The Excretory System Overview: The kidneys are one of the three organs of excretion (lungs + skin). The kidneys remove ammonia, urea, and uric acid, and maintain water balance. Structures of Excretion:

1. ______________:

• produce urine.

2. ______________:

• transport urine from kidneys to bladder

3. ______________:

• Storage of urine 4. _____________:

• Elimination of urine from the bladder 5. _______________________ (outer layer of kidney)

1. Contains glomerulus, proximal and distal tubules 2. Pressure filtration, selective reabsorption and tubular excretion 3. Responsible for most of the work of the kidney

6. ___________________________ (inner loop of kidney)

• Contains loop of Henle, and most of the collecting duct

• Responsible for H2O reabsorption and salt balance = concentrates urine

The Nephron and Urine Formation The kidney is composed of millions of tiny separate units called nephrons. _____________________

• capillary network in which a large portion of blood plasma filters from the blood vessels into the Bowman’s capsule.

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• Collection area for blood plasma from (filtrate) glomerulus. Blood is pressure filtered. Proteins, cells, large stuff stays in, salts sugar, waste etc. filter out)

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• blood vessel entering the glomerulus. Normal O2 blood.

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• Blood vessel leaving the glomerulus. Goes to the capillary network. Thick blood (less plasma etc)

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• Network of capillaries around the rest of the tubules. Reabsorbs nutrients, water, etc.

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• Sugar and salts are ACTIVELY TRANSPORTED OUT of the filtrate (tubule) back into the blood (capillary network). Uses ATP and oxygen. Walls are covered in microvilli.

• The filtrate is now less concentrated (hypotonic) than the blood, so water moves from the filtrate to the blood by diffusion.

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• Dips down into medulla (into a hypertonic environment)

• More water is absorbed into blood from descending limb

• Na+ and Cl- also reabsorbed

• COUNTER CURRENT EXCHANGE – salt (Na+ Cl-) is diffused and pumped out of the ascending limb and diffuses back into the descending limb. Also, urea diffuses out to the collecting duct and into the ascending limb. This adds to the hypertonic solution of the medulla and allows the kidney to excrete very hypertonic urine (concentrated)

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• Some tubular excretion into tubule from blood

• A few materials are actively excreted from blood into tubules. I.e. penicillin, histamines, H+, etc)

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• Common duct which collects filtrate from many distal tubules (nephron) – goes back through the medulla (hypertonic environment)

• Very important in regulating the overall water content of urine (under the control of hormones)

• Can control from very dilute urine (lots of water) to very concentrated urine (very little water, most reabsorbed)

The Effect of the Nephron on Blood Composition Substance Renal Artery Renal Vein -Glucose 100 mg/L 98 mg/L -Urea 30 mg/L 25 mg/L Glucose is 100% reabsorbed form the filtrate. The 2 mg/L drop is a result of sugar used to make ATP to fuel active transport. Urea is lower in the renal vein because it is excreted in the filtrate. Some is reabsorbed in the tubules.

The kidneys do not remove all the wastes from the blood; they remove enough to keep the blood at a constant level (homeostasis).

Hormonal Control of Excretion Notes

ADH (Anti Diuretic Hormone) ▪ Controls water balance ▪ ADH is secreted by the posterior pituitary gland ▪ ADH increases the permeability of the distal tubule and collecting duct so

that more water can be reabsorbed back into the blood. ▪ If ADH is secreted:

o Blood volume increases

o Therefore, blood becomes more dilute and urine becomes more concentrated.

o ADH secretion is controlled by water content of the blood. If low ADH is released, if high no ADH is released and little water is reabsorbed.

Aldosterone

▪ Hormone secreted by the adrenal cortex gland (outer layer of the adrenal gland on top of each of the kidneys)

▪ Controls the excretion of NA+ and K+ ▪ Increases reabsorption of Na+ (increases Na+ in blood) ▪ Increases excretion of K+ (decreases K+ in blood) ▪ If blood Na+ level is too low, aldosterone is secreted, reabsorption of Na+

by the kidneys occur and blood Na+ increases. ▪ Controls blood volume (and pressure). Increase in Na+ in blood causes

water to be reabsorbed, increasing blood volume and pressure.

Role of the Hypothalamus

▪ Hypothalamus is the area of the brain directly above the pituitary gland. ▪ The actual concentration of the blood is monitored by the hypothalamus. ▪ If blood is too concentrated hypothalamus triggers the posterior pituitary gland

to release ADH. As blood becomes more dilute, fewer hormones are secreted.

▪ If blood is too dilute, no ADH is released and the kidney tubules stay impermeable, so little water is reabsorbed.

Summary: Blood is too concentrated:

▪ Hypothalamus triggers PPG to secrete ADH ▪ More water is reabsorbed by collecting duct and distal tubule ▪ Blood becomes more dilute

Blood is too dilute:

▪ Hypothalamus causes less ADH to be secreted. ▪ Little water is reabsorbed by collecting duct and distal tubule ▪ Blood becomes more concentrated