Quote from Janine SNLCC
My book is not doing a good job explaining it...can anyone help?
Hmmm... let me see...
The kidneys have a very rich blood supply. They need to have large quantities of blood passing through so that it can be filtered well, and all the waste products removed. A blood supply of about 600-1300 ml/ minute, which is 20-25% of the cardiac output, flows to the two kidneys. That’s about 72 liters per hour.
Inside the kidney, the blood is cleaned (or filtered) through very small networks of tubes called nephrons. Each kidney has about 1 million nephrons.
The nephron is the functional and structural unit of the kidney. Each one can function independently. They are contained in the renal cortex and medulla.
The function of the nephon is to cleanse
the blood by filtration, reabsorption, and secretion.
The nephrons also must concentrate
the urine to allow for filtration with maximal efficiency, with less fluid loss.
As it filters the blood of impurities, the nephron forms urine from the blood. As the blood passes through the blood vessels of the nephron, all unwanted waste is taken away, as the nephrons converge into a collecting duct, which eventually merges into a pyramid and empties via the papilla into a minor calyx.
Any nutrients needed by the body (such as glucose, amino acids, sodium) are kept or returned to the bloodstream by the nephrons. In this way, the kidney helps to regulate the levels of chemicals in the blood such as sodium and potassium. So the right levels are maintained to keep the body healthy.
A nephron is composed of a renal corpuscle and a tubular system.
The renal corpuscle consists of a glomerulus, which is a tuft of capillaries. The glomerulus is completely surrounded by the thin membranous glomerular capsule (or Bowman’s capsule).
Inside the glomerulus, waste products in the blood move by filtration
from the bloodstream into the Bowman’s capsule. The Bowman’s capsule collects the filtrate and channels it into urine-carrying tubes inside the nephron. These tubes are called tubules.
The tubular system consists of the proximal convoluted tubule, the loop of Henle, and the distal convoluted tubule. The distal convoluted tubule ends in one of many collecting ducts.
The nephrons collect and eliminate wastes from the body in a three-step process: glomerular filtration, tubular reabsorption, and tubular secretion.
—filtering the blood that flows through the kidney’s blood vessels, or glomeruli. This is a passive process
, fueled by blood pressure
Filtration is the movement of fluid through a cell or blood vessel membrane because of hydrostatic or water-pushing pressure differences on both sides of the membrane.
Glomerular filtration is non-selective (this means that the body has no control over what is filtered, the glomerulus is like a sieve & any molecules small enough will pass through) and it is passive (it needs no ATP energy expenditure to occur).
—reabsorbing filtered fluid through the minute canals (tubules) that make up the kidney. Reabsorption occurs by active transport
(substances such as glucose, amino acids, bicarb, electrolytes) or by osmosis
(water). Active transport is "against" the concentration gradient and requires expenditure of APT energy. Osmosis is a passive process, the diffusion of water from an area of higher water concentration to an area of lower water concentration.
Tubular reabsorption is the returning of essential materials (such as bicarb) from the urine filtrate back into circulation.
It is the tubular reabsorption of most of the filtrate that keeps normal urine output at 1-3 liters/day and prevents dehydration. The tubules reabsorb more than 99% of all filtered water back into the body. The tubules have the ability to concentrate urine during times of low fluid intake. This allows for maximum excretion of waste products with less fluid loss.
—release of filtered substances by the tubules into the urine. This is the reverse of re-absorption. Solutes move from the peritubular capillaries into the lumen of the nephron, by both active (active transport) and passive transfer (diffusion)
Tubular secretion is excreting nonessential substances (such as hydrogen ions) out of the blood stream into the urine filtrate to be excreted from the body. Tubular secretion regulates electrolytes and pH.
Potassium (K+) and hydrogen ions (H+) are some of the substances secreted from the blood into the tubular filtrate to maintain homeostasis of electrolytes and pH.
Reabsorption and secretion occur along the entire length of the tubule, causing numerous changes in the composition of the glomerular filtrate, according to the needs of the body at the moment, as the filtrate moves through the tubules.
So, you see, you have both active and passive processes occurring... Hope this helps