Biology Levels of Organization and Systems

Tissues are groups of similar cells that work together to perform a specific function in the body.

Epithelial tissue covers and lines surfaces, connective tissue binds and supports, muscular tissue allows movement, and nervous tissue receives and transmits information.

Organ systems are groups of organs that work together to perform a complex function in the body.

The integumentary system includes skin, nails, hair, and associated glands. It protects the body, receives sensory information, helps regulate temperature, and synthesizes vitamin D.

The circulatory system includes the heart, blood vessels, and blood. It transports blood, nutrients, gases, and waste, fights infections, and helps maintain homeostasis.

The lymphatic system includes lymphatic vessels, lymph nodes, and lymphocytes. It manages fluid balance, absorbs fats, and fights infections.

The digestive system includes the mouth, esophagus, stomach, intestines, and associated organs. It ingests and digests food, absorbs nutrients, and eliminates waste.

The respiratory system includes the lungs and airways. It allows breathing, exchanges gases, and contributes to pH balance.

The urinary system includes the kidneys, bladder, and tubes. It eliminates metabolic wastes, helps with fluid balance, and regulates pH balance.

The nervous system includes the brain, spinal cord, and nerves. It receives sensory information, processes information, sends motor signals, and coordinates other systems.

The musculoskeletal system includes the skeletal system (bones) and the muscular system. It supports the body, protects internal organs, allows movement, stores minerals, produces blood cells, maintains posture, and generates heat.

The endocrine system includes glands that produce hormones. It coordinates other systems, responds to stress, regulates fluid and pH balance, and controls metabolism.

The reproductive system produces gametes, transports them, and produces sex hormones. In females, it also nourishes and gives birth to offspring.

The digestive system processes food through ingestion, digestion, and elimination.

Mechanical digestion breaks down food physically (like chewing), while chemical digestion uses enzymes to break down molecules.

The digestive tract is a long tube that starts at the mouth and ends at the anus. It includes the mouth, pharynx, esophagus, stomach, small intestine, and large intestine.

Teeth chew food, the tongue forms a bolus, the roof of the mouth prevents food from entering the nasal cavity, tonsils fight infection, and salivary glands produce saliva containing enzymes that can digest starch.

The pharynx is a passageway that allows air from the nose and food from the mouth to travel to other organs.

Swallowing is a reflex action that involves closing off the nasopharynx, covering the glottis, and directing food into the esophagus.

The esophagus is a long, muscular tube that uses peristalsis (muscle contractions) to move food from the mouth to the stomach.

The stomach receives food from the esophagus, mixes it with gastric juice (for chemical digestion), and churns it (for mechanical digestion). It then releases chyme (a thick, soupy food mixture) into the small intestine.

The small intestine receives chyme from the stomach, completes digestion of food, and absorbs nutrients into the bloodstream.

The duodenum (top part) receives bile and pancreatic juice, the jejunum (middle part) continues digestion and absorption, and the ileum (lower part) has immune tissues that fight infection.

Villi are finger-like projections in the small intestine that increase surface area for nutrient absorption. Microvilli are even smaller projections on the villi that further increase surface area.

The large intestine absorbs water, salts, and some vitamins. It also stores undigested material until it is eliminated as feces.

The cecum is the starting point of the large intestine, it has the appendix, the colon is divided into ascending, transverse, descending, and sigmoid parts, the rectum is the final section, and the anus is the opening for defecation.

The defecation reflex is triggered by feces entering the rectum. It involves rectal contraction and expulsion of feces from the body.

The pancreas produces pancreatic juice (for digestion) and hormones like insulin and glucagon (for blood sugar regulation).

The liver is the largest gland, detoxifies blood, makes proteins, regulates blood sugar, produces bile for fat digestion, and creates urea for waste removal.

The gall bladder stores bile from the liver and releases it into the small intestine to help digest fats.

Digestive enzymes break down carbohydrates, proteins, nucleic acids, and fats into smaller molecules that can be absorbed.

Salivary amylase breaks down starch into maltose, pancreatic amylase and maltase further break down maltose into glucose. Glucose can be absorbed.

Pepsin breaks down proteins into peptides, trypsin and peptidases further break down peptides into amino acids.

Lipase breaks down fats into glycerol and fatty acids, which are absorbed and repackaged as lipoproteins.

Enzymes work best at their optimal temperature and pH. The digestive system's pH changes as food travels through it, allowing different enzymes to be active.