Urinary System BIO 250. Waste Products of Metabolism Urea Carbon dioxide Inorganic salts Water Heat. Routes of Waste Elimination

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1 Urinary System BIO 250 Waste Products of Metabolism Urea Carbon dioxide Inorganic salts Water Heat Routes of Waste Elimination Skin: Variable amounts of heat, salts, and water; small amounts of urea and carbon dioxide Lungs: Most of the carbon dioxide; some heat and water Alimentary canal: Non-digestible wastes, bile pigments, cholesterol; small amounts of heat, water, salts and carbon dioxide Kidneys: Most of the urea, salts, large and variable amounts of water; some heat 1

2 Urinary System The urinary system consists of the: kidneys ureters urinary bladder urethra Kidneys The kidneys are located on either side of the vertebral column, at the level of T-12 to L-3, high on the posterior wall of the abdominal cavity. They are positioned behind the parietal peritoneum (retroperitoneal), and held in place by adipose and connective tissue. Each kidney is about 4.7 x 2.3 x 1.2 2

3 Kidney structure A kidney contains a hollow renal sinus. The ureter expands into the renal pelvis, which, in turn, is divided into major and minor calyces. Renal papillae project into the renal sinus. Kidney tissue is divided into a medulla and a cortex. 3

4 Functions of the kidneys The kidneys remove metabolic wastes from the blood and excrete them to the outside. They also help regulate : red blood cell production blood pressure calcium ion absorption the volume composition ph of the blood. Nephrons (about 1 million/kidney) A nephron is the functional unit of the kidney and consists of a renal corpuscle and a renal tubule. The corpuscle consists of a glomerulus and a glomerular (Bowman s) capsule. Portions of the renal tubule include the proximal convoluted tubule, the nephron loop (Henle s Loop) (ascending and descending limbs), the distal convoluted tubule, and the collecting duct. 4

5 Juxtaglomerular apparatus The juxtaglomerular apparatus is located at the point of contact between the distal convoluted tubule and the afferent and efferent arterioles. It consists of the macula densa and the juxtaglomerular cells. The juxtaglomerular cells secrete renin (ree nin) Blood supply of a nephron The glomerular capillary receives blood from the afferent arteriole and passes it to the efferent arteriole. The efferent arteriole gives rise to the peritubular (reabsorption) capillary system, which surrounds the renal tubule. 5

6 Urine Formation Nephrons remove wastes from the blood and regulate water and electrolyte concentrations. Urine Ui is the end product of these functions. Kidney function includes nonselective filtration, selective reabsorption from renal tubules, secretion of substances from the peritubular blood. Glomerular filtration Urine formation begins when water and dissolved materials are filtered out of the glomerular capillary. T lfil i 8 9 f Total filtration area 8-9 square feet. The glomerular capillaries are much more permeable than the capillaries in other tissues. 6

7 Filtration pressure Filtration is due mainly to hydrostatic pressure inside the glomerular capillaries. The composition of the filtrate is similar to that of interstitial (tissue) fluid. Filtration rate The rate of filtration varies with the filtration pressure. Filtration pressure changes with the diameters of the afferent and efferent arterioles. Filtration rate varies with the rate of blood flow through the glomerulus. Filtration rate The kidneys produce about 125 milliliters of glomerular fluid per minute (180 liters/day), most of which is reabsorbed (all but liters/day) (99.2%). The volume of filtrate varies with the surface area of the glomerular capillary. Chronic renal failure reduces filtration rate. 7

8 Tubular reabsorption Substances are selectively reabsorbed from the glomerular filtrate. The peritubular capillary is adapted for reabsorption. It carries low pressure blood. It is very permeable. Most reabsorption occurs in the proximal convoluted tubule, where the epithelial cells possess microvilli (about 65% of salt and water). Tubular reabsorption Obligatory reabsorption of water by osmosis occurs in proximal convoluted tubule and loop of Henle. The water reabsorbed cannot be adjusted here. (about 80%) Facultative reabsorption of water by osmosis occurs in the distal convoluted tubule and collecting ducts. The water reabsorption here can be controlled. (about 19%) Tubular reabsorption Different modes of transport reabsorb substances in particular segments of the renal tubule. Glucose and amino acids are reabsorbed b by active transport. Water is reabsorbed by osmosis. Proteins are reabsorbed by pinocytosis. 8

9 Active transport mechanisms have limited transport capacities. If the concentrations of a substance in the filtrate exceeds its renal plasma threshold, the excess is excreted in the urine. Substances Sb that remain in the filtrate are concentrated as water is reabsorbed. Sodium ions are reabsorbed by active transport. Renal Plasma Thresholds High threshold substances can normally be completely reabsorbed unless the blood levels are very high, e.g. glucose, amino acids Low or non-threshold substances are reabsorbed in limited quantities, e.g. urea, uric acid, creatinine Variable threshold substances are regulated by hormones, e.g. H 2 0, Na + and Ca ++ Active transport As positively charged sodium ions are transported out of the filtrate, negatively charged ions accompany them (passive transport). Hydrogen ions are actively transported. Water is passively reabsorbed by osmosis as sodium ions are actively reabsorbed (Water follows the osmotic gradient produced as sodium is actively reabsorbed). 9

10 Variable reabsorption of water The distal tubule and collecting ducts are impermeable to water, which therefore is excreted in urine. ADH from the posterior pituitary gland increases the permeability of the distal tubule and collecting duct, promoting water reabsorption. ADH is secreted when concentration of solutes in CSF increases. Urea and uric acid excretion Urea is a by-product of amino acid deamination (ammonia and CO 2 ). It is reabsorbed passively by diffusion. About 50% of the urea is excreted in urine. Uric acid results from the metabolism of nucleic acids. Most is reabsorbed by active transport. Some is secreted into the renal tubule. Has antioxidant (anti-aging) properties Tubular secretion Tubular secretion transports certain substances from the plasma to the tubular fluid. Some substances are secreted actively. These include various organic compounds and hydrogen ions. The proximal and distal segments of the renal tubule secrete hydrogen ions. 10

11 Urine composition Urine is about 95% water, and it usually contains sodium chloride, urea, uric acid, and creatinine. It may contain a trace of amino acids and varying amounts of electrolytes, depending upon dietary intake. The volume of urine varies with the fluid intake and with certain environmental factors (Range: liters/day; Average 1.4l/day). ph: Range: ; Average: 6.0 Urine composition About 46 grams of solids excreted per day; 33 grams organic / 13 grams inorganic Obligatory loss by kidneys is about 440 ml per day. Normally about 99.2% of filtrate reabsorbed by the kidney. About 99.8% is maximum for human kidneys Diuresis Polyuria 11

12 Elimination of Urine The ureter is a tubular organ that extends from each kidney to the urinary bladder. Its wall has mucous, muscular, and fibrous layers. Peristaltic waves in the ureter force urine to the bladder. Obstruction in the ureter stimulates strong peristaltic waves and a reflex that decreases urine production. Urinary bladder The urinary bladder is a distensible organ that stores urine and forces it into the urethra. The openings for the ureters and urethra are located at the three angles of the trigone in the floor of the urinary bladder. Muscle fibers in the wall form the detrusor muscle. A portion of the detrusor muscle forms an internal urethral sphincter. 12

13 Micturition Micturition is the process of expelling urine. It involves contraction of the detrusor muscle and relaxation of the external urethral sphincter. Micturition reflex Distension stimulates stretch receptors in the bladder wall. The micturition reflex center in the sacral portion of the spinal cord sends parasympathetic motor impulses to the detrusor muscle. 13

14 Micturition reflex As the bladder fills, its internal pressure increases, forcing the internal urethral sphincter open. A second reflex relaxes the external urethral sphincter, unless its contraction is voluntarily controlled. Nerve centers in the brain stem and cerebral cortex aid control of urination. Urethra The urethra conveys urine from the bladder to the outside. In females, it empties between the labia minora. In males, it conveys products of reproductive organs as well as urine. Three portions of the male urethra are prostatic, membranous, and penile. The urethra empties at the tip of the penis. 14