S

Excretion Continued

Composition of Urine

The kidneys remove waste from the plasma and concentrate them in the urine

Ratio of the concentration of substances in urine to concentration of substances in plasma on next slide (ratios, not amounts):

Urine Plasma

Na+ 1 1K+ 12 1CA2+ 1 1Mg2+ 5 1H2PO4-HPO4

2-25 1

glucose 0 1urea 70 1uric acid 14 1

Urination

As bladder fills, stretch receptors send impulses to spinal cord

Impulses from spinal cord cause bladder to contract and urine is released

Usually this reflex is controlled by the brain

Regulatory functions of the kidney 1: water balance

Blood volume (Water balance) Blood volume is directly controlled by kidneys Increased reabsorption of water, increased

blood volume Decreased reabsorption of water, decreased

blood volume Combination of hormones and nervous system

Hormonal control

ADH = anti-diuretic hormone Released by posterior pituitary gland Causes upper DCT and collecting duct

to be more permeable to water at the DCT (more water is reabsorbed, which can increase or maintain blood volume)

Changes in OP detected by osmoreceptors in hypothalamus

Alcohol inhibits release of ADH because it is a diuretic

Brain break!!!

Hormonal control cont’d

Aldosterone (hormone) Released by adrenal gland (on

top of kidney) Causes increased reabsorption

of Na+ from DCT This causes blood to be

hypertonic, causing more water to be reabsorbed into blood from the nephron

Juxtaglomerular Apparatus and BP

Region of afferent arteriole that monitors BP Increase in fluid loss will result in lowered BP If BP is too low for glomerular filtration, the

juxtaglomerular cells release renin (enzyme) Renin stimulates the conversion of

Angiotensinogen (from liver) to Angiotensin

Angiotensin causes vasoconstriction and stimulates the release of aldosterone from the adrenal cortex

Vasoconstriction will help increase BP Aldosterone causes increase in Na+

Increased Na+ retention results in increased blood volume which will also increase BP

Juxtaglomerular Apparatus and BPCont’d

Regulatory functions of the kidney 2: Maintenance of

pH Chemoreceptors in the carotid and aortic

bodies stimulate the respiratory centre when blood pH decrease

But resp. centre can only deal with minor fluctuations in pH

Kidneys help maintain the blood’s constant pH If blood pH decreases, H+ ions are excreted

with ammoniaNH3 + H+ NH4

+ (excreted)

And bicarbonate ions are reabsorbed:

HCO3- + H+ H2CO3

If blood is too basic (alkaline), less NH4

+ will be excreted and less HCO3

- will be reabsorbed Phosphate will also buffer blood