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EXCRETION. Objectives. Define and explain the importance of excretion. Identify the various parts of the urinary system and the function of each parts. Explain the function of human kidney in the removal of urea and water from the body.
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Objectives • Define and explain the importance of excretion. • Identify the various parts of the urinary system and the function of each parts. • Explain the function of human kidney in the removal of urea and water from the body. • Describe the process of dialysis using kidney machines.
9.1 Structure and function of kidneys 9.2 Kidney dialysis Learning outcomes Candidates should be able to: (a) define excretion as the removal of toxic materials and the waste products of metabolism from organisms (b) describe the removal of carbon dioxide from the lungs (c) identify on diagrams and name the kidneys, ureters, bladder, urethra and state the function of each (the function of the kidney should be described simply as removing urea and excess salts and water from the blood; details of kidney structure and nephron are not required) (d) describe dialysis in kidney machines as the diffusion of waste products and salts (small molecules) through a membrane; large molecules (e.g. protein) remain in the blood.
Metabolism • Metabolism refers to all chemical activities within the body. • These chemical activities are called catabolic activities and anabolic activities. • Catabolic activities refers to chemical processes which cause the breakdown of complex substances into simpler ones.
Metabolism • Anabolic activities refers to the building up of simpler substances into more complex ones. • Therefore, Anabolism + Catabolism = Metabolism
What is excretion? Excretion is the process by which metabolic waste products and toxic materials are removed from the body of an organism.
The human urinary system • The urinary system comprises of: - 2 kidneys (10-13 cm long, 5-7cm wide) - 2 ureters - 1 urinary bladder - 1 urethra
Left kidney Right kidney Right ureter Left ureter Bladder Sphincter Urethra Front view of the urinary system Left kidney Right kidney Right ureter Left ureter Bladder Spincter Urethra
The kidneys • Bean-shaped organs. • Attached to the dorsal body wall, one on each side of the vertebral column. • The kidneys lie just above the waistline. • Left kidney is slightly higher than the right one.
The kidneys • Concave surface of each kidney faces the vertebral column. • At the centre of this surface is a depression called the hilus. (Renal Artery, renal vein, nerves are connected to the kidney at hilus) • From the hilus, a narrow tube called the ureter emerges. • Ureters carry urine from the kidney to the urinary bladder.
The kidneys • Theurinary bladder is an elastic, muscular bag (located in front of the rectum) where urine is collected and stored temporarily. • At the bottom of the bladder is a sphincter muscle. • When the bladder is full, the sphincter muscle relaxes to allow the urine to flow into the urethra and pass out of the body.
Structure of the kidney • Longitudinal section of a mammalian kidney consists of: • An outer dark region called the cortex. • Inner pale region called the medulla- renal pyramids are located in this region. • Renal pyramids are conical structures 12-16 : radial stripes indicate nephrons • An inner region leading to the ureter, called the renal pelvis ( enlarged portion of ureter inside the kidney)
Nephron (functional unit of a kidney) page 226 • Kidneys are made up of tiny tubules called nephrons ( about 1 million) • Each nephron begins in the cortex and runs down to the pelvis. • It is in the nephron that ultrafiltration and selective reabsorption takes place and urine is formed.
Structure of a nephron page 227 • Each nephron comprises of: • Bowman’s capsule (cup-like structure) and glomerulus (mass of blood capillaries) –together called renal crorpuscle/Malpighian corpuscle • Proximal (first) and distal (second) convoluted tubules • Loop of Henlé (u shaped part of tubule) • Collecting duct (tubule opens into this- runs straight through medulla and opens into renal pelvis)
Formation of urine • Two main processes are involved in the formation of urine within each tubule: • Ultrafiltration • Selective re-absorption
Ultrafiltration • In each glomerulus, mechanical filtration occurs. This is usually called ultrafiltration. • Blood vessels bringing blood to each glomerulus (afferent arteriole) is quite wide, but the blood vessels taking blood away (efferent arteriole) is narrow. • Hence, the blood pressure in the glomerulus is high.
Ultrafiltration • Only small molecules such as water, salt, glucose and urea can go through into the tubule. • Most protein molecules, blood cells and platelets are too big to be filtered off. They remain in the glomerular capillaries.
Selective re-absorption • In a normal adult, about 120 cm3 of filtrate is formed in the kidney every minute. • To prevent dehydration, a selective re-absorption of useful materials occurs. • Wrapped around each kidney tubule are blood capillaries. They reabsorb the useful substances back to the kidney tubule.
Selective re-absorption • Most of the water is re-absorbed by osmosis. • Some mineral salts, all of the glucose and amino acids are re-absorbed in a healthy individual. • The selective re-absorption process requires energy and this process is called active transport.
Selective re-absorption • The remaining fluid continues on its way along the tubule. • By the time the fluid reaches the collecting tubule and into the renal pelvis, it is mostly water, mineral salts and nitrogenous waste products (urea). • This is called urine.
The normal constituents of urine • Average composition of normal human urine (g/100 cm3) is approximately:
The normal constituents of urine • Composition of normal urine varies considerably depending on several factors, such as diet and weather. • Abnormal constituents of urine are found in certain disease. For example, in sugar diabetes (diabetes mellitus), the urine contains a considerable amount of glucose.
Kidneys as osmoregulators • Water potential of the blood has to be kept relatively constant. • Control of water content of the body is known as osmoregulation. • The kidney, together with the hypothalamus and pituitary gland, are responsible for osmoregulation in humans.
Kidneys as osmoregulators • The hypothalamus continuously monitors the concentration of the blood. • The pituitary gland secretes the hormone called vasopressin or anti-diuretic hormone (ADH). • Anti-diuretic hormone (ADH) increases water re-absorption by the kidney tubules.
Kidney failure • Sometimes, a person’s kidney may stop working properly. This might be due to an infection in the kidney. • Prompt medical treatment have to be given and this can be done with a kidney machine. • The process involved is called dialysis.
Kidney failure • In dialysis, blood is drawn from an artery in the patient’s arm and allowed to flow through the tubing in a dialysis machine. • The dialysis machine contains fluid consisting of water and other substances such as salt.
Kidney failure • As the patient’s blood passes through the dialysis machine, it is separated from the fluid by a partially permeable membrane. • Small molecules such as urea diffused through the membrane into the fluid. • Big molecules, such as proteins and blood cells, remains in the tubing. Video clip
Kidney failure • The filtered blood is then returned to a vein in the patient’s arm. • Dialysis can be inconvenient and expensive for the patients. • A better long-term solution for kidney failure is a kidney transplant. However, there are drawbacks as well.
Summary • Excretion is the removal of metabolic waste products and toxic substances from the body. • Kidney serves as the excretory organ involved in osmoregulation, regulation of pH in blood and regulation of blood plasma composition.
Summary • Each nephron consists of a renal (Bowman’s) capsule, the glomerulus, first (proximal) convoluted tubule, loop of Henlé and second (distal) convoluted tubule). • Urine is formed by ultrafiltration and selective reabsorption. • The hormone ADH regulates the amount of water reabsorbed by the kidney tubules.
