Anatomy & physiology · Study guide

Homeostasis, cells and tissues study guide

The cells-and-tissues unit of an anatomy and physiology course on one page: homeostasis and feedback loops with the textbook examples, the four tissue types with every subtype and where it is found, the cell organelles, membrane transport, mitosis and meiosis, flashcards and 20 practice questions with every answer explained. Free, and for study only.

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1 · Homeostasis

Homeostasis: definition and feedback loops

The idea the whole course keeps coming back to. The definition first, then the four terms, the three parts of every feedback loop, and the textbook examples of negative and positive feedback.

What homeostasis means

Homeostasis is the steady state that the body’s systems maintain: internal conditions such as temperature, blood glucose and fluid balance are held within a narrow, healthy range even while the outside world and the body’s own activity keep changing. Keeping it up means the body monitors its internal conditions all the time and corrects every drift.

Set point: The ideal value for a physiological measure, such as a body temperature of about 37 °C (98.6 °F). The actual value fluctuates a little above and below it.
Normal range: The band of values around the set point that does not cause the control center to react. Only a drift beyond it switches on a response.
Negative feedback: A mechanism that stabilizes an upset by reversing it: the response works against the change, and as the value returns to range the response is switched off. It runs continuously all over the body.
Positive feedback: A mechanism that intensifies a change instead of reversing it. It is normal only where there is a definite end point, such as childbirth or the sealing of a damaged blood vessel.

The three parts of a feedback loop

Step 1
Sensor (or receptor)
Monitors a physiological value, such as temperature or blood glucose, and reports it to the control center.
Step 2
Control center
Compares the reported value with its normal range. If it has drifted too far, it activates an effector.
Step 3
Effector
Causes a change that reverses the drift, so the value returns to its normal range.

Negative vs positive feedback

What it does to the change
Negative feedback: Reverses it: the response works against the drift
Positive feedback: Intensifies it: the response makes the change bigger
How it ends
Negative feedback: Switches off by itself as the value returns to its normal range
Positive feedback: Ends only when an outside event stops it (the birth, the sealed vessel)
How common
Negative feedback: The body’s usual way of keeping values in range, running continuously
Positive feedback: Rare; normal only where there is a definite end point
Textbook examples
Negative feedback: Body temperature; blood glucose after a meal
Positive feedback: Childbirth (oxytocin); blood clotting

The textbook examples

Negative feedback · reverses the change

Body temperature

The set point is about 37 °C (98.6 °F). Sensors report the temperature to a control center in the brain.
Too warm: the brain’s heat-loss center responds. Blood vessels in the skin widen so heat radiates away, sweat glands increase their output so evaporation cools the skin, and breathing deepens.
Too cold: the brain’s heat-gain center responds. Blood flow to the skin drops so heat stays at the core, shivering starts (muscle activity makes heat), thyroid hormone raises metabolic heat production, and epinephrine release breaks glycogen down into glucose for fuel.
Either way, once the temperature is back in range the response switches off. That is what makes it negative feedback.

Negative feedback · reverses the change

Blood glucose

After a meal, glucose in the blood rises. Beta cells in the pancreas detect the extra glucose and release insulin.
Insulin signals skeletal muscle fibers, fat cells and liver cells to take glucose in, so the blood level falls.
As the level comes back into its normal range, the stimulus is gone and insulin release stops.

Positive feedback · strengthens the change

Childbirth

Stretch-sensitive cells in the cervix signal the brain, which has the pituitary gland release oxytocin.
Oxytocin makes the uterine muscle contract harder, which pushes the baby further down and stretches the cervix more, which releases more oxytocin.
The cycle strengthens itself until an outside event ends it: the birth of the baby.

Positive feedback · strengthens the change

Blood clotting

After an injury, the wall of the damaged vessel releases substances that start the clotting process.
Each step stimulates the release of more clotting substances, so the response builds on itself until the damaged area is sealed.

For study only. These are the textbook loops, written to explain the idea, not advice about anyone’s health.

2 · Tissues

The four tissue types and every subtype

A tissue is a group of cells that share a common embryonic origin and work together on one function. Histology is the microscopic study of how tissues look, how they are organized and what they do. Every tissue in the body belongs to one of four types.

Epithelial tissue
Sheets of tightly packed cells that cover body surfaces, line cavities and passageways, and form the glands.
Arises from all three germ layers (ectoderm, mesoderm and endoderm).
Connective tissue
Binds the body’s cells and organs together and gives protection, support and a framework; blood and bone are both connective tissues.
Arises from mesoderm.
Muscle tissue
Excitable tissue that contracts to produce movement. Three kinds: skeletal, cardiac and smooth.
Arises from mesoderm.
Nervous tissue
Excitable tissue that carries electrochemical impulses between regions of the body: neurons and the neuroglia that support them.
Arises mainly from ectoderm.

Epithelial tissue: every type, where it is found and what it does

Epithelia are named for two things. The cell shape: squamous (flat and thin), cuboidal (box-like, about as wide as it is tall) or columnar (taller than wide). The layers: simple (one layer, every cell resting on the basal lamina), stratified (two or more layers, only the bottom layer on the basal lamina; named for the shape of the cells in the top layer), pseudostratified (one layer of irregularly shaped cells that looks layered) or transitional (a stratified epithelium whose cells change shape as the organ stretches).

Simple squamous epithelium
Layers and shape: One layer of flat, thin cells
Where it is found: Air sacs (alveoli) of the lungs; the lining of the heart, blood vessels and lymphatic vessels
What it does: Lets materials pass by diffusion and filtration; secretes lubricating substances
The thinnest epithelium, built for exchange. The simple squamous layer of a serous membrane is called the mesothelium, and it secretes the serous fluid (ch. 4.1).
Simple cuboidal epithelium
Layers and shape: One layer of box-like cells
Where it is found: Ducts and secretory portions of small glands; the tubules of the kidney
What it does: Secretion and absorption
Simple columnar epithelium
Layers and shape: One layer of tall, rectangular cells
Where it is found: Ciliated: larger bronchioles, the uterine tubes and the uterus. Non-ciliated: the lining of the stomach and intestines
What it does: Absorption; secretion of mucus and enzymes
Pseudostratified columnar epithelium
Layers and shape: One layer of irregularly shaped cells that looks like several layers
Where it is found: Trachea, bronchi and much of the upper respiratory tract
What it does: Secretes mucus; its cilia move the mucus along
Every cell touches the basal lamina, so it is a simple epithelium despite the look.
Stratified squamous epithelium
Layers and shape: Several layers; the top layer is flat cells
Where it is found: Non-keratinized (moist): the lining of the mouth and esophagus. Keratinized (dry): the surface of the skin
What it does: Protection against abrasion
In the keratinized form on the skin the outermost cells are dead and filled with keratin, which protects against drying and pathogens (ch. 4.1). The mouth lining is the non-keratinized form.
Stratified cuboidal epithelium
Layers and shape: Two or more layers; the top layer is box-like cells
Where it is found: Ducts of the sweat glands, salivary glands and mammary glands
What it does: Protective lining
Stratified columnar epithelium
Layers and shape: Several layers; the top layer is tall cells
Where it is found: Parts of the urethra; some gland ducts
What it does: Secretion and protection
Uncommon in the body.
Transitional epithelium
Layers and shape: Several layers of cells that change shape, from rounded to flat, as the organ fills
Where it is found: Urinary bladder, ureters and part of the urethra
What it does: Lets the urinary organs expand and stretch

What makes a tissue epithelial: five features

Highly cellular: The cells sit tightly together with little or no extracellular material between them, joined by cell junctions.
Polarity: Each cell has an apical surface that faces the outside or an open space and a basal surface that faces the tissue beneath; the two surfaces carry different proteins and organelles.
Basement membrane: The epithelium secretes a basal lamina (glycoproteins and collagen); the connective tissue below adds a reticular lamina. Together they form the basement membrane that anchors the sheet.
Avascular: No blood vessels cross the basement membrane. Nutrients reach the cells by diffusion from the tissue beneath or by absorption from the surface.
Regeneration: Most epithelia replace damaged and dead cells quickly; surface epithelia such as the airway and gut linings shed worn cells continually.

Glands are epithelium too

Endocrine gland
Ductless. Releases hormones into the surrounding fluid, which diffuse into the blood and reach target cells with matching receptors.
For example: Anterior pituitary, thymus, adrenal cortex
Exocrine gland
Releases its secretion through a duct onto an epithelial surface.
For example: Sweat, saliva, mucus and milk glands
Merocrine
Vesicles release the product by exocytosis and the cell stays intact. The most common mode.
For example: Eccrine sweat glands; watery mucus
Apocrine
The apical part of the cell, with the product inside it, pinches off.
For example: Apocrine sweat glands of the armpit
Holocrine
The whole cell ruptures and becomes the secretion; new cells grow in from the surrounding tissue.
For example: Sebaceous (oil) glands of the skin and hair

Cell junctions

Tight junction
Seals neighboring cells together so nothing slips between them; makes the sheet a selective barrier and keeps the apical and basal sides separate.
Anchoring junction
A strong, flexible connection on the side and bottom surfaces. A desmosome links two cells (cadherins tied to intermediate filaments); a hemidesmosome anchors a cell to the basal lamina (integrins); an adherens junction uses actin-linked proteins.
Gap junction
A channel between two cells that lets ions and small molecules pass, coupling the cells electrically and metabolically so a group of cells can act together.

Connective tissue: every type

The most varied of the four: bone, blood, fat, tendons and cartilage are all connective tissue. What they share is cells sitting in an extracellular matrix of ground substance and protein fibers. What it does:

Supports and connects other tissues and organs: the sheaths around muscles, the tendons, the skeleton.
Protects, through fibrous capsules and bone.
Defends against microorganisms, through specialized cells.
Transports fluid, nutrients, waste and chemical messengers, as blood and lymph.
Stores energy and insulates, as fat.

Cells: Fibroblasts, the most common, make the fibers and ground substance. Others: adipocytes (fat cells), mesenchymal stem cells, and immune cells such as macrophages and mast cells.
Ground substance: A matrix of polysaccharides (including hyaluronic acid) and proteins that trap water. Usually fluid or gel-like; mineralized in bone.
Collagen fibers: Long, straight, flexible and very strong under tension; they give tendons and ligaments their strength.
Elastic fibers: Made mostly of elastin; they return to their original shape after being stretched or compressed.
Reticular fibers: Thin, branching fibers of the same protein as collagen; they form a supportive net inside soft organs.

Loose connective tissue
Areolar
Made of: All the cell types and all three fiber types, loosely and randomly arranged
Where it is found: Under most epithelia; around blood and lymph vessels; between muscle fibers
What it does: Fills space and supports the organs around it
Loose connective tissue
Adipose
Made of: Mostly adipocytes storing fat, with little matrix and many capillaries
Where it is found: White fat around the kidneys and behind the eyes; brown fat in the neck and collarbone region
What it does: Stores lipid, insulates and cushions; brown fat makes heat
Loose connective tissue
Reticular
Made of: Reticular cells and a net of reticular fibers
Where it is found: Lymphatic tissue, the spleen and the liver
What it does: A supportive framework for soft organs
Dense connective tissue
Dense regular
Made of: Collagen fibers in parallel bundles
Where it is found: Tendons and ligaments
What it does: Tensile strength: resists stretching along the fiber direction
Dense connective tissue
Dense irregular
Made of: Collagen fibers running in every direction
Where it is found: Dermis of the skin
What it does: Strength in all directions
Dense connective tissue
Dense elastic
Made of: Elastin fibers with collagen
Where it is found: Some ligaments, such as those between the vertebrae and in the vocal folds; the walls of arteries
What it does: Returns to its original length after stretching
Cartilage
Hyaline cartilage
Made of: Short, scattered collagen fibers and a large amount of proteoglycan; chondrocytes in small spaces called lacunae
Where it is found: The costal cartilage of the rib cage, the nose, the joint surfaces of bones, the growth plates, and the template of the embryonic skeleton
What it does: Strong but flexible support
Cartilage
Fibrocartilage
Made of: Thick bundles of collagen
Where it is found: The menisci of the knee; the intervertebral discs
What it does: Absorbs pressure; compressible
Cartilage
Elastic cartilage
Made of: Elastic fibers with collagen and proteoglycans
Where it is found: The external ear
What it does: Firm support that springs back
Bone
Compact bone
Made of: Collagen fibers in a matrix mineralized with hydroxyapatite; osteocytes in lacunae
Where it is found: The solid part of a bone
What it does: Greatest structural strength
Bone
Spongy (cancellous) bone
Made of: The same mineralized matrix, arranged as a lattice of struts called trabeculae
Where it is found: Inside some bones and at the ends of long bones
What it does: Lighter; a sponge-like structure
Fluid connective tissue
Blood
Made of: Red blood cells, white blood cells and platelets in a liquid matrix, the plasma
Where it is found: Circulates in the blood vessels
What it does: Transports gases, cells, nutrients, salts and wastes; red cells carry oxygen and some carbon dioxide, white cells defend, platelets clot
Fluid connective tissue
Lymph
Made of: White blood cells in a liquid matrix
Where it is found: The lymphatic vessels
What it does: Returns fluid and large molecules to the blood; carries absorbed fats from the intestine

Mesenchyme: The first connective tissue to form in the embryo and the stem-cell source of every other connective tissue. Scattered mesenchymal cells stay in adult tissue to supply cells for repair.
Mucous connective tissue: Forms in the umbilical cord and is gone after birth.

Muscle tissue: the three types

Skeletal muscle
Cells: Long, cylindrical fibers. Striated; many, at the edge of the fiber
Control: Voluntary
Where it is found: Attached to bones; around the entrances to the body
What it does: Voluntary movement; produces heat; protects organs
Cardiac muscle
Cells: Short, branched cells joined end to end by intercalated discs. Striated; one, in the center
Control: Involuntary
Where it is found: The heart
What it does: Contracts to pump blood
Smooth muscle
Cells: Short, spindle-shaped cells. No visible striations; one per fiber
Control: Involuntary
Where it is found: The walls of major organs and passageways
What it does: Involuntary movement: moves food and secretions along, and helps control breathing

Nervous tissue: neurons and neuroglia

Neurons carry information as electrochemical impulses called action potentials. The neuroglia (glial cells) were once thought to be simple support cells; they also regulate how neurons signal.

The parts of a neuron

Cell body (soma): Holds the nucleus and most of the cytoplasm and organelles.
Dendrites: Thin branches off the cell body that receive signals and carry them toward the soma.
Axon: A long extension that carries the action potential away from the cell body to another excitable cell.

The neuroglia

Astrocytes: Regulate ion concentrations between cells, take up or break down some neurotransmitters, and form the blood-brain barrier.
Microglia: Protect the nervous system against infection.
Oligodendrocytes: Make myelin, the insulating wrap around axons, in the brain and spinal cord.
Schwann cells: Make myelin in the peripheral nervous system.

Tissue membranes: mucous, serous, cutaneous and synovial

A tissue membrane is a thin sheet of epithelium on connective tissue (or, for the synovial membrane, connective tissue alone) that covers a surface or lines a cavity.

Mucous membrane (mucosa)
Type: Epithelial membrane
Where it is found: Lines the cavities and passageways that open to the outside: the digestive, respiratory, excretory and reproductive tracts
What it is: An epithelium on a layer of connective tissue, the lamina propria, which supports it; glands keep the surface wet with mucus
Serous membrane (serosa)
Type: Epithelial membrane
Where it is found: Lines the closed body cavities and covers the organs inside them: the pleura around each lung, the pericardium around the heart, the peritoneum around the abdominal organs
What it is: A simple squamous layer (the mesothelium) on connective tissue. The part lining the cavity wall is the parietal layer; the part covering the organ is the visceral layer; the serous fluid between them cuts friction as the organs move (ch. 1.6)
Cutaneous membrane
Type: Epithelial membrane
Where it is found: The skin
What it is: A stratified squamous epithelium on connective tissue; its exposed surface is dead, keratinized cells that protect against drying and pathogens
Synovial membrane
Type: Connective tissue membrane
Where it is found: Lines the cavity of a freely movable joint
What it is: Made of connective tissue only; it releases hyaluronan, which helps form the synovial fluid that lubricates the joint

3 · The cell

Cell organelles: what each one is and does

The parts of a human cell as the textbook lists them, from the membrane in. The cytoplasm is everything inside the membrane other than the nucleus: the cytosol plus the organelles.

Cell (plasma) membrane
What it is: Two layers of phospholipids with cholesterol and proteins embedded in them
What it does: Separates the cell from its surroundings and is selectively permeable: only substances that meet its criteria pass unaided (ch. 3.1)
Cytosol
What it is: The jelly-like fluid inside the cell; with the organelles it makes up the cytoplasm
What it does: The medium for the cell’s chemical reactions
Nucleus
What it is: The cell’s central organelle, enclosed by a double-membrane nuclear envelope with pores
What it does: Holds the cell’s DNA as chromatin; the pores let proteins, RNA and solutes pass between nucleus and cytoplasm (ch. 3.3)
Nucleolus
What it is: A dark-staining region inside the nucleus
What it does: Makes the RNA needed to build ribosomes; the ribosomal subunits leave through the nuclear pores (ch. 3.3)
Ribosome
What it is: A small particle, free in the cytosol or attached to the rough ER
What it does: The site of protein synthesis
Rough endoplasmic reticulum
What it is: A network of membranes studded with ribosomes
What it does: Makes and modifies proteins meant for the cell membrane or for export
Smooth endoplasmic reticulum
What it is: The same network without ribosomes
What it does: Makes phospholipids and steroid hormones; stores calcium; handles some carbohydrate metabolism; breaks down certain toxins
Golgi apparatus
What it is: A stack of flattened membrane sacs
What it does: Sorts, modifies and ships the products of the rough ER; also makes new lysosomes
Lysosome
What it is: A membrane sac of digestive enzymes
What it does: Breaks down and digests unneeded cell components; under certain conditions it can trigger the cell’s own controlled death (apoptosis)
Peroxisome
What it is: A small membrane sac of enzymes
What it does: Lipid metabolism and chemical detoxification; its enzymes neutralize harmful by-products such as hydrogen peroxide
Mitochondrion
What it is: Two membranes, the inner one folded into cristae with a large surface area
What it does: The cell’s energy transformer: converts the energy in nutrients into ATP, the cell’s usable energy
Vesicle
What it is: A small membrane-bound sac
What it does: Carries materials around the cell, such as proteins from the rough ER to the Golgi apparatus
Microtubules
What it is: Hollow tubes of the protein tubulin
What it does: Keep the cell’s shape, resist compression, position organelles and lay the tracks that chromosomes move along in cell division
Microfilaments
What it is: Thin fibers of the protein actin
What it does: With myosin, produce muscle contraction; form the cleavage furrow that pinches a dividing cell in two
Intermediate filaments
What it is: Fibers of tough proteins such as keratin, between the other two in thickness
What it does: Resist tension; anchor organelles and link cells together
Centrioles
What it is: A pair of short cylinders near the nucleus
What it does: The origin point of the microtubules that extend out as cilia or flagella, and of the spindle in cell division (ch. 3.5)
Cilia
What it is: Many short, hair-like extensions
What it does: Beat rhythmically to move materials along a surface, such as dust and mucus along the airways
Flagellum
What it is: A single long whip-like extension
What it does: Cell locomotion; in humans only sperm cells have one

4 · Membrane transport

Diffusion, osmosis and active transport

Passive transport moves a substance down its concentration gradient and costs the cell no energy. Active transport moves it against the gradient and costs ATP, directly or through an ion gradient a pump built. Vesicular transport moves whole packages in or out.

Passive
Simple diffusion
How it works: A substance moves down its concentration gradient, from higher to lower, straight through the lipid bilayer. No energy needed. Only small, uncharged molecules such as oxygen and carbon dioxide cross this way (and very small polar ones, such as water).
Example: Oxygen diffuses into cells; carbon dioxide diffuses out
Passive
Facilitated diffusion
How it works: Still down the gradient and still without energy, but through a membrane protein, because the substance cannot cross the bilayer on its own. Channel proteins form pores; carrier proteins bind one kind of molecule and pass it across.
Example: Sodium channels; the glucose transporter, a carrier that moves glucose into the cell
Passive
Osmosis
How it works: The diffusion of water through a semipermeable membrane, toward the side with more dissolved solute.
Example: Water moving into or out of a cell depending on the solution around it (see tonicity below)
Active
Primary active transport
How it works: A pump uses ATP directly to move a substance against its gradient.
Example: The sodium-potassium pump moves 3 Na⁺ out of the cell and 2 K⁺ in for each ATP, which keeps the negative resting charge of nerve cells
Active
Secondary active transport
How it works: Uses the energy stored in an ion gradient that a primary pump built. A symporter moves two substances the same direction; an antiporter moves them in opposite directions.
Example: The sodium-glucose symporter brings glucose in with sodium; the sodium-hydrogen antiporter moves H⁺ out as Na⁺ comes in
Vesicular
Endocytosis
How it works: The membrane wraps around material outside the cell and pinches off as a vesicle inside it. Phagocytosis (“cell eating”) takes in large particles; pinocytosis (“cell drinking”) takes in fluid; receptor-mediated endocytosis takes in a specific substance after it binds a membrane receptor.
Example: Immune cells engulfing pathogens (phagocytosis); iron bound to the protein transferrin entering cells (receptor-mediated)
Vesicular
Exocytosis
How it works: A vesicle inside the cell fuses with the membrane and releases its contents outside.
Example: Pancreatic cells releasing digestive enzymes; endocrine cells releasing hormones

Tonicity: what osmosis does to a cell

Isotonic
Solute outside the cell: Same solute concentration as inside the cell
Water moves: Moves in and out equally
The cell: Keeps its normal shape
Hypotonic
Solute outside the cell: Less solute outside than inside
Water moves: Moves into the cell
The cell: Swells, and can burst
Hypertonic
Solute outside the cell: More solute outside than inside
Water moves: Moves out of the cell
The cell: Shrivels

5 · Cell division

The cell cycle, mitosis and meiosis

A body cell spends most of its life in interphase, growing and copying its DNA, then divides in the mitotic phase. Meiosis is the other kind of division, used only to make gametes.

The cell cycle

G₁ (first gap)
The cell grows and carries out its normal functions. It can last hours or many days.
S (synthesis)
The cell replicates its DNA, so each chromosome becomes two identical sister chromatids joined at the centromere. Replication is semiconservative: each new DNA molecule keeps one original strand and one new strand (ch. 3.3).
G₂ (second gap)
The cell keeps growing and prepares for mitosis.
G₀
A resting state outside the cycle for cells that have stopped dividing, for a while or for good, such as nerve cells.

Mitosis, step by step

Prophase
Chromatin condenses into visible chromosomes, the nucleolus disappears and the nuclear envelope breaks down. The centrosomes move apart and the mitotic spindle forms between them; its microtubules attach to the kinetochores at the centromeres.
Metaphase
The chromosomes line up along the metaphase plate, a plane through the middle of the cell.
Anaphase
The sister chromatids separate and are pulled to opposite ends of the cell.
Telophase
Two new nuclei form, one at each end; the chromosomes loosen back into chromatin and the spindle breaks down.
Cytokinesis
The cytoplasm divides. A cleavage furrow, a contractile band of actin microfilaments around the middle of the cell, pinches it into two cells.

Mitosis produces two identical diploid cells. A human body cell has 46 chromosomes in 23 homologous pairs, and each daughter cell gets the full set.

Mitosis vs meiosis

Number of divisions
Mitosis: One
Meiosis: Two (meiosis I and meiosis II)
Daughter cells
Mitosis: Two
Meiosis: Four
Chromosome number
Mitosis: Same as the parent cell: diploid (46 in humans, 23 pairs)
Meiosis: Half the parent’s: haploid, one set of chromosomes
Genetically
Mitosis: Identical to the parent cell
Meiosis: Not identical: crossing over in prophase I swaps segments between homologous chromosomes, and the pairs sort independently in metaphase I
What separates
Mitosis: Sister chromatids, in anaphase
Meiosis: Homologous pairs in meiosis I (the reductional division); sister chromatids in meiosis II
Where and why
Mitosis: Body (somatic) cells: growth, maintenance and repair
Meiosis: Germ cells only: makes the gametes for sexual reproduction

6 · Flashcards

54 cells and tissues flashcards

Homeostasis, every epithelium, the connective tissues, muscle, nervous tissue, the membranes, the organelles, transport and cell division. Tap a card (or press Enter) to flip it, use the arrows or the left and right keys to move, and shuffle when the order starts to feel familiar.

See all 54 cards as a list

HomeostasisThe steady state the body’s systems maintain: internal conditions kept within a narrow, healthy range
Set point vs normal rangeSet point: the ideal value. Normal range: the band around it that does not trigger the control center
Sensor (receptor)Monitors a value and reports it to the control center
Control centerCompares the value with its normal range and activates an effector when it drifts too far
EffectorCauses the change that reverses the drift
Negative feedbackReverses a change and switches off as the value returns to range: body temperature, blood glucose
Positive feedbackIntensifies a change until an outside event ends it: childbirth (oxytocin), blood clotting
Too warm: the heat-loss centerSkin blood vessels widen, sweat output rises, breathing deepens
Too cold: the heat-gain centerLess blood to the skin, shivering, thyroid hormone raises heat production, epinephrine frees glucose
The four tissue typesEpithelial, connective, muscle, nervous
Simple squamous epitheliumAir sacs (alveoli) of the lungs; the lining of the heart, blood vessels and lymphatic vessels. Lets materials pass by diffusion and filtration; secretes lubricating substances
Simple cuboidal epitheliumDucts and secretory portions of small glands; the tubules of the kidney. Secretion and absorption
Simple columnar epitheliumCiliated: larger bronchioles, the uterine tubes and the uterus. Non-ciliated: the lining of the stomach and intestines. Absorption; secretion of mucus and enzymes
Pseudostratified columnar epitheliumTrachea, bronchi and much of the upper respiratory tract. Secretes mucus; its cilia move the mucus along
Stratified squamous epitheliumNon-keratinized (moist): the lining of the mouth and esophagus. Keratinized (dry): the surface of the skin. Protection against abrasion
Stratified cuboidal epitheliumDucts of the sweat glands, salivary glands and mammary glands. Protective lining
Stratified columnar epitheliumParts of the urethra; some gland ducts. Secretion and protection
Transitional epitheliumUrinary bladder, ureters and part of the urethra. Lets the urinary organs expand and stretch
Keratinized vs non-keratinized stratified squamousKeratinized: the skin, dry, dead keratin-filled top cells. Non-keratinized: moist linings such as the mouth
Merocrine, apocrine, holocrineExocytosis with the cell intact; the cell top pinches off; the whole cell becomes the secretion (sebaceous glands)
Tight, anchoring and gap junctionsSeal between cells; strong flexible attachment (desmosome, hemidesmosome, adherens); a channel for ions and small molecules
Areolar connective tissueAll cell and fiber types, loosely arranged; under most epithelia; fills space and supports
Adipose tissueFat-storing adipocytes; stores lipid, insulates, cushions; brown fat makes heat
Dense regular vs dense irregularParallel collagen in tendons and ligaments (strength one way) vs collagen in every direction in the dermis (strength every way)
Hyaline, fibrocartilage, elastic cartilageRib cage, nose, joint surfaces, growth plates; knee menisci and intervertebral discs; the external ear
Compact vs spongy boneSolid mineralized matrix with the most strength vs a lighter lattice of trabeculae inside bones and at the ends of long bones
Blood and lymphThe fluid connective tissues: cells in a liquid matrix; blood carries gases, nutrients and wastes, lymph returns fluid to the blood and carries absorbed fats
Collagen, elastic and reticular fibersStrong under tension; springs back after stretching; thin branching net in soft organs
Skeletal muscleLong cylindrical striated fibers, many nuclei at the edge, voluntary; attached to bones
Cardiac muscleShort branched striated cells, one central nucleus, intercalated discs, involuntary; the heart
Smooth muscleShort spindle-shaped cells, no striations, one nucleus, involuntary; walls of organs and passageways
Neuron: dendrites, soma, axonDendrites receive signals and carry them to the cell body; the axon carries the action potential away
NeurogliaAstrocytes (ions, neurotransmitters, blood-brain barrier), microglia (defense), oligodendrocytes (myelin in the CNS), Schwann cells (myelin in the PNS)
Mucous vs serous membraneMucous lines passageways open to the outside; serous lines closed cavities (pleura, pericardium, peritoneum) and secretes serous fluid
Parietal vs visceral layerParietal lines the cavity wall; visceral covers the organ; serous fluid between them cuts friction
Synovial membraneThe connective tissue membrane lining a freely movable joint; releases hyaluronan for the synovial fluid
Rough ER vs smooth ERRough (ribosomes): makes and modifies proteins. Smooth: phospholipids, steroid hormones, calcium storage, detox
Golgi apparatusSorts, modifies and ships the products of the rough ER; makes lysosomes
Lysosome vs peroxisomeLysosome: enzymes that digest unneeded parts. Peroxisome: lipid metabolism and detox, neutralizes hydrogen peroxide
MitochondrionDouble membrane, inner folds called cristae; converts nutrient energy into ATP
NucleolusThe region of the nucleus that makes the RNA for ribosomes
Microtubules, microfilaments, intermediate filamentsTubulin: shape and chromosome tracks. Actin: contraction and the cleavage furrow. Keratin-type proteins: resist tension
Cilia vs flagellumMany short, beat to move mucus along airways vs one long, for locomotion (sperm only)
Simple vs facilitated diffusionBoth down the gradient without energy; facilitated goes through a channel or carrier protein (glucose transporter)
OsmosisDiffusion of water through a semipermeable membrane toward the side with more solute
Isotonic, hypotonic, hypertonicCell keeps its shape; cell swells and can burst; cell shrivels
Sodium-potassium pumpPrimary active transport: 3 Na⁺ out, 2 K⁺ in per ATP; keeps the negative resting charge of nerve cells
Symporter vs antiporterSecondary active transport: same direction (sodium-glucose) vs opposite directions (sodium-hydrogen)
Phagocytosis, pinocytosis, receptor-mediated endocytosis; exocytosisLarge particles in; fluid in; a specific bound substance in; a vesicle releases its contents out
Interphase: G₁, S, G₂Growth and normal work; DNA replication; growth and preparation for mitosis. G₀ = resting, not dividing
Prophase, metaphase, anaphase, telophaseChromosomes condense and the spindle forms; line up in the middle; sister chromatids pulled apart; two nuclei form
CytokinesisThe cytoplasm divides: a cleavage furrow of actin microfilaments pinches the cell in two
Mitosis vs meiosisOne division, two identical diploid cells, body cells vs two divisions, four non-identical haploid gametes, germ cells
Meiosis I vs meiosis III separates homologous pairs after crossing over (reductional); II separates sister chromatids

7 · Practice questions

20 practice questions

Homeostasis and the feedback loops, the epithelia, connective tissue, muscle and nervous tissue, the organelles, transport and mitosis, multiple choice, one at a time. Every answer is explained, and your score is free: no sign-up needed.

Question 1 of 20

Homeostasis is best defined as

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A tissue-types chart you can print: all 8 epithelia, the 13 connective tissues, the three muscle types, nervous tissue and the four membranes, each with what it is made of, where it is found and what it does, on one sheet for the histology unit.
30 more practice questions, every answer explained: the feedback loops, the epithelia, the connective tissues, glands and junctions, muscle, neuroglia, the membranes, tonicity and the phases of mitosis.

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Where the facts come from

Every fact on this page is reworded from one of these public sources; the chapter number sits next to each fact. Check anything important against your course materials.

Questions

Common questions

Homeostasis is the steady state that the body’s systems maintain: internal conditions such as temperature and blood glucose are kept within a narrow, healthy range even as the outside world and the body’s own activity change. Each value has a set point, and a feedback loop (sensor, control center, effector) pulls it back when it drifts.

Negative feedback reverses a change: when body temperature rises, sweating and wider skin vessels bring it back down, and the response switches off as the value returns to range. Positive feedback intensifies a change until an outside event ends it: in childbirth, cervix stretch releases oxytocin, which strengthens contractions, which stretch the cervix more, until the baby is born. Negative feedback runs all the time; positive feedback is normal only where there is a definite end point.

A single layer of flat, thin cells, the thinnest epithelium. It lines the air sacs of the lungs, the heart and the blood and lymphatic vessels, where its job is to let materials pass by diffusion and filtration and to secrete lubricating substances.

Wherever a surface takes abrasion. The non-keratinized (moist) form lines the mouth and esophagus; the keratinized (dry) form is the surface of the skin, where the outermost cells are dead and filled with keratin.

Epithelial (sheets that cover surfaces, line cavities and form glands), connective (cells in a matrix that bind, support and protect: bone, blood, fat, tendons, cartilage), muscle (skeletal, cardiac and smooth, which contract to produce movement) and nervous (neurons and neuroglia, which carry impulses). Part 2 of this page lists every subtype with its location and function.

The thin membranes that line the closed body cavities and cover the organs inside them: the pleura around each lung, the pericardium around the heart and the peritoneum around the abdominal organs. Each has a parietal layer on the cavity wall and a visceral layer on the organ, with serous fluid between them to cut friction as the organs move.

No. This is a study aid for people in anatomy and physiology prerequisite courses and healthcare programs. It describes textbook structure and function; it does not diagnose, treat or say anything about any person’s health. Your textbook and your instructor come first.

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Homeostasis, cells and tissues study guide

Homeostasis: definition and feedback loops

What homeostasis means

The three parts of a feedback loop

Sensor (or receptor)

Control center

Effector

Negative vs positive feedback

What it does to the change

How it ends

How common

Textbook examples

The textbook examples

Body temperature

Blood glucose

Childbirth

Blood clotting

The four tissue types and every subtype

Epithelial tissue

Connective tissue

Muscle tissue

Nervous tissue

Epithelial tissue: every type, where it is found and what it does

Simple squamous epithelium

Simple cuboidal epithelium

Simple columnar epithelium

Pseudostratified columnar epithelium

Stratified squamous epithelium

Stratified cuboidal epithelium

Stratified columnar epithelium

Transitional epithelium

Glands are epithelium too

Cell junctions

Connective tissue: every type

Areolar

Adipose

Reticular

Dense regular

Dense irregular

Dense elastic

Hyaline cartilage

Fibrocartilage

Elastic cartilage

Compact bone

Spongy (cancellous) bone

Blood

Lymph

Muscle tissue: the three types

Skeletal muscle

Cardiac muscle

Smooth muscle

Nervous tissue: neurons and neuroglia

The parts of a neuron

The neuroglia

Tissue membranes: mucous, serous, cutaneous and synovial

Mucous membrane (mucosa)

Serous membrane (serosa)

Cutaneous membrane

Synovial membrane

Cell organelles: what each one is and does

Cell (plasma) membrane

Cytosol

Nucleus

Nucleolus

Ribosome

Rough endoplasmic reticulum

Smooth endoplasmic reticulum

Golgi apparatus

Lysosome

Peroxisome

Mitochondrion

Vesicle

Microtubules

Microfilaments

Intermediate filaments

Centrioles

Cilia

Flagellum

Diffusion, osmosis and active transport

Simple diffusion