Excretion The removal of metabolic wastes from

the body Kidneys are the primary organ of this

process

1. Excretion of Metabolic Wastes Removing metabolic waste products, notably

nitrogenous waste from breakdown of amino acids (aa):

Urea Breakdown of amino acids releases ammonia which is

toxic to cells) Liver converts the ammonia to urea so it be safely

transported in the blood to the kidney where it is eliminated in urine

Creatinine Uric acid

if too much uric acid crystals can form in joints This is known as gout

2. Maintenance of Water-Salt Balance Blood volume is associated with salt

balance in body Salt (NaCl) can cause water diffusion into blood ↑ salt = ↑ blood volume = ↑ blood pressure

3. Maintenance of Acid-Base Balance Kidneys regulate acid-base in blood Excretion of hydrogen ions and

reabsorption of bicarbonate to keep pH around 7.4

Urine often has a pH of 6

4. Secretion of Hormones Renin

Leads to secretion of aldosterone Reabsorption of sodium ions

Erythropoetin Stimulates red blood cell production

Activates vitamin D from the skin Helps promote calcium absorption from the

digestive tract.

16-9

Fig. 16.1

Kidneys Two bean-shaped organs Located near lumbar region on either side

of vertebral column Has a depression called the

hilum where the Renal artery Renal vein are located

Ureters Conduct urine from the kidneys to the

bladder Small, muscular tube 25cm long, 5 mm

thick

Urinary Bladder Stores urine until it is expelled from the

body Located in the pelvic cavity 3 openings (orifices):

2 for the ureters 1 for urethera

Urinary Bladder con’t... Expandable

Detrusor Muscle – bladder wall Sphincter Muscles – surround bladder neck

and urethra

While the bladder is filling: the detrusor muscle is relaxed so the

bladder wall can expand and the sphincter muscles are contracted to keep the urethra closed

When the bladder empties: the detrusor muscle contracts to squeeze

the urine out, and the sphincter muscles relax so the urethra can open up

http://www.aboutkidshealth.ca/en/howthebodyworks/kidneysandbladderoverview/bladderfillingandemptying/pages/default.aspx

Urethra Small tube that extends from the urinary

bladder to an external opening females

4 cm long Increases likelihood of urinary tract infection

Not connected to reproductive system Males

20 cm long Common pathway for reproductive and urinary

tracts

16-17

Micturition When the bladder fills to 250ml

Stretch receptors in the bladder wall send sensory impulse to the spinal cord

Motor impulses from spinal cord send a signal the bladder

Bladder contraction and sphinter to relax

This is called an involuntary reflex Present in infants and young children

In older children and adults the brain can control this reflex and delay urination until a suitable time Stretch receptors in the bladder send a

sensory signal to micturition center in the pons

If not appropriate it will send an inhibitory signal to keep the sphinter closed

This is called a voluntary response https://www.youtube.com/watch?v=US0vNoxsW-k