Urinary system
Nephron
From the renal artery, an afferent arteriole transports blood to the
glomerulus, a knot of capillaries inside the glomerular capsule. Blood
leaving the glomerulus is carried away by the efferent arteriole. Blood
pressure is higher in the glomerulus because the efferent arteriole is
narrower than the afferent arteriole. The efferent arteriole divides and
forms The peritubular capillary network, which surrounds the rest of
the nephron. Blood from the efferent arteriole travels through the
peritubular capillary network. Thenthe blood goes into a venule that
carries blood into the renal vein
Each nephron is made up of several parts Some functions are shared by
all parts of the nephron. However, the specifi c structure of each part is
especially suited to a particular function First, the closed end of the
nephron is pushed in on itself to form a cuplike structure called the
glomerular capsule (Bowman’s capsule). The outer layer of the
glomerular capsule is composed of squamous epithelialcells. The inner
layer is made up of podocytes that have long cytoplasmic extensions. The
podocytes cling to the capillary walls of the glomerulus and leave pores
that allow easy passage of small molecules from the glomerulus to the
inside of the glomerular capsule. This process ,called glomerular
filtration, produces a filtrate of the blood. Next, there is a proximal
convoluted tubule. The cuboidal epithelial cells lining this part of the
nephron have numerous microvilli, about 1 μm in length, that are tightly
packed and form a brush border
A brush border greatly increases the surface area for the tubular
reabsorption of filtrate components. Each cell also has many
mitochondria, which can supply energy for active transport of molecules
from the lumen to the peritubular capillary network. the tube narrows and
makes a U-turn called the loop of the nephron (loop of Henle). Each
loop consists of a descending limb and an ascending limb. The
descending limb of the loop allows water to diffuse into tissue
surrounding the nephron The ascending limb actively transports salt from
its lumen to interstitial tissue. As we shall see, this activity
facilitates the reabsorption of water by the nephron and collecting duct
The cuboidal epithelial cells of the distal convoluted tubule have
numerous mitochondria, but they lack microvilli This means that the
distal convoluted tubule is not specialized for reabsorption. Instead, its
primary function is ion exchange. During ion exchange, cells reabsorb
certain ions, returning them to the blood. Other ions are secreted from
the blood into the tubule. The distal convoluted tubules of several
nephrons enter one collecting duct. Many collecting ducts carry urine to
the renal pelvis
Urine Formation
1-Glomerular filtration. Due to glomerular blood pressure, water and
small molecules move from the glomerulus to the inside of the
glomerular capsule. large molecules and formed elements are unable to
pass through glomerulus . The glomerular fi ltrate inside the glomerular
capsule now contains the fi lterable blood components. nephrons in the
kidneys filter 180 liters of water per day, along with a considerable
amount of small molecules (such as glucose) and ions (such as sodium).
2-Tubular reabsorption occurs as molecules and ions are passively and
actively reabsorbed from the nephron into the blood of the peritubular
capillary network. The osmolarity of the blood is maintained by the
presence of plasma proteins and salt. When sodium ions (Na+) are
actively reabsorbed, chloride ions (Cl–) follow passively. The
reabsorption of salt (NaCl) increases the osmolarity of the blood
compared with the fi ltrate. Therefore, water moves passively from the
tubule into the blood. About 65% of Na+ is reabsorbed at the proximal
convoluted tubule. Nutrients such as glucose and amino acids return to
the peritubular capillaries almost exclusively at the proximal convoluted
tubule This is a selective process because only molecules recognized by
carrier proteins are actively reabsorbed. Glucose is an example of a
molecule that ordinarily is completely reabsorbed because there is a
plentiful supply of carrier proteins for it. However, every substance has a
maximum rate of transport. After all its carriers are in use, any excess in
the fi ltrate will appear in the urine. In diabetes mellitus, the blood
glucose level is above normal and glucose appears in the urine. The
presence of excess glucose in the fi ltrate raises its osmolarity. Therefore,
less water is reabsorbed into the peritubular capillary network. The
frequent urination and increased thirst experienced by people with
untreated diabetes are due to less water being reabsorbed from the fi ltrate
into the blood.
3-Tubular secretion is a second way by which substances are removed
from blood and added to the tubular fluid. Hydrogen ions (H+),
creatinine, and drugs such as penicillin are some of the substances moved
by active transport from blood into the kidney tubule. In the end, urine
contains substances that have undergone glomerular filtration but have
not been reabsorbed and substances that have undergone tubular
secretion.
Immunity
. Immunity is the ability of the body to defend itself against infectious
agents, foreign cells, and even abnormal body cells, such as cancer cells.
Thereby, the internal environment has a better chance of remaining
stable. Immunity includes nonspecific and specific defenses. The
four types of nonspecific defenses—barriers to entry, the inflammatory
reaction, natural killer cells, and protective proteins— are effective
against many types of infectious agents
-1Barriers to Entry
1-Skin and Mucous Membranes The intact skin is generally an
effective physical barrier thatprevents infection. Mucous membranes
lining the respiratory, digestive, reproductive, and urinary tracts are
also physical barriers to entry by pathogens. For example, the
ciliated cells that line the upper respiratory tract sweep mucus and
trapped particles up into the throat, where they can be swallowed,
spit, or coughed out.
2-Chemical Barriers The chemical barriers to infection include the
secretions of sebaceous (oil) glands of the skin. These secretions
contain chemicals that weaken or kill certain bacteria on the skin.
Perspiration, saliva, and tears contain an antibacterial enzyme called
lysozyme. Saliva also helps to wash microbes of the teeth and tongue,
and tears wash the eyes. Similarly, as urine is voided from the body,
it fl ushes bacteria from the urinary tract.The acid pH of the stomach
inhibits growth or kills many types of bacteria. At one time, it was
thought that no bacterium could survive the acidity of the stomach.
But now we know that ulcers are caused by the bacterium
Helicobacter pylori
2 Natural Killer Cells
Natural killer (NK) cells kill virus-infected cells and tumor cells by cellto-cell contact. They are large, granular lymphocytes. They have no
specificity and no memory. Their number is not increased by
immunization.
3 Protective Proteins The complement system, often simply called
complement, is a number of plasma proteins designated by the letter C
Complement is activated when pathogens enter the body, it is involved in
and amplifies the inflammatory response because
1- complement proteins attract phagocytes to the scene.
2- Some complement proteins bind to the surface of pathogens already
coated with antibodies, which ensures that the pathogens will be
phagocytized by a neutrophil or macrophage
3- Certain other complement proteins join to form a membrane attack
complex that produces holes in the walls and plasma membranes of
bacteria. Fluids and salts then enter the bacterial cell to the point that they
burst
4- inflammatory response (as demonsterated in diagram below)
Specific Defenses
When nonspecific defenses have failed to prevent an infection specific
defenses come into play. An antigen is any foreign substance (often a
protein or polysaccharide) that stimulates the immune system to react to
itLymphocytes are capable of recognizing an antigen because they have
antigen receptors—plasma membrane receptor proteins whose shape
allows them to combine with a specific antigen.
. Immunity is primarily the result of the action of the B lymphocytes and
the T lymphocytes. B lymphocytes mature in the bone marrow,1 and T
lymphocytes mature in the thymus gland. B lymphocytes, also called B
cells, give rise to plasma cells, which produce antibodies, proteins
shaped like the antigen receptor and capable of combining with a specific
antigen. These antibodies are secreted into the blood, lymph, and other
body fluids. In contrast, T lymphocytes, also called T cells, do not
produce antibodies.
Instead, certain T cells directly attack cells that bear nonself antigens.
Other T cells regulate the immune response
Immunity occurs naturally through infection or is brought about artifi
cially by medical intervention. The two types of acquired immunity are
active and passive. In active immunity the individual alone produces
antibodies against an antigen. Active immunity sometimes develops
naturally after a person
is infected with a pathogen. However, active immunity is often induced
when a person is well to prevent future infection. Artifi cial exposure to
an antigen through immunization
can prevent future disease. The United States is committed to immunizing
all children against the common types of childhood disease
Immunization involves the use of vaccines, substances that contain an
antigen to which the immune system responds. Passive Immunity
Passive immunity occurs when an individual is given prepared antibodies
or immune cells to combat a disease These antibodies are not produced
by the individual’s plasma cells, so passive immunity is temporary. For
example newborn infants are passively immune to some diseases because
IgG antibodies have crossed the placenta from the mother’s blood). These
antibodies soon disappear, and within a few months, infants become more
susceptible to infections. Breast-feeding prolongs the natural passive
immunity an infant receives from the mother because IgG and IgA
antibodies are present in the mother’s milk