Human Biology Study Guide: Muscular, Respiratory, Digestive Systems & Genetics

Human Muscular System

Muscle Contraction

The process of muscle contraction is fundamental to movement and body function. It involves the interaction of specialized proteins within muscle cells.

• Sarcomere: The basic functional unit of a muscle, which shortens during contraction.

• Actin: Thin filament pulled toward the center of the sarcomere during contraction.

• Myosin: Thick filament that pulls on actin to cause contraction.

• Sliding Filament Theory: Explains how muscles contract:

  1. Nerve signal reaches muscle.

  2. Acetylcholine (ACh) is released.

  3. Muscle cell receives an electrical signal.

  4. Calcium is released.

  5. Calcium enables myosin to attach to actin.

  6. Myosin pulls actin.

  7. Sarcomere shortens, resulting in muscle contraction.

• Important: Actin and myosin do not shorten; they slide past each other.

Motor Neuron & Action Potential

Muscle contraction is initiated by signals from motor neurons.

• Action Potential: An electrical signal traveling down a neuron, triggered when threshold is reached.

• Sequence at the muscle:

  1. Action potential arrives.

  2. Ca2+ enters neuron.

  3. ACh is released.

  4. Muscle receives electrical signal.

  5. Ca2+ released in muscle cell.

  6. Contraction occurs.

• Acetylcholine (ACh): Neurotransmitter released by motor neuron, signals muscle to contract.

• Sarcoplasmic Reticulum: Stores and releases calcium in response to muscle signals.

• T-tubules: Carry electrical signals deep into the muscle cell.

Muscle Heat Production

Muscle activity is a major source of body heat.

• Muscle contractions use energy; some is released as heat.

• Shivering produces heat through rapid muscle contractions.

Major Muscle Groups

Muscles are classified by location and function.

Muscle	Location/Function
Biceps	Front of upper arm; bends elbow
Triceps	Back of upper arm; straightens elbow
Deltoid	Shoulder
Pectorals	Chest
Abdominals	Stomach area
Gluteus maximus	Buttocks
Quadriceps	Front of thigh
Hamstrings	Back of thigh
Gastrocnemius	Calf

Isotonic vs. Isometric Contractions

Muscle contractions can be classified based on whether they produce movement.

• Isotonic: Muscle changes length, causing movement (e.g., lifting a dumbbell).

• Isometric: Muscle length remains the same, creating tension without movement (e.g., holding a plank).

Remember: Isotonic = movement; Isometric = no movement.

Human Respiratory System

Purpose of Breathing

Breathing is essential for gas exchange and energy production.

• Brings oxygen into the body.

• Removes carbon dioxide.

• Oxygen is required for cellular respiration to produce energy.

Respiratory Structures

The respiratory system is divided into upper and lower regions, each with specialized structures.

• Upper Respiratory System:

  • Nose/Nasal cavity: Entry point; filters, warms, and moistens air.

  • Pharynx: Passageway for air.

  • Larynx: Voice box; contains vocal cords.

  • Epiglottis: Covers airway during swallowing, preventing food from entering lungs.

• Lower Respiratory System:

  • Trachea: Windpipe; carries air toward lungs.

  • Bronchi: Two main branches entering lungs.

  • Bronchioles: Smaller airways inside lungs.

  • Alveoli: Tiny air sacs where gas exchange occurs.

  • Diaphragm: Main muscle used for breathing.

Mechanics of Breathing: Inspiration vs. Expiration

Breathing involves changes in lung volume and pressure.

• Inspiration (breathing in):

  1. Diaphragm contracts and moves down.

  2. Rib cage moves up and out.

  3. Lung volume increases.

  4. Lung pressure decreases.

  5. Air moves into the lungs.

  More space → less pressure → air IN

• Expiration (breathing out):

  1. Diaphragm relaxes and moves up.

  2. Rib cage moves down and in.

  3. Lung volume decreases.

  4. Lung pressure increases.

  5. Air moves out.

  Less space → more pressure → air OUT

Intercostal Muscles

These muscles assist breathing by moving the rib cage.

• Located between the ribs.

• Inhalation: ribs move up and out.

• Exhalation: ribs move down and in.

Blood Flow Through the Heart

The heart pumps blood through a specific pathway to ensure oxygenation.

1. Body → right atrium → right ventricle → pulmonary arteries → lungs → pulmonary veins → left atrium → left ventricle → aorta → body

• Oxygen-rich blood: Lungs → pulmonary veins → left atrium → left ventricle → aorta → body

• Pulmonary arteries: Carry oxygen-poor blood.

• Pulmonary veins: Carry oxygen-rich blood.

Respiratory Disorders

Several disorders can affect the respiratory system.

Disorder	Description
Asthma	Airways become narrow/inflamed; breathing is difficult.
Emphysema	Damages alveoli; impairs gas exchange.
Bronchitis	Airways inflamed; excess mucus may be produced.
Pneumonia	Infection/inflammation in lungs; breathing is difficult.

Human Digestive System

Main Purpose of Digestion

Digestion breaks down food into smaller molecules for absorption and use by the body.

Digestive Tract

Food passes through a series of organs in a specific order.

1. Mouth → pharynx → esophagus → stomach → small intestine → large intestine → rectum → anus

• Mouth: Chews food; saliva (with amylase) begins digestion of starch.

• Esophagus: Moves food to stomach via peristalsis.

• Stomach: Stores/mixes food; acid aids digestion; begins protein digestion.

• Small Intestine: Most chemical digestion and nutrient absorption occur here.

• Large Intestine: Absorbs water; forms feces.

• Rectum: Stores feces.

• Anus: Removes feces from the body.

Accessory Organs

These organs aid digestion but food does not pass through them.

• Liver: Produces bile; processes nutrients.

• Gallbladder: Stores and releases bile into small intestine.

• Pancreas: Produces digestive enzymes; sends them to small intestine.

Digestive Enzymes

Enzymes break down food into absorbable molecules.

Enzyme	Breaks Down
Amylase	Carbohydrates/starch
Pepsin	Proteins
Lipase	Fats/lipids

• Carbohydrates → simple sugars

• Proteins → amino acids

• Fats → fatty acids + other small fat molecules

Surface Area in Digestion

The small intestine has villi and microvilli to increase surface area for absorption.

• More surface area = more efficient nutrient absorption.

Peristalsis

Peristalsis refers to waves of muscle contractions that push food through the digestive tract.

• Alternating squeeze and push actions move food along.

Metabolic Rate

Metabolic rate measures the energy used by the body.

• Standard Metabolic Rate (SMR): Energy used under standard conditions.

• Basal Metabolic Rate (BMR): Minimum energy needed at rest to stay alive (includes breathing, heartbeat, temperature control).

MyPlate Nutrition Guide

MyPlate is a guide for balanced nutrition.

• Encourages eating fruits, vegetables, grains, protein, and dairy.

• Purpose: promote balanced nutrition.

Human Genetics & Inheritance

DNA, Genes, & Chromosomes

Genetic information is stored and transmitted through DNA, genes, and chromosomes.

• DNA: Molecule storing genetic information.

• Gene: Section of DNA coding for a trait or protein.

• Chromosome: Structure made of DNA and proteins; humans have 46 chromosomes (23 pairs).

Genetics & Genome

Genetics is the scientific study of heredity and variation.

• Genome: All of an organism's genetic material.

Homologous Chromosomes & Karyotype

Homologous chromosomes are matching pairs, one from each parent.

• Carry same types of genes, may have different alleles.

• Karyotype: Picture of chromosomes arranged in pairs; used to identify chromosome number and abnormalities.

• Normal human karyotype: 23 pairs = 46 chromosomes.

Genotype vs. Phenotype

Genotype refers to genetic makeup; phenotype is the observable trait.

• Genotype: Examples: BB, Bb, bb

• Phenotype: Example: brown eyes

• Genotype = genes; Phenotype = what you see

Punnett Squares

Punnett squares predict possible offspring genotypes and phenotypes.

• Example: Bb × Bb

• Possible offspring: BB, Bb, Bb, bb

• Results: 25% BB, 50% Bb, 25% bb

• 75% dominant phenotype, 25% recessive phenotype

Dominant & Recessive Alleles

Alleles can be dominant or recessive.

• Dominant: Expressed when at least one copy is present (e.g., B).

• Recessive: Expressed when two copies are present (e.g., b).

Homozygous vs. Heterozygous

Describes allele combinations.

• Homozygous: Two of the same alleles (BB or bb).

• Heterozygous: Two different alleles (Bb).

• Homo = same; Hetero = different

Laws of Mendel

Mendel's laws explain inheritance patterns.

• Law of Segregation: Two alleles for a gene separate when gametes are made; each egg or sperm gets one allele.

• Law of Independent Assortment: Different genes are usually inherited independently, creating different trait combinations. Exception: linked genes.

Genetic Disorders

Disorders can be caused by dominant or recessive alleles.

• Dominant disorder: One copy of harmful allele causes disorder.

• Recessive disorder: Usually requires two copies; carriers (Aa) do not show disorder.

Incomplete Dominance & Codominance

Some traits do not follow simple dominance.

• Incomplete Dominance: Neither allele completely dominates; phenotype is intermediate (e.g., red + white = pink).

• Codominance: Both alleles are fully expressed (e.g., AB blood type).

Polygenic Inheritance

Traits controlled by multiple genes.

• Examples: height, skin color

Environment & Phenotype

Environment can influence gene expression.

• Nutrition, exercise, sun exposure can affect traits.

• Phenotype = genes + environment

Linked Genes & Crossing Over

Genes close together on a chromosome are more likely to be inherited together.

• Crossing over during meiosis can separate linked genes, creating new combinations.

Meiosis & New Allele Combinations

Meiosis creates genetic diversity.

• Crossing Over: Homologous chromosomes exchange DNA.

• Independent Assortment: Chromosomes randomly separated into gametes.

• Random Fertilization: Any sperm can combine with any egg.

Sex-Linked Genes

Genes located on sex chromosomes, especially the X chromosome.

• Examples: red-green color blindness, hemophilia

• Males have one X chromosome; X-linked recessive alleles are expressed if present.

Nondisjunction

Nondisjunction occurs when chromosomes fail to separate correctly during meiosis.

• Results in gametes with too many or too few chromosomes.

• Example: Down syndrome (trisomy 21).

Last-Minute Things to Memorize

• Muscle: ACh → electrical signal → calcium → actin + myosin → contraction

• Breathing: Inhale = volume ↑, pressure ↓; Exhale = volume ↓, pressure ↑

• Gas exchange: Alveoli = O2 in, CO2 out

• Heart: Body → right heart → lungs → left heart → body

• Digestion: Mouth → esophagus → stomach → small intestine → large intestine → rectum → anus

• Genetics: Genotype = genes; Phenotype = appearance/trait; Homozygous = same; Heterozygous = different; Incomplete dominance = blend; Codominance = both show; Nondisjunction = chromosomes don't separate; Crossing over = chromosomes exchange DNA