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

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

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 & 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

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

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

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

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

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

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

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

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

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.

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

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

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

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+

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

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.