The Kidney Topics 11.3

I. The kidney is involved in excretion A I. The kidney is involved in excretion A. Excretion refers to the removal of liquid and solid metabolic wastes from the body B. All living organisms have a need for excretion as a means of maintaining homeostasis (what goes in must come out) 1. This need extends from the cellular level to the body level 2. Excretory products in plants include oxygen, while in animals they include carbon dioxide and nitrogenous compounds a. nitrogenous wastes include ammonia, uric acid, and urea

- ammonia is a result of deamination of amino acids -ammonia is very toxic and must be excreted rapidly or converted into a less toxic form such as uric acid or urea - freshwater fish can get rid of ammonia although it is highly toxic (due to its bascitiy), because it can be diluted by the readily available water b. Uric acid forms crystals and is excreted in a relatively dehydrated form - primary excretory compound in terrestrial animals such as insects, some reptiles and birds because they cannot carry a lot of water

c. Urea is excreted by amphibians and mammals - the liver synthesizes urea in the urea cycle - urea is highly soluble, and a relatively high amount of water is needed for its storage and excretion

III. The kidneys, urinary bladder, and their ducts make up the urinary system A. The outer portion of the kidney is the cortex; the inner is the medulla 1. The medulla is composed of 8-10 pyramids, each with a renal papilla at the apex, where the collecting ducts empty 2. The urine flows from a papilla to the renal pelvis 3. The ureters carry urine to the urinary bladder, which is a distensible, muscular sac 4. During urination, urine flows through the urethra

B. The nephron is the functional unit of the kidney 1 B. The nephron is the functional unit of the kidney 1. Human kidneys consist of several million nephrons 2. The Bowman’s capsule collects fluid from the glomerulus 3. The renal tublue can be divided into the proximal convoluted tubule, loop of Henle, and distal convoluted tubule 4.The distal convoluted tubule empties into the collecting duct 5. Blood from the arterial system of the kidney enters the nephron via the afferent arterioles a. The glomerulus is a capillary bed within the Bowman’s capsule

6. The efferent arterioles carry blood away from the glomerulus and is smaller in diameter than the afferent arterioles, producing a higher pressure within the glomerulus 7. The efferent arteriole branches into a second set of capillaries; the peritubular capillaries which surround the renal tubules a. The peritubular capillaries receive reabsorbed materials from the tubules an return them to the venous circulation 8. The peritubular capillaries unite to form small veins which ultimately lead to the renal vein

C. Urine is produced by ultrafiltration, reabsorption, and secretion 1 C. Urine is produced by ultrafiltration, reabsorption, and secretion 1. Ultrafiltration is not selective with regard to ions and small molecules a. Small molecules are forced out of the glomerular capillaries into the lumen of the Bowman’s capsule b. Several factors contribute to ultrafiltration - blood pressure in glomerular capillaries is higher than in other capillaries due to the high resistance to outflow of the efferent arteriole, which is smaller in diameter than the afferent arteriole -highly coiled capillaries provide lots of surface area

-glomerular capillaries are greatly permeable due to having small pores called fenestrations c. The wall of Bowman’s capsule consists of specialized epithelial cells called podocytes which are separated by narrow gaps called filtration slits d. Between the fenestrated capillaries and the podocytes lies a gel of glycoproteins called the basement membrane e. The basement membrane is impassable to cells in the blood and most proteins (so it is the structure that is actually doing the filtration of the blood)

f. The amount of fluid filtered per. day is 4. 5 times the amount of f. The amount of fluid filtered per day is 4.5 times the amount of fluid in the entire body; therefore, most is reabsorbed

2. Reabsorption is highly selective a 2. Reabsorption is highly selective a. 99% of the filtrate is reabsorbed into the blood from the renal tubules b. The epithelia of the renal tubules have microvilli which increase surface area, and abundant mitochondria to provide energy for active transport of materials c. Most of the filtrate (65%) is reabsorbed as it passes through the proximal tubule d. Some materials are actively transported into the tubule cells; others follow by passive diffusion e. Water follows by osmosis 1) about 80% of the water in the filtrate is reabsorbed in the proximal tubule

f. The renal threshold is the level above which it will not be reabsorbed, and therefore excreted into the urine - in diabetes mellitus, the amount of glucose in the filtrate exceeds the renal threshold, so excess glucose is present in the urine 3. Some substances are actively secreted from the blood into the filtrate a. some ions and drugs are secreted into the filtrate, particularly by the cells of the distal convoluted tubule b. Controlled secretion of hydrogen ions allows regulation of blood pH c. Aldosterone, produced by the adrenal cortex, is involved in regulation of tubular secretion

D. Osmoregulation 1. Osmoregulation is the control of water balance of the blood, tissue, or cytoplasm of a living organism 2. Urine becomes concentrated as it passes through the renal tubule 3. Cortical nephrons have small glomeruli and short loops of Henle a. produce a relatively dilute urine 4. Juxtamedullary nephrons have large glomeruli and long loops of Henle a. the loop of Henle is specialized to produce a concentrated urine b. The descending loop has walls which are permeable to water but impermeable to sodium and urea

c. The interstitial fluid surrounding the loop has a high concentration of sodium d. The ascending loop has walls which are permeable to salt but less permeable to water e. Filtrate is concentrated as it moves down the descending loop, and diluted as it moves up the ascending loop f. This countercurrent mechanism maintains the high salt concentration in the interstitial fluid g. The vasa recta, a set of capillaries surrounding the loops of Henle in juxtamedullary nephrons, absorbs water

E. The concentration of proteins, glucose, and urea differs amongst glomerular filtrate, blood plasma, and urine 1. Only about 50% of urea is reabsorbed so it is in much higher concentration in the urine than the glomerular filtrate or blood plasma

2. Glucose and amino acids are 100% reabsorbed into the blood so there are equal amounts in the filtrate and blood plasma (just not at the same time) and none in the urine 3. Proteins and other macromolecules should not be filtered in the Bowman’s capsule (too big) and any presence in urine is usually regarded as an indicator of high blood pressure and damage to the basement membrane (nephritis)

F. Kidney function is regulated by hormones 1. Urine volume is regulated by the hormone ADH (antidiuretic hormone) a. ADH is produced by the hypothalamus and is stored in the posterior pituitary. When triggered it will travel to the kidney and targets the collecting ducts, making them more permeable to water, resulting in a more concentrated urine c. Receptors which are stimulated by osmotic changes cause production of ADH and constriction of blood vessels, and a thirst receptor causes increased fluid intake d. diabetes insipidus is a condition caused by a lack of ADH, and in uncontrolled situations, results in a great volume of water being lost

2. Sodium reabsorption is regulated by the hormone aldosterone a 2. Sodium reabsorption is regulated by the hormone aldosterone a. Aldosterone is produced by the adrenal cortex and stimulates the distal convoluted tubules and collecting ducts to increase sodium reabsorption b. Aldosterone secretion is stimulated by a decrease in blood pressure causing the cells of the juxtaglomerular apparatus to produce renin, which activates the renin- angiotensin pathway c. Atrial natriuretic peptide (ANP) is produced by the walls of the atria of the heart, and inhibits aldosterone secretion and renin release (due to a decrease in blood pressure)