Cells

• Multicellular organisms

  • Have systems of organs that work together, carrying out processes that keep us alive
  • Cells make up tissues, tissues make up organs, and organs make up organ systems
  • The function of an organ system depends on the integrated activity of its organs
  • The survival of the organism depends on the integrated activity of all the organ systems, often coordinated by the endocrine and nervous systems
• If you were a single-celled organism and you lived in a nutrient-rich place, staying alive would be pretty straightforward
• Most cells in large multicellular organisms don't directly exchange substances like nutrients and wastes with the external environment, instead, they are surrounded by an internal environment of extracellular fluid
• Specialized systems in multicellular organisms

  • Digestive system
  • Respiratory system
  • Muscular and skeletal systems
  • Reproductive system
  • Excretory system
• All the systems of the body work together to keep an organism up and running
• Cells

  The fundamental units of life
• Tissues

  Groups of similar cells that work together on a specific task
• Organs

  Structures made up of two or more tissues organized to carry out a particular function
• Organ systems

  Groups of organs with related functions
• At each level of organization—cells, tissues, organs, and organ systems—structure is closely related to function
• Four basic tissue types in humans and other large multicellular animals

  • Epithelial tissue
  • Connective tissue
  • Muscle tissue
  • Nervous tissue
• Epithelial tissue

  • Consists of tightly packed sheets of cells that cover surfaces and line body cavities
  • Epithelial cells are polarized, with a top (apical) and bottom (basal) side
  • Epithelial cells are tightly packed, acting as barriers to the movement of fluids and potentially harmful microbes
• Connective tissue

  • Consists of cells suspended in an extracellular matrix
  • Supports and connects other tissues
  • Includes loose connective tissue, dense/fibrous connective tissue, adipose tissue, bone, cartilage, and blood
• Muscle tissue

  • Essential for keeping the body upright, allowing movement, and pumping blood and moving food
  • Contains the proteins actin and myosin, which allow contraction
  • Includes skeletal muscle, cardiac muscle, and smooth muscle
• Nervous tissue

  • Involved in sensing stimuli, processing and transmitting information
  • Consists of neurons (nerve cells) and glia (supporting cells)

  
• Most organs contain all four tissue types
• Nerve impulses or action potentials

  Allow the neurons to convey information very rapidly across long distances
• Glia

  Mainly act to support neuronal function
• Neuron

  • Has projections called dendrites that receive signals
  • Has projections called axons that send signals
• Glial cells

  • Astrocytes regulate the chemical environment of the nerve cell
  • Oligodendrocytes insulate the axon so the electrical nerve impulse is transferred more efficiently
• Organs

  Made up of two or more types of tissue organized to serve a particular function
• Organs

  • Heart
  • Lungs
  • Stomach
  • Kidneys
  • Skin
  • Liver
• Small intestine

  Layered walls provide an example of how tissues form an organ
• The inside of the intestine is lined by epithelial cells, some of which secrete hormones or digestive enzymes and others of which absorb nutrients
• Around the epithelial layer are layers of connective tissue and smooth muscle, interspersed with glands, blood vessels, and neurons
• The smooth muscle contracts to move food through the gut, under control of its associated networks of neurons
• Organ systems

  Organs are grouped into organ systems, in which they work together to carry out a particular function for the organism
• Organ systems

  • Cardiovascular system
  • Respiratory system

  
• Major organ systems of the human body

  • Cardiovascular
  • Lymphatic
  • Digestive
  • Endocrine
  • Integumentary
  • Muscular
  • Nervous
  • Reproductive
  • Respiratory
  • Skeletal
  • Urinary
  • Immune
• Parts of one system may play a role in another system
• There's a lot of functional overlap among the different systems
• Organs in a system work together

  Just like workers on an assembly line, the organs of an organ system must work together for the system to function as a whole
• Digestion

  1. Taking in food

  2. Breaking it down into molecules small enough to be absorbed

  3. Absorbing it

  4. Eliminating undigested waste products
• Mechanical digestion

  Chunks of food are broken into smaller pieces
• Chemical digestion

  Large molecules like proteins and starches are broken into simpler units that can be readily absorbed
• Mechanical digestion and initial chemical digestion

  1. Takes place in the mouth and stomach

  2. Chewing breaks food into smaller pieces

  3. Stomach churns the food up into a fluid mixture

  4. Stomach acts as a storage tank, releasing partially digested food into the small intestine at a rate the small intestine can handle
• Small intestine

  • Major site of chemical digestion, carried out by enzymes released from the pancreas and liver
  • Main site of nutrient absorption; molecules like sugars and amino acids are taken up by cells and transported into the bloodstream for use
• Digestion wouldn't work so well if one of the organs or glands involved decided to take the day off
• Organ systems work together, too
• Respiratory system and circulatory system

  Work closely together to deliver oxygen to cells and to get rid of the carbon dioxide the cells produce