06 Biology and Human Body Systems

Cells and their structures

Human cells are eukaryotic. A plasma membrane surrounds each cell, cytoplasm fills its interior, and a nucleus contains most of its genetic material. The membrane regulates what enters and leaves the cell, helping maintain a stable internal environment.

Important cell structures have specialized roles:

• Nucleus: stores DNA and helps direct cell activity.

• Mitochondria: produce most of the cell’s usable energy, ATP, through cellular respiration.

• Ribosomes: assemble proteins.

• Rough endoplasmic reticulum: helps make and process proteins, especially proteins destined for membranes or export.

• Smooth endoplasmic reticulum: makes lipids and performs other functions, including storing ions.

• Golgi apparatus: modifies, sorts, and packages proteins and lipids.

• Lysosomes: use enzymes to break down materials and cellular waste.

• Cytoskeleton: helps maintain cell shape and supports movement of the cell and its internal parts.

Cell structures support the cell’s particular functions. For example, a muscle cell needs abundant mitochondria because contraction requires energy.

Genetics and cell division

DNA is packaged with proteins into chromosomes in the nucleus. Most human body cells have 46 chromosomes arranged in 23 pairs, while eggs and sperm each have 23 chromosomes. At fertilization, the egg and sperm each contribute one set, restoring the paired number in the resulting cell.

Different forms of a gene are called alleles. An individual’s allele combination is the genotype, while an observable characteristic is the phenotype. In a simple dominant–recessive pattern, a dominant allele can determine the phenotype when paired with a recessive allele. For example, if A is dominant and a is recessive, Aa shows the dominant phenotype, while aa shows the recessive phenotype. This model is useful for basic genetics problems, but many human traits are influenced by multiple genes, the environment, or inheritance patterns that are not simple dominant–recessive.

Mitosis produces two genetically similar body cells for growth and repair. Meiosis produces gametes with one chromosome from each pair. The separation and mixing of chromosomes help explain how alleles pass from parents to offspring.

Body organization and tissues

The body is organized in levels: molecules form organelles, organelles are parts of cells, cells form tissues, tissues contribute to organs, and organs work together in organ systems that make up the organism.

A tissue is a group of cells working together. An organ contains at least two tissue types, and an organ system is a group of organs that cooperate to perform major functions. The heart, for example, contains muscle, connective, epithelial, and nervous tissues and works with blood vessels as part of the cardiovascular system.

The four major tissue types have distinct functions:

• Epithelial tissue covers body surfaces, lines cavities, and forms many glands. Its functions include protection, absorption, and secretion.

• Connective tissue supports, binds, protects, and connects body structures. Examples include bone, blood, cartilage, and fat.

• Muscle tissue contracts to produce movement. Skeletal muscle is generally voluntary, smooth muscle moves material through internal organs, and cardiac muscle contracts in the heart.

• Nervous tissue receives and transmits signals that help coordinate body activity.

Anatomy, physiology, and organ systems

Anatomy describes body structures, while physiology explains how those structures function. Organ systems interact, so a single task such as running depends on several systems at once.

The major systems and their core functions include:

• Integumentary: skin, hair, and nails protect the body and help regulate temperature and sensation.

• Skeletal: bones, joints, and cartilage support and protect the body and work with muscles in movement.

• Muscular: skeletal, smooth, and cardiac muscle produce movement, move substances internally, and generate heat.

• Nervous: the brain, spinal cord, and nerves detect information, process it, and coordinate rapid responses.

• Endocrine: hormone-producing glands, including the thyroid and pancreas, use hormones to regulate processes such as growth and metabolism.

• Cardiovascular: the heart, blood, and blood vessels circulate blood to deliver oxygen and nutrients and carry away wastes.

• Lymphatic/immune: lymphatic vessels, lymph nodes, the spleen, and immune cells return excess tissue fluid to the blood and help defend against pathogens.

• Respiratory: airways and lungs exchange oxygen and carbon dioxide between air and blood.

• Digestive: the digestive tract, liver, pancreas, and gallbladder break down food, absorb nutrients, and eliminate undigested material.

• Urinary: the kidneys, ureters, bladder, and urethra filter blood, remove wastes, and help regulate water and electrolyte balance.

• Reproductive: reproductive organs produce gametes and support reproduction.

System boundaries are not absolute, and an organ can contribute to more than one system. For example, the pancreas has digestive functions and also releases hormones that regulate blood glucose.

Homeostasis and feedback

Homeostasis is the regulation of internal conditions within ranges that support normal function. Most homeostatic control uses negative feedback: a change triggers a response that opposes it. A typical feedback loop includes a sensor, a control center, and an effector.

When body temperature rises, for example, sensors and control centers coordinate responses such as sweating that help lower it. Blood glucose is also regulated through negative feedback. When glucose rises after a meal, the pancreas releases insulin, which promotes glucose uptake and storage; as glucose falls, the stimulus for insulin release decreases.

Positive feedback differs from negative feedback because it intensifies an initial change until a particular endpoint. Contractions during childbirth are an example.