Chapter 11 Urinary System and Excretion Mader, Sylvia S. Human Biology. 13 th Edition. McGraw-Hill, 2014.

Points to Ponder What are the parts and functions of the urinary system? What is the macroscopic and microscopic structure of the kidney? What are the 3 processes in urine formation? How is the kidney involved with regulating water-salt and acid-base balance of blood What are the common disorders of the kidney? How can kidney failure be treated? How is the kidney involved with maintaining homeostasis along with other body systems?

What are the organs of the urinary system Kidneys (2) – bean-shaped, fist-sized organ where urine is formed –Covered by a tough capsule of fibrous connective tissue, called the renal capsule –Concave side, renal artery enters and renal vein and ureter exit Ureters (2) – small, muscular tubes that carry urine from the kidneys to the bladder –Three layers: inner mucosa, smooth muscle layer, and outer fibrous coat of connective tissue Peristaltic contractions cause urine to enter the bladder 10.1 Urinary system

What are the organs of the urinary system Bladder (1) – expandable organ that stores urine until it is expelled from the body –Three openings: two for the ureters and one for the urethra (drains bladder) –Smaller folds of bladder mucosa act like a valve to prevent backward flow –Two sphincters at exit of bladder Internal sphincter: smooth muscle that is under involuntary control External sphincter: skeletal muscle that is under voluntary control –When bladder is full Stretch receptors are stimulated Sensory nerve impulse travels to the spinal cord Motor nerve impulses from spinal cord cause the bladder to contract and sphincters to relax for urination Urethra (1) – tube (longer in men than women) that carries urine from the bladder to the outside of the body –Males: carries urine for urination, sperm during ejaculation

Overview of the urinary system

What are the functions of the urinary system? 1.Excretion of metabolic wastes 2.Maintenance of water-salt balance 3.Maintenance of acid-base balance 4.Hormone secretion: -renin and erythropoietin 10.1 Urinary system

1. Excretion Mostly of nitrogenous wastes of metabolism: 1.Urea made by the breakdown of amino acids in the liver -Releases ammonia which combines with carbon dioxide to produce urea 2. Uric acid made by the breakdown of nucleotides Insoluble, can form crystals 3. Creatinine made by muscle cells from the breakdown of creatine phosphate - High-energy phosphate 10.1 Urinary system

2 & 3. Maintenance of water-salt and acid-base balance Both are homeostatic mechanisms Water-salt balance –helps to maintain blood volume and pressure –NaCl causes osmosis into the blood Greater [NaCl] = Greater blood volume and pressure Kidneys maintain the levels of potassium ions, bicarbonate ion and calcium ions in the blood Acid-base balance –Healthy pH = 7.4 –By excreting hydrogen ions and reabsorbing the bicarbonate ions the kidneys help maintain a healthy pH 10.1 Urinary system

4. Hormone secretion Renin – secreted by the kidneys to allow the adrenal glands to secrete aldosterone to help regulate water-salt balance –Promotes Na+ reabsorption, which consequently promotes water reabsorption Erythropoietin – secreted by the kidneys to stimulate red blood cell production when blood oxygen is low –Secreted when oxygen-carrying capacity of blood is reduced Activation of vitamin D –promote calcium absorption 10.1 Urinary system

What are the 3 regions of the kidney? Renal cortex – –an outer granulated layer –Dip in between inner real medulla layer –Proximal convoluted tubules, distal convoluted tubule, and portion of collection ducts Renal medulla – –cone-shaped tissue masses called renal pyramids –Loop of the nephron and collection ducts Renal pelvis – –central cavity that is continuous with the ureter 10.2 Kidney structure

Anatomy of the kidney

What are nephrons? Microscopic functional unit of the kidney –Produces urine –Urine drains into collecting ducts –Collecting ducts enter renal pelvis > 1 million per kidney Each nephron has own blood supply 1. Renal artery (afferent arteriole) enter kidney at glomerulus 2. Efferent arteriole exit glomerulus and takes blood to peritubular capillary network 3. From capillary network blood feeds into venules 4. Finally blood enters the renal vein 10.2 Kidney structure

Anatomy of a nephron 1. Glomerulus – squamous epithelium –a knot of capillaries inside the glomerular capsule (bowman’s capsule) where pores produce a blood filtrate Pores allow passage of small molecules from the glomerulus to the inside of the glomerular capsule –Glomerular filtration  filtration of blood 2. Proximal convoluted tubule – cuboidal epithelial epithelium –epithelial layer with a brush border of microvilli –allow increased surface area and reabsorption of filtrate components 10.2 Kidney structure

Anatomy of a nephron 3. Loop of nephron – simple squamous epithelium –U-shaped structure descending limb to allow water to leave ascending limb that pushes out salt, no water can be reabsorbed –Facilitates reabsorption of water by nephron and collecting ducts 4. Distal convoluted tubule – –made of epithelial cells rich in mitochondria –important for movement of molecules from the blood to the tubule (tubular secretion) 5. Collecting ducts – several nephrons share a collecting duct which serve to carry urine to the renal pelvis

10. Urine formation

What are the 3 processes in the formation of urine? 1. Glomerular filtration 2. Tubular reabsorption 3. Tubular secretion 10.3 Urine formation

Glomerular filtration Water and small molecules move from the glomerulus to the glomerular capsule due to glomerular blood pressure Large molecules and formed elements remain in the glomerular blood and exit via efferent arterioles 10.3 Urine formation

Tubular reabsorption Many molecules and ions are reabsorbed from the nephron into the blood Actively reabsorbed Na+ causes passive reaborption of Cl- –Increases the osmolarity of the blood compared with the filtrate Water moves passively from tubule into blood via aquaporins –65% of Na+ and water: reabsorbed in proximal convoluted tubules Aquaporins are always open in proximal convoluted tubules Proximal convoluted tubules –Glucose and amino acids return to peritubular capillaries –Selective process involving active reabsorption via carrier proteins Diabetes –Blood glucose level is high –Excess glucose in filtrate raises its osmolarity thus less water is reabsorption Increase urination and increase thirst

Tubular reabsorption and secretion

Tubular Secretion Remove substances such as drugs, H + and creatinine from the blood into the kidney tubles Occurs along the length of the kidney tubule

Maintenance of Blood volume and pressure Reabsorption of salt – increases the blood volume Aldosterone: promotes the excretion of K + and the reabsorption of Na + –Juxtaglomerular apparatus: region of contact between afferent arteriole and distal convoluted tubule If blood volume and pressure insufficient for filtration, apparatus secretes renin Renin leads to secretion of aldosterone Atrial natriuretic hormone (ANH) – secreted by the heart when blood volume increases and inhibits the secretion of aldosterone which promotes the excretion of Na + –Inhibits secretion of renin by the juxtaglomerular apparatus thus less water will be absorbed

Maintenance of Blood volume and pressure Reabsorption of water – –due to the solute gradient water leaves the descending limb of the loop of the nephron Antidiuretic Hormone: promotes the reabsorption of water by opening aquaporins in the distal convoluted tubule –Secreted by posterior pituitary according to the osmolarity of the blood –If water intake is low, ADH is secreted and water moves from the distal convoluted tubule and collecting duct into the blood

Water reabsorption in nephrons

Reabsorption from Nephron Medulla Loop of the Nephron –Descending limb Water and salt reabsorption –Ascending limb Salt passively diffused out of the lower portion [NaCl] is greater is you move deeper into the medulla No water reabsorption because there are not aquaporins Collecting Ducts –Water reabsorption if aquaporins are open, when ADH is present only

Diuretics To increase the amount of urine Alcohol –inhibits ADH secretion and thus increases the amount of urine and dehydration Caffeine –Increases the glomerular filtration rate and decreases the tubular reabsorption of Na+ Diuretic drugs –Decrease tubular reabsorption of Na+ and thus decrease reabsorption of water –Result: decrease blood volume and pressure 10.4 Regulatory functions of the kidneys

How is the acid-base balance maintained? 1.Acid-base Buffer System 2.Respiratory System 3.Kidneys

How is the acid-base balance maintained? Buffers are a chemical or a combination of chemicals that can take up excess H + or excess OH - When H + are added to blood: H + + HCO 3 - H 2 CO 3 When OH- are added to blood: OH - + H 2 CO 3 HCO H 2 O Regulatory functions of the kidneys

How is the acid-base balance maintained? The respiratory center in the medulla oblongata of the brain can increase breathing rate if the buffers cannot maintain the pH Increase exhale CO2  decreases H+ –Help increase blood pH H + + HCO 3 - ↔ H 2 CO 3 ↔ H CO 2

How is the acid-base balance maintained? Kidneys responsible for maintaining blood pH Rids body of acidic and basic substances –Acidic blood  excrete H+, bicarbonate ions are reabsorbed –Basic blood  H+ not excreted, excrete bicarbonate ions

Kidney function disorders Diabetes, hypertension and inherited conditions are the most common cause of renal disease and failure such as: –Urethritis – localized infection of the urethra –Cystitis – infection in the bladder –Pyelonephritis – infection of the kidneys –Kidney stones – hard granules formed in the renal pelvis due to UTI’s, enlarged prostate, pH imbalances or intake of too much calcium –Uremia – high levels of urea and other waste substances in the blood that causes a serious condition when water and salts are retained due to extensive nephron damage

How can kidney failure be treated? Hemodialysis – uses an artificial kidney machine to subtract and add substances to the blood as needed Continuous ambulatory peritoneal dialysis (CAPD) – used the peritoneal membrane to filter the blood and allows a person to go about their normal life without interruption Kidney replacement – single kidney transplant with a high success rate 10.5 Disorders with kidney function

How do the kidneys maintain homeostasis? 1. Excrete wastes –Urea, creatinine and uric acid 2. Water-salt balance of blood –Helps regulate blood volume and pressure 3. Acid-base balance of blood –Helps regulate pH 4. Assistance to other systems –Endocrine, cardiovascular, skeletal, muscular nervous and digestive 10.6 Homeostasis

Urinary system interacts with other systems 10.6 Homeostasis