The Human Excretory System MAJOR POINTS

Major Points: All cells produce waste products. Cells must maintain stable internal environments, called… homeostasis. If cells did not get rid of waste, then as waste levels increase, levels may become toxic, and cells may die. To maintain homeostasis, organisms perform EXCRETION Excretion = the removal from an organism of the toxic waste products of metabolism.

How does waste travel? The waste produced by each individual cell is exported into the blood. The blood then transports waste to specific organs that are specialized for waste removal (excretion). Kidneys are the major organs involved in excretion. You have 2 of them, bean-shaped, located in the small of your back, one behind the stomach, and one behind the liver.

Examples of Excretion Pathways NH3 (ammonia) in cells enters blood and travels… To the liver to be converted to UREA (less toxic). Urea travels… To the kidneys, then ureters that connect to the bladder… The bladder empties into the urethra which moves waste OUT!

Another example of excretion… CO2 in the cells to… excreted into the blood… blood flows to the lungs… and out when you exhale!

Important Beans There are 3 main parts of the kidney: Cortex (outermost part-- 1/3 of kidney) Medulla (inner 2/3 of the kidney) Renal Pelvis is in the center… Funnel-shaped with renal artery and renal vein Renal artery brings blood, wastes, and nutrients into the kidneys so kidneys can take care of waste removal and proper absorption. Kidneys are made of millions of nephrons. Nephrons are the functional units of the kidneys.

Nephrons: The functional unit of the kidney. 3 Important things happen here: Filtration Re-absorption Secretion

(Blood vessels surround nephrons)

The Three Stages: #1: Filtration Materials from the blood are forced out of the glomerulus (pack of blood capillaries) and into the Bowman’s capsule (nephron) during this process. High pressure also forces water, urea, glucose, vitamins, water, and salts through thin capillary walls of the glomerulus into the Bowman’s capsule. About 1/5th of the fluid portion of blood filters into the capsule. The rest remains in the blood, along with big proteins and cells that are too large to fit through capillary walls. In a healthy kidney, the filtrate, the fluid entering the nephron, does not contain large proteins.

#2: Reabsorption Because filtration is non-selective, it is important that small molecules essential to the body be returned to blood fluid. The proximal tubule, the loop of Henle, and part of the distal tubule, are mainly responsible for this. Glucose, sodium, potassium, & calcium are all returned to the blood (by active transport… we’ll learn about this soon). In this region, much of the water in the filtrate returns to the blood (due to osmosis…we’ll learn more about osmosis later too).

#3: Secretion As the filtrate reaches the distal tubule, some bad substances from the blood enter/re-enter the filtrate by a process called secretion. These substances include wastes and toxic materials… we want to get rid of these! The pH of the blood is also adjusted here with the help of hydrogen ions that are secreted from blood to filtrate.

Urine Formation The filtrate and wastes that remain in the distal tubule form urine. Urine from several tubules flow into a collecting duct. More water is reabsorbed back into the blood in this region. The high salt concentration outside of the collecting duct is maintained by the loop of Henle with active transport. Negative Cl ions are pumped out, and positive Na ions follow, keeping the salt gradient higher outside the nephron, allowing water to diffuse back into the blood by osmosis, keeping you more hydrated!

Elimination of Urine Urine from the collecting ducts flows through the renal pelvis and into a tube called a ureter. A ureter leads from each kidney to the urinary bladder. Muscular contractions send urine out of the body through the urethra. At least 500ml of urine must be eliminated every day to get rid of the waste your body makes. A normal adult makes between 1.5 L to 2.3 L of urine a day, depending upon hydration level, and how much water is lost through respiration and perspiration.

MAJOR POINTS

Kidney Versatility If you are very dehydrated, the kidney is able to secrete very concentrated urine in a small volume, making it possible to discharge wastes with minimal water loss. However, if you have consumed an excessive amount of fluid, kidneys can excrete a large amount of dilute urine, making it possible to eliminate a lot of water, while still holding onto essential salts. This versatility is due to nervous and hormonal control.

Antidiuretic Hormone (ADH) When the body becomes dehydrated, more ADH is released from the brain (pituitary gland) and the kidneys reabsorb more water than normal. As you begin to re-hydrate, less ADH is released, until you have plenty of water in the blood fluid, and kidney reabsorb less and less water until homeostasis is achieved! Amazing little organs!!... HOMEOSTASIS!

Blood vessels