The Human Body Systems

1. The Human Body Systems Aliya Deri 5*

2. The Integumentary System What is it made up of? Skin, hair, nails, and sweat and oil glands • What does it do? • protects against infections and injuries • protects against ultraviolet radiation • provides a gateway into the nervous system • maintains body heat Standards? None

3. 1. Protects against infections and injuries. The skin, the largest part of the system, is made of two sections. The epidermis is the thin layer at the top. Cells at thebottom of the epidermis divide quickly and move to the top. As the cells die, they become filled withkeratin, a tough protein. These dead cells form a tough, waterproof barrier. The skin also prevents infections and injuries because the epidermis does not have any blood vessels. Because of this, a small scratch will not cause bleeding and expose the bloodstream to germs. Hair, another part of the integumentary system, also prevents infections and injuries by keeping particles from entering the nose, ears, or eyes (eyelashes).

4. 2. Protects against ultraviolet radiation. The second part of the skin is the dermis, which is much thicker than the epidermis. (In the picture, it is both the dermis and hypodermis.) Cells within the dermis, called melanocytes, produce a dark brown pigment (melanin). This pigment protects the body from sunburn and skin cancer. That is why dark skinned people like African Americans do not sunburn easily and have lower rates of skin cancer than people of European descent. Hair also protects against radiation by shielding the head from the sun. Hair is maintained by the sebaceous glands in the dermis, which secrete an oil that maintains each hair. 3. Provides a gateway into the nervous system. The dermis contains pain, heat, cold, and pressure receptors that transmit messages about the environment to the nervous system.

5. 4. Maintains body heat. On cold days, the blood vessels in the dermis contract to conserve heat. On hot days, they widen to bring blood to the surface of the skin and increase heat loss. The dermis also contains sweat glands that make a watery secretion called sweat. When sweat evaporates from the skin on a hot day, it takes heat away. The skin also provides insulation because it has a layer of fat in the deepest part of the dermis.

6. REAL LIFE SITUATION You decided a few months ago to grow your hair out to shoulder length. But recently you’ve begun to notice that your hair has become dry and split.You ask your sister, who also has long hair, how she keeps her hair from splitting or becoming too dry. She doesn’t really know either, but she explains that she brushes her hair every morning and every night for twenty minutes. Why does brushing help her hair?

7. REAL LIFE SITUATION You have learned that the sebaceous glands in the dermis maintain individual hair strands.The sebaceous glands produce oils that keep hair glossy and healthy. However, as hair grows longer, the oils have to be spread out over the entire hair.Otherwise the ends of the hair will become dry and split. As your hair grew out, you didn’t realize that you needed to brush it more often to spread out the sebaceous oils.

8. The Nervous System What is it made up of? Specialized cells called neurons • What does it do? • controls body functions • responds to internal and external changes • Standards? • b.how the nervous system mediates communication between different parts of the body and the body’s interactions with the environment • Functions of the nervous system and the role of neurons in transmitting electrochemical impulses • Role of sensory neurons, interneurons, and motor neurons in sensation, thought, and response.

9. The Neuron The dendrites, the branches on the body of the neuron, are the “ears” of the neuron. They receive messages from other neurons. The axon, the long section of the neuron, is the “mouth”. It tells other neurons about what its dendrites have received, by sending an electrochemical message that other neurons’ dendrites will receive.

10. Standard DFunctions of the nervous system and the role of neurons in transmitting electrochemical impulses The nervous system controls body functions and responds to the environment by relaying messages between neurons. As an electrical current from another neuron or a change in the environment moves across a neuron’s membrane, it is transferred across the membrane in a domino effect. Eventually the ions of the electrical current reach the axon. The impulse travels down the axon to where it branches out into an axon terminal. Axon terminals are either connected to a cell that is not a neuron, or are very close to a dendrite of another neuron. The axon terminal uses chemicals called neurotransmitters to send the impulse to a cell or another neuron. The place where the axon terminal can send on an impulse is called a synapse. In the image above, the synapses are the highlighted gold parts.

11. Standard ERole of sensory neurons, interneurons, and motor neurons in sensation, thought, and response. There are 3 different types of neurons. Sensory Neurons are found in the sensory organs— the nose, eyes, ears, tongue, and skin. They carry impulses about changes in the environment to the brain or spinal cord ?!! Interneurons are found in the brain and spinal cord. They are a bridge between sensation and response. They receive impulses from the sensory neurons and send the appropriate impulse to motor neurons. Motor Neurons are found in muscles and glands. They receive impulses from the brain or spinal cord and prompt the muscles to move the body. (More in the Muscular System)

12. Standard BHow the nervous system mediates communication between different parts of the body and the body’s interactions with the environment The nervous system is divided into two parts that work together to transmit messages from sense organs to the brain/spinal cord and then to the muscles.This circular method is known as a “reflex arc”. (Standard C) The peripheral nervous system is made up of the nerves and ganglia (groups of neuron cell bodies) that are not in the brain or spinal cord. The neurons in the peripheral nervous system are mainly motor and sensory neurons. The sensory division transmits impulses to and the motor division receives impulses from the central nervous system. The central nervous system is the brain and spinal cord. The neurons in the central nervous system are mainly interneurons. The central nervous system analyzes and processes information from sensory and motor neurons. The spinal cord only handles rapid reflexes that must happen very quickly. The brain takes care of all thinking.

13. REAL LIFE SITUATION It’s 7 A.M. on a Monday morning. As you blearily stumble into the kitchen, you hear the intolerably cheerful humming of your mother as she packs lunches for you and your siblings before rushing off to work. The sharp ding of the toaster goes off a few feet away. “Would you grab the toast?” your mother calls from the kitchen table. But when you reach for the two pieces of bread, you stumble forward and the side of your hand brushes against the hot surface of the toaster. Before you even feel the pain, you yelp and pull your hand away. As your pinky suddenly throbs with pain, you quickly put it in your mouth to cool it. What happened?

14. REAL LIFE SITUATION First, the pain and heat sensory neurons in your finger detected the heat of the toaster (the stimulus) and sent an electrochemical impulse to the central nervous system. Because the danger of the toaster was great, this impulse was processed directly by the spinal cord. The interneurons in the spinal cord sent an impulse to the motor neurons of your arm and chest muscles to pull away from the toaster. You did not feel the pain of the burn at first because the spinal cord cannot analyze information like the brain. However, when an impulse was sent to the brain, AFTER the danger had been taken care of, you felt the pain. The interneurons in your brain “decided” that the best way to stop the pain was to put your hand in your mouth.

15. The Skeletal System What is it made up of? bones and cartilage • What does it do? • supports the body • protects internal organs from injury • allows movement • stores mineral reserves • provides a place where blood cells can be made Standards? None

16. The Bone Bones are made of several layers.The outer layer is a tough tissue called the periosteum. Blood vessels must pass through the periosteum to carry nutrients and oxygen to the bone. Next is a layer of compact bone. Through the compact bone, tiny channels called Haversian canals contain nerves and blood vessels. Another kind of bone, less dense than compact bone, is spongy bone. It is found at the ends of long bones and in the middle of short bones. Spongy bone “adds strength to bone without adding mass” (p. 922, Prentice Hall Biology). Where force is regularly applied to the end of bones, spongy bone is organized into a strong, supportive structure. A bone also has a cavity in its center. The cavity is filled with a tissue called bone marrow. The function of marrow will be discussed later.

17. 1. Supports the body. The bones of your body hold up your body like the walls of a building. Everything in the human body is supported by and built around the skeleton. 2. Protects internal organs from injury. Some bones are designed to protect a certain vital organ. Three good examples are the pelvis (protects the reproductive organs) the ribs (act as a cage around the heart and lungs), and the skull (protects the brain). The combined strength of the periosteum, compact bone, and spongy bone keeps the body’s most important organs from most injuries. Deposits of calcium salts in bones are essential in giving bones strength.

18. 3. Allows movement. Despite the strength and protection that the skeleton gives the body, it still allows free movement. This is because the places where bones attach to each other, called joints, are often movable. Joints can be immovable (the bones in the skull are fused), slightly movable (the joints between adjacent vertebra allow restricted movement), or freely movable (four different types). PivotJoint—elbowLets one bone pivot around the other Ball-and-Socket Joint—shoulderPermits circular movement Saddle Joint—handsBones can slide over each other Hinge Joint—kneesPermits back and forth movement The ends of bones in freely movable joints have cartilage to stop chafing.A joint capsule around the joint helps the bone stay together and move. Bands of connective tissue called ligaments hold the bones together. The capsule also produces synovial fluid, which lubricates the joint. Some freely movable joints, like the knee, have small bags of synovial fluid called bursae. Bursae reduce friction and absorb shock.

19. 4. Stores mineral reserves. Bones are made up of cells called osteocytes. Osteocytes deposit and regulate calcium salts in the bone. Calcium can be removed from the bone if the bloodstream’s calcium level is too low. (Other than strengthening bones, calcium also helps blood clot and ensures proper muscle and nerve function. 5. Provides a place where blood cells can be made The cavity in a bone contains marrow, a soft tissue that produces red and some white blood cells. There are 2 kinds of marrow: red & yellow Red marrow is found in spongy bone, the ends of long bones, and in the ribs, vertebrae, sternum, and pelvis. It makes all red blood cells, white blood cells, and platelets. Yellow marrow is found in most bones, especially the shafts of long bones. Yellow marrow is mostly fat, and it is used as an energy reserve. If the body loses a large amount of blood, yellow marrow can be changed into red marrow.

20. How a Broken Bone is Fixed As discussed previously, a bone is made up of cells called osteocytes, whose main job is to maintain the bone. But what happens when a bone is broken? First of all, the damaged bone is removed by cells called osteoclasts. Osteoclasts tidy the damaged site to create a clean working area. Then, cells called osteoblasts create new bone tissue through a slow process that can take months. Osteoblasts are also the bones that fused bones and changed cartilage into bone while you were a child.

21. REAL LIFE SITUATION Your Aunt Georgia, aged 57, fractured her hip and ribs recently after tripping down a staircase. You asked your doctor why a small fall caused such a bad injury. He said that Aunt Georgia is probably suffering from osteoporosis, a gradual weakening of the bones as they lose density. He also said that osteoporosis affects people after the age of 30, when bone tissue replacement slows. However, he warned you that it is hard to prevent osteoporosis after your mid-twenties. It’s easiest to increase bone density now, as a teenager, by eating healthy foods (especially milk and dairy) and exercising regularly. Why did he advise these two solutions?

22. REAL LIFE SITUATION • A healthy diet, especially milk-based foods, is important to preventing osteoporosis early because it contains a lot of calcium. Calcium salts strengthen bones and provide density. Osteoporosis occurs because osteocytes cannot replace calcium salts efficiently.You can also take calcium pills to make sure that you have enough. 2. Exercise at a young age prevents osteoporosis because applying force to bones actually strengthens them. For example, the leg bones of a runner will be dense and strong because of the constant pounding on them. The spongy bone at the end of his leg bones is arranged to support the compact bone. Note that exercises like weightlifting do not prevent osteoporosis because they do not apply force to the bones.

23. The Muscular System What is it made up of? skeletal, smooth, and cardiac muscle tissue • What does it do? • moves bones, pumps blood, etc. Standards? h. Students know the cellular and molecular basis of muscle contraction, including the roles of actin, myosin, Ca+2, and ATP.

24. Muscle Tissue There are 3 different types of muscle tissue. Skeletal muscle is attached to bones. It moves bones and its movement is voluntary and controlled by the central nervous system. Skeletal muscle cells are large and appear to make stripes, or striations. Smooth muscle is found in hollow organs like the stomach and blood vessels. It moves food through the digestive tract and blood through blood vessels. This movement is involuntary. Most smooth muscles do not need nerves to contract. Instead, impulses travel directly between muscle cells. Cardiac muscle is found in the heart. It is striated like skeletal muscle, but its movement is involuntary like smooth muscle. It pumps blood through the heart.

25. Standard HStudents know the cellular and molecular basis of muscle contraction, including the roles of actin, myosin, Ca+2, and ATP. The striations in a skeletal muscle are made up of alternating thick and thin filaments. The thick filament with the branches is formed by a protein called myosin. The thin filament is made of a protein called actin. For a muscle to contract, each myosin fiber attaches to an actin fiber and uses energy from ATP to pull.

26. Standard H (continued)Students know the cellular and molecular basis of muscle contraction, including the roles of actin, myosin, Ca+2, and ATP. When motor neurons in the muscle receive an impulse from the central nervous system, acetylcholine is released.Acetylcholine makes the muscle release calcium ions (CA2+) . The calcium ions make the myosin fiber bind to the actin fiber. This is called a cross-bridge. The cell in which the contraction is taking place burns ATP through cellular respiration to make energy. The energy is used to bend the cross-bridge and pull the ends of the muscle cell together. As each cell in a muscle shortens, the entire muscle contracts. Click on the animation at left to see this. When the motor neurons stop producing acetylcholine and CA2+ stops being made, the myosin fiber will release the actin fiber and the contraction will stop.

27. REAL LIFE SITUATION You wake up with a start, suddenly remembering the morning soccer practice you now have on Saturday mornings. You roll over and stare at the time. It’s already 6:45, and you only have 15 minutes to get to practice. While hurriedly brushing your teeth, you briefly consider eating breakfast and risking the wrath of your coach. Nah, you think to yourself. I’m not that hungry anyway. You get to the field just in time for practice, and you run up to join your friend Taylor. Later on in practice, you notice that Taylor is sprinting every drill and working harder than anyone else. But the fact doesn’t seem very important you, because your legs and stomach are in excruciating pain.Your abs and hamstrings feel like they’re knotted tightly, and you can barely run. Ten minutes later, both you and Taylor are on the bench watching your teammates. Taylor is complaining of an intense burning in the thighs. What happened?

28. REAL LIFE SITUATION Your problem came from your lack of food. When a muscle cell does not have enough ATP, it remains in constantly contracted because the calcium ions are forcing the myosin fiber to pull on the actin fiber. This causes severe muscle cramps known as muscle fatigue. When your body ran out of ATP, your leg muscles could not relax and you began to feel a very painful cramp. Taylor’s problem, however, came from the sudden increase in exercise. By pushing his muscles beyond their endurance, Taylor put himself in a state of oxygen debt. Taylor’s body was not efficient enough to supply his muscles with enough oxygen. So his muscles needed another way to make energy without oxygen—fermentation. In fermentation, lactic acid is produced as a waste product, and builds up in the muscle. This caused the burning Taylor felt.

29. The Circulatory System What is it made up of? the heart, a network blood vessels, and blood • What does it do? • delivers oxygen and to cells within the body • transports oxygen-poor blood back to the heart to receive more oxygen Standards? a. how the complementary activity of major body systems provides cells with oxygen and nutrients and removes toxic waste products such as carbon dioxide.

30. Blood For a cell to make energy, it needs two things: ATP and oxygen. In the human body, blood is the fluid that provides oxygen and food to every cell. Blood is about 55% plasma, a brownish liquid that is essentially water with dissolved gases, salts, nutrients, waste products, and plasma proteins.There are 3 kinds of plasma proteins: globulins (transport substances and fight infection), albumins (transport substances and control blood pressure & volume), and fibrinogen (clots blood). The remaining 45% of blood consists of red blood cells, white blood cells, & platelets. Red blood cells are the most common blood cell. They are released into the bloodstream without nuclei.Instead, they are filled with an iron-producing protein called hemoglobin that transports oxygen. White blood cells are less common than red blood cells. Their job is to attack and consume foreign substances like viruses.Different types of white blood cells have different ways of destroying foreign cells.

31. Blood Platelets and Clotting Platelets are very small fragments of cytoplasm. Their key function is to clot blood whenever a blood vessel breaks. When a blood vessel breaks, platelets in the bloodstream become sticky and accumulate around the break. The platelets release proteins called clotting factors. These begin a complicated series of reactions that ends with the plasma protein fibrinogen being converted into sticky fibrins. These fibers hold the wound together until it heals.

32. Standard Ahow the complementary activity of major body systems provides cells with oxygen and nutrients and removes toxic waste products such as carbon dioxide. 1. Transporting oxygen to Cells heart The circulatory system provides oxygen to cells within the body by using the heart to pump oxygen-rich blood through arteries and arterioles (smaller arteries). Arteries are blood vessels that take blood from the heart to the body/lungs. Blood is then taken through capillaries, which are extremely tiny blood vessels, which supply cells with oxygen. artery arteriole capillary

33. Standard A (cont.) 1. Replenishing Blood with Oxygen After depleting the oxygen in the blood, cells produce carbon dioxide as a waste product. The blood in the capillaries take this oxygen-poor blood and the carbon dioxide in it to venules and then larger veins. Venules and veins are blood vessels that transport blood back to the heart. In the heart, the oxygen-poor blood is pumped into the lungs, where it releases the carbon dioxide and receives oxygen. Then the oxygen-rich blood goes back to the heart and is pumped through the body, starting the process over again. capillary venule vein heart

34. Standard A (cont.) The Heart The heart is the most important part of the human body. Pulmonary Circulation(Right Side of the heart to the Lungs) The heart receives oxygen-poor blood from the superior and inferior vena cava. The superior vena cava brings the blood from the upper part of the body, the inferior from the lower part. The oxygen-poor blood is put into the right atrium, one of the 4 chambers in the heart.Then it is pumped into the right ventricle.The tricuspid valve keeps blood from flowing back into the right atrium. Then the blood is pumped into the lungs by the pulmonary arteries. The pulmonary valve keeps the blood from flowing back into the right ventricle. NOTE: The pulmonary arteries are called arteries even though they carry oxygen-poor blood. This is because they take blood away from the heart.

35. Standard A (cont.) The Heart The heart is the most important part of the human body. Systemic Circulation(Left Side of the heart to the Body) The lungs replenish the blood with oxygen. Then it sends the oxygen-rich blood back to the heart through the pulmonary veins. NOTE: The pulmonary veins are called veins although they carry oxygen-rich blood. This is because they take blood to the heart. The oxygen-rich blood enters the left atrium. Then it is pumped into the left ventricle. The mitral valve keeps the blood from flowing back into the left atrium. From the left ventricle the blood is pumped into the aorta, a huge artery that carries all oxygen-rich blood from the heart.

36. The Lymphatic System What is it made up of? a network of vessels, the thymus, and the spleen • What does it do? • Returns lost fluid to the bloodstream • Protects against disease • Absorbs nutrients • Destroy damaged red blood cells and platelets Standards? None

37. 1. Returns lost fluid to the bloodstream The lymphatic system is closely linked with the circulatory system. When blood moves through the body, more than 3 liters of fluid leak from blood vessels into the rest of the body per day. The lymphatic system makes the circulatory system more efficient by collecting the lost fluid, known as lymph, and moving it back into the bloodstream. There are several different vessels in the lymphatic system. Lymphatic capillaries are the first vessels to collect lymph from tissue. Then the lymph moves into larger vessels, and eventually into ducts. Ducts return the lymph to the bloodstream through two holes in the superior vena cava. bloodstream lymphatic ducts lymphatic vessels

38. 2. Protects against disease Lymph vessels are lined with small knobs called lymph nodes. Lymph nodes remove bacteria and microorganisms from lymph. When lymph nodes become filled with bacteria, they swell up. Swollen tonsils are a good example of this. Lymph nodes also contain lymphocytes, which protect the body from disease. A certain kind of lymphocyte called a T cell matures in the thymus gland, another part of the lymphatic system. 3. Absorbs nutrients Blood needs to supply cells with food as well as oxygen.The lymphatic system supplies the necessary nutrients. Lymph vessels near the intestines absorb nutrients and carry them into the bloodstream.

39. 4. Destroy damaged red blood cells and platelets The Spleen The spleen is the largest organ in the lymphatic system. It is a mass of lymphatic tissue that destroys damaged blood cells and platelets. Blood cells that are too old are sent to the spleen to be broken down.

40. The Respiratory System What is it made up of? the nose, pharynx, larynx, trachea, bronchi, and lungs • What does it do? • supplies blood with oxygen • removes carbon dioxide from the blood Standards? a. how the complementary activity of major body systems provides cells with oxygen and nutrients and removes toxic waste products such as carbon dioxide.

41. Standard Ahow the complementary activity of major body systems provides cells with oxygen and nutrients and removes toxic waste products such as carbon dioxide. The Need for Air For a cell to make energy, it needs two things: ATP and oxygen. It produces water and carbon dioxide as waste products. In the human body, the respiratory system’s function is to take in oxygen and let out carbon dioxide. Breathing When a person takes a breath, the flat muscle below the lungs (the diaphragm) contracts and increases the lungs’ volume.Air rushes into the vacuum of the expanded lungs. When the diaphragm relaxes, the air in the lungs is squeezed out as their volume decreases.

42. Standard A (cont.) Breathing When a person inhales, air enters through either the nose or mouth. It moves into the throat, or pharynx, which both air and food pass through.Since lung tissue is very delicate, nostril hairs and mucus in the pharynx filter out particles. Air is moved from the pharynx into the windpipe, or trachea. Food is prevented from entering the trachea because a piece of cartilage (the epiglottis) covers the opening when you swallow. Located at the top of the trachea is the larynx, which is made of 2 elastic bands of tissue called vocal cords. As air passes through the larynx, a person can speak. Next the air passes through one of the 2 air passages that lead to the lungs (bronchi).

43. Standard A (cont.) Inside the Lungs Each bronchus separates into smaller bronchi, and then into even smaller bronchioles. The bronchioles divide until they come to a dead-end—millions of minuscule air sacs (alveoli). Each alveolus is connected to a fragile network of capillaries. As you inhale, air is brought into each alveolus. The oxygen in the air diffuses across the alveolus’ membrane and the capillary’s thin wall. At the same time, the carbon dioxide in the blood diffuses into the alveolus. When this exchange occurs and the lungs are filled with carbon dioxide, the diaphragm relaxes and the carbon dioxide is exhaled.

44. The Digestive System What is it made up of? the mouth, pharynx, esophagus, stomach, small intestine, and large intestine. The salivary glands, pancreas, and liver add secretions. • What does it do? • converts food into molecules that cells can use • Standards? • how the complementary activity of major body systems provides cells with oxygen and nutrients and removes toxic waste products such as carbon dioxide. • the individual functions and sites of secretion of digestive enzymes (amylases, proteases, nucleases, lipases), stomach acid, and bile salts.

45. The Need for Food For a cell to make energy, it needs two things: ATP and oxygen. The job of the digestive system is to ingest food and break it down into sugars that cells can convert into ATP. However, food is not needed only for its sugars. There are many other materials in food that the body needs. Water—needed to replenish fluids like blood and lymph, and to replace urine and sweat Carbohydrates—breads and fruit contain sugar used for cellular respiration, and fiber that helps with digestion Fats—needed for protection, insulation, and stored energy. Also help absorb fat-soluble vitamins Proteins—meat, eggs, and beans are needed for skin, muscle, enzymes, and amino acids

46. The Difference between Vitamins and Minerals Vitamins—are organic materials that interact with enzymes to regulate body processes. Some are created within the body and some must be ingested. Vitamin C (found in oranges and tomatoes) maintains cartilage, bone, and gums and helps heal wounds. Vitamin D (found in eggs and made by the skin when exposed to sunlight) promotes bone growth. Minerals—are inorganic materials that are used to a variety of different purposes. Usually they are needed in smaller quantities than vitamins. Calcium (found in dairy) is essential for healthy bones and teeth. Iron (found in leafy vegetables) is needed for hemoglobin in red blood cells.

47. Standard A & F how the complementary activity of major body systems provides cells with nutrients & the individual functions and sites of secretion of digestive enzymes (amylases, proteases, nucleases, lipases), stomach acid, and bile salts Beginning the Digestive Process The mouth—ingestion begins when a person puts food into his/her mouth. Teeth , whose surfaces of enamel are much harder than bone, physically tear and crush the food to pieces. As the teeth grind the food, the salivary glands at the bottom of the mouth secrete saliva, a thick fluid. Saliva contains amylase, an enzyme that breaks down starch into sugar. Saliva also contains an enzyme called lysozyme that breaks down the cell walls of bacteria that are in food. This protects the body from infection.

48. Standard A & F (cont.) Getting to the Stomach After being chewed and broken down, the tongue and throat muscles make the person swallow. The food moves into the esophagus, a long tube that leads to the stomach. As discussed previously, the epiglottis blocks food from entering the trachea. The food is forced down the epiglottis by involuntary muscle contractions. Inside the Stomach The stomach is a large sac lined with smooth muscle. The muscles around the stomach let it churn and mix the food, helping with the physical digestion of food.

49. Standard A & F (cont.) Chemical Digestion in the Stomach The stomach is lined with millions of gastric glands that release fluids into the stomach. Mucus protects the stomach wall from damage. Hydrochloric acid makes the food in the stomach very acidic, and activates an enzyme called pepsin. Pepsin and hydrochloric acid work together to break down proteins into smaller pieces. The combination of chemical and physical digestion in the stomach breaks food into a mixture called chyme. About an hour after ingestion, the pyloric valve between the stomach and small intestine, opens to slowly allow chyme to enter the small intestine.

50. Standard A & F (cont.) The Duodenum The duodenum is the first of the three parts of the small intestine. Inside the duodenum, enzymes and fluids from the liver and pancreas digest the chyme further. The pancreas produces amylase, protease, and lipase, which neutralize the stomach acid and also break down starch, proteins, and fat. The liver produces bile, a fluid filled with lipids and salts. Bile dissolves fats so that they can be broken down. Bile is stored in the gall bladder, a small organ beneath the liver.