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

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 the muscle cell.

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

• Actin: The thin filament pulled toward the center of the sarcomere.

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

• Sliding Filament Theory: Describes how actin and myosin slide past each other, not shorten, to produce contraction.

1. Nerve signal reaches muscle.

2. Acetylcholine (ACh) is released.

3. Muscle cell receives an electrical signal.

4. Calcium is released from the sarcoplasmic reticulum.

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 get shorter; they slide past each other.

Motor Neuron and Action Potential

Motor neurons transmit signals that initiate muscle contraction.

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

• At the muscle: Action potential → Ca2+ enters neuron → ACh released → muscle electrical signal → Ca2+ released → contraction.

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

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

• T-tubules: Carry the electrical signal deeper into the muscle cell.

Muscle Heat Production

Muscle contractions require energy, and some of this energy is released as heat.

• Muscle activity is the main source of body heat.

• Shivering produces heat through rapid muscle contractions.

Major Muscle Groups

Muscles are grouped by location and function. Understanding these groups is essential for anatomy and physiology.

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 stays 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, providing oxygen for cellular respiration and removing carbon dioxide.

• Oxygen is needed to make energy through cellular respiration.

• Breathing removes carbon dioxide, a waste product.

Respiratory Structures

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

• Upper Respiratory System:

  • Nose/Nasal cavity: Air enters, filtered, warmed, and moistened.

  • Pharynx: Passageway for air.

  • Larynx: Voice box, contains vocal cords.

  • Epiglottis: Covers airway during swallowing, prevents 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.

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, located between the ribs, assist in breathing by moving the rib cage.

• 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, ensuring oxygenation and circulation.

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, impacting breathing and gas exchange.

• Asthma: Airways become narrow/inflamed, making breathing difficult.

• Emphysema: Damages alveoli, making gas exchange harder.

• Bronchitis: Airways become inflamed, can cause excess mucus.

• Pneumonia: Infection/inflammation in the lungs, can make breathing difficult.

Human Digestive System

Main Purpose of Digestion

Digestion breaks food into smaller molecules that the body can absorb and use for energy, growth, and repair.

Digestive Tract

The digestive tract is a continuous tube through which food passes and is processed.

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 using peristalsis.

• Stomach: Stores and mixes food; acid helps 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 the small intestine.

• Pancreas: Produces digestive enzymes; sends them into the 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 absorption of nutrients.

Peristalsis

Peristalsis is the wave-like muscle contractions that push food through the digestive tract.

• Squeeze → push → squeeze → push

Metabolic Rate

Metabolic rate refers to the energy used by the body under various conditions.

• 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 healthy eating, promoting balanced nutrition.

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

• Purpose: balanced nutrition.

Human Genetics & Inheritance

DNA, Genes, and Chromosomes

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

• DNA: Molecule that stores genetic information.

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

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

Genetics and Genome

Genetics is the scientific study of heredity and variation; the genome is all of an organism's genetic material.

Homologous Chromosomes

Homologous chromosomes are matching pairs, one from each parent, carrying the same types of genes but possibly different alleles.

Karyotype

A karyotype is a picture of a person's chromosomes arranged in pairs, used to count chromosomes and identify 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, affecting trait expression.

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

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

Homozygous vs. Heterozygous

Describes the combination of alleles for a gene.

• 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 how traits are inherited.

• 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 the harmful allele causes the disorder.

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

Incomplete Dominance

Neither allele completely dominates; phenotype is intermediate/blended.

• Example: Red + White = Pink

• Incomplete dominance = BLEND

Codominance

Both alleles are fully expressed.

• Example: AB blood type

• Codominance = BOTH SHOW

Polygenic Inheritance

Traits controlled by multiple genes.

• Examples: Height, skin color

• Poly = many

Environment & Phenotype

Environment can affect gene expression and phenotype.

• Nutrition affects height.

• Exercise affects muscle development.

• Sun exposure affects skin pigmentation.

• Phenotype = genes + environment

Linked Genes

Genes located close together on the same chromosome are more likely to be inherited together.

• Crossing over can separate linked genes and create new combinations.

Meiosis & New Allele Combinations

New combinations arise from crossing over, independent assortment, and random fertilization.

• Crossing Over: Homologous chromosomes exchange DNA.

• Independent Assortment: Chromosomes are 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, can cause sex-linked traits.

• Examples: Red-green color blindness, hemophilia

• Males have one X chromosome, so X-linked recessive alleles are more likely to be expressed.

Nondisjunction

Nondisjunction occurs when chromosomes fail to separate correctly during meiosis, leading to abnormal chromosome numbers.

• Gametes may have 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