Excretion

Syllabus links Plant Excretion The role of leaves as excretory organsof plants The Excretory System in the Human Role of the excretory system in homeostasis. Function, location and excretory products of the lungs, skin, and urinary system.

Excretion “Excretion is the removal from the body of the toxic waste products of metabolism”

Excretion in plants Excretion is a term normally associated with animals The equivalent term in plants is the loss of waste (gases)

Excretion and homeostasis Excretion helps in homeostasis as follows: Removes excess water, salts and wastes from the body By controlling the salt/water balance it acts to regulate osmosis Organs of excretion in humans: Skin (excretes salts and water as sweat) Lungs (excrete carbon-dioxide and water) Kidneys (excrete water, salts, and urea in the form of urine)

Syllabus links The Excretory System in the Human Macrostructure and basis function of the urinary excretory system in humans (kidney, ureters, urinary bladder, and urethra).

Urinary System

KidneysFunctions Excretion: Nitrogenous wastes (urea, uric acid), excess salts, excess water Osmoregulation: Maintaining the blood at a suitable constant concentration Homeostasis: Maintaining a suitable constant internal environment to sustain efficient metabolism

Syllabus links The Excretory System in the Human Role of the kidney in regulating body fluids. Identification of the site of filtration. Reabsorption in the cortex, in the medulla and renal pelvis. Description of the pathway of urine from the kidney to the urethra.

Kidney StructureLS Kidney Structure LS

Syllabus links H The Nephron as a Unit of Kidney Function The nephron structure and its associated blood supply. Formation of urine: Bowman’s capsule, passage of glomerular filtrate through the proximal convoluted tubule, where reabsorption of required body substances takes place – glucose, amino acids, some salts and water reabsorbed into the blood by osmosis, diffusion, and active transport. More water reabsorbed in the Loop of Henle and the distal convoluted tube. Urine passes into the pelvis of the kidney and to the bladder for storage. Reabsorption of water in the collecting duct is under hormonal influence (ADH). Its action depends on the water content of the blood. (No further details required).

Nephron The nephron or renal tubule is the functional unit of the kidney The nephron has a number of functionally distinct parts Each human kidney has about one million nephrons Urine is manufactured by the nephrons Production of Urine: Filtration and Selective Reabsorption

Details of Urine Formation - Filtration Glomerular filtrate composition is water, glucose, amino acids, vitamins, salts, urea, and uric acid About 20% of the plasma volume passes out of the glomerulus The filtration is much higher than expected The blood pressure is unusually high in the glomerulus The blood pressure is generated by the pumping action of the heart

The high blood pressure in the glomerulus is due to: The efferent arteriole is narrower than the afferent arteriole creating a high pressure The surface area in the capillaries is large increasing the area for filtration The walls of the glomerular capillaries are more porous than normal capillaries The Bowman’s capsule is only 1 cell thick The higher than normal filtration at the glomerular capillaries is known as ultrafiltration 180 litres of glomerular filtrate are formed every 24 hours – 4 to 5 times the fluid content of the body!!

Proximal Convoluted Tubule (PCT) - Reabsorption Total reabsorption of glucose and amino acids Four fifths of the salts and water are reabsorbed Glucose, amino acids and salts are reabsorbed by active transport Water is reabsorbed by osmosis The cells lining the PCT are rich in mitochondria, which supply the ATP for active transport

The Loop of Henle - Reabsorption This structure allows the kidney to reabsorb extra water in times of water stress As a result it is possible for the kidney to produce hypertonic urine, i.e., more concentrated than blood plasma A Loop of Henle is only present in mammals and birds — the only animals able to produce hypertonic urine About 5% of the water from the glomerular filtrate is reabsorbed from the Loop of Henle by osmosis

Distal Convoluted Tubule (DCT) - Reabsorption Reabsorption of water is by osmosis The amount varies depending on the need of the body Water reabsorption by the DCT is under the influence of ADH (antidiuretic hormone) Reabsorption of salt is by active transport The amount of salt reabsorbed depends on the needs of the body The role of the DCT is crucial in osmoregulation Osmoregulation is a major process in homeostasis

Secretion Some substances pass from the blood to the nephron K +, H + and NH 4 + ions These ions are secreted by a combination of diffusion and active transport

Summary of nephron functions Location% Water reabsorbed Salts reabsorbed Proximal tubule80Most Descending limb of loop of Henle 50 Ascending limb of loop of Henle 0Some Distal tubule10Some Collecting duct4.9None

Summary of the main functions of the nephron

Osmoregulation “Blood concentration is kept in check by varying the amount of water and salt reabsorbed by the kidneys nephrons”

Blood Concentration Rising Cause: salty food, water loss due to sweating, inadequate water intake Response: increases water reabsorption, decreases salt reabsorption Blood Concentration Falling Cause: excessive water intake, cold weather (sweating less than usual), diet very low on salt Response: decreases water reabsorption, increases salt reabsorption Note: the greater the excess protein in the diet the greater is the urea content of the urine

Regulation of Body Fluids by the Kidney The kidney maintains the blood at the correct composition and concentration by excretion and osmoregulation As a result all the other body fluids are kept at optimum condition i.e. tissue fluid and cell cytoplasm

Control of urine volume