Biology Chapter: Levels of Organization

Levels of Organization

• Cellular Level
• Tissue Level
• Organ Level
• Organ System Level

Body Plans

• Symmetry
• Segmentation
• Body Cavity

Homeostasis

• Ability of an organism to maintain a stable internal environment despite changes in external conditions
• Internal environment of the body refers to the extracellular fluid (interstitial fluid and plasma) that surrounds body cells

Control of Homeostasis:

• Feedback system (feedback loop) – a cycle of events in which the status of a body condition is monitored, evaluated, changed, re-monitored, and re-evaluated
• Controlled variable – monitored variable (controlled condition), such as body temperature, blood pressure, blood glucose level
• Stimulus – any disruption that changes a controlled condition.

Feedback System: Basic Components

• Receptor – a body structure that monitors changes in a controlled condition and sends input to a control center.
• Afferent Pathway – information flows toward the control center
• Control center – sets the narrow range or set point within which a controlled condition should be maintained, evaluates the input it receives from receptors, and generates commands when they are needed.
• Efferent Pathway – information flows away from the control center
• Effector – a body structure that receives output from the control center and produces a response or effect that changes the controlled condition.

Feedback System: Types of Response

• Negative feedback system – reverses a change in a control condition
  • blood pressure (BP) (controlled condition) – force exerted by blood as it presses against the walls of blood vessels
  • baroreceptors (receptor) – pressure- sensitive nerve cells located in the walls of certain blood vessels, detect the higher pressure
    • send nerve impulses (input) to the brain
  • brain (control center) – interprets the impulses and responds by sending nerve impulses (output) to the heart and blood vessels (the effectors)
  • heart and blood vessels (effectors) – heart rate decreases and blood vessels dilate (widen)
• Positive feedback system – strengthen or reinforce a change
  • stretching of the cervix (controlled condition) – force exerted by blood as it presses against the walls of blood vessels
  • stretch-sensitive nerve cells in the cervix (receptor) – monitor the amount of stretching of the cervix
    • send more nerve impulses (input) to the brain
  • brain (control center) – causes the pituitary gland to release the hormone oxytocin (output) into the blood
  • muscles in the wall of the uterus (effectors) – contract more forcefully

Respiration

• Process of supplying the body with oxygen, and removing carbon dioxide
• Three (3) basic steps:
  • Pulmonary ventilation – Breathing process involving the exchange of air between the atmosphere and lungs (O2 in, CO2 out).
  • External (pulmonary) respiration – Gas exchange between alveoli and blood, where blood gains O2 and loses CO2.
  • Internal (tissue) respiration – Gas exchange between blood and tissue cells, where blood loses O2 and gains CO2 during ATP production (cellular respiration).

Circulation

• Responsible for transporting nutrients, gases, hormones, and waste products throughout the body.
• Invertebrates
  • Open Circulatory System – arthropods (insects, spiders) and mollusks (snails), the heart pumps hemolymph (a mix of blood and interstitial fluid) into open cavities where it bathes the organs directly; no veins or capillaries
  • Closed Circulatory System – annelids (earthworms), the blood is enclosed in vessels, flowing in a continuous circuit driven by a heart
• Vertebrates
  • Fish – single-loop circulatory system where blood flows from the heart to the gills for oxygenation and then to the rest of the body; two-chambered heart