Anatomy Terms Starting With D
Anatomy Glossary: D
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Dermis
/ DER-mis / · Greek derma, skin
Dermis dermis is the thick layer of dense connective tissue beneath the epidermis that gives skin its strength, elasticity, and sensory capacity, and houses hair follicles, sweat glands, and blood vessels.
The dermis is divided into a superficial papillary layer of loose connective tissue and a deeper reticular layer of dense irregular connective tissue rich in collagen and elastin fibers. It contains sensory receptors for touch, pressure, pain, and temperature, as well as the dermal papillae that interlock with epidermal ridges to form fingerprints. Fibroblasts in the dermis synthesize and remodel the collagen matrix, and their declining activity with age contributes to skin wrinkling and reduced wound healing.
The dermis contains collagen and elastic fibers that give skin strength and stretch. It also holds many blood vessels, nerves, hair follicles, and sweat glands.
Integumentary System Facts →All skin layers are made of dead cells. The dermis is living tissue with active cells, blood supply, and sensory structures.
How To Become A Dermatologist? →In elephants, the dermis helps form thick skin with deep folds. Those folds can hold water and mud, which helps cooling at the surface.
Elephant →Diaphysis
/ dy-AF-ih-sis / · Greek diaphyein, to grow between
Diaphysis diaphysis is the elongated central shaft of a long bone, composed primarily of compact bone surrounding a medullary cavity filled with yellow bone marrow in adults.
The cortical bone of the diaphysis provides the mechanical rigidity needed to bear body weight and resist bending forces during locomotion, while the hollow medullary cavity reduces bone mass without sacrificing strength. Its outer surface is covered by periosteum, a fibrous connective tissue sheath containing osteogenic cells responsible for appositional bone growth and fracture repair. In children, the diaphysis is separated from the epiphyses at each end by cartilaginous growth plates that drive longitudinal bone elongation until skeletal maturity.
The diaphysis is the shaft of a long bone and is built mainly from compact bone. Its hollow central cavity helps reduce weight while still allowing strong support.
Fun Facts About the Skeletal System →The whole long bone is called the diaphysis. The diaphysis is only the shaft, while the ends are called epiphyses.
In a horse's metacarpal bone, the diaphysis is long and strong for fast running. Its shape helps carry large forces through the limb.
Diastole
/ dy-AS-toh-lee / · Greek diastole, dilation
Diastole is the phase of the cardiac cycle during which the ventricles relax and fill with blood from the atria, lasting approximately 0.5 seconds at a resting heart rate of 75 beats per minute.
Ventricular relaxation begins with isovolumetric relaxation, during which ventricular pressure falls below aortic pressure, causing semilunar valve closure, followed by passive filling as atrioventricular valves open. Late diastole includes atrial contraction, which contributes approximately 20% of ventricular filling at rest but up to 40% during rapid heart rates when filling time is reduced. Diastolic dysfunction, meaning impaired ventricular relaxation, underlies heart failure with preserved ejection fraction, a condition affecting tens of millions of people worldwide.
Coronary artery perfusion also occurs predominantly during this phase, because myocardial compression during systole compresses the vessels and limits flow.
Most coronary blood flow into the heart muscle occurs during diastole rather than during systole. When the ventricles relax, the compression on intramyocardial vessels is released, and perfusion pressure drives blood through the coronary circulation.
How To Become A Cardiologist? →Diastole means the heart has stopped working. Diastole is an active part of the heartbeat when chambers relax and fill with blood.
In trained endurance athletes such as competitive cyclists, resting heart rates can fall below 40 beats per minute, extending diastolic filling time and allowing stroke volume to increase substantially. This prolonged diastole is one reason athletic hearts pump more blood per beat than untrained hearts.
Distal Tubule
/ DIS-tul TYOO-byool / · Latin distalis, far; Latin tubulus, small pipe
Distal Tubule is the nephron segment between the loop of Henle and the collecting duct that fine-tunes the ionic composition of the tubular fluid under hormonal regulation by aldosterone and parathyroid hormone.
Principal cells of the distal convoluted tubule reabsorb sodium under aldosterone stimulation and secrete potassium, while intercalated cells regulate acid-base balance by secreting protons or bicarbonate. Parathyroid hormone acts on this segment to increase calcium reabsorption, directly linking renal function to systemic calcium homeostasis. Thiazide diuretics, among the most widely prescribed antihypertensive drugs, target the distal tubule by blocking the sodium-chloride cotransporter, reducing sodium reabsorption and lowering blood volume.
Because only about 5 to 10 percent of filtered sodium reaches this segment, the tubule exerts fine regulatory control rather than bulk reabsorption.
The distal tubule was identified as a distinct nephron segment through micropuncture studies pioneered by Alfred Richards in the 1920s and 1930s, in which tiny glass pipettes sampled fluid directly from individual tubule segments in frog kidneys. Those experiments revealed that tubular fluid composition changes substantially between the loop of Henle and the collecting duct.
Endocrine System Fun Facts →The distal tubule simply carries fluid onward with no changes. It actively adjusts the filtrate's sodium, potassium, calcium, and acid-base composition before the fluid reaches the collecting duct.
Urinary System Fun Facts →In the kidneys of the domestic dog (Canis lupus familiaris), distal tubule cells respond to aldosterone by increasing sodium reabsorption when dietary sodium is restricted. This hormonal response helps maintain blood pressure and plasma volume without requiring changes in glomerular filtration rate.
How To Become A Nephrologist? →Duodenum
/ dyoo-oh-DEE-num / · Latin duodeni, twelve each (about 12 finger-widths long)
Duodenum is the first and shortest segment of the small intestine, approximately 25 centimeters long in adults, where acidic gastric chyme mixes with pancreatic enzymes and bile to continue chemical digestion.
As chyme enters from the stomach, duodenal enteroendocrine cells release secretin and cholecystokinin, hormones that stimulate the pancreas to secrete bicarbonate and digestive enzymes and prompt the gallbladder to contract and release bile. Bicarbonate neutralizes the acidic chyme, raising luminal pH to a range that allows pancreatic lipases, proteases, and amylases to function. Bile salts emulsify dietary fats, increasing the surface area available for lipase activity.
The duodenum also absorbs iron and folate, nutrients whose uptake is largely restricted to this proximal intestinal segment.
Peptic ulcers form more often in the duodenum than in the stomach, with duodenal ulcers accounting for roughly 80 percent of all peptic ulcer cases. Most duodenal ulcers are caused by infection with Helicobacter pylori, a bacterium that disrupts the mucous lining and exposes the epithelium to acid damage.
Fun Facts About Digestive System →The stomach finishes most chemical digestion. The duodenum is where many digestive enzymes and bile first meet food after it leaves the stomach, making it the primary site where enzymatic digestion of fats and proteins begins.
In the common bottlenose dolphin (Tursiops truncatus), the duodenum receives pancreatic secretions adapted for digesting a high-protein, high-fat fish diet. Researchers studying cetacean digestion have found that bile salt composition in marine mammals differs from that of terrestrial species, reflecting dietary specialization.