Anatomy Terms Starting With S
Anatomy Glossary: S
Sarcomere
/ SAR-koh-meer / · Greek sarx, flesh; meros, part
Sarcomere is the repeating contractile unit of striated muscle, bounded at each end by a Z-disc and containing interdigitating thick myosin filaments and thin actin filaments whose sliding interaction shortens the unit and generates force.
Under an electron microscope, the sarcomere displays a precise banding pattern: the dark A-band spans the full length of the myosin filaments, including the zones where myosin and actin overlap, while the lighter I-band on either side of each Z-disc contains only actin. At the center of the A-band, the H-zone marks the region occupied solely by myosin, and the M-line bisects it, cross-linking adjacent myosin filaments. During contraction, myosin heads bind actin, pivot in a power stroke, and release in a cycle driven by ATP hydrolysis, drawing the Z-discs closer together and shortening the sarcomere from a resting length of about 2.2 micrometers to as little as 1.6 micrometers.
Titin, the largest known protein, spans from the Z-disc to the M-line and constitutes a molecular spring that resists over-stretch and centers the myosin filament.
A single human biceps brachii muscle fiber can contain more than 100,000 sarcomeres arranged end to end along its length; when the muscle shortens by just a few centimeters, each sarcomere contributes only a fraction of a micrometer to that movement, but the effect accumulates across the entire series.
Muscular System Facts →Muscle filaments physically shorten during contraction. Both actin and myosin filaments maintain their individual lengths throughout contraction; the sarcomere shortens because the filaments slide past each other, increasing their overlap rather than compressing.
In the flight muscle of the hawkmoth (Manduca sexta), sarcomeres cycle at up to 100 contractions per second during sustained hovering, a rate far beyond the 10 to 20 cycles per second typical of human fast-twitch fibers; this speed depends on specialized calcium-handling proteins and an unusually high mitochondrial density that can supply ATP fast enough to sustain each power stroke.
Fun Facts About the Skeletal System →Sinoatrial Node
/ sy-noh-AY-tree-ul NOHD / · Latin sinus, cavity; atrium; Latin nodus, knot
Sinoatrial Node is a small cluster of specialized autorhythmic cells in the right atrial wall that spontaneously generates electrical impulses at 60 to 100 per minute, setting the heart's natural rhythm without requiring external nerve signals.
SA node cells have an unstable resting membrane potential that spontaneously depolarizes through funny current and calcium channel activity, triggering action potentials that spread across the atria and reach the atrioventricular node. Sympathetic stimulation increases the firing rate while parasympathetic stimulation decreases it, allowing the autonomic nervous system to adjust heart rate to match physiological demand. When SA node dysfunction occurs, an artificial electronic pacemaker must be implanted to restore a normal rhythm.
The SA node is not the only tissue in the heart capable of spontaneous depolarization. The atrioventricular node and Purkinje fibers can also generate impulses, but at slower intrinsic rates of 40 to 60 and 20 to 40 beats per minute respectively, so they only take over if the SA node fails.
Fun Facts About the Nervous System →Every heartbeat begins in the brain. The brain can speed or slow the heart, but the normal rhythm originates in the sinoatrial node itself.
In humans, the sinoatrial node sits in the right atrium near the opening of the superior vena cava. Its impulse spreads through the atria before reaching the ventricles, producing the coordinated contraction that pumps blood forward.
Circulatory System Fun Facts →Skeletal Muscle
/ SKEL-eh-tul MUS-ul / · Greek skeletos, dried; Latin musculus, little mouse
Skeletal muscle is the voluntary, striated muscle tissue attached to the skeleton that generates forces for body movement, postural support, heat production, and breathing.
Skeletal muscle fibers form when myoblasts fuse during development, creating long multinucleate cells whose sarcomeres align in register to produce the characteristic cross-striped appearance under a microscope. Each fiber belongs to a motor unit, a group of fibers controlled by a single motor neuron, and the number of fibers per unit determines how finely a movement can be graded. Fiber type also shapes performance: slow-twitch fibers resist fatigue and sustain posture, while fast-twitch fibers generate rapid, powerful contractions suited to sprinting or jumping.
The human body contains more than 600 skeletal muscles, yet the smallest, the stapedius muscle in the middle ear, measures only about 1 mm in length and dampens excessive vibration of the stapes bone to protect hearing.
Muscular System Facts →Skeletal muscle only moves bones. Beyond locomotion, skeletal muscle maintains posture, drives breathing through the diaphragm and intercostal muscles, and generates most of the body's resting heat through continuous low-level contraction.
Fun Facts About the Skeletal System →In cheetahs (Acinonyx jubatus), fast-twitch skeletal muscle fibers dominate the hindlimbs and back, supporting acceleration from rest to roughly 100 km/h in about three seconds. The large epaxial muscles along the spine flex and extend the vertebral column with each stride, effectively lengthening the animal's step and amplifying ground speed.
Skeleton
/ SKEL-eh-ton / · Greek skeletos, dried up body
Skeleton is the internal framework of bones, cartilage, and connective tissue that supports the body, protects vital organs, anchors muscles for movement, stores minerals, and houses hematopoietic bone marrow.
The adult human skeleton contains 206 bones organized into the axial skeleton, which includes the skull, vertebral column, and rib cage, and the appendicular skeleton, which includes the limbs and girdles. Bone tissue stores approximately 99% of the body’s calcium, releasing it into the bloodstream when dietary intake falls short. Continuous remodeling by osteoblasts, osteoclasts, and osteocytes adjusts bone density and architecture in response to mechanical loading, hormonal signals, and nutritional status throughout life.
Newborn humans have roughly 270 to 300 separate bone elements at birth; many of these fuse progressively through childhood and adolescence until the adult count of 206 is reached, with the last growth plates typically closing in the early twenties.
Fun Facts About the Skeletal System →The skeleton is only a frame for body shape. Beyond structural support, it protects organs such as the brain and heart, produces blood cells in red marrow, and regulates blood calcium levels through constant mineral exchange.
In sea stars (class Asteroidea), the skeleton consists of hundreds of small calcium carbonate plates called ossicles embedded in the body wall. These plates interlock loosely, giving the animal rigid support while still permitting the flexible arm movements used during feeding and locomotion.
Skin
/ SKIN / · Old Norse skinn, hide
Skin is a stratified epithelial and connective tissue organ covering approximately 1.7 square meters in adults that provides mechanical protection, waterproofing, thermoregulation, sensory reception, and immune defense.
Skin comprises three layers: the epidermis, dermis, and hypodermis, which together form a physical and immunological barrier against pathogens, UV radiation, chemicals, and dehydration. Eccrine sweat glands distributed across most body surfaces drive evaporative cooling, while sebaceous glands secrete sebum that maintains skin flexibility and surface acidity. Melanocytes in the basal epidermis produce melanin that screens UV radiation and determines skin pigmentation, with output increasing after UV exposure as a protective response.
Human skin completely replaces its outermost epidermal layer roughly every 27 days, shedding approximately 30,000 to 40,000 dead cells per hour. Over a lifetime, this turnover amounts to about 48 kg of shed skin.
Integumentary System Facts →Skin is only a covering. Skin is a living organ containing immune cells such as Langerhans cells, sensory nerve endings, blood vessels, and glands that actively respond to injury, infection, and environmental change.
Fun Facts About the Nervous System →In poison dart frogs (family Dendrobatidae), specialized skin glands synthesize and release alkaloid toxins potent enough to cause cardiac arrest in predators. Bright warning coloration on the skin signals this chemical defense, a strategy known as aposematism.
Small Intestine
/ SMAWL in-TES-tin / · Old English smael; Latin intestinum
Small intestine is the 6 to 7 meter tubular organ between the stomach and large intestine where the majority of chemical digestion and virtually all nutrient absorption occur across its highly folded, villous mucosal surface.
Three sequential segments make up the small intestine: the duodenum, jejunum, and ileum. The duodenum receives bile from the gallbladder and digestive enzymes from the pancreas, which break down fats, proteins, and carbohydrates into absorbable molecules. Circular folds, finger-like villi, and microscopic microvilli on the epithelial surface expand the absorptive area to roughly 250 square meters, about the size of a tennis court, concentrating most amino acid, sugar, and fatty acid uptake in the jejunum.
The ileum, the final segment of the small intestine, contains specialized patches of lymphoid tissue called Peyer's patches that sample gut contents and mount immune responses against ingested pathogens, linking digestion directly to immune surveillance.
Fun Facts About Digestive System →The small intestine is called small because it is short. Its name refers to its diameter, which is narrower than that of the large intestine, even though the small intestine is considerably longer.
In humans, the duodenum receives bile and pancreatic enzymes within centimeters of the stomach's pyloric valve, rapidly lowering the acidity of chyme and activating fat-digesting lipases. Absorption continues through the jejunum and ileum, with vitamin B12 and bile salts recovered specifically in the terminal ileum before the remaining material passes into the large intestine.
Smooth Muscle
/ SMOOTH MUS-ul / · Old English smoth; Latin musculus
Smooth muscle is the non-striated, involuntary muscle tissue found in the walls of hollow organs, including blood vessels, airways, the gastrointestinal tract, and the uterus, that regulates lumen diameter and organ motility.
Smooth muscle cells are spindle-shaped, uninucleate, and lack the sarcomere organization of striated muscle, using dense bodies and intermediate filaments instead of Z-discs to anchor actin filaments. Contraction begins when intracellular calcium binds calmodulin, activating myosin light chain kinase and enabling cross-bridge cycling at a slow rate that sustains tension for hours at low energy cost. Autonomic nerves, circulating hormones such as epinephrine and oxytocin, and local metabolites all modulate smooth muscle tone, allowing organs to adjust their diameter and contractile force without conscious control.
The uterine smooth muscle, called the myometrium, undergoes a dramatic shift in contractile behavior near term pregnancy. Oxytocin receptor density on myometrial cells increases roughly 200-fold in the days before labor, making the tissue far more sensitive to oxytocin and coordinating the powerful contractions of childbirth.
How To Become A Gastroenterologist? →All muscle contractions are voluntary. Smooth muscle operates involuntarily in organs such as the gut, blood vessels, bladder, and uterus, and its activity cannot be consciously started or stopped the way skeletal muscle contractions can.
In human intestines, smooth muscle arranged in circular and longitudinal layers produces coordinated peristaltic waves that propel food along the digestive tract. Circular fibers constrict the lumen behind a food bolus while longitudinal fibers shorten the segment ahead, moving contents forward at roughly 2 to 25 cm per minute depending on the intestinal region.
Fun Facts About Digestive System →Spongy Bone
/ SPUN-jee BOHN / · Old French espongier; Old English ban
Spongy bone is the porous, lattice-like internal bone tissue composed of trabeculae oriented along lines of mechanical stress that provides structural strength with minimal mass and houses red marrow in its interconnected spaces.
The trabecular architecture of spongy bone follows the principal stress trajectories, both compression and tension lines, predicted by engineering analysis, distributing loads efficiently while keeping mass low. Red bone marrow occupying the trabecular spaces in flat bones, vertebrae, and the epiphyses of long bones is the site of active hematopoiesis, producing billions of blood cells each day. Osteoporosis preferentially affects spongy bone because its high surface-area-to-volume ratio exposes more matrix to osteoclast-mediated resorption than the denser cortical shell of compact bone.
The trabecular pattern in the human femoral head so closely mirrors the stress lines calculated by engineer Karl Culmann in 1866 that his analysis of a crane's load distribution, when overlaid on a cross-section of the femur, matched the bone's internal architecture almost exactly, an observation that helped establish the field of bone biomechanics.
How To Become A Hematologist? →Spongy bone is weak because it has spaces. Its trabecular lattice is oriented precisely along the directions of greatest mechanical load, making it highly resistant to the forces a bone routinely experiences while using far less material than solid cortical bone would require.
In the head of a human femur, spongy bone distributes compressive forces from the hip joint across a broad internal network of trabeculae, preventing stress concentration at any single point. The marrow-filled spaces within this network remain active in hematopoiesis well into adulthood, producing red blood cells, platelets, and white blood cells continuously.
Systemic Circulation
/ sis-TEM-ik ser-kyoo-LAY-shun / · Greek systema; Latin circulatio
Systemic circulation is the high-pressure circuit that carries oxygenated blood from the left ventricle through the aorta, arteries, arterioles, and capillaries to all body tissues, returning deoxygenated blood via veins to the right atrium.
Systemic arterial pressure averages 120/80 mmHg and reflects the balance between cardiac output and total peripheral vascular resistance, regulated by the autonomic nervous system, the renin-angiotensin-aldosterone system, and vasoactive hormones. Capillary beds throughout the body are the sites where oxygen and nutrients move from blood into tissues and carbon dioxide and metabolic waste move in the opposite direction. Portal circulation, coronary circulation, and cerebral circulation are specialized components of the systemic circuit, each with distinct anatomical arrangements and regulatory mechanisms suited to the metabolic demands of their target organs.
The total length of blood vessels in the human systemic circuit, including arteries, capillaries, and veins, is estimated at roughly 100,000 kilometers, enough to circle Earth about two and a half times. Most of that length consists of capillaries, whose combined surface area exceeds 500 square meters.
Circulatory System Fun Facts →Systemic circulation includes the lungs. The lung circuit is pulmonary circulation; systemic circulation supplies every other organ and tissue in the body, from the brain to the skin to the skeletal muscles.
Respiratory System Fun Facts →In giraffes (Giraffa camelopardalis), the left ventricle generates systolic pressures of roughly 300 mmHg, nearly twice the human value, to drive oxygenated blood up the roughly 2-meter neck to the brain. Specialized valves and elastic arterial walls prevent that pressure from damaging cerebral vessels when the animal lowers its head to drink.
Systole
/ SIS-toh-lee / · Greek systole, contraction
Systole is the phase of the cardiac cycle during which the ventricles contract and eject blood into the pulmonary artery and aorta, producing the peak arterial pressure recorded as the upper number in a blood pressure reading.
Ventricular systole begins with isovolumetric contraction, during which pressure rises sharply inside the ventricles before the semilunar valves open, followed by rapid ejection and then a reduced ejection phase as ventricular pressure approaches aortic or pulmonary pressure. Left ventricular systolic pressure reaches approximately 120 mmHg in normotensive adults, while right ventricular systolic pressure is only about 25 mmHg, reflecting the shorter, lower-resistance path to the lungs. Systolic dysfunction, defined as impaired ventricular contractility, underlies heart failure with reduced ejection fraction, a condition in which the ventricle ejects less than 40% of its end-diastolic volume per beat.
Atrial systole, the contraction of the upper chambers, contributes roughly 20 to 30% of ventricular filling at rest by pushing a final volume of blood into the ventricles just before ventricular systole begins. At high heart rates, when diastolic filling time shortens, this atrial contribution becomes proportionally more important for maintaining stroke volume.
Circulatory System Fun Facts →Systole means the whole heart contracts at one time. The atria and ventricles contract in a timed sequence: atrial systole precedes ventricular systole by a fraction of a second, with the delay coordinated by the atrioventricular node.
In a resting human, ventricular systole lasts approximately 270 milliseconds at a heart rate of 75 beats per minute, during which roughly 70 mL of blood is ejected from each ventricle. The closing of the aortic and pulmonary valves at the end of systole produces the second heart sound, heard as the "dub" in a stethoscope recording.