A. Lungs: Adaptations for Terrestrial Vertebrates

Lungs are folded within the body and are restricted to one location. Since they are centrally located, a circulatory system must carry oxygen to all cells. Lungs contain a dense net of capillaries. Lungs consist of an inpocketing, branching tube that ends in a multitude of tiny air sacs called alveoli, where the blood and the air are separated by a thin moist membrane. By muscular control of its breathing apparatus and valves of its mouth and pharynx, an animal can control the amount of air that passes through the lungs.

1. First Lungs: Amphibians

Nearly all have lungs and exchange gases in two ways: through bag like lungs and through their moist, highly vascularized skin. These animals represent the transition from aquatic to terrestrial animals. The larval froms of amphibians breathe with gills, which are strictly aquatic structures. A metamorphosis occurs, the lungs expand, and gills are absorbed into their bodies. The adult amphibians then use the lungs and skin to obtain oxygen.

2. Reptiles

The reptile skin is impervious to air, so reptiles are strictly lung breathers. The trachea is subdivided into smaller passages that enter numerous membranous compartments. This increase the surface area of gas exchange membrane. The breathing movements fo pertiles occurs when the muscles around the entire body cavity contract and relax.

3. Birds

A one way, countercurrent circulation of air occurs in the lung. There is not much mixing of old air and new air. Birds breathe through mouth and nostrils but incoming and out going air are separated within the respiratory cavity. The repiratory system is the most complex of all vertebrates. Inhaled air passes in wide bronchi through the lungs and into air sacs, which can be surrounded by gonads, and into the cavities of some bones. When the bird is not flying, muscles on the thorax and abdomen respire for breathing movements. In flight, the rapide beating of wings is all that is required for air movements.

Trachea: Insects

Air is a different medium than water. There is an increase in oxygen and carbon dioxide concentration, and oxygen diffuses much quicker than in water. The respiratory surface, which must be large and moist, always loses water through evaporation. A solution to this problem is to have a respiratory surface that has many folds in the interior of the body.

Trachea are tiny air tubes throughout the insect’s body. The tiniest tubules extend to almost every cell, where the gas is exchanged by diffusion across a moist membrane. The opening into the trachea is a spiracle. Air can enter the trachea through diffusion or rhythmic body movements.

A. Gills

Other organisms use a localized region of the body that is golded or branched, which enlarges the area of the respiratory surface for gas exchange. The expanded respiratory surface for aquatic animals, external and bathed, are gills. A detterent in having water as a respiratory medium is that the concentration of oxygen in the water is lower than in the air; the dissolved oxygen concentratin in water is about .4% compared to about 20% in air. The warmer and saltier the water, the less dissolved oxygen it holds. Gills must be efficient in obtaining oxygen from the water. A process helps is ventilation, increased flow of the respirtory medium over the respiratory surface. Lobsters and crayfish use tiny appendages to beat a current of water over the gills. If ventilation didn’t ovvur, water around the gills would stagnate and become quickly depleted of oxygen.

The arrangement of capillaries in the gills increases gas exchange. Blood flows opposite to the direction in which water passes over the gills, a countercurrent exchange. With countercurrent exchange, gills can remove 80% if the oxygen in the water. Gill baskets are found in tunicates and lancelets. Tunicates hava na open circulatory system; their blood lacks hemoglobin. Gill baskets are highly branched structures with enormous surface area, but they probably don’t fuction in gas exchange. Instead they may strain sea water. Fish have a combination of respiratory and circulatory system. There is a thin walled, finely divided out-pocketing with extensive capillary beds through which blood flows, bringing carbon dioxide and carrying oxygen away. These structures are gills.

Gills in Fishes: In fish, gills have a solely respiratory function. There are five pairs of gills found on most fish. Gills are rows of fingers like rods from which feather filaments arise either side. Each supporting rod hoiuses an artery that sends capillaries into each filament. Oxygen and carbon dioxide can pass across the walls of the capillaries easily. Oxygenated blood passes back to the supporting rod and joins to form a dorsal aorta which branches off to the body for distribution.

Gills are composed if rows of supportive gills arches along with blood vessels and

Gill filaments form these arches. Gill rakers are stiff fingers-like protrusions from gill arches. These keep swallowed foood from passing over the gills. All active aquatic animals must keep water flowing constantly over the respiratory surfaces. Sharks swim constantly with their mouths open for the most part. Fish keep water moving by moving mouth muscles. Gills are closed off to the outside by the opeculum. The fish closes its mouth and swallows, the operculum opens and closes the watere flows out over its gills.

V. Respirtation

A. Introduction

Exchange of carbon dioxide and oxygen between an animal and the environment.

1. Animals

a. require a continuos supple of oxygen for aerobic respiration

b. must expel carbon dioxide

c. must have a wet respiratory surface

the exchange of gases requires structures with two basic charachteristics

1. Permeable surface area of sufficient size

2. moist surface area since gases cant normally cross dry membranes

B. Gas exchange Comparisons

1. Protists

Gases are exchanged by diffusion

2. Sponges to annelids

Gases are exchanged by diffusion

3. Mollusks and enchinoderms

Gases are exchanged through gills

4. Arthropoda

Gases are exchanged through trachea, book lungs, or gills

Some organisms, for example, earthworms and frogs, use their entire outer skin as the respiratory organ. The skin needs to be moist, so they live in water or in damp places.

I. Food as Fuel

The energy content of food is measured in calories

The energy content of:

• Fat=9 kcal/ 1 gram
• Protein= 4 kcal/1 gram
• Carbohydrates= 4 kcal/1 gram

Several processes must occur continually in higher anmals alive, for example, breathing and heart beating. The number of kcal a resting animal needs at a given time is called a basal metabolism rate, or BMR

Nutrition

1. Carbohydrates

Carbohydrates are the most common source of energy used in cellular respiration. Blood glucose must remain constant. Glucose is stored in the muscles as glycogen and used for energy. If not used, carbohydrates are stored as fat.

2. Fats

Unsaturated fats are necessary for cell membrane synthesis. Fats are sources of energy and insulators. Fats also serve as a source for vitamins A, D, E, and K

3. Proteins

Proteins can be stored in the liver and in musce tissue. There is a constant turnover of body protein. Amino acids can build proteins, form nitrogenous bases, can be oxidized for energy, and can be converted into fats and carbohydrates. When amino acids are used, they leave behind a nitrogenous waste which is poisonous and must be removed. Humans can produce 12 of the 20 amino acids. There are 8 amino acids that humans must obtain from food. Brain development requires proteins.

4. Vitamins and Minerals

Vitamins: function in common enzymatic reactions as co-enzymes. There are two types of vitamins: fat soluble and water soluble

5. Mineral

Simple inorganic ions. They may be used in the formation of gross structures, such as bones or may become an active part of functional molecules. Some are necessary for enymatic actions.

E.Digestion in Herbivores: Ruminants

These organisms must digest cellulose. However, mammals don’t produce the necessary enzymes. In order to digest the cellulose, mammals must have large flat teeth for grinding cellulose. Cellulose must be broken down to release plant nutrients. Once broken down the food goes through the esophagus to a four chambered stomach of such ruminant herbivores such as cattle, deer, giraffes, antelopes, and buffalo. The stomachs harbor protozoans and bacteria that break down cellulose.

F.Chemistry of Digestion

1. Carbohydrate digestion

Starch digestion begins in the mouth with amylase that is found in the salivea. The breakdown of carbohyrdrates is stalled in the stomach because the acidity and resumes in the small intestine where amylase from the pancreas converts all starch into maltose. Maltose is broken down by maltase into glucose which is absorbed. Sucrose, which is glucose and fructose, is broken down by sucrase. Lactose, milk sugar, is broken down in the gut by lactase. Lactase is absent in most blacks and Asians and in some whites preventing them from breaking down lactose.

2. Fat digestion

Fats reach the small intestine with little chemical change. Even though the stomach secretes a lipase. Bile separates fats into tiny droplets that are broken down further by lipases into fatty acids and glycerol. These cross the membrane and reform in the villi. Longer chans of fatty acids, more than 12 carbons, enter the lymphatic vessels of the villi. Fatty acids with fewer than 12 carbons go into the capillaries and are carried to the liver. The ones in the lymph system follow the lymphatic system to the thoracic duct near the heart and enter the blood stream. If there is too much fat in the blood after the meal, the blood will appear milky.

3. Protein digestion

Proteins are the most complex food molecule and their digestion is complex. Pepsinogen, secreted by the chief cells is changed by hydrochloric acid into pepsin. Pepsin is a nonspecific endopeptidase and hydrolyzes proteins into smaller peptides. Specific enzymes will break pepitides down further until individual amino acids are left. These amino acids are absorbed into the blood stream and sent to the liver. In the liver, the amino acids are used for eneg and to build proteins.

4. Nucleic Acids

Almost everything we eat contains some nucleic acids. Nucleases produced in the pancreas break nucleic acid bonds. They break down the nucleic acids into small units of either single bases or small chains.

G. Integration and Control of the Digestive Process

The realease of digestive enzymes must be precisely timed. They are controlled mechanically, neurally, and hormonally.

For example, the thought of chocolate stimulates saliva flow. The saliva flow can also be stimulated by chewing. Neural and mechanial stimulants can stimulate saliva flow. Gastric secretions can be stimulated by the presence of food. The neural message is sent along the vagus nerve from the brain to th stomach lining. Food in the stomach stimulates sensory neurons in the stomach and a hormone gastrin is released from stomach to the blood.

1. Small Intestine

Food in liquid form enters the small intestine. The small intestine is about 20 feet long and is broken up into three regions, duodenum, jejunum, and ileum. It is lined with mucin and adapted for absorption of food through a highly molded inner surface. Each fold is called a villus.

a.Functions

1) Chemical digestion

2) Absorption

b. Enzyme Producing Organs

These secrete the enzymes into the small intestine.

1) Liver: The liver secretes bile into the duodenum. Bile is a fat emulsifier. Bile is stored in the gall bladder and reached the duodenum of the small intestine via the bile duct. Where it is joined by basic fluids from the pancreatic duct. Bile reduces the size of fat globules so lipases, enzymes that break down lipids, can break them down. If the bile duct is blocked, one becomes jaundiced because bile ends up in the blood stream.

2) Pancreas: The pancreas is a glandular organ that lies in the first turn of the small intestine. The pancreas secretes bicarbonate which neutralized acid. The pancreas also secretes enzyemes that brea down carbohydrates, proteins, fats, and nucleic acid. These enzymes from the pancreas carry out much of the digestive process. The acid of the chyme activates the stomach to produce secretin, a hormone that stimulates the pancreas to release bicarbonate.

The digestion of starch, begun in the mouth, is continued in the small intestine by the addition of pancreatic amylase, which hydrolyzes starch into the disaccharide maltose. Maltase will break down maltose into glucose monomers.

Proteins are broken down by trypsin and chymotrypsin. These break down the polypeptides into the smaller polypeptides. Carboxypeptidases split one amino acid at a time off the carboxyl end while the aminopeptidases work in the opposite direction. These enzymes are secreted by the pancreas as inactive zymogens. Enterokinase activates zymogens in the small intestine.

Nucleases hydrolyze DNA and RNA

All this occurs in the duodenum of the small intestine. The two remaining regions are specialized for absorption.

c.Absorption in the Small Intestine

Each villus is covered with epithelial cells that contain microvilli. These projections increase the surface area of the small intestine. Within each villus is a capillary bed and a lacteal. The nutrients enter directly into the blood stream from the small intestine.

The small intestine is capable of vigorous movement which mizes food and enzymes together. This movement also helps with absorption.

Nutrients are absorbed across the epithelial layer into the capillaries or lacteals. Sometimes the transport is passive while other times it is active. Amino acids, vitamins, glucose, are pumped against the gradient by the epithelial membranes. The absorption of some nutrients appears to be soupled with the active transport of sodium across the membrane. The sodium is actively transported out of the cell, nutrients are co-transported.

2. Large Intestine

The small intestine joins the large intestine on the right side of the body. Here, the cecum forms a pouch which ha a finger like projection called the appendix. There is a one way valve at the junction of the large and small intestine called the ileocecal valve which insures that there is no backflow into the small intestine.

The large intestine consists of several parts: cecum, ascending colon, transeverse colon, descending colon, sigmoid colon, rectum, anal canal, and anus. The fun tions of the large intestine include the absorption of water and minerals into the blood, preparation of feces, and the housing of bacteria which produce vitamin K, biotin, folic acid, and methane. Approximately 2/3 of fecal matter is dead bacteria. The large intestine secretes mucus to lubricate for easier export. The rectal valves support the feces until defecation. The wastes go through the anal sphincter, under voluntary control and out the anus. It takes 12-24 hours for material to travel the length of the large intestine.

Digestion is the breakdown of complex food molecules into smaller components that are used by the organism. This may occur outside of the organism as in bacteria and fungi, or the organism may have extracellular digestion. Most organisms take food into cells.

Digestion is a series of chemical reactions that use hydrolytic enzymes. The chemical digestion can be preceeded by mechanical fragmentation of food into smaller pieces. Breaking down the food mechanically increases the surface area of the food and makes the digestive enzymes more effective. Once the food is digested the organic molecules can cross the plasma and enter cells. The undigested materials are defecated.

A. Definitions

1.Holotrophs

organisms that ingest other organisms, dead or alive, whole or by the piece, or absorb organic molecules directly.

2. Herbivores

organisms that eat plants or algae

1. Carnivores

Organisms that ingest other animals

2. Omnivores

Organisms that ingest both plant/algae and animals

3. Suspension feeders

Organisms that sift small food particles from the water

4. Substrate feeders

Organisms that live in or on the food source

5. Fluid Feeders

Organisms that suck nutrient rich fluids of the living host

6. Bulk Feeders

Organisms that ingest relatively large pieces of food.

B.Comparative Digestion

The simplest compartment for digestion is a food vacuole. These can digest food without the enzymes mixing with the cytoplasm. Protists take food in through endocytosis and then perform the intracellular digestion.

Sponge: Food particles enter the choanocytes via phagocytosis. Digestion occurs in food vacuoles. The food vacuoles are transferred to amoebocytes which distribute the food to other cells.

Gastrovascular Cavities are digestive sas with a single opening. This cavity functions in both digestion and distribution of nutrients through the body. For example, hydra catches food using cnidocytes on the tentacles, ad then stuff food into a mouth. In the gastrovascular cavity specialized cells produce digestive enzymes that break the food down, flagellated cells spread the food particles in th cavity, and particles are taken into cells by phagocytosis. Planarians also have a gastrovascular cavity with one opening. An alimentary canal is a tube with two openings: mouth and anus. Parts of the tube are specialized, ie. Some parts digest the good and other parts absorb the food. In earthworms, food is taken in by the mouth and pharynx, passes to the esopogus, the crop, the gizzard, the intestine, on to the anus. This is similar to the foraging and digestive structures of fish, amphibians, reptiles and birds.

B.Foraging and Digestive Structures in Mammals

Teeth grow in sockets in the jaw. Usually mammals begin with temporary milk teeth that are replaced by the adult teeth. There are 4 types of teeth in mammals.

1. Incisor

Chisel shaped for cutting

2. Canines

Tearing food and defense

3. Premolars

Ginding food

4. Molars

Grinding food. In herbivores molars are large and flat.

Humans have teeth that are not specialized for any particular type of diet.

D. Human digestive system is a good representative of the Mammalian system

1.Oral cavity

the lips are essential in eating.

a. Close lips to swallow

b. Help food keep inside the mouth

2. Tongue

a. Moves food into position for chewing

b. Tells us when our food can be swallowed

c. Prevents us from swallowing

d. Contains chemoreceptors which can distinguish 4 different types of taste: salt, sour, and bitter. These receptors are in a specific concentration on specific sites of the tongue. The stimulation of the taste buds can also enhance saliva flow.

3.Three pairs of Salivary Glands

a. Parotids

Located in the front and below the ears at the angle of the jaw.

b. Submaxillaries

Found below the angle of the jaw

c. Sublinguals

Found below the tongue.

Saliva: 95% water, ions, lubricating mucus, and starch splitting enzyme amylase which starts digestion and digests the starch caught between the teeth. Dissolved in the saliva is a slippery glycoprotein called mucin. Mucin protects the mouth from abrasions and lubricates the food. Saliva also contains buffers, which help prevent dental cavities by neutralizing acid in the mouth, and antibacterial agents.

4. Pharynx

Near the rear of the oral cavity and forms a common passageway to the naval cavity. Below the tongue the pharynx divides into the larynx and laryngopharynx. When you swallow the top of the larynx moves so that the air passageway is blocked by a cartilaginous flap called epiglottis. This ensures that the bolus stays out of the respiratory system.

5. Esophogus

Moistens the food and mves it to the stomach and is made up of smooth muscle.

The esophagus serves two functions

a. Secretes mucus into the lumen and cavity of the digestive tract.

b. Moves food along- compressing food inta a bolus-muscles contract behind the food and push it along.

6. Stomach

Located on the left side of the abdominal cavity just below the diaphragm. This temporarily stores and helps digest the food.

a. food in the stomach

the stomach stretches easily and any reistance to the stretching will cause cramps. The stomach is closed off at either end by wo sphincters. The pyloric sphincter which is the bottom sphincter that opens to the small intestine. The cardiac sphincter is the top that opnes up into the esophagus. Food is churned without being pushed into the esophagus or intestine. Ulcers occur when the digestive system digests its own stomach or small intestine. The vomit center in the brain allows for the cardiac sphincter to relax, the pyloric sphincter to tighten, and the stomach to contract.

A complex layering of muscles allows for the twisting action and wringing and shortening of the stomach that grinds up the food.

After being ground up, the pyloric sphincter relaxes a bit and the food enters the small intestine.

About every 20 seconds the stomach contents are mixe by the churning of the smooth muscles. When the stomach is empty, the stomach churns and hunger pangs are felt. After food and gastric juices mix, acid chyme is produced. The pyloric sphincter opens and squirts some chyme into the small intestine. It takes 2-6 hours for the stomach to empty after a meal.

b. Chemistry of the stomach

The stomach is lined mucosa, mucus secreting tissue, the tubular glands, which secrete gastric juices. Chief cells secrete hydrochloric acid which activates pepsinogen to be converted to pepsin. Pepsin hydrolyzes proteins and works well in the low ph generated by the HCL. Other glands secrete water mucus, and rennin, which digests milk and gastric lipases. A low ph in the stomach, kills bacteria.

Gastric secretions are controlled by the nervous system and hormones. When we see and smell foods, the brain stimulates the stomach to secrete gastric juices. Certain substances will cause the stomach wall to realease a hormone called gastrin. Gastrin travels through the circulatory system to the stomach, causing the stomach to secrete more gastric juices.