Download Kidney Notes

The Excretory System (chap. 11, p. 374)
The excretory system maintains homeostasis by removing metabolic wastes such as
water, salt, and metabolite concentrations in the blood.
Remember, the excretory system is not all about the kidney. Other excretory organs
include the lungs, the skin, and the liver.
Lungs: removes CO2 from the body
Skin: removes heat
Liver: removes metabolic wastes
Kidneys: Functions to filter blood in order to remove cellular waste products from the
blood. Also controls water balance, pH, and the body’s levels of sodium, potassium, and
bicarbonate.
Facts
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about the Kidney
Humans have two kidneys. They are the size of a fist.
Found in the lower back on each side of the spine.
The major waste product is urine.
At any time, 20% of the body’s blood is in the kidneys.
If for any reason a person only has one kidney, it will increase in size to compensate
for the increased workload.
Removal of nitrogenous wastes
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The main metabolic wastes are urea, uric acid, and creatine, all of which have nitrogen
as a major component.
Urea is produced in the liver from the breakdown of amino acids (protein)
The amine (NH2) is removed so the rest of the amino acid can be converted to
carbohydrate or fat.
The NH2 combines with a hydrogen ion to form ammonia (NH3) which is transported to
the liver where it is converted to the less toxic urea.
Other waste products in the blood are uric acid, which is produced from the
breakdown of nucleic acids, and creatine, the waste product from muscle action.
The Urinary System (Fig 11-17, p. 374)
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Composed of Two kidneys, ureters, a urinary bladder, and urethra.
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Ureters: Tubes that carry urine from the kidneys to the bladder.
Urinary bladder: A temporary storage for urine.
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Can hold about 600 ml.
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When it contains 250 ml. We become aware
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Two sphincters control urine release: one involuntarily controlled by the brain and the
other we learn to control as children.
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Urethra: Urine exits the bladder through the urethra. In males it is approx. 20 cm
long. In females it is approx. 4 cm.
Structure of the kidney (Fig. 11-19, p. 375)
A kidney has three major sections: the cortex, the medulla, and the pelvis.
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Cortex: The outer layer of the kidney
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Medulla: The inner part of the kidney
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Pelvis: Where urine accumulates before it travels to the bladder.
Within the cortex and medulla, there are approx. one million tiny filters known as
nephrons. Each nephron consists of five major parts: The Bowman’s capsule, the
proximal tubule, the loop of Henle, the distal tubule, and the collecting duct.
The Bowman’s capsule:
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Blood enters the cavity of the ball-shaped Bowman’s capsule through an artery that
branches to form a capillary network known as the glomerulus. Here, some plasma and
small particles are forced into the surrounding cavity while larger components remain
in the capillaries. Fluid in the Bowman’s capsule is referred to as nephric filtrate.
Proximal tubule:
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The nephric filtrate enters the proximal tubule where materials required by the body
are returned to the bloodstream by osmosis, diffusion, and active transport. (Water,
glucose, amino acids, and ions) Tiny villi increase the surface area of this tubule.
Loop of Henle:
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Filtrate next moves into the loop of Henle. Its main function is to remove water from
the filtrate by osmosis. The medulla has a high sodium concentration which helps draw
the water out.
Distal tubule:
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Next the filtrate moves to the distal tubule, which is responsible for tubular
secretion, whereby active transport pulls substances such as creatinine and drugs out
of the blood and into the filtrate.
Collecting duct:
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Fluid from many nephrons now empty into a common collecting duct, which carries the
urine to the pelvis.