Homeostasis of body fluid Text: Chapter 8 Human Biology Stage 3

Keywords Hypothalamus Osmoreceptors Thirst centre ADH Adrenal glands Aldosterone Nephron Bowman’s capsule - glomerulus Proximal convoluted tubule Loop of Henle Distal convoluted tubule Collection duct Ureter Bladder 5/22/2019 3HBS

Renal function: osmoregulation (6 min) https://www.youtube.com/watch?v=SDsVx6m219o

Fluid balance – water in Food 1000 ml Fluids 1200 ml Metabolic water (from respiration) 300 ml TOTAL 2500 ml Anthea Sieveking, Wellcome Images

Fluid balance – water out Urine 1200 ml Sweating 750 ml Lungs 400 ml Faeces 150 ml TOTAL 2500 ml Anthea Sieveking, Wellcome Images

Regulation of body fluids The kidneys regulate the level and composition of body fluids Functions Fluid balance Salt balance Removal of wastes (urea) pH balance interdependent 5/22/2019 3HBS

The kidney Proximal convoluting tubule Glomerulus Capsule Cortex Distal convoluting tubule Medulla Renal artery Collecting duct Renal vein Loop of Henle Renal pelvis Capillaries Ureter KIDNEY A NEPHRON

The kidney Renal medulla – loops of Henle Renal artery Renal vein Renal cortex – Bowman’s capsules Ureter – to the bladder 5/22/2019 3HBS

The kidneys The functional unit of the kidney is the nephron It filters waste products out of the blood Each kidney contains about 1.2 million nephrons 5/22/2019 3HBS

The kidneys Blood enters the glomerulus (Bowman’s capsule) under high pressure. Water and small molecules are forced out of the blood vessels This filtrate is collected and passes through several tubules Some water and useful substances are reabsorbed The rest passes through to the ureters and is stored in the bladder for excretion Renal cortex Renal medulla 5/22/2019 3HBS

Fluid regulation There are 2 hormones that regulate the body’s fluid composition: Antidiuretic hormone (ADH) Water reabsorption Pituitary gland Aldosterone Salt reabsorption Hypothalamus & adrenal glands 5/22/2019 3HBS

Selective water re-absorption The second stage of water re-absorption is important if we become dehydrated. It can be divided into two phases (though both are interdependent). The first phase involves the re-absorption of salt under the influence of the hormone aldosterone. The second phase involves the re-absorption of water under the influence of the antidiuretic hormone (ADH).

Fluid regulation ADH acts by increasing the permeability of the walls of the distal convoluted tubule and the collecting duct Aldosterone acts by increasing the activity of the sodium-potassium pumps in the loop of Henle and the distal convoluted tubule ADH – water reabsorption aldosterone – salt reabsorption 5/22/2019 3HBS

Selective reabsorption Process Structure Substance Active/passive Reabsorption PCT Water (60-70%) Salts (60-70%) Glucose (100%) Amino acids (100%) Vitamins (100%) Passive (osmosis) All active Loop of Henle Water (25%) Na+/Cl- (25%) Active DCT Water (5%) Na+/Cl- (5%) Collecting duct

Tubular secretion Tubular secretion PCT & DCT Active H+ Process Structure Substance Active/passive Tubular secretion PCT & DCT H+ NH4+ (ammonium) Creatinine Toxins Drugs Neurotransmitters Active

Reabsorption of salt - aldosterone Stimulus Decreased blood volume → reduced blood pressure Receptor Baroreceptors in Renal artery

Reabsorption of salt - aldosterone Transmission Several chemical messengers ending with release of aldosterone from the adrenal cortex Effector Sodium pumps in DCT and loop of Henle Response Sodium reabsorbed increasing ion concentration in interstitial fluid (creates osmotic gradient)

Negative feedback – aldosterone Pressoreceptors in renal artery Decreased blood volume → reduced blood pressure Stimulus Receptor Hypothalamus & adrenal glands –(aldosterone) More water is reabsorbed because of increased osmotic gradient Negative feedback Regulation Response Effector Sodium reabsorbed Sodium pumps in loop of Henle and distal convoluting tubule

Reabsorption of water - antidiuretic hormone Stimulus Decreased blood volume → reduced blood pressure → increased osmotic pressure Osmotic pressure is the minimum pressure which needs to be applied to prevent the inward flow of water across a semipermeable membrane or The tendency of a solution to take in water by osmosis. Receptor Osmoreceptors in hypothalamus → (activates thirst reflex)

Reabsorption of water - antidiuretic hormone Transmission nerve signal to posterior pituitary gland ADH released into bloodstream Effector DCT and collecting duct Response Increases permeability of above structures water (approx 10%) reabsorbed

Negative feedback – ADH Decreased blood volume → reduced blood pressure → increased osmotic pressure Osmoreceptors in hypothalamus Stimulus Receptor ADH from posterior pituitary gland Osmotic pressure maintained or reduced Thirst reflex Negative feedback Drink Regulation Water reabsorbed Response Effector Increases permeability of DCT and collecting duct DCT and collecting duct

Deamination Definition The stripping of nitrogen from amino acid and nitrogen bases (RNA) Deamination occurs in the liver Nitrogen occurs in the amino (NH2) part of an amino acid. Nitrogen is toxic to the human body and must be removed. The amino group Alanine

Deamination – chemical reaction Amino acid → ammonia + organic compounds (used for respiration) Ammonia (very toxic) + CO2 → urea (H2NCONH2)

Nitrogen wastes Nitrogen compound Source Amount Relative Toxicity Urea Amino Acids 21 g/day Moderate Creatinine Muscle metabolism 1.8 g/day High Uric acid RNA 480 mg/day Weak