Urinary System

• The excretory system removes the body’s metabolic wastes through a process called excretion
• Metabolic wastes are wastes produced by the cells—bilirubin is a good example
• Kidneys are the primary organs of this system. The excretory system is sometimes referred to as the urinary system when the focus is on only the kidneys and their urine production. The nitrogenous wastes removed by the kidneys can be lethal to the body if they are allowed to accumulate in the blood in excessive amounts.
• Ammonia is produced from the breakdown of amino acids. It is extremely toxic, but it is quickly converted by the liver to urea, a less toxic waste. Urea is the most common nitrogenous waste produced in the body, accounting for 50% of that waste. It is ultimately formed from the breakdown of proteins. Uric acid is formed from the breakdown of nucleic acids. Creatinine is formed from the breakdown of creatine phosphate, a stable energy storage molecule (discussed in the muscular system chapter, under “Muscle Metabolism”)
• The kidneys are dark red, bean-shaped organs about the size of a tightly clenched fist.
• Renal fascia (a connective tissue covering) anchors the kidney to the posterior muscle wall of the body’s abdomen.
• the functional unit of the kidney—a nephron —is microscopic.
• Each kidney contains over 1 million nephrons. These structures produce urine. There are two principal parts to a nephron—the renal corpuscle and the renal tubule.
• The renal corpuscle is like an elaborate filter in a cup. It is composed of glomerular capsule (Bowman's capsule) and glomerulus.
• The renal tubule can be divided into three sections on the basis of anatomy and location—the proximal convoluted tubule (PCT), the nephron loop (loop of Henle), and the distal convoluted tubule (DCT).
• Together, the juxtaglomerular cells and the macula densa make up a structure called the juxtaglomerular apparatus.
• The glomerular capsule catches whatever is removed from the blood in the glomerulus. This material collected by the glomerular capsule is called filtrate.
• Materials flow in this direction:

  Glomerular capsule → PCT → nephron loop → DCT → collecting duct → minor calyx
• the efferent arteriole leads to a complex capillary bed surrounding the renal tubule—the peritubular capillaries.
• Urine production involves three processes: filtration, reabsorption, and secretion.
• Filtration occurs between the glomerulus and the glomerular capsule of the renal corpuscle. The thin capillary walls of the glomerulus and the cells that cover them act as a filter that allows materials to cross, depending on size—small molecules may pass through, while larger molecules cannot.
• Reabsorption begins between the proximal convoluted tubule and the peritubular capillaries and continues along the renal tubule. In this process, 100% of the glucose, 100% of the amino acids, and variable amounts of the mineral salts that were filtered out of the blood are actively transported (requiring energy) from the tubules to the capillaries. In addition, 99% of the water that was filtered into the glomerular capsule is reabsorbed by osmosis into the bloodstream.
• In secretion, the nephron removes the rest of the wastes that remain in the blood. In this process, materials move from the peritubular capillaries to the tubules. These materials include the rest of the nitrogenous wastes (those that could not be filtered because of their size), excess hydrogen ions, excess potassium, and the by-products of drug metabolism.
• Respiratory acidosis happens if the respiratory system cannot eliminate sufficient CO2.
• Metabolic acidosis happens if there is decreased kidney elimination of hydrogen ions or increased production of acidic substances through metabolism.
• Blood pH must stay within a narrow range of 7.35–7.45 to maintain normal body functions.
• Respiratory alkalosis occurs during hyperventilation. In this case, too much CO2 is being blown off and homeostasis cannot be maintained.
• Metabolic alkalosis is relatively rare, but it can occur if there is prolonged vomiting, which results in the repeated loss of stomach acids
• There are three main hormones—antidiuretic hormone (ADH), aldosterone, and atrial natriuretic hormone (ANH)—that regulate urine production in the kidneys
• Antidiuretic Hormone As you may recall from the endocrine system chapter, antidiuretic hormone (ADH) is produced by the hypothalamus, but it is stored and then released from the posterior pituitary when commanded by the hypothalamus.
• Aldosterone You studied this hormone in the endocrine system chapter. As you may recall, aldosterone is a mineralocorticoid produced in the adrenal cortex that targets the kidneys.
• • Blood pressure falls (hypotension).

  • The level of sodium in the blood is too low (hyponatremia).

  • The level of potassium in the blood is too high (hyperkalemia).
• Cells in the right atrium of the heart produce atrial natriuretic hormone (ANH) when the blood pressure in the right atrium is too high.
• A diuretic is anything that increases urine volume. (e.g. alcohol, caffeine, and diuretic drugs)
• The ureters carry urine from the renal pelvis to the urinary bladder.
• The urinary bladder functions to store urine until its release. The maximum amount the bladder can hold is 700 to 800 mL. A feeling of fullness is typically felt at 500 mL.
• Three openings at the base of the bladder—two ureters and a urethral opening—define a triangular area called the trigone. This area is often the site of infection in the urinary bladder.
• Three layers of smooth muscle make up the detrusor muscle of the bladder’s walls. This muscle is very important in the physiology of passing urine. At the base of the bladder, the detrusor muscle thickens to form the internal urethral sphincter
• The urethra is a tube that delivers urine from the urinary bladder to the outside. It begins at the internal urinary sphincter at the urinary bladder’s base. The urethra passes through the pelvic floor, where it is encircled by a skeletal muscle called the external urethral sphincter.
• Micturition is the passing or voiding of urine
• Functions of Excretory System:

  • Removal of metabolic wastes.
  • Maintenance of the body’s fluid and electrolyte balance.
  • Maintenance of the body’s acid–base balance.
  • Regulation of blood pressure.