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Biology
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2. 38–2 The Process of Digestion
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Digestion
What are the organs of the digestive system?
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Digestion
The digestive system includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.
Other structures add secretions to the digestive system, and aid in digestion. These include the salivary glands, pancreas, and liver.
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5. 38–2 The Process of Digestion
The Digestive System
Mouth
Pharynx
Salivary glands
Esophagus
Liver
Stomach
The digestive system includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. Because the pancreas and most of the gallbladder are behind other organs, their locations are indicated by dotted lines.
Pancreas
Gallbladder
Large intestine
Small intestine
Rectum
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The Mouth
What is the function of the digestive system?
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The Mouth
The function of the digestive system is to help convert foods into simpler molecules that can be absorbed and used by the cells of the body.
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The Mouth
The Mouth
Chewing begins mechanical digestion, which is the physical breakdown of large pieces of food into smaller pieces.
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The Mouth
The teeth cut, tear, and crush food into small fragments.
As the teeth cut and grind the food, salivary glands secrete saliva, which moistens food and makes it easier to chew.
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The Mouth
Saliva helps ease the passage of food through the digestive system and also begins the process of chemical digestion.
Saliva contains amylase, an enzyme that breaks the chemical bonds in starches and releases sugars.
Saliva also contains lysozyme, an enzyme that fights infection.
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The Esophagus
The Esophagus
From the throat, the chewed food passes through the esophagus, or food tube, into the stomach.
Food is moved along by contractions of smooth muscle.
These contractions, known as peristalsis, squeeze the food through the esophagus into the stomach.
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The Esophagus
Esophagus
Bolus
Peristalsis
Muscles contracted
Stomach
Muscles in the walls of the esophagus contract in waves. Each wave pushes the chewed clump of food, or bolus, in front of it. Eventually, the bolus is pushed into the stomach.
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The Esophagus
The cardiac sphincter closes the esophagus after food has passed into the stomach.
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The Stomach
The Stomach
Food from the esophagus empties into the stomach.
The stomach continues mechanical and chemical digestion.
Alternating contractions of three smooth muscle layers churn food.
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The Stomach
Chemical Digestion
The stomach lining has millions of gastric glands that release substances into the stomach.
Some glands produce mucus, which lubricates and protects the stomach wall.
Other glands produce hydrochloric acid, which makes the stomach contents very acidic.
Other glands produce pepsin, an enzyme that digests protein.
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The Stomach
Pepsin and hydrochloric acid begin protein digestion.
Pepsin breaks proteins into smaller polypeptide fragments.
Other enzymes are denatured (broken down) by stomach acid.
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The Stomach
Digestive enzymes break down foods and make nutrients available to the body.
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The Stomach
Mechanical Digestion 
The stomach contracts to churn fluids and food, gradually producing a mixture known as chyme.
After 1–2 hours, the pyloric valve between the stomach and small intestine opens and chyme flows into the small intestine.
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The Small Intestine
The Small Intestine
As chyme is pushed through the pyloric valve, it enters the duodenum.
The duodenum is the first of three parts of the small intestine, and is where most digestive enzymes enter the intestine.
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The Small Intestine
Most chemical digestion and absorption of food occurs in the small intestine.
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The Small Intestine
Accessory Structures of Digestion
Liver
Bile duct
Pancreas
Gallbladder
Pancreatic duct
Duodenum
Accessory structures, including the liver and pancreas, add secretions to the digestive system. The pancreas secretes enzymes that help break down carbohydrates, proteins, and fats.
To rest of small intestine
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The Small Intestine
Accessory Structures of Digestion
Just behind the stomach is the pancreas.
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The Small Intestine
During digestion, the pancreas:
produces enzymes that break down carbohydrates, proteins, lipids, and nucleic acids.
produces sodium bicarbonate, a base that neutralizes stomach acid so that these enzymes can be effective.
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The Small Intestine
Assisting the pancreas is the liver, which produces bile.
Bile dissolves and disperses droplets of fat in fatty foods. This enables enzymes to break down smaller fat molecules.
Bile is stored in the gallbladder.
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25. Absorption in the Small Intestine
The small intestine is adapted for the absorption of nutrients.
The folded surfaces of the small intestine are covered with fingerlike projections called villi.
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26. Absorption in the Small Intestine
Villus
Small intestine
Circular folds
Epithelial cells
Villi
Capillaries
Lymph vessel
The lining of the small intestine consists of folds that are covered with tiny projections called villi. Within each villus there is a network of blood capillaries and lymph vessels that absorb and carry away nutrients. 
Vein
Artery
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27. Absorption in the Small Intestine
Cell surfaces of villi have more projections called microvilli.
These provide an enormous surface area for the absorption of nutrient molecules.
Slow, wavelike contractions of smooth muscles move the chyme along this surface.
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28. Absorption in the Small Intestine
Nutrient molecules are absorbed into the cells lining the small intestine.
Most products of carbohydrate and protein digestion are absorbed into the capillaries in the villi.
Molecules of undigested fat are absorbed by lymph vessels.
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The Large Intestine
The Large Intestine
When the chyme leaves the small intestine, it enters the large intestine, or colon.
The large intestine removes water from the chyme.
Water is absorbed quickly, leaving undigested materials behind.
Concentrated waste material passes through the rectum and is eliminated from the body.
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30. Digestive System Disorders
Stomach acids sometimes damage the organ’s own lining, producing a hole in the stomach wall known as a peptic ulcer. Most peptic ulcers are caused by the bacterium H. pylori.
Other digestive disorders include diarrhea and constipation.
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38–3 The Excretory System
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32. Functions of the Excretory System
Every cell produces metabolic wastes.
The process by which these wastes are eliminated is called excretion.
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33. Functions of the Excretory System
The skin excretes excess water and salts in the form of sweat.
The lungs excrete carbon dioxide.
The kidneys also play a major role in excretion.
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34. Functions of the Excretory System
The Kidneys
What are the functions of the kidneys?
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35. Functions of the Excretory System
The kidneys:
remove waste products from the blood.
maintain blood pH.
regulate the water content of the blood and, therefore, blood volume.
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The Kidneys
The kidneys are located on either side of the spinal column near the lower back.
A tube, called the ureter, leaves each kidney, carrying urine to the urinary bladder.
The urinary bladder is a saclike organ where urine is stored before being excreted.
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The Kidneys
Structure of the Kidneys
Kidney
Nephron
Kidneys are made up of nephrons. Blood enters the nephron, where impurities are filtered out and emptied into the collecting duct. The purified blood leaves the nephron through the renal vein.
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The Kidneys
Blood enters the kidney through the renal artery.
The kidney removes urea, excess water, and other waste products and passes them to the ureter.
The clean, filtered blood leaves the kidney through the renal vein and returns to circulation.
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The Kidneys
Kidney Structure 
A kidney has two distinct regions:
The inner part is called the renal medulla.
The outer part is called the renal cortex.
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The Kidneys
Cortex
Medulla
Renal artery
Renal vein
Ureter
To the bladder
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The Kidneys
The functional units of the kidney are called nephrons.
Nephrons are located in the renal cortex, except for their loops of Henle, which descend into the renal medulla.
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The Kidneys
Capillaries
Bowman’s capsule
Glomerulus
Kidneys are made up of nephrons. Blood enters the nephron, where impurities are filtered out and emptied into the collecting duct. The purified blood leaves the nephron through the renal vein.
Collecting duct
Vein
Artery
To the ureter
Loop of Henle
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The Kidneys
Capillaries
Each nephron has its own blood supply:
an arteriole
a venule
a network of capillaries connecting them
Collecting duct
Vein
Artery
To the ureter
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The Kidneys
Capillaries
Each nephron releases fluids to a collecting duct, which leads to the ureter.
Collecting duct
Vein
Artery
To the ureter
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The Kidneys
How is blood filtered?
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The Kidneys
As blood enters a nephron through the arteriole, impurities are filtered out and emptied into the collecting duct.
The purified blood exits the nephron through the venule.
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The Kidneys
The mechanism of blood purification involves two distinct processes: filtration and reabsorption.
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The Kidneys
Filtration 
Passing a liquid or gas through a filter to remove wastes is called filtration.
The filtration of blood mainly takes place in the glomerulus.
The glomerulus is a small network of capillaries encased in the top of the nephron by a hollow, cup-shaped structure called Bowman's capsule.
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The Kidneys
Fluid from the blood flows into Bowman’s capsule.
The materials filtered from the blood include water, urea, glucose, salts, amino acids, and some vitamins.
Plasma proteins, cells, and platelets remain in the blood because they are too large to pass through the capillary walls.
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The Kidneys
Reabsorption 
Most of the material removed from the blood at Bowman's capsule makes its way back into the blood.
The process in which liquid is taken back into a vessel is called reabsorption.
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The Kidneys
Almost 99% of the water that enters Bowman’s capsule is reabsorbed into the blood.
When the filtrate drains in the collecting ducts, most water and nutrients have been reabsorbed into the blood.
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The Kidneys
Remaining material, called urine, is emptied into a collecting duct.
Urine is primarily concentrated in the loop of Henle.
The loop of Henle is a section of the nephron tubule in which water is conserved and the volume of urine minimized.
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The Kidneys
As the kidney works, purified blood is returned to circulation while urine is collected in the urinary bladder.
Urine is stored here until it is released from the body through a tube called the urethra.
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54. Control of Kidney Function
The activity of the kidneys is mostly controlled by the composition of the blood.
In addition, regulatory hormones are released in response to the composition of blood.
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55. Control of Kidney Function
When you drink a liquid, it is absorbed into the blood through the digestive system.
As a result, the concentration of water in the blood increases.
As the amount of water in the blood increases, the rate of water reabsorption in the kidneys decreases.
Less water is returned to the blood, and excess water is sent to the urinary bladder to be excreted as urine.
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56. Control of Kidney Function
When the kidneys detect an increase in salt, they respond by returning less salt to the blood by reabsorption.
The excess salt the kidneys retain is excreted in urine, thus maintaining the composition of the blood.
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Kidney Disorders
Kidney Disorders
Humans have two kidneys, but can survive with only one.
If both kidneys are damaged by disease or injury, there are two options:
a kidney transplant
kidney dialysis
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Kidney Disorders
Kidney dialysis works as follows:
Blood is removed by a tube and pumped through special tubing that acts like nephrons.
Tiny pores in the tubing allow salts and small molecules to pass through.
Wastes diffuse out of the blood into the fluid-filled chamber, allowing purified blood to be returned to the body.
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Kidney Disorders
Kidney Dialysis
Blood in tubing flows through dialysis fluid
Blood pump
Vein
Artery
Shunt
Used dialysis fluid
For people with damaged kidneys, dialysis machines can perform many of the functions of the kidneys.
Dialysis machine
Fresh dialysis fluid
Compressed air
Air detector
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60. END OF SECTION

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38–2
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38–2
Food is moved through the esophagus into the stomach by
air pressure.
muscle contractions.
gravity.
swallowing.
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38–2
A gland that has both endocrine and exocrine functions is the
liver.
spleen.
pancreas.
gallbladder.
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38–2
The enzyme in saliva that begins the digestion of starch is
amylase.
pepsin.
lysozyme.
peptidase.
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38–2
Stomach muscles contract to churn and mix stomach fluids and food, producing a mixture known as
chyme.
amylase.
bile.
acid.
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38–2
Absorption of vitamins, minerals, and digested food molecules takes place in the
stomach.
small intestine.
large intestine.
duodenum.
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