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A midsagittal (median) plane runs vertically through the midline and produces equal right and left halves. Transverse and frontal planes divide the body differently, so only the midsagittal plane satisfies the 'equal right and left' criterion.
Anterior means toward the front of the body, and the stomach lies in the ventral cavity while the spinal cord lies dorsally. Therefore the stomach is anterior to the spinal cord. Distal is used for limb positions, not for truncal organs.
Simple squamous epithelium is a single layer of flat cells, producing a minimal diffusion distance ideal for exchange in the alveoli and capillary walls. Cilia, microvilli, and stratified layers are features of other epithelial types serving different roles.
Cardiac muscle is striated, involuntary, and joined by intercalated discs that contain gap junctions and desmosomes. Skeletal muscle is voluntary, smooth muscle lacks intercalated discs, and dense regular connective tissue is not muscle at all.
The small intestine, especially the jejunum, provides the vast surface area of villi and microvilli where most digestion products are absorbed. The stomach absorbs limited substances, and the large intestine mainly reclaims water and electrolytes.
Pulmonary veins return oxygenated blood from the lungs to the left atrium. This is the classic exception to the rule that veins carry deoxygenated blood. The pulmonary artery carries deoxygenated blood away from the heart to the lungs.
A tendon is dense regular connective tissue that attaches muscle to bone and transmits contractile force. Ligaments join bone to bone, while cartilage and fascia serve different structural roles.
Dendrites are branched processes that receive synaptic input and carry graded signals toward the soma. The axon conducts impulses away from the cell body, and myelin insulates the axon to speed conduction.
Hyperventilation blows off carbon dioxide and produces respiratory alkalosis. To compensate, the kidneys reduce bicarbonate reabsorption so that more bicarbonate is excreted, lowering pH back toward normal. Retaining more hydrogen ions would also help, but the kidneys normally excrete hydrogen rather than reabsorb it during alkalosis.
As ventricular pressure rises during systole, it exceeds atrial pressure and closes the AV valves, producing the first heart sound. When ventricular pressure then exceeds arterial pressure, the semilunar valves open and blood is ejected.
The posterior pituitary stores and releases antidiuretic hormone and oxytocin, which are synthesized in the hypothalamus. Thyroid-stimulating hormone, ACTH, and prolactin are produced and secreted by the anterior pituitary.
The proximal convoluted tubule reabsorbs roughly two-thirds of filtered water and sodium, aided by abundant microvilli and mitochondria. Later segments perform finer adjustments under hormonal control.
Contraction of the diaphragm flattens it and increases thoracic volume, accounting for most of the air moved during quiet inspiration. External intercostals assist, while internal intercostals and abdominal muscles act mainly in forced expiration.
Erythrocytes contain hemoglobin, whose iron-containing heme groups bind oxygen in the lungs and release it in tissues. Leukocytes defend against pathogens, platelets aid hemostasis, and plasma proteins serve transport and osmotic roles.
A reflex arc begins with a receptor detecting the stimulus, followed by a sensory neuron carrying the impulse to the integration center, typically the spinal cord. A motor neuron then transmits the response to the effector, which contracts or secretes.
The myocardium is the middle muscular layer, and it is thickest in the left ventricle because that chamber generates the highest pressure for systemic circulation. The endocardium lines the chambers and the epicardium is the outer serous layer.
A hinge joint is a uniaxial synovial joint that allows flexion and extension in a single plane, as at the elbow and knee. Ball-and-socket and saddle joints permit motion in multiple planes, and pivot joints allow rotation.
Beta cells of the islets of Langerhans synthesize and release insulin in response to rising blood glucose. Alpha cells secrete glucagon, delta cells secrete somatostatin, and acinar cells produce digestive enzymes.
The epiglottis is a cartilaginous flap that folds over the laryngeal opening during swallowing, directing the bolus into the esophagus. The glottis is the opening itself, and the uvula and soft palate help close the nasopharynx.
The duodenum is the first segment of the small intestine and receives bile from the common bile duct and pancreatic juice from the pancreatic duct. The jejunum and ileum continue absorption, and the cecum belongs to the large intestine.
Corticospinal fibers descend and cross at the medullary pyramids, so the left motor cortex controls the right side of the body. A left-sided cortical lesion therefore produces contralateral, right-sided weakness.
Dilation of the afferent arteriole increases renal blood flow and glomerular capillary pressure, helping restore filtration rate. Constricting the afferent arteriole would lower pressure further, and ANP promotes sodium loss rather than preserving filtration.
Fick's law states that diffusion is inversely proportional to membrane thickness, so a thickened alveolar-capillary membrane slows oxygen and carbon dioxide transfer. Increased surfactant, bronchodilation, and higher blood flow generally support rather than impair exchange.
The sodium-potassium pump maintains the concentration gradients that underlie the resting potential. Blocking it allows sodium to leak in and potassium to leak out, so gradients dissipate and the membrane gradually depolarizes over time.
The second heart sound occurs at the start of diastole when ventricular pressure falls below arterial pressure and the semilunar valves snap shut. The first heart sound results from AV valve closure at the start of systole.
A sagittal plane runs vertically front to back, and the midsagittal plane specifically passes through the midline, splitting the body into equal right and left portions. Other planes divide the body along different axes and therefore cannot produce equal left and right halves.
The thoracic cavity is the rib-enclosed space above the diaphragm, and it houses the lungs, heart, and great vessels. The abdominal and pelvic cavities lie below the diaphragm, while the cranial cavity encloses the brain.
Transitional epithelium is a stratified epithelium whose surface cells flatten and slide over one another, allowing the lining to stretch without rupturing. This property is why it is found in the bladder, ureters, and renal pelvis.
Ribosomes bound to the rough endoplasmic reticulum translate proteins that are threaded into the organelle and processed for secretion or membrane insertion. Smooth endoplasmic reticulum, by contrast, handles lipid and steroid synthesis and detoxification.
Sebaceous glands are holocrine glands attached to hair follicles that release sebum, an oily mixture that lubricates skin and hair. Sweat glands, not sebaceous glands, produce sweat for thermoregulation.
The pectoral girdle consists of the clavicle and scapula on each side, connecting the upper limb to the axial skeleton. The humerus and radius belong to the upper limb proper, and the ilium and ischium form part of the pelvis.
The hypothalamus contains nuclei that act as the body's homeostatic control center, governing temperature, thirst, hunger, and endocrine regulation of the pituitary. The medulla controls vital reflexes such as breathing and heart rate rather than these set points.
Water-soluble hormones such as peptides and catecholamines cannot cross the lipid bilayer, so they bind surface receptors and trigger intracellular second messengers like cyclic AMP. Steroid and thyroid hormones instead act on intracellular receptors.
Neutrophils make up roughly 60 percent of circulating leukocytes and are the first phagocytes recruited to acute bacterial infections. Lymphocytes handle specific immunity, while eosinophils and basophils are far less numerous.
Ventricular filling begins when the atrioventricular valves open after ventricular pressure falls below atrial pressure. Semilunar valve closure marks the start of isovolumetric relaxation, which precedes filling.
The left anterior descending artery supplies the anterior wall and apex of the left ventricle, so occlusion threatens that myocardium. The right coronary artery typically supplies the right atrium and inferior wall, and circumflex branches supply the lateral left ventricle.
A rise in hydrogen ion concentration lowers pH and reduces hemoglobin's affinity for oxygen, shifting the curve right and promoting release. The Bohr effect also includes rising carbon dioxide and temperature as rightward-shifting factors.
Residual volume is the air that cannot be exhaled even after maximal effort, and it keeps alveoli open and prevents collapse. Vital capacity, by contrast, is the maximum volume that can be exhaled after a maximal inhalation.
Villi are fingerlike projections of the small intestinal mucosa, and their microvilli further multiply surface area for nutrient absorption. Rugae and haustra are folds of the stomach and colon respectively and serve different mechanical roles.
Dehydration raises plasma osmolarity, triggering antidiuretic hormone release, which inserts aquaporins in the collecting duct. More water is reabsorbed, so a smaller volume of more concentrated urine is produced.
Schwann cells wrap individual peripheral axons to form myelin, and each cell myelinates a single segment. Oligodendrocytes perform the equivalent role in the central nervous system but extend processes to several axons.
Acetylcholinesterase normally hydrolyzes acetylcholine to end its signal, so inhibiting the enzyme prolongs the neurotransmitter's presence and its stimulation of the muscle membrane. This can cause prolonged contraction and is the basis of certain insecticides and nerve agents.
Parathyroid hormone responds to low blood calcium by promoting osteoclast-mediated bone resorption, renal calcium reabsorption, and activation of vitamin D. Calcitonin has the opposite effect and lowers blood calcium.
Sperm produced in the seminiferous tubules travel to the epididymis, where they mature and are stored until ejaculation. The vas deferens transports mature sperm, and the prostate contributes seminal fluid.
The lower number is the diastolic pressure, measured while the ventricles relax and the arteries recoil. The higher value is systolic pressure, generated during ventricular contraction.
Cardiac output equals stroke volume multiplied by heart rate, so 70 mL times 80 beats per minute gives 5,600 mL per minute, or 5.6 L/min. This falls within the typical adult resting range of roughly 5 L/min.
Fick's law states that net diffusion rate rises with a larger concentration gradient and a thinner membrane, and also with greater surface area. Combining a large gradient with a thin barrier therefore maximizes the rate.
Parietal cells secrete hydrochloric acid and intrinsic factor, and they are stimulated by acetylcholine from the vagus as well as by gastrin and histamine. Chief cells secrete pepsinogen, which requires the acid to become active pepsin.
Pain and temperature travel in the spinothalamic tract, which crosses the midline near its level of entry, so a left-sided cord lesion produces right-sided loss. The dorsal column pathway carries touch and proprioception and would remain intact in this pattern.
The body's levels of organization proceed from chemical to cellular, tissue, organ, organ system, and organism. An organ is defined as a structure composed of two or more tissue types that cooperate to carry out a particular function, such as the stomach combining epithelium, muscle, and connective tissue.