Urinary System Chapter 24

Organs of Excretion Kidneys are the most important organs of excretion Eliminate nitrogenous waste, water, electrolytes, toxins and drugs Other organs include Sweat glands (nitrogen, water, electrolytes) Lungs (carbon dioxide and water) Intestines (digestive waste, bile pigments)

Urinary System Includes Kidneys (form urine from blood) Ureters (carry urine from kidneys to bladder) Urinary bladder (store urine until eliminated) Urethra (carries urine from bladder to outside of body) Ren/o and nephr/o both refer to kidneys Ur/o refers to urine or urinary system

Kidneys Located in the posterior abdomen, behind the peritoneum; retroperitoneal Protected by the ribs, renal fascia and adipose tissue

Kidneys Structure: reddish-brown, beanlike shape, enclosed in a tough fibrous capsule; three distinct regions: Renal cortex (outer region) Renal medulla (deeper, triangular structures) Renal pelvis (collection area for urine, forms the upper end of the ureter)

Kidneys (cont’d.) Blood supply: renal artery, which arises from the abdominal aorta; renal vein, which empties into the inferior vena cava 20-25% of cardiac output is sent to the kidneys Nerve supply: renal nerves; primarily sympathetic nerves

Kidneys (cont’d.) Functions: Excrete nitrogenous waste (urea, uric aid, ammonia, and creatinine) Regulate blood volume by determining the amount of water excreted Help regulate the electrolyte content of the blood

Kidneys (cont’d.) Functions: Play a major role in the regulation of acid-base balance by controlling the excretion of H+ Play a role in regulation of blood pressure Play a role in regulation of RBC production

Urine Making: The Nephron Unit Nephron: functional unit of the kidney; two parts: Renal tubules: Bowman’s capsule, proximal convoluted tubule, loop of Henle, distal convoluted tubule, collecting duct Renal blood vessels: renal artery, afferent arteriole, glomerulus, efferent arteriole, peritubular capillaries, renal vein

Renal tubular structures

Renal vascular structures

Urine Formation Urine is formed as water and dissolved substances move between the vascular and tubular systems Includes three processes: Glomerular filtration Tubular reabsorption Tubular secretion

Glomerular Filtration Water and dissolved substances move from the glomerulus into Bowman’s capsule Occurs because blood pressure in the glomerulus is higher than the pressure within Bowman’s capsule Pressure difference provides the force behind filtration

Glomerular Filtration Pores in the walls of the glomerulus determine which substances can move across the membrane into Bowman’s capsule Water, sodium, potassium, chloride, glucose, uric acid and creatinine move across easily RBCs and proteins are large and do not easily cross (remain in the blood)

Glomerular Filtration Glomerular Filtration Rate (GFR) refers to the rate at which glomerular filtration occurs The amount of filtrate normally formed is 125 ml/min or 180 L in 24 hours Only about 1.5 L/day is excreted; the rest is reabsorbed

Tubular Reabsorption Most of the filtrate from the glomerulus is reabsorbed and returned to circulation Tubular reabsorption is the process by which water and dissolved substances move from the tubules into the blood of the peritubular capillaries Most reabsorption occurs in the proximal convoluted tubule

Tubular Reabsorption Diuresis refers to the excess secretion of urine; a diuretic is a drug that increases the production of urine Most diuretics block tubular reabsorption of sodium and, therefore, the reabsorption of water. They both remain in the tubules and are eliminated as urine. Water follows sodium wherever it goes

Tubular Secretion Tubular secretion is a second process that moves very small amounts of substances from the blood into the tubules Involves active secretion of substances from the blood into the tubules Secreted substances: potassium ions (K+), hydrogen ions (H+), uric acid, ammonium ions, and drugs

Hormones That Work on the Kidneys Hormones regulate water and electrolyte excretion; affect blood volume, blood pressure and electrolyte levels of the body Aldosterone Antidiuretic hormone (ADH) Natriuretic peptides Atrial natriuretic peptide (ANP) Brain natriuretic peptide (BNP) Parathyroid hormone

Hormones That Work on the Kidneys (cont’d.) Aldosterone: Stimulates the reabsorption of sodium and water Stimulates the excretion of potassium Acts primarily on the distal tubule Increases blood volume and blood pressure Release is stimulated by angiotensin II

Aldosterone Secreted by the adrenal cortex; acts primarily on the distal tubule Stimulates reabsorption of sodium and water and the excretion of potassium “salt retaining hormone”; causes an increase in blood volume and increase in blood pressure

Aldosterone Aldosterone is released in response to renin Renin stimulates the renin-angiotensin-aldosterone system; renin is released by cells in the juxtaglomerular apparatus in response to low blood pressure or low blood volume…. Then sets off a series of events

Renin-angiotensin-aldosterone system Renin activates angiotensinogen (inactive) to form angiotensin I Converting enzyme changes angiotensin I to angiotensin II Angiotensin II stimulates the adrenal cortex to release aldosterone Aldosterone stimulates the distal tubule to reabsorb Na and H2O and excrete K

Angiotensin II Angiotensin II is also a powerful vasopressor Causes vasoconstriction and an elevation in blood pressure ACE inhibitors prevent the production of angiotensin II and aldosterone, so blood pressure is reduced -pril drugs are ACE inhibitors (lisinopril, captopril)

Antidiuretic Hormone Stimulates the reabsorption of water Works primarily on the collecting duct Plays a role in determining blood volume and blood pressure Release is stimulated by a decrease in blood volume and an increase in the concentration of solutes in the plasma

Hormones That Work on the Kidneys (cont’d.) Natriuretic peptides: cause natriuresis, excretion of sodium (Na+); decrease the secretion of aldosterone by the adrenal cortex Atrial natriuretic peptide (ANP): secreted by the walls of the atria of the heart in response to an increase in the volume of blood Brain natriuretic peptide (BNP): secreted by the walls of the ventricles in response to elevated ventricular pressure

Hormones That Work on the Kidneys (cont’d.) Parathyroid hormone (PTH): Secreted by the parathyroid glands Plays an important role in the regulation of two electrolytes Stimulates the renal tubules to reabsorb calcium and excrete phosphate Release stimulated by low plasma level of calcium

Characteristics of Urine Amount (volume): average 1500 ml/24 hours pH: average 6.0 Specific gravity: slightly heavier than water (1.001 to 1.035) Color: yellow (amber, straw colored, deep yellow in dehydration, pale yellow with overhydration)

Characteristics of Urine (cont’d.) Some abnormal constituents of urine: Albumin: (albuminuria) indicates an increased permeability of the glomerulus; sometimes induced by exercise or pregnancy Glucose: (glycosuria) usually indicates diabetes mellitus Red blood cells: (hematuria) bleeding in the urinary tract; indicates inflammation, trauma, or disease

Characteristics of Urine (cont’d.) Some abnormal constituents of urine (cont’d.): Hemoglobin: (hemoglobinuria) indicates hemolysis White blood cells: (pyuria) indicates infection within the kidney or urinary tract Ketone bodies: (ketonuria) usually indicates uncontrolled diabetes mellitus Bilirubin: (bilirubinuria) usually indicates disease involving the liver and/or biliary tree

When the Parts Don’t Work Nephrotic syndrome: excretion of large amounts of protein in the urine, causing hypoalbuminemia Glomerulonephritis: autoimmune reaction to streptococcus Acute tubular necrosis (ATN): consequence of renal tubular damage Uremia and dialysis: urine in the blood can be prevented by dialysis, an artificial method of cleansing the blood

Hemodialysis

Hemodialysis Access

Hydronephrosis

Your Plumbing Urinary tract: Ureters: connect the kidneys and bladder Urinary bladder: temporary reservoir for storage of urine Urethra: tube that carries urine from the bladder to the outside Urination: process of expelling urine from the bladder; also called micturition or voiding Micturition reflex: gives rise to a sense of urgency

Nephrolithiasis

Kidney stone shapes Shock wave lithotripsy

Percutaneous Nephroscopic Lithotripsy Ureteroscopy

Image credit: Laura Glick Kidney Transplant