If asked to name the primary function of the urinary system, many people will answer that the primary function of the kidneys is to eliminate wastes from the body. Elimination of waste is an important role for the kidneys, but the primary function of the kidneys is to maintain a stable balance of water and solutes in the fluid of the body. The urinary system regulates blood volume and blood pressure, controls levels of electrolytes and metabolites, and regulates blood pH.
At every moment approximately one fourth of the blood pumped by the heart is passing through the kidneys. The kidneys process the blood, removing some substances and adding others. The complex mechanics of filtration, reabsorption, and secretion constitute the subject matter of urinary physiology, which has major overlap with important topics from general chemistry. The urinary system is a long-standing favorite subject for MCAT passages.
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Conceptual Vocabulary for The Urinary System
The urinary system is the organ system that produces, stores, and eliminates urine. In humans it includes two kidneys, two ureters, the bladder, and the urethra.
Urine is a liquid produced through the kidney, and is collected in the bladder and excreted through the urethra.
The kidneys are organs that filter wastes, such as urea, from the blood and excrete them, along with water, as urine.
The urinary bladder is a hollow, muscular, and distensible organ that collects urine excreted by the kidneys prior to disposal by urination.
Urination, known by physiologists as micturition ot voiding, is the process of disposing urine from the urinary bladder through the urethra.
A nephron is the basic structural and functional unit of the kidney.
The proximal tubule is the portion of the duct system of the nephron leading from Bowman's capsule to the loop of Henle.
Bowman's capsule is a cup like sac at the beginning of the tubular component of a nephron in the kidney. A glomerulus is enclosed in the sac.
A glomerulus is a capillary tuft surrounded by Bowman's capsule in nephrons of the vertebrate kidney which receives its blood supply from an afferent arteriole of the renal circulation.
In the kidney, the loop of Henle is the portion of the nephron that leads from the proximal convoluted tubule to the distal convoluted tubule. The main function of this structure is to reabsorb water and ions from the urine.
The distal convoluted tubule is a portion of kidney nephron between the loop of Henle and the collecting duct system.
The collecting duct system of the kidney consists of a series of tubules and ducts that connect the nephrons to the ureter.
The urethra is a tube which connects the urinary bladder to the outside of the body.
The bicarbonate buffering system is the most important buffer solution for maintaining a relatively constant pH in the plasma.
The ureters are the ducts that carry urine from the kidneys to the urinary bladder.
The renal cortex is the outer portion of the kidney between the renal capsule and the renal medulla.
The renal medulla is the innermost part of the kidney.
Countercurrent exchange is a mechanism used to transfer some property of a fluid to another across a semipermeable membrane.
Ultrafiltration occurs at the barrier between the blood and the filtrate in the renal corpuscle or Bowman's capsule in the kidneys.
The glomerular filtration rate is the volume of fluid filtered from the renal glomerular capillaries into the Bowman's capsule per unit time.
Diuresis is increased production of urine by the kidney.
Renal compensation is a mechanism by which the kidneys can regulate the plasma pH.
The juxtaglomerular cells are cells that synthesize, store, and secrete the enzyme renin.
The juxtaglomerular apparatus is a microscopic structure in the kidney which regulates the function of each nephron.
Renal pyramids are cone-shaped tissues of the kidney within the renal medulla, which is made up of 8 to 18 of these conical subdivisions.
A renal corpuscle is the initial filtering component of a nephron consisting of a glomerulus and a Bowman's capsule.
Aquaporins are a class of major intrinsic proteins that form pores in the membrane of biological cells which selectively conduct water molecules in and out, while preventing the passage of ions and other solutes.
Renal blood flow is the volume of blood delivered to the kidneys per unit time.
The renal clearance of a substance is the inverse of the time constant that describes its removal rate from the body divided by its volume of distribution.
The cells of the macula densa within the juxtaglomerular apparatus are sensitive to the ionic content and water volume of the fluid in the distal convoluted tubule within the kidney.
The renal papilla is the location where the Medullary pyramids empty urine into the renal pelvis.
A juxtamedullary nephron is one where the proximal convoluted tubule and its associated loop of Henle occur at a deep position compared to most other nephrons.
Sodium-glucose transport proteins are a family of glucose transporter found in the intestinal mucosa of the small intestine and the proximal tubule of the nephron.
Peritubular capillaries are tiny blood vessels that travel along side nephrons allowing reabsorption and secretion between blood and the inner lumen of the nephron.
The glomerular basement membrane is the basal laminal portion of the glomerulus which performs the actual filtration with the renal corpuscle, separating the blood on the inside from the filtrate on the outside.
Podocytes are cells of the visceral epithelium in the kidneys and form a crucial component of the glomerular filtration barrier, contributing size selectivity and maintaining a massive filtration surface.
Foot processes of podocytes of the glomerulus interdigitate with one another forming filtration slits that, in contrast to those in the glomerular endothelium, are spanned by diaphragms.
The trigone is a smooth triangular region of the internal urinary bladder formed by the two ureteral orifices and the internal urethral orifice.
The minor calyx is a structure surrounding the apex of the malpighian pyramids into which urine formed in the kidney passes before passing into the major calyx.
The major calyx is a structure surrounding the apex of the malpighian pyramids into which urine formed in the kidney passes after passing into the minor calyx.
Anion Exchanger 1 or Band 3 is a transport protein responsible for catalysing the electroneutral exchange of chloride for bicarbonate across a plasma membrane.
The Davenport Diagram is a graphical tool that allows a clinician or investigator to describe blood bicarbonate concentrations and blood pH following a respiratory and/or metabolic acid-base disturbance.
Titratable acid is a term to describe acids such as phosphoric acid, sulfuric acid which are involved in renal physiology, a term used explicitly to exclude ammonium as a source of acid.
The renal capsule is a tough fibrous layer surrounding the kidney and covered in a thick layer of perinephric adipose tissue.
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