Human Body Systems

Cells: The basic structural and functional units of life. They are the smallest units that can carry out all life processes. Examples include muscle cells, nerve cells, and epithelial cells.
Tissues: Groups of similar cells that work together to perform a specific function. There are four main types of tissues in animals:
- Epithelial Tissue: Covers body surfaces and lines cavities; functions in protection, absorption, and secretion.
- Connective Tissue: Provides support, binds tissues together, and stores energy (e.g., bone, blood, adipose tissue).
- Muscle Tissue: Responsible for movement; three types: skeletal, cardiac, and smooth muscle.
- Nervous Tissue: Comprises neurons and glial cells; responsible for transmitting signals throughout the body.
Organs: Structures composed of at least two types of tissues that perform specific functions. Examples include the heart, liver, and lungs.
Body Systems: Groups of organs that work together to perform complex functions. Examples include the circulatory system, respiratory system, and digestive system. Each system plays a critical role in maintaining homeostasis.

Mouth: Begins the process of digestion through mechanical (chewing) and chemical (saliva) means.
Esophagus: Transports food from the mouth to the stomach through peristalsis (muscle contractions).
Stomach: Mixes food with gastric juices for chemical digestion; it has a highly acidic environment that aids in breaking down food and killing pathogens.
Small Intestine: Site of most chemical digestion and nutrient absorption. It has three parts: duodenum, jejunum, and ileum. Enzymes from the pancreas and bile from the liver further digest food here.
Large Intestine: Absorbs water and electrolytes; forms and stores feces.
Rectum: Stores feces until they are expelled.
Anus: The exit point for feces from the body.

Physical Digestion: The mechanical breakdown of food into smaller pieces. This includes chewing (mastication) and the churning of food in the stomach.
Chemical Digestion: The enzymatic breakdown of food into smaller molecules. This process involves various digestive enzymes that break down carbohydrates, proteins, and fats into absorbable units (e.g., amino acids, fatty acids, monosaccharides).

Heart: The muscular organ that pumps blood throughout the body. It has four chambers: two atria and two ventricles.
Blood Vessels: Includes arteries (carry blood away from the heart, usually oxygenated), veins (carry blood to the heart, usually deoxygenated), and capillaries (smallest vessels where gas exchange occurs).
Blood: The fluid that circulates in the body, composed of red blood cells, white blood cells, platelets, and plasma.

Red Blood Cells (Erythrocytes): Contain hemoglobin, which binds oxygen in the lungs and transports it to tissues. They also carry carbon dioxide from tissues back to the lungs.
White Blood Cells (Leukocytes): Part of the immune system; fight infections and foreign invaders.
Platelets (Thrombocytes): Important for blood clotting; prevent bleeding by accumulating at injury sites and forming clots.
Plasma: The liquid component of blood that carries cells, nutrients, hormones, electrolytes, and waste products throughout the body.

Nasal Cavity: Filters, warms, and moistens air as it enters the lungs. Contains mucus and cilia to trap particles and pathogens.
Pharynx: A shared passageway for air and food.
Larynx (Voice Box): Contains the vocal cords and acts as a passageway for air.
Trachea (Windpipe): The tube that connects the larynx to the bronchi; has cartilage rings to keep it open.
Bronchi and Bronchioles: Branching tubes that carry air into each lung; bronchioles lead to alveoli.
Alveoli: Microscopic air sacs where gas exchange occurs; oxygen enters the blood, and carbon dioxide is expelled.

Roots: Anchor the plant, absorb water and minerals from the soil, and store energy.
Stems: Support the plant, transport nutrients and water between roots and leaves, and store energy.
Leaves: Key sites for photosynthesis; contain chlorophyll that absorbs light energy.
Flowers: Reproductive structures that produce seeds.
Tissues: Include dermal (outer protective layer), vascular (transport), and ground tissues (support and storage).

Stomata: Small openings on the surface of leaves that regulate gas exchange. They allow carbon dioxide to enter the leaf for photosynthesis and facilitate the release of oxygen.
Stomatal opening and closing is controlled by guard cells that respond to environmental conditions (e.g., light, humidity).
Stomata play a critical role in maintaining the balance of water and gas exchange within the plant, affecting photosynthetic efficiency.