The Nephron as a Unit of Kidney Function Honours Objectives – What you will need to know from this section Outline the structure & associated blood supply & draw a diagram. Explain urine formation, including: Bowman's capsule/glomerulus/ proximal convoluted tubule/Loop of Henle/distal convoluted tubule/pelvis/bladder Outline the sites & action of reabsorbing glucose/amino acids/salts/water State that reabsorbing water occurs in the collecting duct & is under the influence of ADH.

OUR EXCRETORY SYSTEM

The Nephron as a Unit of Kidney Function Honours The urinary system consists of the kidneys, the bladder and some ducts (tubes).

The Nephron as a Unit of Kidney Function Honours A section through the kidney shows an outer darker region (cortex) and a lighter inner zone (medulla). Cortex Medulla Ureter Pelvis

– Identify Cortex, Medulla, Pelvis, Ureter LEARNING CHECK – Identify Cortex, Medulla, Pelvis, Ureter D A B C A = Cortex B = Medulla C = Ureter D = Pelvis

KIDNEY LS

H 3.4.8 The Nephron as a Unit of Kidney Function The structural and functional unit of the kidney is the nephron. The renal artery divides into afferent arterioles and then into a capillary network (glomerulus) at the top of each nephron.

NEPHRON

1. Filtration The renal artery divides into afferent arterioles and then into a capillary network (glomerulus) at the top of each nephron. A cup-shaped funnel (Bowman’s capsule) surrounds each glomerulus and it is here that smaller molecules in the blood are forced, under pressure, out of the plasma and into the lumen of Bowman’s capsule, forming the glomerular filtrate. The blood pressure is high because the efferent (before) arteriole is narrower than the afferent (after) arteriole, so force-filtering the plasma. Everything except large proteins and blood cells gets filtered.

2. Reabsorption The body cannot afford to lose useful chemicals like food and water, so as the glomerular filtrate passes from the Bowman's capsule into the proximal convoluted tubule, glucose, amino acids, some salts and water are reabsorbed back into the blood. The food molecules, including most of the salt ions, are taken back by active transport (against the concentration gradient, so energy is needed for this). Most of the water is reabsorbed by osmosis—from the Loop of Henle and convoluted tubules.

Urea and other wastes, along with some water, are not reabsorbed Urea and other wastes, along with some water, are not reabsorbed. They pass, as urine, into the pelvis of the kidney and to the bladder for storage. Of the 180L of blood filtered each day, about 99 % of the filtrate is reabsorbed.

LEARNING CHECK Name the parts of the nephron Where does filtration takes place? What does not get filtered Where does reabsorption take place? What substances are reabsorbed? What are not reabsorbed? List some substances present in urine.

The Nephron as a Unit of Kidney Function Honours The kidney regulates the amount of water in the body by varying the amount of urine produced. This is known as osmoregulation, and it is an example of homeostasis. ADH [Anti-diuretic hormone] controls whether the distal tubule and collecting ducts reabsorb water or not.

If you drink a great deal of water the hypothalamus in the brain detects the diluted blood and turns off ADH production. Less water is reabsorbed, so more water is allowed to escape to the bladder, and a larger volume of dilute urine is produced. When the body is low on water ADH is secreted from the pituitary gland. More water is reabsorbed and only a small volume of urine is produced.

When the body has excess water, ADH is NOT secreted from the pituitary gland. Less water is reabsorbed and so a larger volume of urine is produced. If Water Levels Rise in the Body …..

When the body is low on water, ADH is secreted from the pituitary gland. More water is reabsorbed and only a small volume of urine is produced. If Water Levels Fall in the Body…..

LEARNING CHECK What is ADH? Where is it made and where does it act? Explain what happens in a nephron where we take in a lot of fluid. Explain what happens in each nephron when we have not taken in fluid for many hours.