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Renal – Kidneys
Excretion and Osmoregulation
Chapter 18.6 page 513
Excretion and Osmoregulation
In mammals the removal of metabolic waste materials (excretion)
and much of the regulation of body fluids (osmoregulation) are
achieved by the excretory system (Renal – Kidneys)
The separation of excretory substances from blood and the
conservation of water are performed in the kidneys.
The Kidneys
The concentrations of chemicals dissolved and suspended in the
cytoplasm, intercellular fluid and circulating fluid must be
maintained within a particular range. This is achieved by
osmoregulation and the excretion of metabolic wastes. Most
animals use an excretory system to maintain the homeostasis.
Much metabolic waste material is removed from the animal
through surfaces in contact with the environment.
Carbon dioxide diffuses from the blood to the alveoli of the lungs
and through skin surfaces.
Salts may be removed through the skin; for example, by sweat in
mammals.
Salts are also secreted into the large intestine.
The kidneys have a significant role in the regulation of salt levels in
the body, along with fluid levels.
Removal of Ammonia
Amino acids derived from cell replacements and those taken in as
surplus to the body’s requirements cannot be stored, and are
broken down by the liver in vertebrates. In this process the nitrogen
containing amino group is removed. The products either enter the
respiratory pathway or are converted to a storage material and
ammonia. Ammonia is highly toxic to animal cells and must either
be rapidly removed from the body or converted to a less toxic
compound such as urea or uric acid.
Removal of Ammonia
Since mammals are primarily terrestrial animals, mammals must
conserve water and thus their nitrogenous waste material must be in a
relatively soluble, non-toxic form – they excrete urea. The separation of
excretory substances from blood and the conservation of water are
performed in a pair of kidneys
Formation of Urine
Waste materials dissolved in water (urine) passes from the kidneys
through the ureters to be stored in the bladder. The urine eventually
passes to the external environment through the urethra.
STRUCTURE AND
FUNCTION OF THE
HUMAN KIDNEY
Chapter 18.6 page 514
The excretory system consists of a pair of kidneys which remove
excretory products from the blood plasma. The urine so formed
passes through the ureters to the bladder for storage. When the
bladder is distended with urine, it releases the urine through a
sphincter of striated muscle to a median urethra which discharges
to the exterior.
Structure of the kidney
The two kidneys are found at either side of the mid-dorsal line of
the abdominal cavity. The kidney is a bean-shaped organ. It is
enclosed by a capsule of connective tissue and composed of two
layers of tissue: an outer, darker-coloured cortex and an inner,
lighter-coloured medulla, surrounding a central cavity.
Structure of the kidney
The Kidneys are connected to the circulatory system via the renal
arteries and veins and to the bladder via the ureters. Within the
central cavity the top of the ureter expands to form the pelvis. The
medulla is divided into lobes of approximately conical shape which
are called the pyramids.
Structure of the Nephron
Nephrons are functional units of the kidney. There are between 1-2
million nephrons per kidney.
Each nephron consists of an elongated tubule closely associated at
one end with a group of blood capillaries (the glomerulus) via a
cup-shaped Bowman’s capsule and opening at the other end into a
collecting duct. Many nephrons open into each collecting duct,
which drains into the pelvis and thus out through the ureter.
The nephron tubule has distinct portions. The Bowman’s
capsule, wrapping around the glomerulus, opens into the
proximal convoluted tubule which is highly coiled. This
leads to the U-shaped loop of Henle and on to the coiled
distal convoluted tubule, which opens into the collecting
duct. Only the loop of Henle and the collecting duct are in
the medulla. The rest of the nephron is contained within the
cortex of the kidney.
Proximal Convoluted Tubule
The proximal tubule as a part of the nephron can be divided into an
initial convoluted portion and a following straight (descending)
portion. Fluid in the filtrate entering the proximal convoluted
tubule is reabsorbed into the peri-tubular capillaries, including
approximately two-thirds of the filtered salt and water and all
filtered Organic solutes (primarily Glucose and Amino acids).
The Loop of Henle
The Loop of Henle, also called the nephron loop or the loop of
Hundley, is a U-shaped tube that extends from the proximal tubule.
It consists of a descending limb and ascending limb. It begins in the
cortex, receiving filtrate from the proximal convoluted tubule,
extends into the medulla as the descending limb, and then returns
to the cortex as the ascending limb to empty into the distal
convoluted tubule. The primary role of the loop of Henle is to
concentrate the salt in the tissue surrounding the loop.
The Descending Limb - Loop of Henle
Considerable differences distinguish the descending and ascending
limbs of the loop of Henle. The Descending Limb is permeable to
water and noticeably less impermeable to salt, and thus only
indirectly contributes to the concentration of the interstitium. As
the filtrate descends deeper into the hypertonic interstitium of the
renal medulla, water flows freely out of the descending limb
by osmosis until the tonicity of the filtrate and interstitium
equilibrate. Longer descending limbs allow more time for water to
flow out of the filtrate, so longer limbs make the filtrate more
hypertonic than shorter limbs.
The Ascending Limb - Loop of Henle
Unlike the descending limb, the thin ascending limb of loop of
Henle is impermeable to water, a critical feature of
the countercurrent exchange mechanism employed by the loop.
The ascending limb actively pumps sodium out of the filtrate,
generating the hypertonic interstitium that drives countercurrent
exchange. In passing through the ascending limb, the filtrate
grows hypotonic since it has lost much of its sodium content. This
hypotonic filtrate is passed to the distal convoluted tubulein the
renal cortex.
The Distal convoluted tubule
The distal convoluted tubule has a different structure and function
to that of the proximal convoluted tubule. Cells lining the tubule
have numerous mitochondria to produce enough energy (ATP) for
active transport to take place. Much of the ion transport taking
place in the distal convoluted tubule is regulated by the endocrine
system. Significant concentration of urine can occur in the DCT.