Anatomy and Physiology Chapter 18: The Urinary System PDF

Anatomy and Physiology Chapter 18 The Urinary System Urinary System ◼ 2 kidneys ◼ 2 ureters ◼ Urinary bladder ◼ Urethra Formation of urine is the function of the kidneys. Elimination is the function of the rest of the structures. Other Functions ◼ Body cells produce wast...

Anatomy and Physiology Chapter 18 The Urinary System Urinary System ◼ 2 kidneys ◼ 2 ureters ◼ Urinary bladder ◼ Urethra Formation of urine is the function of the kidneys. Elimination is the function of the rest of the structures. Other Functions ◼ Body cells produce waste products such as urea, creatinine, and ammonia,)(by product of which must be removed prior to becoming toxic. ◼ Regulation of volume of blood by excretion or conservation of water ◼ Regulation of the electrolyte content of the blood by the excretion or conservation of minerals. ◼ Regulation of acid-base balance of the blood by excretion or conservation of ions such as H+ ions (hydrogen) or HCO3- ions (bicarbonate) ◼ Regulation of all the above in tissue fluid Kidneys ◼ Located in the upper abdominal cavity on either side of the vertebral column, behind the peritoneum. ◼ Upper portions rest on the lower surface of the diaphragm ◼ Enclosed and protected by the lower rib cage. ◼ Embedded in adipose tissue that acts as a cushion ◼ Fibrous connective tissue called renal fascia hold kidneys in place. ◼ Each kidney has an indentation called a hilus on the medial side. Medial means middle ◼ The renal artery enters the kidney, and the renal vein and ureter emerge at the hilus. ◼ Renal artery is a branch of the abdominal aorta. ◼ Renal vein returns blood to the inferior vena cava. ◼ Ureter carries urine from the kidney to the bladder. Internal Structure of the Kidney ◼ Outer tissue layer is the renal cortex ◼ Inner tissue layer is the renal medulla ◼ Renal Pelvis – cavity formed by the expansion of the ureter within the kidney at the hilus. Nephron ◼ Structural and functional unit of the kidney. ◼ 1 million nephrons in each kidney ◼ The nephrons in association with blood vessels, that urine is formed. ◼ 2 major parts: ◼ Renal corpuscle ◼ Renal tubule Renal Corpuscle ◼ Consists of a glomerulus surrounded by a Bowman’s capsule. ◼ Glomerulus is a capillary network that arises from afferent arterioles and empties into efferent arterioles. ◼ Efferent is smaller than afferent to maintain high blood pressure in glomerulus. ◼ Bowman’s capsule is the expanded end of a renal tubule; it encloses the glomerulus. ◼ The inner layer is permeable because of podocytes ◼ The outer layer is not permeable. ◼ The space between the inner and outer layers of Bowman’s capsule contain renal filtrate, the fluid formed from the blood in the glomerulus and will eventually become urine. Renal Tubule ◼ Continues from the Bowman’s capsule and consists of : ◼ Proximal Convoluted Tubule (renal cortex) ◼ Loop of Henle (renal medulla) ◼ Distal convoluted tubule (renal cortex) ◼ The distal convoluted tubules from several nephrons empty into a collecting tubule. ◼ Several collecting tubules unite to form a papillary duct that empties urine into a calyx of the renal pelvis. ◼ The microvilli in the proximal convoluted tubule provide for efficient exchanges of materials. ◼ All parts of the renal tubule are surrounded by peritubular capillaries, which arise from the efferent arteriole. ◼ These capillaries will receive materials reabsorbed by the renal tubules. Blood Vessels in the Kidney ◼ Blood branches from the abdominal aorta to the renal artery. It then branches into smaller arteries. The smallest arteries give rise to afferent arterioles in the renal cortex. ◼ From the afferent arterioles, blood flows into the glomeruli, to efferent arterioles, to peritubular capillaries, to veins within the kidney, to the renal vein, and to the inferior vena cava. Blood Pathway Starts at the abdominal aorta- >renal artery->smaller arteries- >afferent artertioles->glomerulus- >bowman’s capsule(filtered)- >efferent arterioles->peritubular capillaries->renal vein->inferior vena cava-> heart and lungs ◼ 2 sets of capillaries where exchanges take place. These will form urine from the blood plasma. Formation of Urine ◼ Glomerular filtration ◼ Tubular reabsorption ◼ Tubular secretion Glomerular Filtration ◼ Blood pressure forces plasma, dissolved substances, and small proteins out of the glomeruli and into Bowman’s capsules. ◼ This fluid is no longer plasma but called renal filtrate. ◼ The blood pressure is higher in these capillaries, about 60 mmHg. ◼ The pressure in Bowman’s capsule is very low, and very permeable. 20-25% of blood becomes renal filtrate in Bowman’s capsules. ◼ Blood cells and proteins are to large, so they stay in blood. ◼ Waste products, nutrients and minerals are also present in renal filtrate. ◼ Glomerular filtration rate (GFR) is the amount of renal filtrate formed by the kidneys in 1 minute, averages 100-125 ml. ◼ Altered if rate of blood flow changes. ◼ More is more, less is less. Tubular Reabsorption ◼ Takes place from the renal tubules into the peritubular capillaries. ◼ In a 24 hour period, the kidneys form 150-180 liters of filtrate, and normal output in that time is 1-2 liters. ◼ 99% is reabsorbed into the blood by the peritubular capillaries. ◼ Most reabsorption and secretion take place in the proximal convoluted tubules, whose cells have microvilli ◼ Distal Convoluted tubules and collecting tubules are important for reabsorption. Urine pathway Starts with abdominal aorta, goes into afferent arterioles Mechanisms of Reabsorption ◼ Active Transport – cells of the renal tubule use ATP to transport most of the useful materials from filtrate to blood. These include: glucose, amino acids, vitamins, and positive ions. ◼ Threshold level ◼ Reabsorption of Ca ◼ Reabsorption of Na and excretion of K ◼ Passive transport ◼ Osmosis ◼ Pinocytosis Tubular Secretion ◼ Substances are actively secreted from blood in peritubular capillaries into the filtrate in the renal tubules. ◼ Waste products such as ammonia, creatinine, meds, and hydrogen to adjust pH. Hormones that Influence Reabsorption of Water ◼ Aldosterone ◼ Atrial Natriuretic Hormone ◼ Antidiuretic Hormone Summary of Urine Formation ◼ The kidneys form urine from blood plasma. Blood flow through the kidneys is a major factor in determining kidney output. ◼ Glomerular filtration is the first step in urine formation. Selective in terms of size only. High BP forces plasma, dissolved materials, and small proteins into Bowman’s capsules; the fluid is now called renal filtrate. ◼ Tubular reabsorption is selective in terms of usefulness. Nutrients are reabsorbed by active transport and may have threshold levels. Positive ions by active transport and negative ions by passive transport. Water is absorbed by osmosis, and small proteins by pinocytosis. ◼ Reabsorption takes place from the filtrate in the renal tubules to the blood in the peritubular capillaries. ◼ Tubular secretion takes place from the blood in the peritubular capillaries to the filtrate in the renal tubules and can ensure that wastes are put into urine ◼ Hormones influence reabsorption of water and help maintain normal blood volume and blood pressure. ADH influences concentration or dilution of urine. ◼ Waste products remain in renal filtrate and are excreted in urine Kidney’s Acid-Base Balance ◼ If too acidic, kidneys will secrete more Hydrogen into renal filtrate and return more bicarbonate to blood. ◼ If too alkaline, kidneys will secrete more bicarbonate to renal filtrate and return more Hydrogen to blood. Other Functions of Kidneys ◼ Secretion of Renin ◼ Secretion of Erythropoietin ◼ Activation of Vitamin D Elimination of Urine ◼ Ureter extends from the hilus of a kidney to the lower, posterior side of the urinary bladder. ◼ Retroperitoneal, or behind the peritoneum ◼ Smooth muscle contracts in peristaltic waves to propel urine toward bladder ◼ As the bladder fills, it expands and compresses the lower ends of the ureters to prevent the backflow of urine. Urinary Bladder ◼ Muscular sac below the peritoneum and behind the pubic bones. ◼ In women, inferior to the uterus. ◼ In men, superior to the prostate gland. ◼ Reservoir for accumulating urine, and it contracts to eliminate urine. ◼ Mucosa is transitional epithelium, which permits expansion without tearing. ◼ When empty they appear wrinkled (rugae) ◼ Floor contains a triangular area called trigone, which has no rugae and does not expand. ◼ The points of the trigone are the openings of the two ureters and that of the urethra. ◼ Smooth muscle layer is called the detrusor muscle. ◼ Around the opening of the urethra the muscle fibers of the detrusor form the internal urethral sphincter. Urethra ◼ Carries urine from the bladder to the exterior. ◼ The external urethral sphincter is made of skeletal muscle and is under voluntary control. ◼ Women 1-1.5 inches long and anterior to the vagina. ◼ Men 7-8 inches long. First part just outside bladder is surrounded by prostate gland so it’s called prostatic urethra. ◼ The next inch is the membranous urethra, around which is the external urethra sphincter. ◼ The longest portion is the cavernous urethra which passes through the cavernous tissue of the penis. ◼ The male urethra carried semen as well as urine. Urination Reflex ◼ Urination is also called micturition or voiding. ◼ Spinal cord reflex over which voluntary control can be exerted. ◼ Stimulus is stretching of the detrusor. ◼ Bladder can hold 800 ml urine but the reflex is activated before this point. ◼ When the level reaches 200-400 ml the stretching generates sensory impulses that travel to the spinal cord. Motor impulses return to detrusor causing contraction. ◼ At the same time the external urethral sphincter is voluntarily relaxed, urine flows into the urethra, and the bladder is emptied. ◼ Voluntary control is not possible beyond a certain point. Characteristics of Urine ◼ Amount ◼ Color ◼ Specific Gravity ◼ pH ◼ Constituents ◼ Nitrogenous Wastes Aging and Urinary System ◼ With age number of nephrons in kidneys decreases, often to half the original number by 70-80 and kidneys lose some concentrating ability ◼ Glomerular filtration rate also decreases r/t arteriosclerosis and diminished renal blood flow. Removal of wastes is usually adequate. ◼ Urinary bladder decresaes in size and tone of detrusor muscle decreases. May lead to frequent urination. ◼ Incontinence is not a usual consequence of aging ◼ More at risk for UTIs