The Excretory System

• Excretion is the removal of the waste products of metabolism from living organisms.

• The accumulation of these waste products would be toxic if they were not eliminated.

• In plants and simple animals, waste products are removed by diffusion. Plants, for example, excrete O2, a product of

photosynthesis.

METABOLIC WASTE A BY-PRODUCT OF…

Water Dehydration synthesis and cellular respiration

Carbon dioxide Cellular respiration

Salts Neutralization reactions

Urea Deamination (removal of amino group from amino acids

Uric acid Nucleic acid breakdown

Organs of Excretion

• There are four major organs involved in excretion:

1. The lungs2. The liver3. The skin4. The kidneys

THE LUNGS• Cellular respiration occurs in every living cell in

your body. • Carbon dioxide is produced as a waste product. 

As the carbon dioxide accumulates in body cells, it eventually diffuses out of the cells and into the bloodstream, which eventually circulates to the lungs.

• In the alveoli of the lungs, carbon dioxide diffuses from the blood, into the lung tissue, and then leaves the body every time we exhale.  Some water vapor also exits the body during exhalation.

THE SKIN• As you already know, sweat comes out of

pores in your skin.  • Sweat is a mixture of three metabolic wastes:

water, salts, & urea.•   So as you sweat, your body accomplishes

two things:

1) sweating has a cooling effect on the body,

2) metabolic wastes are excreted.

• The skin is made up of two layers:

1. The thin epidermis at the top

2. The thicker dermis below.  This is where oil glands, hair follicles, fatty layers, nerves, and sweat glands are found.

• The sweat gland is a tubular structure tangled with capillaries (the smallest of blood vessels).

• This close association of tubes allows wastes (water, salts & urea) to diffuse from the blood  & into the sweat gland.

• When body temperature rises, the fluid (sweat) is released from the gland, travels through the duct, and reaches the skin surface through openings called pores.

THE LIVER• The liver is a large, important organ.  In fact

it is the largest internal organ in our bodies.  Its numerous functions make it "part" of the circulatory, digestive, and excretory systems.

• In excretion, the liver removes the NH2 from amino acids in proteins. This is called deamination

• The by-product of deamination is ammonia, a water-soluble gas.

• Ammonia is extremely toxic so it is combined with CO2 to make urea.

• Urea is much less toxic and can dissolve in the blood for excretion through the sweat or through urine.

• Uric acid is formed by the breakdown of nucleic acids in the liver.

THE KIDNEY• The kidney is the major organ of excretion.• The kidney is also a major regulatory organ.• Through the formation of urine, it is responsible

for the following: 1. Removal of organic wastes: urea, uric acid and

the breakdown products of hemoglobin and hormones

2. Regulation of concentrations of important ions: sodium, potassium, calcium, magnesium, sulfate and phosphate ions

3. Regulation of pH balance of body: control levels of H+, HCO3

- and NH4+

4. Regulation of Red Blood Cell production. The kidneys release erythropoietin, which regulates the production of RBCs in the bone marrow.

5. Regulation of blood pressure: regulation of fluid volume of the body.

6. Limited control of blood glucose and blood amino acid concentration: eliminate excess amounts

Structure of the Kidney

• Renal Capsule: A smooth semitransparent membrane that adheres tightly to the outer surface of the kidney.

• Renal Cortex: The region of the kidney just below the capsule. This part of the kidney is rich is arterioles and venules.

• Renal Medulla: The region below the cortex that is segregated into triangular regions. The triangular regions are the renal pyramids, which are striated (or striped) in appearance due to the collecting ducts running through them.

• Renal Pelvis: A cavity within the kidney that is continuous with the ureter. The pelvis has portions that extend towards the renal pyramids. These extensions are called calyces.

The Nephron

• The functional unit of the kidney is the nephron. Each kidney contains over one million of these microscopic filters.

• About 20% of the total blood pumped by the heart each minute will enter the kidneys to undergo filtration. This is called the filtration fraction

The nephron

Urine Formation

• Occurs in the nephron through three processes:

1. Glomerular Filtration

2. Tubular Reabsorption

3. Tubular Secretion

Glomerular filtration• The transfer of fluid and solutes from the

glomerular capillaries into Bowman’s capsule.• Blood enters the glomerulus under pressure. This

causes water, small molecules (urea, glucose, amino acids) and ions to filter through the capillary walls into Bowman’s capsule.

• Large blood components (RBCs, WBCs, platelets and proteins) cannot filter through.

• The fluid in the Bowman's capsule appears very much like interstitial fluid without the proteins. It is called the nephric filtrate

• The glomerular capillaries are substantially more (100 to 1000X) leaky than regular capillaries and have 2-3 times more pressure than regular capillaries.

• Glomerular filtration rate is fairly constant. (130 ml/min or 7.8 l/hr). This means that about 190 L of filtrate is formed every 24 hours by both kidneys

Tubular Reabsorption• Occurs in the Proximal convoluted tubule, the

Loop of Henle and the Distal convoluted Tubule.• Materials which are required by the organism are

returned to the bloodstream: water, ions, glucose, amino acids etc…

• The kidney does not work by a process of identifying what is bad; rather it works by identifying those things that are good for the body

• Much of the urea is lost simply because the kidney chooses not to recover it after it has been filtered.

• Any small foreign molecule that has entered our blood, even if it has not existed in human evolutionary history (drugs, new pollutants, etc) can be removed by the kidney.

Proximal convoluted tubule

• The cells of the tubule are lined with microvilli. Why?

• Reabsorption of glucose, amino acids, and most inorganic salts occurs here.

• Na+ ions are transported out of the tubule by active transport, through carrier molecules.

• Cl- ions and HCO3- ions follow by

charge attraction.• As these solutes move out of the

tubule, they create an osmotic gradient and water moves out of the tubule and back into the blood, through osmosis.

• About 80-85% of the water in the filtrate is reabsorbed in the proximal tubule.

• Glucose and amino acids attach to carrier molecules and are transported out by active transport.

• This requires a lot of energy so there are many mitochondria in the cells of the proximal tubule.

• There is a limit to the amount of sodium, glucose and amino acids that can be reabsorbed by the carrier molecules: the threshold limit. When this limit is reached, these substances are excreted in the urine.

• H+ ions are secreted into the proximal tubule. This helps regulate pH.

• About 50% of the urea that was in the nephric filtrate is reabsorbed in the tubule. This is a passive mechanism. The rest is excreted in the urine.

The loop of Henle• The descending loop of Henle

is permeable to water. Water is reabsorbed into the peritubular capillaries by osmosis.

• The filtrate decreases in volume,but increases in osmotic concentration.

• Salt (NaCl) becomes concentrated in the filtrate as the loop penetrates the inner medulla of the kidney.

• The ascending loop of Henle is permeable to salt but not to water. Sodium is actively transported out of the filtrate and chlorine follows by charge attraction.

• The volume of the filtrate does not change, but the concentration decreases.

• The peritubular capillaries ensure a rich blood supply for reabsorption

The Distal Convoluted Tubule

• More sodium is reclaimed by active transport, and still more water follows by osmosis.

• Although 97% of the sodium has already been removed, it is the last 3% that determines the final balance of sodium.

• This determines the water content and blood pressure in the body.

• The reabsorption of sodium in the distal tubule and the collecting tubules is closely regulated, chiefly by the action of the hormone aldosterone.

The Collecting Duct.• The filtrate now flows

into the collecting duct, which gathers fluid from many nephrons.

• Urine flows from the collecting ducts into the renal pelvis to the ureters and into the bladder.

• Water regulation occurs here.

• The plasma membranes of the cells of the distal tubule and the collecting duct have transmembrane channels made of a protein called aquaporin.

• When these channels are open, water can pass through very quickly. These water channels are responsive to levels of antidiuretic hormone (ADH).

Tubular secretion

• This is the third mechanism of excretion.

• Secretion occurs when substances are transported from the blood directly into the distal tubule.

• Hydrogen, potassium, and ammonium ions are actively secreted into the tubule. This helps to regulate pH.

• If the pH of the blood becomes too acid, more H+ ions are secreted into the tubule.

• If the pH of the blood becomes too alkaline, then secretion of H+ is reduced.

• The pH of urine can vary from 4.5 to 8.5

• Certain drugs, such as penicillin, are also secreted into the tubule for excretion.

• Secretion occurs in the proximal tubule, the distal tubule and the collecting duct.

• While we think of the kidney as an organ of excretion, it is more than that.

• It does remove wastes, but it also removes normal components of the blood that are present in greater-than-normal concentrations and reclaim these components when they are present in the blood in less-than-normal amounts.

• Thus the kidney continuously regulates the chemical composition of the blood within narrow limits. The kidney is one of the major homeostatic devices of the body.

Urine Testing• Color: Normal urine will vary from light

straw to amber color.

• The color of normal urine is due to a pigment called urochrome, which is the end product of hemoglobin breakdown: 

• Hemoglobin hematin bilirubin urochromogen urochrome

• The following changes in colour have pathological implications:a. Milky: presence of pus, bacteria, fat.b. Reddish amber: presence of urobilinogen or porphyrin. Urobilinogen is produced in the intestine by the action of bacteria on bile pigments. Porphyrin may be evidence of liver cirrhosis, jaundice, Addison's disease, and other conditions.c. Brownish yellow or green: presence of bile pigments.  d. Red to smoky brown: presence of blood and blood pigments.

 

• Carrots may cause increased yellow color due to carotene, while beets cause reddening and rhubarb may cause the urine to become brown. These food items and certain drugs may color the urine, yet have no pathological significance.

Glucose• glucose is not normally present in the urine

because all of it is usually reabsorbed from the renal tubules into the blood.

• When the glucose concentration of the filtrate is within the normal limits (70-110 mg per 100 ml), there is a sufficient number of carrier molecules in the renal tubules to transport all the glucose back into the blood.

• However, if the blood glucose level exceeds its threshold (for glucose, about 180 mg per 100 ml), there will not be enough carrier molecules to reabsorb all of the glucose.

• The untransported glucose will end up in the urine and result in a condition known as glycosuria.

• The main cause of glycosuria is diabetes mellitus

Ketones (Ketonuria)• Normally, no ketones are present in urine.• Detectable levels of ketone may occur in

urine during physiological stress conditions such as fasting, pregnancy, and frequent strenuous exercise.

• When there is carbohydrate deprivation, such as in starvation or high-protein diets, the body relies increasingly on the metabolism of fats for energy.

• This pattern is also seen in people with diabetes mellitus, where the lack of insulin prevents the body cells from utilizing the large amounts of glucose available in the blood.

• When the production of the intermediate products of fatty acid metabolism (ketone bodies) exceeds the ability of the body to metabolize these compounds, they accumulate in the blood and spill over into the urine (ketonuria).

pH

• Freshly voided urine is usually acidic (around pH 6) but the normal range is between 4.8 and 7.5.

• The pH will vary with the time of day and diet. High acidity is present in acidosis, fevers, and high protein diets. Excess alkalinity may be due to urine retention in the bladder, chronic cystitis, anemia, and obstructing gastric ulcers.

Protein • Since proteins are very large molecules, they are

not normally present in measurable amounts in the glomerular filtrate or the urine.

• The detection of proteins in the urine, therefore, may indicate that the permeability of the glomerulus is abnormally increased.

• This may be caused by renal infections (nephritis), or it may be caused by other diseases that affect the kidney,such as diabetes mellitus, jaundice, or hyperthyroidism.