1. Chapter 11: Human Organization

2. Types of Tissues
A tissue is composed of specialized cells that perform a function in the body.
The human body has four major types of tissues:
Epithelial tissue
Connective tissue
Muscular Tissue
Nervous Tissue
Epithelial tissue covers body surfaces and lines body cavities. Connective tissue binds and supports body parts. Muscular tissue moves body parts. Nervous tissue receives stimuli and conducts impulses from one body part to another.

3. Cancers are classified by the type of tissue from which they arise:
Carcinomas – cancers of epithelial tissue
Sarcomas – cancers of connective tissue
Leukemias – cancers of blood
Lymphomas – cancers of lymphatic tissue
Cancers are more likely to arise in tissues in which cells divide rapidly.
Carcinomas and leukemias are common types of cancer due, in part, to the fact that they arise in tissue with a high rate of cell division.

4. Epithelial Tissue
Epithelial tissue (epithelium) is made of highly packed cells that line the body surface and inner body cavities.
Epithelial tissue functions in protection, secretion, absorption, excretion, and filtration.
Epithelial tissue is classified according to cell type.

5. Squamous epithelium is composed of flattened cells and is found lining the lungs and blood vessels.
Cuboidal epithelium contains cube-shaped cells and is found lining the kidney tubules.
Columnar epithelium has elongated cells with nuclei at the bottom of cells and is found in the digestive tract.
Ciliated columnar epithelium is found lining the oviducts.

6. Epithelial tissue is also classified according to the number of layers in a tissue.
Simple means the tissue has a single layer of cells.
Stratified means the tissue has layers of cells piled on top of one another.
Pseudostratified means the epithelium appears layered but is not.

7. Simple squamous epithelium
Simple squamous epithelium has flattened cells, occurs in air sacs of lungs, walls of capillaries, and lining of blood vessels. This type of epithelium functions in protection, diffusion, and filtration. The tiniest blood vessels, called capillaries, are made up of highly permeable simple squamous epithelium.

8. Simple cuboidal epithelium
Simple cuboidal epithelium has cube-shaped cells, occurs in the lining of the kidney tubules and on the surfaces of ovaries. It functions in protection, secretion, and absorption.

9. Simple columnar epithelium
Simple columnar epithelium has rectangle-shaped cells, occurs in the linings of the intestines and uterus, and functions in protection, secretion, and absorption.

10. Pseudostratified ciliated columnar epithelium
Pseudostratified ciliated columnar epithelium appears to be layers but is not. Instead, nuclei appear at various positions within cells, giving the appearance of layers. This type of epithelium occurs in the lining of the respiratory tract where a secreted covering of mucus traps foreign particles, and the upward motion of cilia carries the mucus to the back of the throat where it may be swallowed or expectorated. Smoking inhibits secretion of mucus and inhibits ciliary action, and can result in bronchitis.

11. Some epithelial cells are glandular and secrete a product.
A basement membrane joins epithelium to an underlying layer of connective tissue.
Some epithelial cells are glandular and secrete a product.
A gland may be a single cell or contain many cells.
Mucus-secreting digestive glands are single goblet cells.
Exocrine glands secrete products into ducts, while endocrine glands secrete directly into the bloodstream.
The basement membrane consists of glycoprotein from epithelial cells and collagen fibers from the connective tissue.

12. Junctions Between Epithelial Cells
Junctions that occur between cells help cells function as a tissue.
A tight junction forms an impermeable barrier between cells.
A gap junction allows material to pass from one cell to the next.
Adhesion junctions adhere cells together so tissues can stretch.

13. Epithelial tissues are held tightly together by tight junctions (a); (b) gap junctions that allow materials to pass from cell to cell; and (c) adhesion junctions that allow tissues to stretch.
A tight junction forms an impenetrable barrier because adjacent plasma membrane proteins actually join in a zipper-like fashion. In the intestine, tight junctions prevent gastric juices from entering the body, and in the kidneys, urine stays within kidney tubules because epithelial cells are joined by tight junctions.
A gap junction forms when two adjacent plasma membrane channels join. This lends strength, but it also allows certain ions and small molecules to pass between two cells. Gap junctions in the heart and smooth muscle ensure coordinated contraction.
In an adhesion junction, also called a desmosome, the adjacent plasma membranes do not touch but are held together by intercellular filaments firmly attached to button-like thickenings. In some organs, such as the heart, stomach, and urinary bladder where tissues are stretched, adhesion junctions hold cells together.

14. Connective Tissue
Connective tissue binds organs together, provides support and protection, fills spaces, produces blood cells, and stores fat.
Connective tissue cells are separated by noncellular matrix that ranges from solid to semifluid.
The matrix houses fibers of three possible types.

15. White collagen fibers contain the protein collagen; these fibers are flexible and strong.
Reticular fibers are very thin, highly branched collagen fibers that form delicate supporting networks.
Yellow elastic fibers contain the protein elastin; these fibers are more elastic and not as strong as collagen fibers.

16. Loose Fibrous and Dense Fibrous Tissues
Loose fibrous and dense fibrous connective tissues have cells called fibroblasts in a matrix containing collagen and elastic fibers.
Loose fibrous connective tissue supports epithelium and many internal organs.
Dense fibrous connective tissue, packed with collagen fibers, is found in tendons and ligaments.
The presence of loose fibrous connective tissue in lungs, arteries, and the urinary bladder allows these organs to expand. It forms a protective covering enclosing many internal organs, such as muscles, blood vessels, and nerves.
Dense fibrous connective tissue is found in tendons, which attach muscles to bones, and in ligaments, which connect bones to other bones at joints.

17. Loose fibrous connective tissue
Loose fibrous connective has space between components, occurs beneath skin and most epithelial layers, and it functions in support and binds organs.

18. Adipose Tissue and Reticular Connective Tissue
In adipose tissue, fibroblasts enlarge and store fat to be used for energy, insulation, and organ protection.
Adipose tissue is found beneath the skin and around certain internal organs.
Reticular connective tissue forms the supporting meshwork of lymphoid tissue in lymph nodes, the spleen, thymus, and bone marrow.
All types of blood cells are produced in red bone marrow, but a certain type of lymphocyte (T lymphocyte) completes its development in the thymus. The lymph nodes store lymphocytes.

19. Adipose tissue
Adipose tissue has cells filled with fat, occurs beneath the skin and around organs, including the heart, and functions in insulation and fat storage.

20. Cartilage
Cartilage cells lie in small chambers called lacunae separated by a solid but flexible matrix.
Hyaline cartilage contains very fine collagen fibers and is found at the ends of bones, in respiratory passages, and in the nose.
The fetal skeleton is made of hyaline cartilage and is replaced by bone.
The matrix of hyaline cartilage has a white, translucent appearance. Cartilage in general lacks a direct blood supply and thus heals very slowly.

21. Hyaline cartilage
Hyaline cartilage has cells in scattered lacunae. This type of tissue occurs in the nose and walls of respiratory passages, and at the ends of bones, including the ribs. It functions in support and protection.

22. Flexible elastic cartilage has abundant elastic fibers and is found in the framework of the outer ear.
Fibrocartilage has a matrix with strong collagen fibers.
Fibrocartilage is found in structures that withstand tension and pressure, such as the pads between the vertebrae in the backbone and the wedges in the knee joint.

23. Bone
Bone is the most rigid connective tissue with its matrix of calcium and other inorganic salts and protein fibers.
Compact bone makes up the shafts of long bones and consists of cylindrical units called osteons.
Osteons contain a central canal through which blood vessels carry blood and nutrients to bone tissue.

24. Compact bone
Compact bone has cells in concentric rings of lacunae. This type of tissue occurs in bones of the skeleton and functions in support and protection.

25. In compact bone, cells lie within lacunae which are interconnected by tiny nutrient-delivering canals called canaliculi.
The ends of long bones contain spongy bone.
Spongy bone contains many bony bars and plates located along lines of stress, separated by irregular spaces.
Spongy bone is strong yet lightweight.

26. Blood
Blood is a fluid connective tissue containing blood cells in liquid plasma.
Blood has many functions:
Keeps body chemistry within limits
Transports nutrients and wastes to cells
Distributes heat
Keeps pH and ions in balance
Protects against blood loss and disease
Blood is unlike other types of connective tissue in that the matrix (i.e., plasma) is not made by the cells. Some people prefer to classify blood separately as vascular tissue rather than grouping it with connective tissue as is done here.

27. Blood, a fluid tissue
If blood is placed in a test tube and prevented from clotting, it separates into two layers. The upper, liquid layer, called plasma, represents about 55% of the volume of whole blood and contains a variety of inorganic and organic substances dissolved or suspended in water.
The lower layer consists of red blood cells (erythrocytes), white blood cells (leukocytes), and blood platelets (thrombocytes). Collectively, these are called the formed elements and represent 45% of the volume of whole blood. Formed elements are manufactured in the red bone marrow of the skull, ribs, vertebrae, and ends of the long bones.

28. Components of Blood Plasma
Inorganic ions: (electrolytes)
Gases:
Plasma proteins:
Organic nutrients:
Nitrogenous wastes:
Na+, Ca2+, K+, Cl2, HCO3-, HPO42+
O2, CO2
Albumins, globulins, fibrinogen
Glucose, lipids, amino acids, phospholipids
Urea, ammonia

29. Formed elements of the blood include:
Red blood cells – small, biconcave, and lacking nuclei, with hemoglobin that transports oxygen
White blood cells – larger, nucleate, and fight infection and produce antibodies
Platelets – fragments of larger cells that form a plug in damaged blood vessels, thus helping in the clotting process
Hemoglobin is composed of four units; each is composed of the protein globin and a complex iron-containing structure called heme. The iron forms a loose association with oxygen, and in this way red blood cells transport oxygen.
White blood cells differ in appearance because of the way they take up stain during preparation for viewing through the microscope. Some white blood cells are phagocytic and engulf infectious pathogens, while other white blood cells produce antibodies, molecules that combine with foreign substances to inactivate them.
Platelets are not complete cells; rather they are fragments of giant cells present only in bone marrow.

30. Formed elements of blood

31. Muscular Tissue
Muscular tissue is made up of cells called muscle fibers.
All muscular tissue contains actin filaments and myosin filaments; the interaction of these accounts for movement.
Three types of vertebrate muscle are skeletal, smooth, and cardiac.
Muscle fibers are whole cells; fibers found in connective tissues are protein fibers and not cells.

32. Skeletal muscle, under voluntary control, is attached by tendons to bones and allows for movement.
Skeletal muscle fibers are long and cylindrical with many nuclei just inside the plasma membrane.
Actin filaments and myosin filaments form a striated appearance in skeletal muscle.

33. Skeletal muscle
Skeletal muscle has striated cells with multiple nuclei. It occurs in muscles attached to the skeleton and functions in voluntary movement of the body.

34. Smooth (visceral) muscle is involuntary and nonstriated.
Long, tapered cells, each with a single nucleus, form layers within the smooth muscle.
Smooth muscle is found in the walls of the digestive tract and in blood vessels.
Smooth muscle contracts more slowly than skeletal muscle but can remained contracted for a longer period.
Smooth muscle is responsible for the movement of food through the lumen of the digestive tract. When the smooth muscle of the blood vessels contracts, blood vessels constrict, helping to raise blood pressure.

35. Smooth muscle
Smooth muscle has spindle-shaped cells, each with a single nucleus. Cells have no striations. Smooth muscle functions in the movement of substances in lumens of the body. This type of muscle is involuntary.

36. Cardiac muscle has striations but is involuntary.
Cardiac muscle is found only in the walls of the heart and functions to pump blood.
Cardiac muscle has striations but is involuntary.
Cardiac muscle fibers are branched and have a single nucleus.
Cells are bound end to end at intercalated disks, areas where plasma membranes between cells contain adhesion junctions and gap junctions.
Cardiac muscle combines the features of both smooth muscle and skeletal muscle.

37. Cardiac muscle
Cardiac muscle has branching striated cells, each with a single nucleus. It occurs only in the walls of the heart and functions in the pumping of blood. Cardiac muscle is involuntary.

38. Nervous Tissue
Nervous tissue found in the brain and spinal cord is made up of cells called neurons.
Neurons have three parts:
Dendrites – carry impulses to the neuron
Cell body – houses nucleus
Axon – carries impulse away from cell
Axons are insulated with myelin; axons are bound together to form nerves.
Myelin is a white, fatty substance that covers long axons. The term fiber is used to refer to an axon along with its myelin sheath, if it has one. Outside the spinal cord and brain, fibers bound together by connective tissue form nerves.
The nervous system has three functions: sensory input, integration of data, and motor output. Nerves conduct impulses from sensory receptors to the spinal cord and the brain where integration occurs. The phenomenon called sensation occurs only in the brain, however. Nerves also conduct impulses away from the spinal cord and brain to the muscles and glands, causing them to contract and secrete, respectively. In this way, a coordinated response to the stimulus is achieved.

39. Three types of neuroglia are found in the brain:
In addition to neurons, nervous tissue contains neuroglia, cells that support and nourish neurons.
Three types of neuroglia are found in the brain:
Microglia support neurons and engulf bacteria and cellular debris
Astrocytes provide nutrients and secrete a hormone called glia-derived growth factor
Oligodendrocytes form myelin.
Neuroglia are cells that outnumber neurons nine to one and take up half the volume of the brain. Although the primary function of neuroglia is to support and nourish neurons, research is currently being conducted to see how much they directly contribute to brain function.
Glia-derived growth factor, produced by astrocytes, may one day be used as a cure for Parkinson disease and other diseases caused by neuron degeneration.
Neuroglia don’t have a long process, but even so, researchers are now beginning to gather evidence that they do communicate among themselves and with neurons.

40. A neuron and some types of neuroglia
Neurons conduct nerve impulses. Microglia become mobile in response to inflammation and phagocytize debris. Microglia are phagocytes that clean up debris. Astrocytes lie between neurons and a capillary; therefore, substances entering neurons from the blood must first pass through astrocytes. Oligodendrocytes form the myelin sheaths around fibers in the brain and spinal cord.

41. Body Cavities and Body Membranes
The human body is divided into the ventral cavity and the dorsal cavity.
During development, the coelom becomes the ventral cavity, which is divided into thoracic and abdominal cavities.
The thoracic cavity contains the pleural cavities each containing a lung, and the pericardial cavity housing the heart.

42. Mammalian body cavities
The side view of the human body shows its body cavities. The dorsal (toward the back) cavity contains the cranial cavity and the vertebral canal. The brain is in thet cranial cavity, and the spinal cord is in the vertebral canal. The well-developed ventral (toward the front) cavity is divided by the diaphragm into the thoracic cavity and the abdominal cavity. The heart and the lungs are in the thoracic cavity, and most other internal organs are in the abdominal cavity.

43. This shows the front view of the thoracic cavity
This shows the front view of the thoracic cavity. Membranes divide the thoracic cavity into the pleura cavities, containing the right and left lungs, and the pericardial cavity, containing the heart.

44. The thoracic cavity is separated from the abdominal cavity by the diaphragm.
The upper abdominal cavity contains the stomach, liver, spleen, gall bladder, and most of the intestines.
The lower abdominal cavity contains the rectum, urinary bladder, and the rest of the large intestine.
The dorsal cavity contains the cranial cavity that houses the brain, and the vertebral canal that contains the spinal cord.
The diaphragm is a horizontal sheet of muscle.
Males have an external extension of the abdominal wall, called the scrotum, containing the testes.

45. Body Membranes
Here the term “membrane” refers to a thin lining of epithelium overlying a layer of loose connective tissue.
Body membranes line cavities and internal spaces of organs and tubes that open to the outside.
There are mucous membranes, serous membranes, synovial membranes, and meninges.

46. Mucous membranes line the tubes of digestive, respiratory, urinary, and reproductive systems.
Mucus secreted by goblet cells in the epithelial layer of mucous membrane protects the body from invasion by bacteria and viruses.
Mucus also protects the lining of the stomach from digestive juices.

47. Serous membranes have specific names according to their location:
Serous membranes line the thoracic and abdominal cavities and the organs they contain and secrete a watery fluid that lubricates the membranes.
Serous membranes have specific names according to their location:
Pleura - line pleural cavity and cover lungs
Pericardium - lines pericardial cavity and covers heart
Peritoneum - lines abdominal cavity where it forms a double-layered mesentery.
Serous membranes support the internal organs and compartmentalize the large thoracic and abdominal cavities. This helps hinder the spread of infection.
Within the abdominal cavity, peritonitis is a life-threatening infection of the peritoneum that may occur if an inflamed appendix bursts before it is removed.

48. Synovial membranes line freely movable joint cavities.
They secrete lubricating synovial fluid into the joint cavity that helps bones move freely.
The meninges are membranes in the dorsal cavity that protect the brain and spinal cord.
Meninges are composed of connective tissue only.
In rheumatoid arthritis, the synovial membrane becomes inflamed and grows thicker, restricting movement.
Meningitis is a life-threatening infection of the meninges.

49. Organ Systems
Organs work together in organ systems; all body organ systems work together.
A single organ may be part of more than one organ system.
The integumentary system is made up of the skin, including an outer epidermis and an inner dermis.
Skin covers and protects the body, houses sensory receptors, and helps in temperature regulation.

50. The integumentary system includes nails, glands, sensory receptors, and hairs.
The digestive system consists of the mouth, esophagus, stomach, small intestine, and large intestine along with associated organs.
The digestive system receives and breaks down food into nutrient molecules that are distributed to cells.
The cardiovascular system, made up of the heart and blood vessels, distributes nutrients, oxygen, and heat throughout the body, and helps to remove wastes.
Associated organs of the digestive system include the teeth, tongue, salivary glands, liver, gallbladder, and pancreas.
Nondigested materials are eventually eliminated.

52. The lymphatic system consists of lymphatic vessels that transport lymph, lymph nodes, and other lymphoid organs,
This system helps protect again disease by producing and storing lymphocytes, collects excess tissue fluid, and absorbs fats from digestion.
The immune system consists of all body cells that protect against disease.
The respiratory system consists of the lungs and branched tubes that carry air to them.
The respiratory system brings in oxygen and removes carbon dioxide from the body, and helps regulate pH.

54. The urinary system, made up of kidneys, urinary bladder, and tubes that convey urine, rids the body of nitrogenous wastes, and regulates fluid balance and pH.
The bones skeletal system protect the body and aid in movement; they also store minerals and calcium and produce blood cells in bone marrow.
The muscular system, made up of the skeletal muscles, provides movement and generates heat for the body.
Smooth muscle and cardiac muscle provide movement of internal organs.

56. The nervous system, made up of the brain, spinal cord, and nerves, receives and processes information, and causes the body to react to stimuli; the nervous system regulates the activities of the other organ systems of the body.
The endocrine system consists of hormone-secreting glands, helps to regulate the functioning of other body systems.
The reproductive systems produce sperm and egg cells, allowing humans to produce more of their own kind.

58. Integumentary System
The skin and its accessory organs make up the integumentary system.
Skin plays a significant role in homeostasis by protecting underlying tissues from trauma, infection, and water loss, and by helping to regulate temperature.
The skin synthesizes vitamin D and houses sensory receptors.

59. Regions of the Skin The skin has two regions:
Outer epidermis made up of stratified squamous epithelium, with waterproof keratin, and pigment-producing melanocytes
Inner dermis made up of fibrous connective tissue with collagen and elastic fibers, blood vessels, and sensory receptors.
A subcutaneous layer, composed of loose connective and adipose tissues, connects the dermis to underlying organs.
Within the epidermis, new cells derived from basal cells become flattened and hardened as they push to the surface. Hardening takes place because the cells produce keratin, a waterproof protein. Dandruff occurs when the rate of keratinization in the skin of the scalp is two or three times the normal rate. A thick layer of dead keratinized cells, arranged in spiral and concentric patterns, forms fingerprints and footprints. Specialized cells in the epidermis called melanocytes produce melanin, the pigment responsible for skin color.
Fat in the subcutaneous layer beneath the skin insulates the body and provides protective padding.

60. Human skin anatomy
Skin consists of two regions, the epidermis and the dermis. A subcutaneous layer lies below the dermis.

61. Accessory Organs of the Skin
Nails, glands, and hair are structures of epidermal origin even though they are located in the dermis.
Nails are a protective covering of the ends of the fingers and toes; these increase dexterity.
Nails grow from cells in the nail root; these become keratinized as they grow over the nail bed.

62. Nail anatomy
Cells produced by the nail roots become filled with keratin, forming the nail body.

63. Hair follicles are in the dermis and extend through the epidermis; arrector pili muscles allow the hair to become erect.
Each hair follicle has oil (sebaceous) glands that secrete moisturizing sebum.
Sweat (sudoriferous) glands are present in all regions of the skin and play a role in temperature regulation.
Epidermal cells form the root of hair, and their division causes a hair to grow. The cells become keratinized as they are pushed farther from the root.
If sebaceous glands fail to discharge their oily sebum, blackheads and whiteheads can result. The color of blackheads is due to oxidized sebum. Acne is an inflammation of the sebaceous glands that occurs usually during adolescence due to hormonal changes.
Sweat absorbs body heat as it evaporates.

64. Homeostasis
Homeostasis is the relative constancy of the body’s internal environment.
Internal conditions fluctuate slightly in dynamic equilibrium.
Illness results if internal conditions vary to a great degree.
A homeostatic mechanism in the body has a sensor, a regulatory center, and an effector.
A sensor detects a change in the internal environment; the regulatory center activates the effector; the effector reverses the changes and brings conditions back to normal again. Now, the sensor is no longer activated.

65. Homeostasis
Because of homeostatic mechanisms, large external changes cause only small internal changes in such parameters as body temperature and pH of the blood.

66. A sensor detects an internal environmental change and signals a regulatory center. The center activates an effector, which reverses this change.

67. Negative Feedback
Negative feedback is the primary homeostatic mechanism that keeps a variable close to its set point.
A home heating system, with its temperature set at 68oF (the set point), operates by negative feedback; many negative feedback mechanisms in the body function in a similar manner.
A set point is a particular value targeted by the homeostatic mechanism.

68. Negative feedback
When the room is cool, a thermostat that senses the room temperature signals the furnace to turn on. Once the room is warm, the furnace turns off.

69. When blood pressure falls, special sensory receptors in blood vessels signal a regulatory center in the brain. The brain signals the arteries to constrict, and blood pressure rises to normal. This response reverses the change. Once the blood pressure rises to normal, the system is inactivated.

70. Regulation of Body Temperature
The regulatory center for body temperature is located in the brain’s hypothalamus.
When body temperature is below normal, the hypothalamus sends nervous impulses to the skin blood vessels and they constrict, conserving heat.
If cooling continues, skeletal muscles are signaled, shivering ensues, generating heat and raising body temperature.
Temperature rises to normal, and the regulatory center is inactivated. Temperature is controlled by negative feedback in this manner.

71. When body temperature is higher than normal, the regulatory center directs skin blood vessels to dilate, radiating heat to the outside.
Sweat glands are also activated, and evaporation helps to lower body temperature.
Temperature drops to normal, and the regulatory center is inactivated.

72. Homeostasis and body temperature regulation
Negative feedback mechanisms control body temperature so that it remain relatively stable at 37C. These mechanisms return the temperature to normal when it fluctuates above or below this set point.

73. Positive Feedback
Positive feedback mechanisms bring about an ever greater change in the same direction and does not achieve relative stability; they may have a cut-off point.
Examples of positive feedback:
Childbirth and the hormone oxytocin
Blood clotting
Digestion of protein in the stomach
Certain fevers
The first three example do have cut-off points; fever may not have a cutoff point and without treatment can lead to death in certain cases. Death occurs at a body temperature of 45C because cellular proteins denature at this temperature and metabolism stops.
When a woman is delivering a baby, the head of the baby presses against the cervix, stimulating receptors there. When nerve impulses reach the brain, the brain causes the pituitary gland to secrete the hormone oxytocin. Oxytocin travels in the blood and causes the uterus to contract. As labor continues the cervix is ever more stimulated and uterine contractions become stronger until birth occurs.
Positive feedback loops like those involved in childbirth, blood clotting, and the stomach’s digestion of protein assist the body in completing a process that has a definite cut-off point.

74. Homeostasis and Body Systems
The internal environment of the body consists of blood and tissue fluid.
The chemical composition of tissue fluid remains constant only as long as blood composition remains constant.
All systems of the body contribute toward maintaining homeostasis and therefore a relatively constant internal environment.
Examples of how body systems contribute to homeostasis are many; here are two.
The digestive system takes in and digests food, providing nutrient molecules that enter the blood and replace the nutrients that are constantly being used by body cells.
The chief regulators of blood composition are the liver and the kidneys. They monitor the chemical composition of plasma and alter it as required. Immediately after glucose enters the blood, it can be removed by the liver for storage as glycogen. Later, the glycogen can be broken down to replace the glucose used by the body cells; in this way, the glucose composition of the blood remains constant. The hormone insulin, secreted by the pancreas, regulates glycogen storage. The liver also removes toxic chemicals, such as ingested alcohol and other drugs. The liver makes urea, a nitrogenous end product of protein metabolism. Urea and other metabolic waste molecules are excreted by the kidneys. Urine formation by the kidneys is extremely critical to the body, not only because it rides the body of unwanted substances, but also because it offers and opportunity to carefully regulate blood volume, salt balance, and pH.

75. Regulation of tissue fluid composition
Cells are surrounded by tissue fluid, which is continually refreshed because oxygen and nutrient molecules constantly exit, and carbon dioxide and waste molecules continually enter the bloodstream.

76. Chapter Summary Human tissues are organized into four groups.
Epithelial tissues cover the body and line its cavities.
Connective tissues bind body parts together.
Muscle tissue allows the enter body or its internal organs to move and contract.
Nervous tissue conducts nerve impulses.

77. The internal organs occur within cavities.
Major body cavities include the ventral cavity, with thoracic and abdominal portions, and the dorsal cavity, which includes the cranial cavity and vertebral canal.
Body membranes line body cavities and the internal spaces of organs, and include mucous, serous, and synovial membranes, and the meninges.
Organ systems provide coordinated functions for the body.

78. Processing and transporting functions are provided by the digestive, cardiovascular, lymphatic, respiratory, and urinary systems.
The musculoskeletal system supports the body and permits movement.
The nervous system detects changes and responds to stimuli and controls the activities of other organ systems.
The endocrine system produces hormones, some of which affect the reproductive system.

79. Skin covers and protects the body, houses sensory receptors, and helps in temperature regulation.
Homeostasis is the relative constancy of the internal environment.
Most homeostasic mechanisms are by negative feedback, although some use positive feedback.
All organ systems contribute to the homeostasis of tissue fluid and blood.