Conservation of bicarbonate in kidney.

Conservation of bicarbonate in kidney.

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.

WikiPremed Resources

The Urinary System Images
Image gallery for study with links to larger teaching JPEGs for classroom presentation

Question Drill for The Urinary System
Conceptual Vocabulary Self-Test

Basic Terms Crossword Puzzle

Basic Puzzle Solution

Learning Goals


Be able to describe the overall structure and primary functions of the urinary system.

Understand how transamination reactions as well as the alanine-glucose cycle and urea cycle enable the excretion of nitrogenous waste in mammals in the form of urea.

Be able to describe how the gross anatomical features of kidney relate to its functions in filtering the blood and producing urine.

Master a clear narrative of changes to the filtrate as it passes from Bowman's capsule, to the proximal convoluted tubule, the loop of Henle, distal convoluted tubule, and collecting duct.

Be prepared to describe the narrative of filtration and urine formation with regards to the reabsorption and secretion of water, ions, urea, ammonia, glucose and amino acids.

Be able to describe how countercurrent flow works with reference to differences in permeability to water and active transport mechanisms in the descending and ascending limbs of the loop of Henle.

Understand the mechanism by which ADH regulates water reabsorption in the collecting duct.

Be able to describe how the blood pressure and reabsorption of sodium is regulated by the action of the hormones atrionatriuretic factor, renin, angiotensin II and aldosterone and how reabsorption of calcium and secretion of phosphate is regulated by parathyroid hormone.

Have a good understanding of the mechanisms of renal regulation of acid-base balance including bicarbonate reabsorption, ammonium production and transfer into the filtrate, and acid excretion.

Suggested Assignments

Review the terminology for the urinary system using the question server. Complete the fundamental terms crossword puzzle. Here is the solution to the puzzle.

Complete the advanced crossword puzzle for the urinary system. Here is the solution to the puzzle.

Read pp. 162-169 in ExamKrackers Biology II. Perform practice items 113-120 on pg. 170.

Review the web resources for the urinary system.

Conceptual Vocabulary for The Urinary System

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.
Urinary bladder
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.
Proximal tubule
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
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.
Loop of Henle
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.
Distal convoluted tubule
The distal convoluted tubule is a portion of kidney nephron between the loop of Henle and the collecting duct system.
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.
Bicarbonate buffering system
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.
Renal cortex
The renal cortex is the outer portion of the kidney between the renal capsule and the renal medulla.
Renal medulla
The renal medulla is the innermost part of the kidney.
Countercurrent exchange
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.
Glomerular filtration rate
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
Renal compensation is a mechanism by which the kidneys can regulate the plasma pH.
Juxtaglomerular cell
The juxtaglomerular cells are cells that synthesize, store, and secrete the enzyme renin.
Juxtaglomerular apparatus
The juxtaglomerular apparatus is a microscopic structure in the kidney which regulates the function of each nephron.
Renal pyramids
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.
Renal corpuscle
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
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.
Macula densa
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.
Renal papilla
The renal papilla is the location where the Medullary pyramids empty urine into the renal pelvis.
Juxtamedullary nephron
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
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
Peritubular capillaries are tiny blood vessels that travel along side nephrons allowing reabsorption and secretion between blood and the inner lumen of the nephron.
Glomerular basement membrane
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.
Advanced terms that may appear in context in MCAT passages
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.
Filtration slits
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.
Trigone of urinary bladder
The trigone is a smooth triangular region of the internal urinary bladder formed by the two ureteral orifices and the internal urethral orifice.
Minor calyx
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.
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.
Band 3
Anion Exchanger 1 or Band 3 is a transport protein responsible for catalysing the electroneutral exchange of chloride for bicarbonate across a plasma membrane.
Davenport diagram
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
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.
Renal capsule
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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