Urinary System

Urinary System Contributes to Homeostasis Excretion: processes that remove wastes and excess materials from the body Digestive system: excretes food residues and wastes produced by the liver Respiratory system (lungs): excretes carbon dioxide Integumentary system (skin): excretes water, salt Urinary system (kidneys): excretes nitrogenous wastes, excess solutes, and water

Digestive tract Skin Respiratory system Circulatory system Liver Figure 15.1 Food, water Water, salt O2 Water CO2 Digestive tract Skin Respiratory system Nutrients, water Water, salt O2 CO2 Waste Water, solutes, wastes Circulatory system Urinary system cells Transport all to Metabolic products, toxins Waste Liver Elimination of food residues Elimination of waste, excess solutes, and water

The Kidneys Regulate Water Levels To maintain homeostasis, Water Intake = Water Output Kidneys adjust water output as necessary Water input: food, drink, metabolism Water output: lungs, skin, feces Kidneys modify output based on intake and loss Output varies from 1/2 liter/day to 1 liter/hour

Table 15.1

The Kidneys Regulate Nitrogenous Wastes and Other Solutes Protein metabolism produces nitrogenous wastes Initially, NH3 (ammonia) is produced during breakdown of amino acids Liver detoxifies NH3, producing urea Urea is transported from liver to kidneys for disposal Other solutes regulated by kidneys Sodium, chloride, potassium, calcium, hydrogen ions, creatinine

Table 15.2

Organs of the Urinary System Kidneys Principal organ of urinary system Cortex: outer portion of the kidney Medulla: inner region of the kidney Renal pelvis: hollow space in center of kidney where urine collects

The components of the urinary system. Figure 15.2 Kidney Cortex Renal artery Renal vein Nephrons Aorta Cortex Inferior vena cava Medulla Renal pelvis Medulla Collecting duct Ureter Bladder Ureter Urethra The components of the urinary system. Internal structure of the kidney. The cortex and medulla of the kidney are composed of numerous nephrons.

Organs of the Urinary System Ureters Muscular tube that transports urine from kidneys to bladder Urinary bladder Three layers of smooth muscle, lined with epithelial cells Stores urine (600–1,000 ml) Urethra Carries urine from bladder to outside of body Two sphincters control urination

The male. The female. Figure 15.3 Urinary bladder Rectum Internal urethral sphincter Prostate gland Uterus Vagina External urethral sphincter Penis Rectum Urethra Testis The male. The female.

Nephrons Produce Urine Nephron: functional unit of the kidney Two functional parts: Tubule Associated blood supply 1 million nephrons per kidney Each nephron consists of a long thin hollow tube (tubule) plus associated blood supply Role of nephrons: remove approximately 180 liters of fluid from the blood daily, and return most of it, minus the wastes that are excreted

Distal tubule Efferent arteriole Glomerular capsule Glomerulus Figure 15.4 Distal tubule Efferent arteriole Glomerular capsule Glomerulus Proximal tubule Afferent arteriole Cortex Descending limb Medulla Loop of Henle Ascending limb Collecting duct Renal pelvis

The Tubule Filters Fluid and Reabsorbs Substances Nephron structure Glomerular capsule: cuplike end of nephron tubule surrounding glomerulus (network of capillaries)—this is where filtration occurs Four distinct regions of tubule 1. Proximal tubule: extends from glomerular capsule to renal medulla 2. Loop of Henle: extends into medulla then back into cortex 3. Distal tubule 4. Collecting duct: shared by several nephrons, empties into renal pelvis

Special Blood Vessels Supply the Tubule Renal artery supplies the kidney Blood vessels associated with tubules Arterioles Afferent: enters the glomerular capsule Efferent: leaves the glomerular capsule Capillaries Glomerular: network within the glomerular capsule Peritubular: surround proximal and distal tubule Vasa recta: parallels the loop of Henle Renal vein: collects filtered blood from kidneys

Figure 15.5 Collecting duct Efferent arteriole Glomerulus Afferent arteriole Afferent arteriole Peritubular capillaries Cortex Efferent arteriole Renal vein Medulla Renal artery Vasa recta Pelvis Main blood vessels in the kidney. The renal artery and renal vein branch many times to deliver blood to each glomerulus. Post-glomerular blood vessels. The efferent arteriole of most nephrons, such as the one shown here on the left, divides to become the peritubular capillaries, which supply proximal and distal tubules in the cortex. In a few nephrons, such as the one on the right, the efferent arteriole descends into the medulla to become the vasa recta,which supply loops of Henle.

Formation of Urine: Filtration, Reabsorption, and Secretion 1. Glomerular filtration: movement of protein-free solution of fluid and solutes from blood into the glomerular capsule 2. Tubular reabsorption: return of most of the fluid and solutes into the blood 3. Tubular secretion: addition of certain solutes from the blood into the tubule

Figure 15.6 3 Tubular secretion: Some drugs, waste products, and ions (primarily hydrogen, ammonium, and potassium) are actively secreted from the peritubular capillaries primarily into the distal tubule but also in other nephron segments. Glomerular capsule Afferent arteriole Efferent arteriole Glomerulus 1 Glomerular filtration: Water, ions, glucose, amino acids, bicarbonate, and waste products (urea, creatinine) are filtered from the glomerular capillaries into the space within the glomerular capsule. Proximal tubule Distal tubule 2 Collecting duct Tubular reabsorption: Water, amino acids, glucose, most ions (including bicarbonate), and some urea are reabsorbed back into the peritubular capillaries, primarily in the proximal tubule but also in other nephron segments. Artery Vein Urine

Glomerular Filtration Filters Fluid from Capillaries Filters protein-free plasma fluid from capillaries into glomerular capsule Large volume filtration, yet highly selective Impermeable to large proteins and cells Filtration is driven by high blood pressure in glomerular capillaries Rate of filtration Resting rate under local chemical control Stress causes sympathetic nervous system to reduce blood flow to kidneys

The outer surface of several glomerular capillaries. Figure 15.7 Blood flow Glomerular capsule Glomerular space Glomerulus Movement of glomerular filtrate Afferent arteriole Efferent arteriole The outer surface of several glomerular capillaries. Podocyte Filtrate Proximal tubule Capillary wall A highly magnified view of the inner surface of a single glomerular capillary, revealing its porous sievelike structure.

Tubular Reabsorption Returns Filtered Water and Solutes to Blood 100% of filtered glucose, amino acids, and bicarbonate and 50% of urea are reabsorbed Most tubular reabsorption occurs in proximal tubule Reabsorption of sodium begins the process Sodium moved by active transport from tubule cells to interstitial fluid and diffuses to capillaries Chloride passively accompanies sodium (balanced charge) Water reabsorbed with salts Water moves through aquaporins (water channels) Movement of sodium provides energy to transport glucose and amino acids from tubule into surrounding cells Glucose, amino acids then diffuse to the interstitial fluid

Proximal tubule cell Interstitial fluid Capillary Tubule lumen Figure 15.9 Proximal tubule cell Interstitial fluid Capillary Glucose, amino acids Na+ Na+ ATP Glucose, amino acids ADP + Pi Tubule lumen Cl− H2O Active transport Diffusion

Tubular Secretion Removes Other Substances from Blood Involves the movement of materials from the peritubular capillaries or vasa recta to the tubule Purpose Regulation of chemical levels in body Excretion of harmful chemicals Substances secreted Penicillin, cocaine, marijuana, pesticides, preservatives, hydrogen ions, ammonium, potassium

Producing Dilute Urine: Excreting Excess Water Kidneys respond to excess water by excreting it Mechanism Distal tubule is impermeable to water so water is not reabsorbed here NaCl is reabsorbed without the concurrent reabsorption of water High volume dilute urine is produced

Cortex Medulla NaCl NaCl Isotonic NaCl NaCl NaCl NaCl NaCl H2O H2O H2O Figure 15.10 NaCl NaCl Isotonic Cortex Medulla NaCl NaCl NaCl NaCl NaCl H2O H2O H2O H2O NaCl H2O Urea Very concentrated Large volume of dilute urine

Producing Concentrated Urine: Conserving Water Too little water can lead to lower blood volume, declining blood pressure, risk of dehydration of body cells Kidneys respond by conserving water and producing a more concentrated urine Mechanism Mediated by ADH (antidiuretic hormone) from the posterior pituitary gland ADH increases permeability of the collecting ducts to water and increases conservation of water Counter-current exchange mechanism

Cortex Medulla NaCl NaCl Isotonic NaCl H2O NaCl NaCl NaCl H2O NaCl H2O Figure 15.11 NaCl NaCl Isotonic Cortex Medulla NaCl H2O NaCl NaCl NaCl H2O NaCl H2O H2O H2O H2O H2O H2O NaCl H2O H2O Urea Very concentrated Low volume of concentrated urine

Counter- current flow of blood Figure 15.12 Kidney surface Blood from efferent arteriole To vein Cortex Isotonic Medulla Solutes Solutes H2O H2O Counter- current flow of blood Solutes Solutes H2O H2O Solutes Solutes H2O H2O Very concentrated Renal pelvis Position of the vasa recta relative to the tubule Movement of water and solutes across the walls of the vasa recta

Urination Depends on a Reflex Micturition reflex: neural reflex that enables emptying of the bladder Responds to stretch receptors in bladder wall Internal urethral sphincter Smooth muscle External urethral sphincter Skeletal muscle, under voluntary control Brain can override the micturition reflex and control the timing of urination Voluntary control becomes increasingly difficult as the bladder gets very full

ADH (Antidiuretic Hormone) Regulates Water Balance Involves the following organs; Hypothalamus: synthesizes ADH Posterior pituitary gland: releases ADH Kidneys: respond to ADH Negative feedback loop regulates solute concentration of the blood Involves increasing or reducing ADH secretion, which will modify water reabsorption by kidneys Involves increasing or decreasing thirst

ADH Regulates Water Balance If blood solute concentration is too high (water concentration too low): ADH released ADH causes: Increase in permeability of collecting duct to water Increase in water reabsorbed by kidney Decrease in urine production Increase in thirst

ADH Regulates Water Balance If blood solute concentration is too low (water concentration too high): ADH secretion is reduced Decrease in permeability of collecting duct to water Decrease in water reabsorbed by kidney Increase in urine production Decrease in thirst

ADH Regulates Water Balance Diuresis: high urine flow rate Diuretic: any substance that increases the formation and excretion of urine Lasix (furosemide): medication that reduces blood volume and blood pressure Used in treatment of congestive heart failure and hypertension Caffeine: inhibits sodium reabsorption Alcohol: inhibits ADH release

Blood solute concentration Figure 15.13 Blood solute concentration Increase Set point Decrease Hypothalamus Rate of Water Posterior pituitary renal intake Thirst water loss ADH Kidneys: collecting duct permeability to water

Aldosterone Regulates Salt Balance Blood volume control is dependent on salt balance Aldosterone: adrenal hormone that regulates sodium excretion Mechanism: increases Na+ reabsorption from distal tubule and collecting duct Aldosterone secretion is controlled by the renin- angiotensin system

The Renin-Angiotensin System Controls Blood Volume and Blood Pressure Aldosterone release is stimulated indirectly by decreased blood volume or blood pressure Decreased blood volume/blood pressure causes release of renin (enzyme) from juxtaglomerular apparatus (region where afferent and efferent arterioles are in close contact with distal tubule) Renin cleaves inactive angiotensinogen (produced by liver), releasing angiotensin I Angiotensin converting enzyme (ACE) in lungs converts antiotensin I (inactive peptide) to angiotensin II (biologically active peptide)

The Renin-Angiotensin System Controls Blood Volume and Blood Pressure Effects of angiotensin II Constricts arterioles which raises blood pressure Stimulates release of aldosterone from adrenal glands Aldosterone: increases sodium reabsorption by distal tubules and collection ducts ANH (atrial natriuretic hormone), released by atria in response to stretching due to high blood volume, inhibits Na+ reabsorption

Juxtaglomerular apparatus Figure 15.14 Distal tubule Afferent arteriole Glomerulus Glomerular capsule Juxtaglomerular apparatus Efferent arteriole Proximal tubule Renin- secreting cells

Angiotensin- converting enzyme Figure 15.15 ADH Blood volume Increase Set point Save water Decrease Save salt Kidneys Kidneys Aldosterone Adrenal cortex Angiotensin- converting enzyme Renin Lungs Angiotensin II Angiotensin I Angiotensinogen Vasoconstriction, blood pressure Liver

Atrial Natriuretic Hormone Protects Against Blood Volume Excess High blood volume stretches atria of heart Atria secrete ANH (atrial natriuretic hormone) in response to stretching ANH inhibits Na+ reabsorption in distal tubules and collecting ducts Na+ excretion increases Water follows the Na+ Effect of ANH is opposite to that of aldosterone

Kidneys Help Maintain Acid-Base Balance and Blood pH Blood pH must stay between 7.35 and 7.45 pH regulated by kidneys, buffers, lungs Role of kidneys in pH maintenance Reabsorption of filtered bicarbonate Excretion of acid as ammonium (NH4+)

Erythropoietin Stimulates Production of Red Blood Cells Decrease in amount of oxygen is detected by certain cells throughout the kidney O2 sensitive cells in kidney secrete hormone, erythropoietin, in response to decrease in oxygen Erythropoietin triggers increase in red blood cell production in the bone marrow

O2 availability Increase Set point Decrease Figure 7.6 O2 availability Increase Set point Decrease O2-sensitive cells in kidneys respond to a decline in O2 availability by increasing erythropoietin production Increased number of RBCs returns O2 availability to normal Erythropoietin stimulates increased RBC production by stem cells in bone marrow

Kidneys Activate Vitamin D Exposure of skin to sunlight causes production of an inactive form of vitamin D from a precursor found in the skin Inactive form of vitamin D is transported to liver, where it is modified Inactive form of vitamin D is then converted to active form by kidneys Conversion to active vitamin D in kidneys is influenced by activity of PTH (parathyroid hormone)

Summary -- W.W.K.D. Contribute to maintenance of water balance Contribute to maintenance of salt balance Secrete an enzyme involved in the control of blood volume and blood pressure Maintain acid-base balance and blood pH Regulate red blood cell production via erythropoietin Activate an inactive form of vitamin D

Disorders of the Urinary System Kidney stones Crystallized minerals Block urine flow Urinary tract infections (UTI) Usually caused by bacteria More common in women than men because of shorter urethra If untreated, bladder infections may ascend to involve kidneys

Disorders of the Urinary System Acute renal failure Short-term impairment, may be reversible Potential causes: sustained very low blood pressure, large kidney stones within renal pelvis, infections, transfusion reactions, severe injury, toxin exposure, drug reactions

Disorders of the Urinary System Chronic renal failure Also known as end stage renal disease (ESRD) ESRD: long term, irreversible damage •60% reduction in functioning nephrons See causes of acute renal failure above 40% of people with type I diabetes will develop renal failure

Dialysis Cleanses Blood Artificially Dialysis: attempts to duplicate function of healthy kidneys CAPD: continuous ambulatory peritoneal dialysis Can be done at home Uses peritoneal cavity for waste, ion removal Risk of infection Hemodialysis Requires several visits/week to a dialysis center Blood is circulated through a kidney machine

Dialysis Cleanses Blood Artificially Problems with dialysis Dialysis cannot achieve complete homeostasis of ions and wastes Dialysis does not replace renal hormones