Osmoregulation and Excretion Chapter 44 Osmoregulation and Excretion

Concept 44.3: Diverse excretory systems are variations on a tubular theme Excretory systems regulate solute movement between internal fluids and the external environment Excretory Processes Most excretory systems produce urine by refining a filtrate derived from body fluids Key functions of most excretory systems: Filtration: pressure-filtering of body fluids Reabsorption: reclaiming valuable solutes Secretion: adding toxins and other solutes from the body fluids to the filtrate Excretion: removing the filtrate from the system

Get rid of the bad that was missed the first time Filtration - Filter out the bad Capillary Excretory tubule Filtrate Reabsorption - Reclaim the goods Secretion Get rid of the bad that was missed the first time General schematic view of excretory function Excretion - Release the bad Urine

Survey of Excretory Systems Systems that perform basic excretory functions vary widely among animal groups They usually involve a complex network of tubules Invertebrates: Planarians have flame cells Earthworm/Annelids have nephridia Insects have Malpighian tubules

Protonephridia: Flame-Bulb Systems A protonephridium is a network of dead-end tubules lacking internal openings The smallest branches of the network are capped by a cellular unit called a flame bulb/cell These tubules excrete a dilute fluid and function in osmoregulation

Interstitial fluid filters through membrane where cap cell and Cilia Interstitial fluid filters through membrane where cap cell and tubule cell interdigitate (interlock) Tubule cell Flame bulb Protonephridia (tubules) Tubule Nephridiopore in body wall

Metanephridia Each segment of an earthworm has a pair of open-ended metanephridia Metanephridia consist of tubules that collect coelomic fluid and produce dilute urine for excretion

Coelom Capillary network Bladder Collecting tubule Nephridiopore Nephrostome has cilia around its opening that brush fluid into the nephrostome. As the fluid passes the tubules water and solutes diffuse between the tubules and the capillaries. Since earthworms inhabit damp soil, they absorb water by osmosis through their skin and must get rid of their excess water. Produce dilute urine Collecting tubule Nephridiopore Nephrostome Metanephridium

Malpighian Tubules In insects and other terrestrial arthropods, Malpighian tubules remove nitrogenous wastes from hemolymph and function in osmoregulation Insects produce a relatively dry waste matter, an important adaptation to terrestrial life

Digestive tract Rectum Hindgut Intestine Midgut (stomach) Malpighian tubules Salt, water, and nitrogenous wastes Feces and urine Anus Malpighian tubule Rectum Reabsorption of H2O, ions, and valuable organic molecules HEMOLYMPH

Vertebrate Kidneys Kidneys, the excretory organs of vertebrates, function in both excretion and osmoregulation

Concept 44.4: Nephrons and associated blood vessels are the functional unit of the mammalian kidney The mammalian excretory system centers on paired kidneys, which are also the principal site of water balance and salt regulation Each kidney is supplied with blood by a renal artery and drained by a renal vein Urine exits each kidney through a duct called the ureter Both ureters drain into a common urinary bladder where it is stored Urine leaves the bladder through the urethra 1. Human urinary system: 2. Kidney, 3. Renal pelvis, 4. ureter, 5. urinary bladder, 6. urethra 7. adrenal gland Vessels:  8. Renal artery and vein, 9. inferior vena cava, 10. abdominal aorta, 11. Common iliac artery and vein With transparency: 12. liver, 13. large intestine, 14. pelvis

Posterior vena cava Renal artery and vein Kidney Renal medulla Aorta Renal cortex Ureter Renal pelvis Urinary bladder Urethra Excretory organs and major associated blood vessels Ureter Section of kidney from a rat Kidney structure Juxta- medullary nephron Cortical nephron Afferent arteriole from renal artery Glomerulus Bowman’s capsule Proximal tubule Renal cortex Peritubular capillaries Collecting duct SEM 20 µm Efferent arteriole from glomerulus Renal medulla Distal tubule To renal pelvis Branch of renal vein Collecting duct Descending limb Loop of Henle Nephron Ascending limb Vasa recta Filtrate and blood flow

Structure and Function of the Nephron and Associated Structures The mammalian kidney has two distinct regions: an outer renal cortex and an inner renal medulla

The nephron, the functional unit of the vertebrate kidney, consists of a single long tubule and a ball of capillaries called the glomerulus which is cupped by the Bowman’s capsule

Long tubule after the Bowman’s capsule consists of 4 major regions Proximal convoluted tubule Loop of Henle Distal convoluted tubule Collecting duct Convoluted = twisted, coiled

Blood Vessels Associated with the Nephrons Each nephron is supplied with blood by an afferent arteriole, a branch of the renal artery that divides into the capillaries The capillaries converge as they leave the glomerulus, forming an efferent arteriole The vessels divide again, forming the peritubular capillaries, which surround the proximal and distal tubules

Filtration of the Blood Filtration occurs as blood pressure forces fluid from the blood in the glomerulus into the lumen of Bowman’s capsule glomerulus & Bowman’s capsule make up the renal corpuscle Fluid that enters the Bowman’s capsule in now called the filtrate Rest of the blood enters the efferent arteriole and into the peritubular capillaries

Pathway of the Filtrate From Bowman’s capsule, the filtrate passes through three regions of the nephron: the proximal tubule, the loop of Henle, and the distal tubule Fluid from several nephrons flows into a collecting duct

From Blood Filtrate to Urine: A Closer Look Filtrate becomes urine as it flows through the mammalian nephron and collecting duct Secretion and reabsorption in the proximal tubule greatly alter the filtrate’s volume and composition

From Blood Filtrate to Urine: A Closer Look Filtrate becomes urine as it flows through the mammalian nephron and collecting duct Secretion and reabsorption in the proximal tubule greatly alter the filtrate’s volume and composition Reabsorption of water continues as filtrate moves into the descending limb of the loop of Henle In the ascending limb of the loop of Henle, salt diffuses from the permeable tubule into the interstitial fluid

The distal tubule regulates the K+ and NaCl concentrations of body fluids The collecting duct carries filtrate through the medulla to the renal pelvis and reabsorbs NaCl

Proximal tubule Distal tubule NaCl Nutrients H2O HCO3– H2O K+ NaCl HCO3– H+ NH3 K+ H+ CORTEX Descending limb of loop of Henle Thick segment of ascending limb Filtrate H2O Salts (NaCl and others) HCO3– H+ Urea Glucose; amino acids Some drugs NaCl H2O OUTER MEDULLA NaCl Thin segment of ascending limb Collecting duct Key Urea Active transport Passive transport NaCl H2O INNER MEDULLA

Regulation of Kidney Function The concentration of the urine is regulated by nervous and hormonal control of water and salt reabsorption in the kidneys Antidiuretic hormone (ADH) increases water reabsorption in the distal tubules and collecting ducts of the kidney An example of an ADH hormone is vasopressin

STIMULUS osmoreceptor cells in the hypothalamus detect an increase in Osmoreceptors in hypothalamus Thirst Hypothalamus Drinking reduces blood osmolarity to set point ADH Increased permeability Pituitary gland Distal tubule H2O reab- sorption helps prevent further osmolarity increase STIMULUS osmoreceptor cells in the hypothalamus detect an increase in the osmolarity of the blood Collecting duct Homeostasis: Blood osmolarity

Regulation of Kidney Function The osmolarity of the urine is regulated by nervous and hormonal control of water and salt reabsorption in the kidneys Antidiuretic hormone (ADH) increases water reabsorption in the distal tubules and collecting ducts of the kidney An example of an ADH hormone is vasopressin Lack of vasopressin is diabetes insipidus Lots of dilute urine is produced Drinking alcohol inhibits the release of vasopressin and mimics diabetes insipidus  frequent urination

The renin-angiotensin-aldosterone system (RAAS) is part of a complex feedback circuit that functions in homeostasis Purpose: Maintain blood volume and pressure

apparatus (JGA) responds Homeostasis: Blood pressure, volume Increased Na+ and H2O reab- sorption in distal tubules STIMULUS: The juxtaglomerular apparatus (JGA) responds to low blood volume or blood pressure (due to dehydration or loss of blood) Aldosterone Arteriole constriction Adrenal gland Angiotensin Distal tubule Angiotensinogen JGA Renin production Renin

Another hormone, atrial natriuretic factor (ANF), opposes the RAAS Inhibits renin secretion and thus the production of angiotensin, and stimulates aldosterone release. Its effect is increased excretion of water and sodium and a lowering of blood pressure, which reduces the workload of the heart.