Unit-12 Excretion

04.27.2020
08:56:32
Biology/ grade-9- Today’s topic - The human excretory system-: course book- pages 156 and 157.
Main points-
The kidney tissue consists of many capillaries and tiny tubes, called renal tubules, held together with connective tissue.
The cortex is the dark, outer region.
The medulla is the lighter, inner zone.
A renal artery carries blood to the kidney and a renal vein carries it away.
The ureter carries urine from the kidney to the bladder.
Where the ureter joins the kidney there is a space called the pelvis.


The renal artery divides up into a great many arterioles and capillaries, mostly in the cortex.
Each arteriole leads to a glomerulus. This is a capillary repeatedly divided and coiled, making a knot of vessels.
Each glomerulus is almost entirely surrounded by a cup-shaped organ called a renal capsule, which leads to a coiled renal tubule.
This tubule, after a series of coils and loops, joins a collecting duct, which passes through the medulla to open into the pelvis.
A nephron is a single glomerulus with its renal capsule, renal tubule and blood capillaries.


Function of the kidneys:
The blood pressure in a glomerulus causes part of the blood plasma to leak through the capillary walls.
The red blood cells and the plasma proteins are too big to pass out of the capillary, so the fluid that does filter through is plasma without the protein.
The fluid thus consists mainly of water with dissolved salts, glucose, urea and uric acid.
The process by which the fluid is filtered out of the body by the glomerulus is called ultrafiltration.
The filtrate from the glomerulus collects in the renal capsule and trickles down the renal tubule.
As it does so, the capillaries that surround the tubule absorb back into the blood these substances which the body needs.
First, all the glucose is reabsorbed, with much of the water.
Then some of the salts are taken back to keep the correct concentration in the blood.
The process of absorbing back the substances needed by the body is called selective reabsorption.
The molecules which are not selectively reabsorbed continue along the nephron tubule as urine .

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