DAT

Questions on organ systems focus on human physiology: how structure supports function, how systems are regulated by negative feedback, and how systems interact to maintain homeostasis. Four tissue types build every organ: epithelial, connective, muscle, and nervous.

Integumentary System

The skin is the body's largest organ. It is a physical barrier to pathogens and water loss, regulates temperature, synthesizes vitamin D in response to UV light, and houses sensory receptors.

Layer	StructureandFunction
Epidermis	Stratifiedsquamousepithelium avascular;keratinocytesareproducedinthestratumbasaleandbecome ,
	keratin-filled dead cells in the stratum corneum Melanocytes make melanin (UV protection); Langerhans .
	cellspresentantigens .
Dermis	Denseconnectivetissue(collagen elastin)withbloodvessels nerves hairfollicles sweatglands , , , ,
	(evaporativecooling) andsebaceousglands(sebum) , .
Hypodermis	Subcutaneousadiposetissue;insulation energystorage andanchoring , , .

Thermoregulation: in heat, dermal arterioles dilate and sweat glands secrete; in cold, arterioles constrict, arrector pili muscles raise hairs, and skeletal muscles shiver.

Skeletal System

The skeleton supports and protects, anchors muscles, stores calcium and phosphate, and houses red marrow for hematopoiesis. Compact bone is organized into osteons (Haversian systems) with a central canal surrounded by concentric lamellae; spongy bone has trabeculae containing red marrow. Bone matrix is collagen (flexibility) plus hydroxyapatite, a calcium phosphate mineral (hardness).

Long bones lengthen at the epiphyseal (growth) plate by endochondral ossification of a cartilage model; flat bones of the skull form by intramembranous ossification. Ligaments connect bone to bone; tendons connect muscle to bone. Synovial joints (knee, shoulder) are freely movable; cartilage cushions articulating surfaces.

Muscular System

Feature	Skeletal	Cardiac	Smooth
Striations Nuclei	Yes Many peripheral(multinucleatedfibers) ,	Yes Oneortwo central ,	No 1 One central ,

Threetypesofmuscle(Continued)

Feature	Skeletal	Cardiac	Smooth
Control	Voluntary(somatic)	Involuntary;intrinsicpacemaker	Involuntary(autonomic hormones) ,
Cellconnections	None;eachfiber	Intercalateddiscswithgap	Gap junctions in single-unit smooth
	innervated	junctions	muscle
Ca2+regulation	Troponin–tropomyosin	Troponin–tropomyosin	Calmodulin myosin light-chain kinase →
Location	Attachedtobones	Heartwall	Wallsofgut vessels airways uterus , , , ,
			bladder

A skeletal muscle is made of fibers (cells) containing myofibrils, which are chains of sarcomeres running Z line to Z line. Thick filaments are myosin; thin filaments are actin with the regulatory proteins tropomyosin and troponin.

Excitation–Contractioncouplingandtheslidingfilamentmodel

Action potential travels

Ca2+ binds troponin;

Myosin binds actin; Pi

ATP binding detaches

➜

➜

➜

➜

down T-tubules; SR

myosin; hydrolysis re-

ACh binds nicotinic

tropomyosin shifts off

release powers the

receptors; the

releases Ca2+

actin

stroke

cocks the head

sarcolemma depolarizes

ATP is needed for myosin to release actin, not only for the power stroke. Without ATP, cross-bridges stay locked (rigor mortis). Relaxation also requires ATP, which powers the SR Ca2+-ATPase that pumps Ca2+ back into the sarcoplasmic reticulum.

Force is graded by motor unit recruitment (more motor neurons active) and by frequency: rapid stimulation causes summation and, at high rates, sustained tetanus. Slow-twitch (type I) fibers are oxidative, fatigue-resistant, and rich in myoglobin; fast-twitch (type II) fibers contract rapidly and fatigue sooner.

Circulatory System

Humans have a closed, double circulation: the pulmonary circuit carries blood to the lungs, and the systemic circuit carries it to the body. Deoxygenated blood returns through the venae cavae to the right atrium, passes the tricuspid valve into the right ventricle, and exits through the pulmonary valve to the pulmonary arteries. Oxygenated blood returns through the pulmonary veins to the left atrium, passes the mitral (bicuspid) valve into the left ventricle, and exits through the aortic valve to the aorta.

Conduction: SA node (pacemaker) → AV node (delay lets the ventricles fill) → bundle of His → bundle branches → Purkinje fibers. Sympathetic input speeds the rate; the vagus nerve (parasympathetic) slows it.

Cardiac cycle: systole = ventricular contraction; diastole = relaxation and filling. The first heart sound is AV valve closure; the second is semilunar valve closure.

Vessels: arteries have thick elastic, muscular walls; arterioles are the main site of resistance and blood-pressure control; capillaries are one endothelial cell thick for exchange; veins have thin walls, low pressure, and valves, aided by skeletal muscle contraction.

Capillary exchange (Starling forces): hydrostatic pressure pushes fluid out at the arteriole end; plasma-protein (oncotic) pressure draws fluid back at the venule end; the excess enters lymphatics.

CO = HR × SV   |   MAP ≈ CO × TPR

Example: 70 beats/min × 70 mL/beat ≈ 4.9 L/min. TPR = total peripheral resistance, set mainly by arteriolar radius.

Component	DescriptionandFunction
Plasma(≈55%)	Water electrolytes nutrients wastes hormones andproteins:albumin(oncoticpressure) globulins , , , , , ,
	(antibodies transport) fibrinogen(clotting) , , .
Erythrocytes	Biconcave and anucleate in mammals; hemoglobin carries O 2; carbonic anhydrase aids CO 2 transport;
	productionstimulatedbyerythropoietinfromthekidney .
Leukocytes	Neutrophils(mostabundant;phagocytes) lymphocytes(B T NKcells) monocytes(become , , , ,
	macrophages) eosinophils(parasites allergy) basophils(histamine) , , , .
Platelets	Cell fragments from megakaryocytes; form plugs and trigger the clotting cascade (prothrombin →
	thrombin; fibrinogen fibrin) → .

Lymphatic System

Lymphatic vessels collect excess interstitial fluid (lymph), filter it through lymph nodes, and return it to the venous circulation at the subclavian veins via the thoracic duct. Lacteals in intestinal villi absorb dietary fats packaged as chylomicrons. Lymphoid organs include the bone marrow (B-cell maturation) and thymus (T-cell maturation), which are primary organs, plus the spleen, lymph nodes, tonsils, and Peyer's patches, which are secondary organs where lymphocytes meet antigen. Blocked lymph drainage causes edema.

Immune System

Immediate, nonspecific, no memory. Barriers (skin, mucus,

Specific, slower on first exposure, with memory. Humoral: B

lysozyme, stomach acid); phagocytes (neutrophils,

cells, activated with help from TH cells, become plasma cells

macrophages, dendritic cells) recognizing pathogen patterns

that secrete antibodies. Cell-mediated: cytotoxic T cells (CD8)

through toll-like receptors; natural killer cells that destroy cells

kill infected or cancerous cells; helper T cells (CD4) release

lacking MHC I; complement proteins; interferons; inflammation

cytokines that coordinate both arms; regulatory T cells

(histamine from mast cells causes vasodilation and leaky

suppress responses.

capillaries); fever.

MHC class I is on all nucleated cells and presents internal (e.g., viral) peptides to CD8 T cells; MHC class II is on antigen-presenting cells (dendritic cells, macrophages, B cells) and presents engulfed antigens to CD4 T cells.

Clonal selection: an antigen activates only the lymphocytes whose receptors bind it; they proliferate into effector and memory cells. The secondary response is faster, larger, and mainly IgG.

Antibodies have two heavy and two light chains; variable regions bind antigen, and the constant region sets the class. They neutralize, opsonize (tag for phagocytosis), agglutinate, and activate complement.

Active immunity (infection or vaccination) creates memory; passive immunity (maternal IgG across the placenta, IgA in breast milk, antibody injections) is immediate but temporary.

Class	KeyFeatures
IgG	Most abundant in serum; main secondary-response antibody; crosses the placenta .
IgM	Pentamer;firstantibodymadeinaprimaryresponse;strongcomplementactivator .
IgA	Dimerinsecretions(saliva tears mucus breastmilk);mucosaldefense , , , .
IgE	Binds mast cells and basophils; allergy and anti -parasite responses .
IgD	B -cell surface receptor on naive B cells .

Digestive System

Digestion combines mechanical breakdown (chewing, churning, segmentation) with enzymatic hydrolysis. The tract runs mouth → pharynx → esophagus (peristalsis) → stomach → small intestine (duodenum, jejunum, ileum) → large intestine

(brush border) multiply its surface area. The large intestine absorbs water and electrolytes and houses bacteria that make vitamin K.

Enzyme	Source	SiteofAction	Substrate Product →
Salivaryamylase	Salivaryglands	Mouth	Starch maltose and oligosaccharides →
Pepsin Pancreaticamylase	Chiefcells(aspepsinogen , activatedbyHCl) Pancreas	Stomach Smallintestine	Proteins peptides → Starch maltose →	1

Digestiveenzymes(Continued)

Enzyme	Source	SiteofAction	Substrate Product →
Trypsin chymotrypsin ,	Pancreas(aszymogens;	Smallintestine	Proteins and peptides smaller peptides →
	enteropeptidaseactivates
	trypsinogen andtrypsin ,
	activatestherest)
Carboxypeptidase	Pancreas	Smallintestine	Removes C -terminal amino acids
Pancreaticlipase	Pancreas	Smallintestine	Triglycerides → fatty acids + monoglycerides
			(afterbileemulsifiesfat)
Nucleases	Pancreas	Smallintestine	Nucleic acids nucleotides →
Brush -border enzymes	Intestinalepithelium	Smallintestine	Disaccharides monosaccharides; peptides → →
(maltase sucrase , ,			aminoacids
lactase aminopeptidase , ,
dipeptidase)

Hormone	SourceandStimulus	Action
Gastrin	StomachGcells;foodin	StimulatesHClsecretionbyparietalcellsandgastricmotility
	stomach
Secretin	Duodenum;acidicchyme	Pancreasreleasesbicarbonatetoneutralizeacid
Cholecystokinin	Duodenum;fatsandproteins	Gallbladdercontracts(bilerelease);pancreasreleasesenzymes;slows
(CCK)		gastricemptying

The liver makes bile (bile salts emulsify fats), stores glycogen, detoxifies drugs, converts ammonia to urea, and synthesizes plasma proteins; the gallbladder stores and concentrates bile. Nutrients absorbed into capillaries travel by the hepatic portal vein to the liver; fats enter lacteals as chylomicrons.

Respiratory System

Air moves through the nasal cavity (warmed, filtered, humidified), pharynx, larynx, trachea (C-shaped cartilage rings; ciliated mucous epithelium), bronchi, and bronchioles to the alveoli, where gas exchange occurs by diffusion across a very thin membrane. Type II alveolar cells secrete surfactant, which lowers surface tension and prevents alveolar collapse.

Ventilation is negative-pressure breathing: the diaphragm contracts and flattens and external intercostals lift the ribs, enlarging the thoracic cavity; intrapulmonary pressure falls below atmospheric pressure and air flows in (Boyle's law). Quiet exhalation is passive elastic recoil. Control: medullary centers set the rhythm. Central chemoreceptors respond mainly to CO2 (via H+ in cerebrospinal fluid); peripheral chemoreceptors in the carotid and aortic bodies also respond to low O2. Rising CO2 is the strongest drive to breathe.

CO2 transport: about 70% as bicarbonate (CO2 + H2O ⇌ H2CO3⇌ H+ + HCO3

−, catalyzed by carbonic anhydrase in

red cells), about 20% bound to hemoglobin (carbaminohemoglobin), and about 10% dissolved.

Acid–base: hypoventilation retains CO2 and lowers blood pH (respiratory acidosis); hyperventilation raises pH (respiratory alkalosis).

Urinary System

The kidneys filter blood, remove nitrogenous waste (urea), and regulate blood volume, pressure, osmolarity, electrolytes, and pH. The nephron is the functional unit.

Segment	Function
GlomerulusandBowman's	Filtrationdrivenbybloodpressure;water ions glucose aminoacids andureapass;bloodcellsand , , , ,
capsule	largeproteinsdonot .
Proximalconvolutedtubule	Reabsorbsabout65%offilteredNa+andwaterandessentiallyallglucoseandaminoacids
	(secondaryactivetransport);secretesH+ drugs andtoxins , , .
LoopofHenle	Countercurrentmultiplier:waterleavesthedescendinglimb;NaClispumpedfromthethick
	ascendinglimb creatingahyperosmoticmedulla , .
Distalconvolutedtubule	Fine-tunes Na+ K+ and Ca2+ (PTH increases Ca2+ reabsorption) , , .
Collectingduct	ADHinsertsaquaporins allowingwatertoleaveintothehyperosmoticmedullaandproducing ,
	concentratedurine;aldosteroneincreasesNa+reabsorptionandK+secretion .

When blood pressure falls, juxtaglomerular cells release renin, which converts angiotensinogen to angiotensin I; ACE (mainly in the lungs) converts it to angiotensin II, which constricts arterioles and stimulates aldosterone and ADH release. Atrial natriuretic peptide (ANP) opposes this system, promoting Na+ and water excretion. Glucose appears in urine only when filtered load exceeds the transport maximum of the PCT carriers, as in uncontrolled diabetes mellitus.

Nervous System

Neurons receive input on dendrites and the cell body, integrate it at the axon hillock, and conduct action potentials down the axon to synaptic terminals. Glial cells support neurons: oligodendrocytes (CNS) and Schwann cells (PNS) form myelin; astrocytes regulate the extracellular environment and help form the blood–brain barrier; microglia are immune phagocytes.

The resting membrane potential (about −70 mV) results from the Na+/K+-ATPase and, above all, the high resting permeability to K+ through leak channels; K+ is concentrated inside the cell and Na+ outside.

Depolarization: reaching threshold (about −55 mV) opens voltage-gated Na+ channels; Na+ rushes in (positive feedback). Repolarization: Na+ channels inactivate and voltage-gated K+ channels open; K+ flows out. Hyperpolarization: slow K+ channel closure briefly overshoots the resting potential.

Refractory periods: absolute (Na+ channels inactivated; no second spike possible) and relative (a stronger stimulus is required). They make propagation one-directional.

Saltatory conduction: in myelinated axons the impulse jumps between nodes of Ranvier, greatly increasing speed. Larger axon diameter also increases speed.

➜

➜

➜

➜

Action potential

Voltage-gated Ca2+

Vesicles release

Receptors open

Enzymatic breakdown

depolarizes the axon

neurotransmitter into the

channels: EPSP or IPSP

(acetylcholinesterase),

channels open

terminal

cleft

reuptake, or diffusion

Postsynaptic potentials sum in space and time. Major neurotransmitters include acetylcholine (neuromuscular junction; parasympathetic), norepinephrine (sympathetic postganglionic), dopamine, serotonin, glutamate (main CNS excitatory transmitter), and GABA (main CNS inhibitory transmitter).

Division	ComponentsandFunctions
Cerebrum	Cortexlobes:frontal(planning voluntarymotorcontrol speechproduction) parietal , , ,
	(somatosensory) temporal(hearing memory) occipital(vision) , , , .
Diencephalon	Thalamusrelayssensoryinput;hypothalamuscontrolshomeostasis(temperature hunger thirst)and , ,
	thepituitary .
Cerebellum	Coordination balance andfinemotorlearning , , .
Brainstem	Midbrain pons andmedullaoblongata;themedullaregulatesbreathing heartrate andblood , , , ,
	pressure .
Spinalcord	Conducts signals to and from the brain; mediates reflex arcs (receptor → sensory neuron →
	interneuron → motor neuron → effector) . Sensory input enters dorsally; motor output leaves ventrally .
SomaticPNS	Voluntarycontrolofskeletalmuscle;onemotorneuronfromCNStoeffector;ACh .
SympatheticPNS	"Fightorflight":thoracolumbarorigin;raisesheartrate dilatespupilsandbronchioles inhibits , ,
	digestion releasesglucose;adrenalmedullasecretesepinephrine , .
ParasympatheticPNS	"Restanddigest":craniosacralorigin(vagusnerve);slowsheartrate constrictspupils stimulates , ,
	digestionandsalivation;AChatbothsynapses .

Sensory Systems

Sensory receptors transduce stimuli into receptor potentials: mechanoreceptors (touch, hearing, balance), chemoreceptors (taste, smell, blood chemistry), photoreceptors, thermoreceptors, and nociceptors (pain).

Light passes the cornea, pupil (size set by the iris), and lens

Sound waves vibrate the tympanic membrane and the ossicles

(shape adjusted by ciliary muscles for accommodation) to the

(malleus, incus, stapes), which push on the oval window of the

retina. Rods are very sensitive, work in dim light, and give

fluid-filled cochlea. Hair cells in the organ of Corti bend

black-and-white vision; cones give color and high acuity and

against the tectorial membrane and depolarize; high

are concentrated in the fovea. Light converts retinal from cis to

frequencies stimulate the base of the basilar membrane and

trans in rhodopsin, which hyperpolarizes the photoreceptor.

low frequencies the apex. The semicircular canals detect

Signals pass through bipolar and ganglion cells to the optic

rotation, and the utricle and saccule detect linear acceleration

nerve.

and gravity.

Endocrine System

Endocrine glands secrete hormones into the blood. Peptide and amine hormones (except thyroid hormone) are water- soluble, bind surface receptors, and act quickly via second messengers. Steroid hormones and thyroid hormone are lipid-soluble, bind intracellular receptors, and alter transcription, acting more slowly but for longer. The hypothalamus links the nervous and endocrine systems: it sends releasing hormones through the hypophyseal portal system to the anterior pituitary and sends ADH and oxytocin down axons to the posterior pituitary for release.

Hypothalamus

TRH, CRH, GnRH, GHRH;

Regulate anterior pituitary secretion

dopamine and somatostatin (inhibitory)

Anterior pituitary

GH

Growth, protein synthesis, fat breakdown; stimulates IGF-1 from the liver

Anterior pituitary

TSH, ACTH

Stimulate thyroid hormone and cortisol release

Anterior pituitary

FSH, LH

Gamete production and sex steroid secretion; LH surge triggers ovulation

Anterior pituitary

Prolactin

Milk production

Posterior pituitary

ADH (vasopressin)

Water reabsorption in collecting ducts; vasoconstriction

Posterior pituitary

Oxytocin

Uterine contraction and milk ejection (positive feedback during labor)

Thyroid

T3, T4 (iodine-containing)

Raise basal metabolic rate; required for normal development

Thyroid (C cells)

Calcitonin

Lowers blood Ca2+ by inhibiting osteoclasts

Raises blood Ca2+: bone resorption, renal Ca2+ reabsorption, activation of

Parathyroids

PTH

vitamin D (calcitriol) for intestinal absorption

Adrenal cortex

Cortisol (glucocorticoid)

Raises blood glucose (gluconeogenesis); suppresses immune response and inflammation

Adrenal cortex

Aldosterone (mineralocorticoid)

Na+ reabsorption and K+ secretion in the distal nephron

Adrenal medulla

Epinephrine, norepinephrine

Fight-or-flight response; glycogenolysis

Pancreas (β cells)

Insulin

Lowers blood glucose: glucose uptake (GLUT4 in muscle and fat), glycogen and fat synthesis

Pancreas (α cells)

Glucagon

Raises blood glucose: glycogenolysis and gluconeogenesis in the liver

Pineal gland

Melatonin

Circadian rhythms

Gonads

Estrogen, progesterone,

Secondary sex characteristics; gametogenesis; progesterone maintains

testosterone

the endometrium

Most hormonal axes use negative feedback: high cortisol suppresses CRH and ACTH; high T3/T4 suppresses TRH and TSH. To locate a defect, compare the tropic hormone with its target hormone. Low T4 with high TSH points to a thyroid (primary) problem; low T4 with low TSH points to the pituitary or hypothalamus.

Reproductive System

Seminiferous tubules of the testes produce sperm; Sertoli cells

Ovaries produce oocytes and hormones. Oogenesis begins

nourish developing sperm (stimulated by FSH), and Leydig

before birth; primary oocytes arrest in prophase I until puberty,

cells make testosterone (stimulated by LH). Sperm mature in

and the secondary oocyte arrests in metaphase II until

the epididymis and travel through the vas deferens,

fertilization. Each meiosis yields one ovum and polar bodies.

ejaculatory duct, and urethra. Seminal vesicles (fructose-rich

Fertilization usually occurs in the oviduct (fallopian tube);

fluid), the prostate, and bulbourethral glands add semen.

implantation occurs in the uterine endometrium.

Spermatogenesis produces four sperm per primary spermatocyte, continuously from puberty.

Theovarianandmenstrualcycles(About28Days)

Phase	Ovary	Hormones	Uterus
Follicular(days1–13)	FSHstimulatesfollicle	Risingestrogenfromgranulosacells	Menses (days 1–5) then ,
	growth		proliferationofthe
			endometrium
Ovulation(≈day14)	Secondaryoocyte	Highestrogentriggerspositive	Endometriumthickened
	released	feedback → LH surge
Luteal(days15–28)	Corpusluteumformsfrom	Progesteroneandestrogenhigh;FSH	Secretoryphase;endometrium
	therupturedfollicle	andLHsuppressed	readyforimplantation
Nopregnancy	Corpusluteum	Progesteronefalls	Endometriumsheds
	degenerates		(menstruation)
Pregnancy	Corpusluteummaintained	hCGfromthetrophoblastsustains	Endometriummaintained
		progesteroneuntiltheplacentatakes
		over

Sliding filaments: Ca2+ binds troponin, tropomyosin moves,

Pancreatic enzymes do most digestion in the small intestine;

myosin binds actin; ATP is required for detachment. The A

secretin triggers bicarbonate, CCK triggers bile and

band never shortens.

enzymes.

Blood path: RA → tricuspid → RV → pulmonary artery →

ADH adds aquaporins to the collecting duct; aldosterone raises Na+ reabsorption; the loop of Henle builds the

lungs → pulmonary veins → LA → mitral → LV → aorta. CO = HR × SV.

medullary gradient.

A right shift of the O2–Hb curve (↑CO2, ↑H+, ↑temperature,

Action potentials: Na+ in (depolarize), K+ out (repolarize);

↑2,3-BPG) unloads O2; most CO2 travels as bicarbonate.

myelin gives saltatory conduction.

Peptide hormones use surface receptors and second

Innate immunity is fast and nonspecific; adaptive immunity is

messengers; steroid hormones act on intracellular receptors

specific with memory. MHC I → CD8 cells; MHC II → CD4

to change transcription. The LH surge triggers ovulation.

cells.