BIOLOGY 12 - EXCRETION: CHAPTER NOTES - Eckert

BIOLOGY 12 - EXCRETION: CHAPTER NOTES 

Chemical reactions produce wastes that must be removed from cells and from 

your body. Many of these wastes are small, water-soluble molecules that become 

dissolved in your blood along with other small molecules that are not wastes. 

Your body must separate and excrete waste products of Metabolism? 

EXCRETION: is the process that rids body of METABOLIC 

WASTES. (especially Nitrogenous wastes) 

• Excretion is performed by: 

• KIDNEYS: excrete Nitrogenous Wastes (Ammonia, Urea, Uric Acid, Creatinine) 

• LIVER: excrete Bile Pigments 

• LUNGS: excrete CO2, 

• SKIN: Sweat 

• Excretion is not the same as DEFECATION, which is the process which rids the 

body of UNDIGESTED, UNABSORBED food remains.

Nitrogenous Wastes End Products: what are they? 

• AMMONIA = NH3: from amino groups of amino acids. VERY TOXIC 

to tissues, so in land mammals NH3 converted to UREA in liver. 

Structure Of Urea 

• Urea is water-soluble - excreted in URINE 

Other Excreted Substances (besides Nitrogenous wastes) 

kidneys also excrete HCO3 - (bicarbonate ion) 

IONS: Salts K+, Na+, Ca++, Mg++, Fe+ 

• WATER: metabolic end product, maintains blood pressure, consumed 

with food 

• UROCHROME from breakdown of heme, gives urine its yellow colour. 

• URINE is composed mainly of UREA (~3%), SALTS ~(2%), H2O (95%). 

• 

URINARY SYSTEM CONSISTS OF THESE PARTS! 

• RENAL VEIN : carries blood from kidneys back to hearty 

• RENAL ARTERY : carries blood to kidneys 

• URETER : muscular tubes, move urine from kidneys to bladder via peristalsis 

• BLADDER: holds up to 600 ml to 1000 ml urine, can expand/contract. 

Has stretch receptors that indicate when it is full, notifies the brain. 

• URETHRA: tube connecting bladder to outside. 

• the urethra of a man is about 6 inches long (extends through penis). In the man, 

the urethra also transports semen (not at the same time). For women, the 

urethra is only ~1 inch (which is why get more infections here). 

KIDNEYS: Excrete urine, regulate blood volume, pH

KIDNEYS - the main organ of excretion 

• Structurally, kidneys have 3 major divisions: CORTEX 

(outer layer), MEDULLA (middle, striated), PELVIS (inner 

cavity). 

Polycystic Kidneys ~18lbs

• NEPHRONS - are the functional units of the kidney. 

They filter wastes from the blood, and retain water and 

other needed materials. There are about 1 million 

nephrons per kidney.

STRUCTURE OF THE NEPHRON 

• BOWMAN'S CAPSULE - Cup-like end of nephron where wastes are forced 

out of the blood and into the nephron. The blood enters a capillary tuft called 

the GLOMERULUS. 

• AFFERENT ARTERIOLE - carries blood to glomerulus 

• EFFERENT ARTERIOLE - carries blood from glomerulus 

• From capsule, nephron narrows into PROXIMAL CONVOLUTED 

TUBULE, which makes a turn to FORM LOOP OF HENLE, which 

is surrounded by the PERITUBULAR CAPILLARY NETWORK. 

Loop leads to the DISTAL CONVOLUTED TUBULE, which finally 

enters a COLLECTING DUCT.

URINE FORMATION: YOU MAKE ABOUT 1 mL OF URINE PER MINUTE! 

Urine formation consists of 3 STEPS 

1. PRESSURE FILTRATION: occurs inside Bowman's capsule as 

molecules are forced through the glomerulus. 

2. SELECTIVE REABSORPTION: occurs in the proximal 

convoluted tubule (Na+,Cl-, H2O) 

3. URINE CONCENTRATION: Occurs in the loop of Henle 

4. TUBULAR EXCRETION: occurs in distal convoluted tubule 

KIDNEY PROCESSES 

1. PRESSURE FILTRATION 

high blood pressure in GLOMERULUS (~60mm Hg) forces SMALL 

MOLECULES [*H2O, nitrogenous wastes, *nutrients, *ions (salts)] into 

BOWMAN'S CAPSULE. 

large molecules are unable to pass (i.e. blood cells, platelets, 

proteins). the molecules that are forced into Bowman's capsule form 

FILTRATE. 

• high blood pressure is necessary for filtration.

2. SELECTIVE REABSORPTION 

• the molecules that are reabsorbed move from the 

proximal convoluted tubule to the peritubular 

capillary network (i.e. back into the blood). 

• WHAT GETS REABSORBED?: most H2O, nutrients, 

some salts (Na+, Cl-) 

• WHAT DOESN’T GET REABSORBED: some H2O, 

wastes, excess salts 

• Reabsorption of nutrients is both ACTIVE

3. URINE CONCENTRATION 

Active reabsorption of salt at the Loop of Henle results in 

the passive transport of water at both the Loop of Henle 

and Collecting Duct and thus urine concentration.

3. TUBULAR EXCRETION (=TUBULAR SECRETION) 

• This is an ACTIVE PROCESS by which other nonfilterable 

wastes can be added to the tubular fluid so that 

these wastes will also be excreted in the urine. 

• Occurs in the DISTAL CONVOLUTED TUBULE: 

secreted substances include some chemicals (e.g. 

penicillin, histamine) H+ ions, NH3 

• The tubular fluid, which we can now call URINE passes 

from duct into pelvis of kidney, and enters ureter for 

transport to bladder.

REGULATORY FUNCTION OF KIDNEYS: the kidneys 

do much more than just filter the blood! 

REGULATE VOLUME OF BLOOD (i.e. water volume). This is 

done by two HORMONES: ADH and ALDOSTERONE. 

ADH (ANTIDIURETIC HORMONE): Hormone released by pituitary 

gland promotes reabsorption of water from collecting duct and 

distal convoluted tubule 

Here is how ADH does it’s job: 

1. cells in hypothalamus detect low H2O content of blood 

2. ADH (made in the hypothatlamus and released by the posterior 

pituitary) released into blood, acts on DISTAL CONVOLUTED 

TUBULE and COLLECTING DUCT 

3. more H2O reabsorbed, volume of urine decreases 

4. therefore, blood volume increases 

**DIURETIC DRUGS, inhibits ADH cause increased urination 

**ALCOHOL also inhibits ADH therefore causes increased urination

ALDOSTERONE 

• this is a hormone released by ADRENAL CORTEX 

(adrenal glands sit on top of kidneys). Aldosterone acts 

on kidney to RETAIN Na+ and EXCRETE K+. 

• [Na+] in blood results in kidneys ability to reabsorb H2O 

• if [Na+] in blood too low, too little H2O is reabsorbed, 

results in HYPOTENSION. 

• if [Na+] in blood too high, results in HYPERTENSION

BIOLOGY 12 - EXCRETION: CHAPTER NOTES - Eckert

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