The Digestive System; The Excretory System

Question 1

Basic anatomy of the digestive tract

Answer 1

Mouth, esophagus, stomach, small intestine (duodenum, ileum, jejunum), large intestine (ascending/transverse/descending/sigmoid colons), rectum, anus

Question 2

a-amylase

Answer 2

Digestion begins in the mouth with a-amylase in saliva.

Starch is the main carbohydrate in the human diet– a-amylase begins breaking down the long, straight chains of starch into polysaccharides.

Question 3

Esophagus

Answer 3

A bolus of food gets pushed into the esophagus by swallowing, and then moved down the esophagus via peristaltic action, a wave motion performed by smooth muscle.

Note that no digestion occurs in the esophagus.

Question 4

Stomach and functions

Answer 4

The stomach is a very flexible pouch that mixes and stores food, reducing it to a semifluid mass called chyme.

Has 2 exocrine glands, which have gastric pits.

Another important function is to begin protein digestion with the enzyme pepsin.

Low pH assists this process by denaturing the proteins. The low pH also helps to kill ingested bacteria. A full stomach has a pH of 2.

Question 5

4 major cell types in the stomach

Answer 5

Mucous, chief (peptic), parietal (oxynytic), and G cells.

Question 6

Mucous cells

Answer 6

Contain rough ER and Golgi to synthesize and secrete mucus. Some also secrete a small amount of pepsinogen. Line the stomach wall and the necks of the endocrine glands.

Mucus- composed of sticky glycoprotein + electrolytes- lubricates the stomach wall so food can slide along its surface without causing damage. Protects the epithelial lining from the acidic environment of the stomach.

Question 7

Chief cells

Answer 7

Found deep in the exocrine glands. Secrete pepsinogen.

Question 8

Pepsinogen

Answer 8

Zymogen precursor to pepsin. Secreted by chief cells (and a little by mucous cells) in the stomach. Pepsinogen is activated to pepsin by the low pH in the stomach.

Once activated, pepsin begins protein digestion.

Question 9

Parietal cells

Answer 9

Found in the exocrine glands of the stomach. Parietal cells secrete HCl, which requires a lot of energy. They also secrete intrinsic factor, which helps the ileum absorb B12.

Net result is to lower the pH of the stomach and raise the pH of the blood.

Question 10

HCl

Answer 10

Secreted by parietal cells in the stomach.

(Process involves CO2, which makes carbonic acid inside the cell. The hydrogen from the carbonic acid is expelled to the lumen side of the cell, while the bicarbonate ion is expelled to the interstitial fluid side. )

Net result is to lower the pH of the stomach and raise the pH of the blood.

Question 11

G cells

Answer 11

Secrete gastrin into the interstitium.

Question 12

Gastrin

Answer 12

A large peptide hormone secreted by the G cells of the stomach. Absorbed into the blood and stimulates parietal cells to secrete HCl.

Question 13

Major hormones of the stomach

Answer 13

The major hormones affecting the secretion of the stomach juices are acetylcholine, gastrin, and histamine.

Acetylcholine increases the secretion of all cell types.

Gastrin and histamine mainly increase HCl secretion.

Question 14

Small intestine

Answer 14

Where 90% of digestion and absorption occurs. Has a pH of 6-7.

Divided into:
Duodenum (digestion - pH of 6 due to bicarbonate ion secreted by the pancreas)
Jejunum, ileum (absorption)

The wall of the small intestine’s outermost layer contains fingerlike projections called villi, which increase surface area of the intestinal wall, allowing for greater digestion and absoprtion.

Question 15

Lacteal

Answer 15

Within each villus (singular of villi) is a capillary network and a lymph vessel, called a lacteal.

Nutrients absorbed through the wall of the small intestine pass into the capillary network and the lacteal.

Question 16

Microvilli/brush border

Answer 16

On the apical (lumen side) surface of the cells of each villus are much smaller finger-like projections called microbilli. These increase the surface area of the intestinal wall still further.

Under a light microscope the microvilli appear as a fuzzy covering, called the brush border. The brush border contains membrane bound digestive enzymes, such as:

• carbohydrate-digesting enzymes (dextrinase, maltase, sucrase, lactase),
• protein digesting enzymes (peptidases),
• and nucleotide-digesting enzymes (nucleosidases).

Question 17

Goblet cells

Answer 17

Some of the epithelial cells of the small intestine. Secrete mucus to lubricate the intestine and help protect the brush border from mechanical and chemical damage.

Question 18

Lysosyme

Answer 18

Helps regulate the bacteria within the intestine. Secreted by intestinal exocrine glands called the crypts of Lieberkuhn.

Question 19

Pancreas

Answer 19

Semifluid chyme is squeezed out of the stomach through the pyloric sphincter and into the duodenum. The fluid in the duodenum has a pH of 6 due to bicarbonate ion secreted by the pancreas.

Also acts as an exocrine gland, releasing enzymes into the duodenum.

Question 20

Major pancreatic enzymes

Answer 20

Trypsin, chemotrypsin, pancreatic amylase, lipase, ribonuclease, deoxyribonuclease

Note that all enzymes are released as zymogens.

Typsin is activated by an enzyme called enterokinase, located in the brush border. Activated trypsin then activates the other enzymes.

Question 21

Trypsin and chemotrypsin

Answer 21

Degrade proteins into small polypeptides. Most proteins reach the brush border as small polypeptides. Here they are reduced to di- and tri-peptides before being absorbed by enterocytes and all are reduced to amino acids.

Question 22

Pancreatic amylase

Answer 22

Much more powerful than salivary amylase. Hydrolyzes polysaccharides to di- and tri-saccharides.

Degrades nearly all the carbohydrates from the chyme into oligosaccharides. The brush border enzymes finish degrading these polymers into their respective monosaccharides before they are absorbed.

Question 23

Lipase

Answer 23

Degrades fat, particularly triglycerides. However- fat clumps together (this is why bile is necessary).

Question 24

Bile

Answer 24

Produced by the liver, stored in the gallbladder. Emulsifies fat (breaks it up into small pieces without changing it chemically). This increases fat’s surface area, allowing lipase to degrade it further (into fatty acids and monoglycerides). These products are shuttled to the brush border in bile micelles, and then absorbed by enterocytes.

Much bile is reabsorbed by the small intestine and transported back to the liver.

Question 25

Large intestine

Answer 25

(“Whenever you get a large intestine question on the MCAT, think water reabsorption.”)

AKA the colon. Has 4 parts: ascending, transverse, descending, and sigmoid colons. The major functions are water absorption and electrolyte absorption. When these fail, you get diarrhea. The large intestine also contains E. coli, a bacteria which produces vitamin K, B12, thiamin, and riboflavin.

There is a mutualistic symbiosis between humans and bacteria in the large intestine. Bacteria get our leftovers, we get certain vitamins from them. We have more bacteria in our gut than we have cells. They are not good anywhere else– but in the large intestine, are NECESSARY for survival.

Question 26

Digestion

Answer 26

The body eats to gain energy in the form of food. The digestive system breaks down food so it can be absorbed in the body. One problem is that the food may move too fast through the tract and come out undigested. The stomach stores food and releases small amounts at a time to be digested and absorbed by the intestine, allowing the body to eat a large amount of food at once and take a long time to digest it.

One of the jobs of the many gastrointestinal hormones is to regulate this process.

Question 27

Glycogenesis

Answer 27

The formation of glycogen. Occurs in the liver. The liver absorbs all carbohydrates and converts nearly all the galactose and fructose into glucose, and then into glycogen for storage.

Question 28

Glycogenolysis

Answer 28

When the blood glucose level decreases, glycogenolysis takes place in the liver, and glucose is returned to the blood.

Question 29

Carbohydrates

Answer 29

Are processed by the liver. Broken down into galactose and fructose, which are converted into glucose. Most glucose is stored for later use. When the glycogen stores are full, the glucose is converted into fat, a long-term form of energy storage. The conversion of glucose to fat takes place in the liver and adipocytes and is stored in the adipocytes.

Question 30

Urea

Answer 30

Nearly all ammonia is converted to urea by the liver and then excreted in the urine by the kidney.

Question 31

Protein absorption

Answer 31

Virtually all dietary protein is broken down into amino acids before being absorbed into the blood. Any protein that is not broken down completely may cause allergic reactions.

Also, when you think proteins, think nitrogen. Nitrogen is one of the compounds that make up amino acids. In terms of ammonia- a nitrogen containing compound- which is converted into urea and excreted.

Question 32

Albumin

Answer 32

From adipose tissue, most fatty acids are transported in the form of free fatty acids, which combine immediately in the blood with albumin. a single albumin molecule typically carries 3 fatty acid molecules, but is capable of carrying up to 30.

Question 33

Fat storage

Answer 33

Fat is insouble in water and typically requires a carrier (ie, a lipoprotein or albumin). For the MCAT, you shoudl associate fat with efficient long-term energy storage, lots of calories (energy) with little weight.

Question 34

Vena cava

Answer 34

The liver receives blood from the capillary beds of the intestines, stomach, spleen and pancreas. All blood received by the liver moves through large flattened spaces called the heaptic sinusoids and collects in the hepatic vein, which leads to the vena cava.

Question 35

Functions of the liver

Answer 35

1. Blood storage
2. Blood filtration
3. Carbohydrate metabolism
4. Fat metabolism
5. Protein metabolism
6. Detoxification
7. Erythrocyte destruction
8. Vitamin storage

Note that these are interrelated.

Question 36

Blood storage in the liver

Answer 36

The liver can expand to act as a blood reservoir for the body

Question 37

Blood filtration in the liver

Answer 37

Special cells phagocytize bacteria picked up from the intestines

Question 38

Carbohydrate metabolism in the liver

Answer 38

The liver maintains normal bloog glucose levels through gluconeogenesis, glygogenesis, and storage of glycogen.

Question 39

Fat metabolism in the liver

Answer 39

The liver synthesizes bile from cholesterol and converts carbohydrates and proteins into fat. It oxidizes fatty acids for energy, and forms most lipoproteins.

Note that when the liver mobilizes fat or protein for energy, the blood acidity increases.

Question 40

Protein metabolism in the liver

Answer 40

The liver deaminates amino acids, forms urea from ammonia in the blood, synthesizes plasma proteins like fibrinogen, prothromibn, albumin, and most globulins, and synthesizes nonessential amino acids.

Question 41

Detoxification in the liver

Answer 41

Detoxified chemicals are excreted by the liver as part of bile or polarized so they may be excreted by the kidney.

Question 42

Erythrocyte destruction and the liver

Answer 42

Special cells destroy irregular erythrocytes (these are also destroyed by the spleen).

Question 43

Vitamin storage and the liver

Answer 43

The liver stores vitamins such as vitamins A, D, and B12. The liver also stores iron, combining it with the protein apoferritin to form ferritin.

Question 44

Important liver proteins

Answer 44

Prothrombin and fibrinogen are important clotting factors.

Albumin is the major osmoregulatory protein in the blood.

Globulins are a group of proteins that include antibodies (antibodies are made by plasma cells, which are made by B cells).

Question 45

Kidney and expelling of urine

Answer 45

The function of the kidney is:

1. To excrete waste products such as urea, uric acid, ammonia, and phosphate,
2. To maintain homeostasis of the body’s fluid volume and solute composition and,
3. To help control plasma pH.

There are 2 kidneys, each is a fist-sized organ made up of an outer cortex and an inner medulla.

The amt of filtrate is related to the hydrostatic pressure of the glomerulus.

Urine is created by the kidney and emptied into the renal pelvis, which is emptied by the ureter, which carries urine to the bladder. The bladder is drained by the urethra.

Question 46

Nephron

Answer 46

The functional unit of the kidney.

Question 47

Renal corpuscle

Answer 47

Blood flows into the first capillary bed of the nephron (the glomerulus). Together, Bowman’s capsule and the glomerulus make up the renal corpuscle.

The fluid that gets into Bowman’s capsule is called filtrate or primary urine.

Question 48

Fenestrations

Answer 48

Hydrostatic pressure forces plasma through fenestrations of the glomerular endothelium and into Bowman’s capsule. Like a sieve, the fenestrations screen out blood cells and large proteins from entering Bowman’s capsule.

Question 49

Proximal tubule

Answer 49

Filtrate moves from Bowman’s capsule to the proximal tubule– where most reabsorption takes place. The net result of the pt is to reduce the amount of filtrate in the nephron while changing the solute composition– without changing the osmolarity.

Secondary active transport proteins in the apical membranes of the pt cells are responsible for the reabsorption of nearly all glucose, most proteins, and other solutes.

These transport proteins can become saturated (at transport maximum). Once saturated, any more solute is washed into the urine.

Water is resabsorbed into the renal interstitium of the pt across tight junctions due to the favorable osmotic gradient.

Question 50

The proximal tubule and secretion

Answer 50

Drugs, toxins, and other solutes are secreted into the filtrate by the cells of the pt.

Hydrogen ions are secreted through an antiport system with sodium. Proton crosses membrane in the opposite direction to sodium.

Uric acid, bile pigments, antibiotics, and other drugs are also secreted into the pt.

Question 51

Loop of Henle

Answer 51

From the pt, the filtrate flows into the loop of Henle, which dips into the medulla.

The function of the LOH is to increase the solute concentration, and thus the osmotic pressure, of the medulla. As filtrate descends into the medulla, water passively diffuses out of the LOH and into the medulla. The descending LOH has low permeability to salt, so filtrate osmolarity goes up. As the filtrate rises out of the medulla, salt diffuses out of the ascending LOH, passively, then actively. The ascending LOH is nearly impermeable to water.

Question 52

Vasa recta

Answer 52

A second capillary bed which surrounds the LOH and helps to maintain the concentration of the medulla.

Question 53

Distal tubule

Answer 53

Reabsorbs Na+ and Ca2+ while secreting K+, H+ and HCO3-. Aldosterone acts on the distal tubule cells to increase sodium and potassium membrane transport proteins.

The net affect of the dt is to lower the filtrate osmolarity. At the collecting tubule, ADH acts to increase the permeability of the cells to water. Therefore, in the presence of ADH, water flows from the tubule, concentrating the fitrate.

Question 54

Collecting duct

Answer 54

The distal tubule empties here. Carries the filtrate into the highly osmotic medulla. Impermeable to water, but sensitive to ADH. In presence of ADH, becomes permeable to water, allowing it to passively diffuse into the medulla, concentrating the urine. Empties into the renal pelvis.

Question 55

Juxtaglomerular apparatus

Answer 55

Monitors filtrate pressure int the distal tubule. Special cells secrete renin, while initiates a cascade to produce angiotensins I-III, which stimulates the adrenal cortex to secrete aldosterone.

Question 56

What happens if proteins don’t get fully metabolized/broken down?

Answer 56

Any protein that is not broken down completely may cause allergic reactions.

Question 57

When you think protein metabolism, think _____…

Answer 57

Nitrogen. Nitrogen is contained in ammonia, which makes up urea, nitrogenous waste that gets expelled in urine.