The Excretory System

• The excretory system balances the volume and composition of bodily fluids.
• It is composed of the kidneys, linked blood vessels, and tubules.
• The tubules channel fluid formed in the kidneys out of the body

Necessity of Excretion

• Cells must intake water and molecules to survive.
• Cells would get bigger if they only took in molecules.
• The cells must also export molecules.
• Exported molecules include signaling molecules like hormones, glucose, and waste produced by cellular metabolism.

Kidney Functions

• Waste excretion
• Breakdown of excess salts and toxins
• Deamination - the breakdown of amino acids, which releases nitrogen.
• Urea filtering from the body

Components of the Excretory System

• The excretory system includes tissues and organs that remove waste.
• The urinary system excretes water, nitrogenous wastes, and salts, which exit the body as urine.
• The digestive system eliminates water, some salts, bile (stomach acid), and indigestible food matter, combining to form feces.
• The respiratory system eliminates carbon dioxide and water vapor.
• The integumentary system (skin) excretes sweat containing water, salts, and small amounts of nitrogenous wastes.

The Urinary System

• The kidneys are the major organs involved in human excretion.
• They remove waste from blood, balance body fluids, and form urine.
• Kidneys filter blood, remove unnecessary substances, and reabsorb usable ones.
• The body's two kidneys are located in the lower back.
• Nephrons are located inside the kidneys.
• Nephrons are microscopic, coiled sets of tubes that contain a tiny capillary-filled bulb at one end.
• The nephrons are the basic functional unit of the kidney for excretion.
• Each nephron extends from the cortex, the outer part of the kidney, to the medulla, the inner part.
• Each kidney contains approximately one million nephrons.
• Nephrons contain long, coiled tubules.
• Bowman's capsule is located at one end of the tubule.
• The Glomerulus surrounds the Bowman's capsule, which is a bowl of capillaries.
• The other end of the tubule opens into collecting ducts.
• The collecting ducts converge in the central cavity of the kidney called the renal pelvis.

Nephron Functions

• Nephrons specialize in these three excretory functions:
• Filtration
• Re-absorption
• Secretion
• Filtration occurs where the glomerulus and Bowman's capsule connect.
• Re-absorption occurs in the proximal tubule.
• The nephron loop, or loop of Henle, forms a U-shape where tubule and capillary walls merge.
• About 20% of re-absorption occurs in the loop of Henle.
• Secretion occurs in the distal tubule, which extends from the loop of Henle to the collecting duct.
• Urine passes from each nephron via the collecting duct into the renal pelvis.
• Urine then empties into the ureter.
• Blood enters the kidneys through the renal artery and exits through the renal vein.
• Kidneys process the blood supply roughly every 3 minutes.
• Approximately 5-L of blood passes through your kidneys about 20 times per hour.

Urine

• Blood exiting the kidneys lacks excess water, salts, and metabolic wastes.
• Urine constitutes a waste fluid composite of filtered substances.
• Urine exits the kidney through the ureter, a duct connecting to the urinary bladder.
• The urinary bladder, a holding tank that temporarily stores urine, empties through the urethra.

Composition of Urine

• Urea and uric acid that contain large amounts of nitrogen
• Salts or electrolytes
• Yellow coloring from bile, which are digestive juices

What Urine Should Not Contain

• Glucose
  • This may mean cells are not absorbing sugar; may be a sign of diabetes
• Blood
  • Indicates kidney disease or urinary system infection

Factors that Affect Kidney Function

• Antidiuretic hormone (ADH) prevents excess water loss.
• Alcohol inhibits ADH secretion, resulting in a higher urine volume.
• Aldosterone prevents excess sodium and water loss.
• Caffeine increases the rate of salt and water excretion.
• Increased blood pressure increases the rate of water loss.

Summary of Structures and functions

• Structures: two kidneys, two ureters, a bladder, a urethra.
• Functions: removal of solid and liquid waste, achievement of isotonic equilibrium in blood.