1. Human Health & Physiology
11.3 – The kidney
Human Health & Physiology

2. Kidney Structure & Function
Excretion: 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 kidney
Osmoregulation: the control of water balance of the blood, tissue or cytoplasm of a living organism

3. 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

4. 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

6. Nephron Structure & Function
Afferent 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

8. Nephron Structure & Function
Bowman’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

10. Nephron Structure & Function
Loop 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

12. Nephron Structure & Function
Distal 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 balanced
Collecting duct: collects urine and carries it into the renal pelvis to the ureter
This is where the final water balance of the blood occurs

13. Formation of urine
Blood enters the glomerulus through the afferent arteriole under high pressure
This forces water, amino acids, small proteins, glucose, and ions into the Bowman’s capsule
The product is the filtrate that enters the nephron
Filtrate flows into the PCT
Glucose, amino acids, and ions are reabsorbed into the bloodstream through active transport
Small proteins are reabsorbed by pinocytosis

14. Formation of urine Filtrate flows into the descending loop of Henle
The descending loop is permeable to water but not to salt
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

16. Formation of urine
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
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

17. Formation of urine
Filtrate moves into the collecting duct, where water may be further reabsorbed if needed
This is controlled by anti-diuretic hormone (ADH)
ADH is secreted by the pituitary gland and increases the permeability of the DCT and collecting duct to water

18. Formation of urine
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
If blood is high in water content, ADH is not secreted and no more water is reabsorbed
Dilute urine is formed

20. Comparing Solute Concentrations
Molecule
Amount in blood plasma (mg/100mL)
Amount in glomerular filtrate (mg/100mL)
Amount in urine (mg/100mL)
proteins
> 700
glucose
> 90
urea 30
> 1800
Damon, A., McGonegal, R., Tosto, P., & Ward, W. (2007). Higher Level Biology. England: Pearson Education, Inc.

21. Diabetes & Glucose in Urine
People 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

22. Nephron Structure & Function
Glomerulus
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

23. Nephron Structure & Function
Proximal tubule
Reabsorption
Active reabsorption of all nutrients, including glucose and amino acids
Active reabsorption of positively charged ions such as Na+, K+, Ca2+
Passive reabosrption of water by osmosis
Passive reabsorption of negatively charged ions such as chloride and bicarbonate by electrical attraction to positively charged ions
Secretion
Active secretion of hydrogen ions

24. Nephron Structure & Function
Descending loop of Henle
Reabsorption
Passive reabsorption of water by osmosis
Ascending loop of Henle
Active reabsorption of Na+ ions
Passive reabsorption of Cl–, K+

25. Nephron Structure & Function
Distal tubule
Reabsorption
Active reabsorption of Na+ ions
Passive reabsorption of water by osmosis
Passive reabsorption of negatively charged ions such as Cl– and bicarbonate
Secretion
Active secretion of hydrogen ions
Passive secretion of K+ ions by electrical attraction to chloride ions
Collecting duct

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