, which is used to further break down protein. These protease are secreted as their zymogens, which are trypsinogen and chymotrypsinogen respectively. Trypsinogen is activated by the intestinal brush border enzyme enteropeptidase, to form trypsin. Trypsin then activates the rest of the trypsinogen, as well as chymotrypsinogen, the latter which forms chymotrypsin. Trypsin and chymotrypsin are also endopeptidases (as just defined ). Pancreas also secretes an exopeptidase, which cleaves a single amino acid from the end
    • Nuclease, which breaks down nucleic acids
    • Lipase, which breaks down lipids into its constituent fatty acids, for example, triglyceride into monoglycerides and fatty acids. Because fat coagulates in the presence of the water soluble digestive tract, the surface area lipase can work on is reduced. Liver secretes bile stored in gallbladder, through the common bile duct, and through the Ampulla of Vater [per the pancreatic duct too], into the small intestine. Bile emulsifies fats. Fat emulsification is the physical “breaking” of fat into a liquid, as they normally don’t mix. Bile is amphipathic, thereby aggregating around fat droplets to form spherical micelle, which are fat-soluble facing in, and water-soluble facing out
  • Jejunum, ileum, which is involved with absorption, which is the diffusion of nutrients across the epithelial wall of the small intestine, into blood. As a result, jejunum and ileum is covered with small, finger-like projections, known as villi. The epithelium of villi is known as enterocyte, which have microvilli on their apical surface (side facing the lumen, as mentioned ). Both villi and microvilli function to increase surface area of the small intestine, and therefore the ability to uptake nutrient into the enterocyte. Because each villi contains many microvilli, it appears as a fuzzy fringe, known as brush border, which contain digestive enzymes to further break down content
    • Brush border enzymes further break down disaccharide and trisaccharide into monosaccharide for absorption. Glucose and galactose is absorbed by the sodium-glucose symporter (secondary active transport, mentioned ). Fructose is absorbed via facilitated diffusion. Because fructose must be absorbed along its concentration gradient, it is not absorbed as efficiently. Glucose, galactose and fructose transport out of the enterocyte across the basolateral membrane (facing the rear, as defined ), via facilitated diffusion, and absorbed into the capillary network [of each villus, discussed ]. In the liver, fructose and galactose is then metabolized into glucose in the liver. The liver stores unrequired glucose as glycogen
    • Exopeptidase (just mentioned ) is another brush border enzyme, which further breaks down peptides into amino acids. Amino acids, di- and tripeptides are absorbed by the sodium-dependent amino acid cotransporter (also, secondary active transport). Some amino acids can be absorbed via facilitated diffusion. Although some di- and tripeptides can be absorbed, brush border enzymes assure they are broken down into amino acid before transporting out of the basolateral membrane of the enterocyte
    • Micelle permits fat to be transported to the brush border [of enterocyte]. As fat is fat-soluble, it can be absorbed by simple diffusion through the enterocytelipid bilayer membrane. Once in the enterocyte, the products of fat digestion are reversed in the smooth endoplasmic reticulum, for example, monoglycerides and fatty acid reformed back into triglyceride. The fats (triglycerides, phospholipids, cholesterol) are packaged into chylomicrons, which are released via exocytosis from the basolateral membrane of the enterocyte, and absorbed into lacteals. Lacteal is a lymphatic capillary network in the villi [of small intestine], which drains into the thoracic duct, which as stated , in turn drains into the left jugular vein [of the neck], eventually draining into the heart. In blood, chylomicron transports its constituent lipids to the liver, and even adipose tissue

Large intestine (consisting of ascending colon, transverse colon, descending colon, sigmoid colon), which functions to absorb water. Accordingly, diarrhea, which is the condition of liquid excrete, is a symptom of problems in the large intestine. Due to loss of fluid, diarrhea causes dehydration. Various bacteria have established a mutualistic relationship with, and therefore resides, in the large intestine. Mutualism is a relationship in which both species derive a benefit. For example, E. coli benefits humans by producing vitamin K. Other intestinal bacteria produce Vitamin B12, thiamine, and riboflavin. In return, bacteria benefit from a stable supply of nutrients



Liver receives oxygen rich blood via the hepatic artery. Liver also has another supply of blood, but this time rich with nutrients glucose, amino acid, monosaccharide, from the capillary networks of the villi, via the hepatic portal vein. Liver has a type of capillary known as sinusoid, which have large pores which increase permeability of the wall to nutrients, including protein. Sinusoid can also store blood. Sinusoid walls have Kupffer cells, which are specialized macrophages, can phagocytize old or defective RBC, and bacteria that have entered bloodstream from digestive tract. Liver responds to hormones to regulate blood sugar levels, by carbohydrate metabolism, including gluconeogenesis. Although glucose absorption in the small intestine [as well as kidney] is by secondary active transport, most tissue uptake glucose by facilitated diffusion. As expressed , facilitated diffusion must be in the direction of concentration gradient, meaning if glucose concentration in blood is lower than glucose concentration in cells, glucose will not only be unable to be taken up by cells, but will actually diffuse out of the cell. Additionally, liver is involved with protein metabolism. Liver is also involved with lipid metabolism, including synthesizing bile from cholesterol, converting unrequired carbohydrates into fat, and synthesis of a bulk of the lipoproteins. Liver is also involved with detoxification.

2 Excretory system

Kidney excretes waste such as urea, ammonia, uric acid, and phosphate (when relating to bone, as mentioned ); salt and water balance between the blood and interstitium; and maintaining the pH of blood. The latter two functions are homeostasis, which means regulation of an internal environment. Kidneys are paired structures, meaning there are two, each approximately the size of a fist. Kidney can be divided into the renal cortex, and renal medulla. Analogous to adrenal cortex and adrenal medulla, renal cortex is the outside portion, and renal medulla is the central portion.

Nephron is the functional unit of kidney. Nephrons can be either cortical or juxtamedullary, the majority the former, but irrespective, all nephron have their glomerulus in the cortex.


Nephron can be divided into, in the order of movement:

  • Renal corpuscle, which is composed of glomerulus and Bowman’s capsule. Blood flows into glomerulus, which is a capillary network which filters blood through its fenestration, into Bowman’s capsule, to become to-be urine fluid known as filtrate. Fenestration (from Latin “fenestra” meaning “window”) are small pores, which are large enough to permit water, glucose, amino acid, ion, and some protein passage; but restrict larger and/or charged molecules, such as all cells, including RBC, due to size; and protein albumin due to both size and charge. Albumin is involved with regulating osmotic pressure. Glomerular blood pressure provides the driving force for filtration out into Bowman’s capsule, and therefore determines the amount of filtrate produced, known as glomerular filtration rate (GFR). Osmotic pressure also affects GFR. Filtrate then moves to the renal tubule
  • Renal tubule, which is composed of:
    • Proximal [convoluted] tubule (PCT), which is where the majority of the reabsorption, and secretion [of medication], occurs. Reabsorption is the absorption of solute by the epithelial cells of the tubule, from the to-be urine filtrate, back into blood. Secretion is the secretion of solute by the epithelial cells of the tubule, from the blood (evidently, not successfully filtrated by the glomerulus), into the to-be-urine filtrate. Reabsorption occurs to recycle wanted solute (such as glucose, amino acid, protein), and secretion occurs to remove unwanted solute. Analogous to enterocyte [of small intestine], proximal tubule utilizes sodium-glucose symporter protein to reabsorb glucose. As active transport doesn’t have concentration gradient requirements, [in a healthy person] all glucose should be reabsorbed. Analogous again to enterocyte, as amino acid can be transported either by cotransporter (active transport) or facilitated diffusion, as facilitated diffusion has concentration gradient requirements, only 99% of amino acid is reabsorbed. Protein (small, as bottlenecked by glomerulus) is reabsorbed by pinocytosis (which as expressed , is a type of active transport)
    • Loop of Henle, which is U-shaped, and in the juxtamedullary nephron (from Latin “juxta” meaning “near”), stretches deep into the medulla [whereas cortical do not, staying at the cortical level only]. Loop of Henle includes descending limb, thin ascending limb, and thick ascending limb. Descending limb has high permeability to water, but has low permeability to ions, causing water to passively diffuse out of the [to-be urine] filtrate. Both thin and thick ascending limb are impermeable to water, but permeable to ions. Thick ascending limb is distinct however, in that it can also reabsorb by active transport. Loop of Henle thus changes the osmolarity [until now, unchanged] of the filtrate [to the outside], concentrating filtrate going down the loop of Henle, but diluting filtrate going up the loop of Henle. Note the purpose of Loop of Henle is not to concentrate filtrate, but to create a concentration gradient in the medulla
    • Distal [convoluted] tubule (DCT), which is impermeable to water, but involved with further reabsorption, therefore further diluting filtrate. As mentioned , ADH acts on the distal tubule [in addition to the collecting duct] to permit water reabsorption (reiterated ). As also mentioned , aldosterone acts on the distal tubule, to reabsorb sodium, and secrete potassium

Collecting duct [system] receives from the distal tubule [of both cortical and juxtamedullary nephron], and stretches deep into the medulla. Like distal tubule, collecting duct is largely impermeable to water without the action of hormones. ADH creates pores in the [distal tubule, as well as the] collecting duct, to permit water to passively diffuse through the membrane, which can be memorized with the mnemonic that ADH stands for “always digging holes”. Because the medulla is concentrated due to the loop of Henle effect, water passively diffuses [out of the to-be filtrate into blood], thereby concentrating urine.

Although only present in juxtamedullary nephron, vasa recta is a capillary network, which traverses the length of the loop of Henle. As vasa recta mimics the structure of loop of Henle, it too has both a descending and ascending limb, and therefore concentrating effect (like loop of Henle). Although vasa recta runs in the same direction as loop of Henle, as the structures lie side-by-side, note that the descending limb [of Henle] and ascending limb [of vasa recta] run in opposite directions [thereby, furthermore, creating an additional concentrating effect].

Macula densa cells lining the distal tubule, monitor sodium and water concentration. If macula densa cells detect that water concentration is low, the efferent arteriole is constricted, which increases hydrostatic pressure in the glomerulus, which promotes increased filtration [thus filtrate created, and as defined earlier, known as GFR]. Efferent arteriole is the continuation of the glomerulus capillary network, following traversing the distance of Bowman’s capsule. Additionally, decreased water concentration (low blood pressure) encourages more reabsorption [of sodium], resulting in decreased concentration of sodium [in to-be-urine filtrate]. As just stated , decreased sodium is detected at the distal tubule. This causes juxtaglomerular cells to release the renin into blood.

As a part of the renin-angiotensin-aldosterone system (RAAS), renin is an enzyme which cleaves angiotensinogen to form angiotensin I, which is further converted to angiotensin II. Angiotensin II (a peptide hormone) constricts the efferent arteriole [further], to further increase [hydrostatic pressure, per  and thus] GFR. Angiotensin II also stimulates the release of aldosterone (which as just mentioned , increases reabsorption of sodium in the distal tubule).


“Oh, so many jokes made in class regarding the Zac Nephron- see what I did there Jamie?” Mandy giggled.

Urine leaves the medullary pyramids by the renal papilla, emptying into the renal calyces, renal pelvis, and then the tubular ureter into the bladder. Bladder stores urine before it is disposed by urination via the urethra.

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Biology - Pre-med science - MR. SHUM'S CLASSROOM