Urinary System and Excretion Organs Urine Formation Homeostatic Mechanisms

Path of Urine through the Urinary System Kidneys produce urine Ureters transport urine Urinary bladder stores urine Urethra passes urine to outside

Functions of the Urinary System Excretion: removal of metabolic wastes Urea: breakdown product of amino acids Creatinine: breakdown product of creatine phosphate, a high energy molecule in muscles Uric acid: breakdown product of nucleotides Ammonium from amino acids used for energy

Structure of the Kidney Renal Cortex: outer layer Renal Medulla: inner layer Renal Pelvis: central space Nephrons are microscopic tubules that produce urine

Structure of the Nephron Proximal convoluted tubule Distal convoluted tubule Glomerular capsule Peritubular capillary network Glomerulus (capillaries) Collecting Duct Descending limb Ascending limb Loop of the nephron

Urine Formation: Glomerular Filtration Cells, cell fragments and proteins remain in the bloodstream Formation of a filtrate containing some components from whole blood Water and small molecules enter glomerular capsule from the glomerulus Proteins, cell fragments and cells are excluded from the filtrate Water, nutrients, salts, nitrogenous wastes enter nephron

Urine Formation: Tubular Reabsorption Water, nutrients, salt reenter the bloodstream Molecules and ions are reabsorbed into the blood, either actively or passively Reabsorption occurs from the proximal convoluted tubule into the peritubular capillary network Water, nutrients and salts are reabsorbed into the bloodstream Nitrogenous waste, excess salts, some water remain in nephron Sodium ions are actively reabsorbed, chloride ions follow passively. This increase in NaCl increases the salt concentration of the blood, so water moves passively into the blood. Nutrients such as glucose and amino acids return to the blood by a selective process, they must be recognized by carrier molecules in order to be actively absorbed. Glucose is ordinarily completely reabsorbed due to a large quantity of carrier molecules. In diabetes, the kidneys cannot absorb the excess levels of glucose in the blood so glucose appears in the urine. Caffeine increases glomerular filtration rate and decreases tubular reabsorption of Na. Peritubular capillary network

Urine Formation: Tubular Secretion H+, creatinine, drugs enter the nephron Substances are actively transported from the blood (peritubular capillary network) into the kidney tubule Involves transport of hydrogen ions, creatinine, penicillin and other drugs Peritubular capillary network In the end, urine contains substances that have undergone glomerular filtration but have not been reabsorbed, and substances that have undergone tubular secretion.

Urine Formation: Water Reabsorption Water is reabsorbed into the bloodstream from all parts of the nephron, and the collecting duct Occurs by osmosis through aquaporins, membrane channel proteins Controlled by Reabsorption of salt Gradient of NaCl in medulla tissue surrounding the loop ADH, antidiuretic hormone, released from pituitary when water intake is low, stimulates aquaporin activity in distal convoluted tubule and collecting duct release is inhibited by alcohol Descending limb: permeable to water but less permeable to Na and Cl Thick ascending limb: actively transports Cl, with Na following passively, from tubule into the surrounding tissue fluid. Not permeable to water. NaCL concentration increases in tissue fluid Thin ascending limb: allows Na and Cl to move passively into surrounding tissue fluid; not permeable to water ADH is released by posterior lobe of pituitary and increases the permeability of the collecting duct. In presence of ADH, more water is reabsorbed at collecting duct and urine volume decreases. If not drinking much, ADH released to retain water. If drinking excess water, ADH is not released and a greater volume of urine is produced. Alcohol inhibits the secretion of ADH. Diuretic drugs inhibit active transport of Na+ in the loop of the nephron. These are used to counteract high blood pressure. Concentration Gradient formed by diffusion and active transport of NaCl in ascending limb Descending limb and collecting duct are permeable to water Reabsorption causes increased concentration of urine compared to blood plasma Urine has a lower water concentration than blood

Urine Formation: Salt Reabsorption Sodium: >99% reabsorbed reabsorption in PCT, DCT, collecting duct active transport in ascending limb of loop of nephron Other reabsorbed or excreted ions potassium ions bicarbonate ions magnesium ions

Hormonal Control of Sodium Reabsorption If blood pressure decreases Angiotensinogen Angiotensin I Renin secretes Angiotensin II Aldosterone (from adrenal glands) stimulates secretion of constricts blood vessels Juxtaglomerular apparatus is a region of contact between the afferent arteriole and the distal convoluted tubule. When blood volume, and therefore blood pressure, is not sufficient to promote glomerular filtration, the JGA secretes renin. Renin is an enzyme that changes angiotensinogen (a large plasma protein produced by the liver) into angiotensin I. Later, angiotensin I is converted to antiotensin II, a powerful vasoconstrictor that also stimulates the adrenal cortex to release aldosterone. Aldosterone promotes the excretion of potassium ions and the reabsorption of sodium ions. The reabsorption of sodium ions is followed by the reabsorption of water. Therefore, blood volume and blood pressure increase. Stimulates sodium reabsorption from DCT, water follows sodium

Hormonal Control of Sodium Excretion If blood pressure increases prevents renin secretion Angiotensinogen Angiotensin I X Renin secretes inhibition of secretion X Angiotensin II Heart atrial cells Atrial Natriuretic Hormone (ANH) secrete Juxtaglomerular apparatus is a region of contact between the afferent arteriole and the distal convoluted tubule. When blood volume, and therefore blood pressure, is not sufficient to promote glomerular filtration, the JGA secretes renin. Renin is an enzyme that changes angiotensinogen (a large plasma protein produced by the liver) into angiotensin I. Later, angiotensin I is converted to antiotensin II, a powerful vasoconstrictor that also stimulates the adrenal cortex to release aldosterone. Aldosterone promotes the excretion of potassium ions and the reabsorption of sodium ions. The reabsorption of sodium ions is followed by the reabsorption of water. Therefore, blood volume and blood pressure increase. Sodium is excreted in urine, water follows Aldosterone (from adrenal glands)

Functions of the Urinary System Maintenance of Water-Salt Balance important for blood pressure and blood volume Water reabsorption Loop of the Nephron Collecting Duct Sodium reabsorption or excretion Proximal convoluted tubule Distal convoluted tubule

Functions of the Urinary System Maintenance of Acid-Base Balance Blood pH is controlled by the bicarbonate buffering system Buffer: substance that resists pH change When lungs release carbon dioxide, H+ has been used to produce water, so pH remains about neutral (7.4) bicarbonate ion hydrogen ion carbonic acid carbon dioxide water When blood pH decreases, chemoreceptors in the carotid bodies (in carotid arteries) and in aortic bodies (aorta) stimulate the respiratory center and the rate and depth of breathing increase. When blood pH begins to rise, the respiratory center is depressed and the level of bricarbonate ions increases in the blood.

Functions of the Urinary System Maintenance of Acid-Base Balance Kidney excretes excess hydrogen ions (H+) reabsorbs bicarbonate ions (HCO3-)

Functions of the Urinary System Assistance with Hormone Release and Secretion of Hormones Renin (enzyme) influences secretion of aldosterone from adrenal glands located at top of kidneys Aldosterone regulates water/salt balance of blood by influencing ion exchange at the distal convoluted tubule Erythropoietin: stimulates red blood cell production

Functions of the Urinary System Assisting other body systems Kidneys regulate the level of calcium ions in blood Convert Vitamin D to a form that assists with calcium absorption Regulate the excretion of calcium Kidneys regulate the sodium and potassium ions in blood, important for nerve conduction, heart and skeletal muscle contraction