Human Health & Physiology

11.3 – The kidney

Kidney Structure & FunctionExcretion: the process of removing

metabolic waste from the cells, tissue fluid, and blood of living organisms The main organ of excretion in mammals

is the kidneyOsmoregulation: the control of

water balance of the blood, tissue or cytoplasm of a living organism

Kidney Structure & Function Functions:

Produces urine Maintain water balance Maintain blood pH Maintain blood pressure

The functional unit of the kidney is the nephron

There are more than 1 million nephrons in a human kidney

On average 120mL/min of fluid passes through the kidney

Kidney Structure & Function Roles of the nephron:

Ultrafiltration Reabsorption Secretion

For the kidney you should be able to label: Cortex Medulla Pelvis Ureter Renal blood vessels

Nephron Structure & FunctionAfferent arteriole: brings blood

into the glomerulus from the renal artery

Efferent arteriole: takes blood out of the glomerulus into the surrounding capillary network and then into the renal vein

Glomerulus: a ball of capillaries that are fenestrated (have pores) and are surrounded by a basement membrane that filters what passes through into the filtrate

Nephron Structure & FunctionBowman’s capsule: a cup shaped

structure at the end of the nephron that surrounds the glomerulus and collects the filtrate

Proximal convoluted tubule (PCT): lined with microvilli to increase surface area and has many mitochondria to provide ATP for active transport

Nephron Structure & FunctionLoop of Henle: carries the filtrate

from the PCT to the DCT The loop of Henle descends into the

medulla of the kidney The concentration gradient of salt

increases as you move down the medulla of the kidney

Nephron Structure & FunctionDistal convoluted tubule (DCT):

conducts urine from the loop of Henle to the collecting duct It is the final place where blood pH and

ions are balancedCollecting duct: collects urine and

carries it into the renal pelvis to the ureter This is where the final water balance of

the blood occurs

Formation of urine

1. Blood enters the glomerulus through the afferent arteriole under high pressure

2. This forces water, amino acids, small proteins, glucose, and ions into the Bowman’s capsule

The product is the filtrate that enters the nephron

3. Filtrate flows into the PCT Glucose, amino acids, and ions are reabsorbed

into the bloodstream through active transport Small proteins are reabsorbed by pinocytosis

Formation of urine

4. Filtrate flows into the descending loop of Henle

The descending loop is permeable to water but not to salt

5. The loop of Henle is hypotonic (higher salt/urea concentration outside) to the medullary fluid

Water moves out of the nephron by osmosis Therefore the filtrate becomes more

concentrated and hypertonic

Formation of urine

6. The ascending loop of Henle is permeable to salt, but not to water

As the filtrate moves up the ascending loop, salt (Na+ and Cl–) moves out passively at first, then is actively pumped out at the top of the loop

7. Filtrate passes into the DCT where it is adjusted to balance blood pH (by secretion of H+) and ion composition

More water is also reabsorbed

Formation of urine

8. Filtrate moves into the collecting duct, where water may be further reabsorbed if needed

This is controlled by anti-diuretic hormone (ADH)

9. ADH is secreted by the pituitary gland and increases the permeability of the DCT and collecting duct to water

Formation of urine

10.If blood is low in water content, ADH is secreted and more water is reabsorbed from the collecting duct into the blood

Concentrated urine is formed and water is conserved

11.If blood is high in water content, ADH is not secreted and no more water is reabsorbed

Dilute urine is formed

Comparing Solute ConcentrationsMolecule Amount

in blood plasma (mg/100

mL)

Amount in

glomerular

filtrate (mg/100

mL)

Amount in urine (mg/100

mL)

proteins > 700 0 0glucose > 90 > 90 0urea 30 30 > 1800Damon, A., McGonegal, R., Tosto, P., & Ward, W. (2007). Higher Level Biology. England: Pearson Education, Inc.

Diabetes & Glucose in UrinePeople with uncontrolled diabetes

can have a large amount of glucose in their blood

Glucose enters the glomerular filtrate and is reabsorbed by active transport There is a maximum rate at which

reabsorption can occur If there is too much glucose in the

blood, reabsorption of all glucose from the glomerular filtrate cannot be achieved

Nephron Structure & FunctionGlomerulus Filtration

Glomerular blood pressure forces some of the water and dissolved substances from the blood plasma through the pores of the glomerular walls

Bowman’s capsule

Receives filtrate from glomerulus

Nephron Structure & FunctionProximal tubule

ReabsorptionActive reabsorption of all nutrients, including glucose and amino acidsActive reabsorption of positively charged ions such as Na+, K+, Ca2+

Passive reabosrption of water by osmosisPassive reabsorption of negatively charged ions such as chloride and bicarbonate by electrical attraction to positively charged ionsSecretionActive secretion of hydrogen ions

Nephron Structure & FunctionDescending loop of Henle

ReabsorptionPassive reabsorption of water by osmosis

Ascending loop of Henle

ReabsorptionActive reabsorption of Na+ ionsPassive reabsorption of Cl–, K+

Nephron Structure & FunctionDistal tubule

ReabsorptionActive reabsorption of Na+ ionsPassive reabsorption of water by osmosisPassive reabsorption of negatively charged ions such as Cl– and bicarbonateSecretionActive secretion of hydrogen ionsPassive secretion of K+ ions by electrical attraction to chloride ions

Collecting duct

ReabsorptionPassive reabsorption of water by osmosis

References

1. Damon, A., McGonegal, R., Tosto, P., & Ward, W. (2007). Higher Level Biology. England: Pearson Education, Inc.

2. Raven, P.H., Johnson, G.B., Losos, J.B., Mason, K.A., & Singer, S.R. (2008). Biology. (8th ed.). New York: McGraw-Hill Companies, Inc.