The Kidneys

The kidneys are the primary functional organ of the renal system.

They are essential in homeostatic functions such as the regulation of electrolytes, maintenance of acid–base balance, and the regulation of blood pressure (by maintaining salt and water balance).

They serve the body as a natural filter of the blood and remove wastes that are excreted through the urine.

They are also responsible for the reabsorption of water, glucose, and amino acids, and will maintain the balance of these molecules in the body.

In addition, the kidneys produce hormones including calcitriol, erythropoietin, and the enzyme renin, which are involved in renal and hemotological physiological processes.

Anatomical Location

The kidneys are a pair of bean-shaped, brown organs about the size of your fist. They are covered by the renal capsule, which is a tough capsule of fibrous connective tissue.

Right kidney being slightly lower than the left, and left kidney being located slightly more medial than the right.

The right kidneys lie  just below the diaphragm and posterior to the liver, the left below the diaphragm and posterior to the spleen.

Resting on top of each kidney is an adrenal gland (adrenal meaning on top of renal), which are involved in some renal system processes despite being a primarily endocrine organ.

They are considered retroperitoneal, which means that they lie behind the peritoneum, the membrane lining of the abdominal cavity.

The renal artery branches off from the lower part of the aorta and provides the blood supply to the kidneys.

 Renal veins take blood away from the kidneys into the inferior vena cava.

The ureters are structures that come out of the kidneys, bringing urine downward into the bladder.

Internal Anatomy of the Kidneys

There are three major regions of the kidney:

1.         Renal cortex

2.         Renal medulla

3.         Renal pelvis

The renal cortex is a space between the medulla and the outer capsule.

The renal medulla contains the majority of the length of nephrons, the main functional component of the kidney that filters fluid from blood.

The renal pelvis connects the kidney with the circulatory and nervous systems from the rest of the body.

Renal Cortex

The kidneys are surrounded by a renal cortex

The cortex provides a space for arterioles and venules from the renal artery and vein, as well as the glomerular capillaries, to perfuse the nephrons of the kidney. Erythropotein, a hormone necessary for the synthesis of new red blood cells, is also produced in the renal cortex.

Renal Medulla

The medulla is the inner region of the parenchyma of the kidney. The medulla consists of multiple pyramidal tissue masses, called the renal pyramids, which are triangle structures that contain a dense network of nephrons.

At one end of each nephron, in the cortex of the kidney, is a cup-shaped structure called the Bowman's capsule. It surrounds a tuft of capillaries called the glomerulus that carries blood from the renal arteries into the nephron, where plasma is filtered through the capsule.

After entering the capsule, the filtered fluid flows along the proximal convoluted tubule to the loop of Henle and then to the distal convoluted tubule and the collecting ducts, which flow into the ureter. Each of the different components of the nephrons are selectively permeable to different molecules, and enable the complex regulation of water and ion concentrations in the body.

Renal Pelvis

The renal pelvis contains the hilium. The hilum is the concave part of the bean-shape where blood vessels and nerves enter and exit the kidney; it is also the point of exit for the ureters—the urine-bearing tubes that exit the kidney and empty into the urinary bladder. The renal pelvis connects the kidney to the rest of the body.

Supply of Blood and Nerves to the Kidneys

•  The renal arteries branch off of the abdominal aorta and supply the kidneys with blood. The arterial supply of the kidneys varies from person to person, and there may be one or more renal arteries to supply each kidney.

•  The renal veins are the veins that drain the kidneys and connect them to the inferior vena cava.

•  The kidney and the nervous system communicate via the renal plexus. The sympathetic nervous system will trigger vasoconstriction and reduce renal blood flow, while parasympathetic nervous stimulation will trigger vasodilation and increased blood flow.

•  Afferent arterioles branch into the glomerular capillaries, while efferent arterioles take blood away from the glomerular capillaries and into the interlobular capillaries that provide oxygen to the kidney.

•  renal vein

The veins that drain the kidney and connect the kidney to the inferior vena cava.

•  renal artery

These arise off the side of the abdominal aorta, immediately below the superior mesenteric artery, and supply the kidneys with blood.

SPECIAL VISCERAL AFFERENT (SVA) PATHWAYS

Taste

Special visceral afferent (SVA) fibers of cranial nerves VII, IX, and X conduct signals into the solitary tract of the brainstem, ultimately terminating in the nucleus of the solitary tract on the ipsilateral side.

Second-order neurons cross over and ascend through the brainstem in the medial lemniscus to the VPM of the thalamus.

Thalamic projections to area 43 (the primary taste area) of the postcentral gyrus complete the relay.

SVA VII fibers conduct from the chemoreceptors of taste buds on the anterior twothirds of the tongue, while SVA IX fibers conduct taste information from buds on the posterior one-third of the tongue.

SVA X fibers conduct taste signals from those taste cells located throughout the fauces.

Smell

The smell-sensitive cells (olfactory cells) of the olfactory epithelium project their central processes through the cribiform plate of the ethmoid bone, where they synapse with mitral cells. The central processes of the mitral cells pass from the olfactory bulb through the olfactory tract, which divides into a medial and lateral portion The lateral olfactory tract terminates in the prepyriform cortex and parts of the amygdala of the temporal lobe.

These areas represent the primary olfactory cortex. Fibers then project from here to area 28, the secondary olfactory area, for sensory evaluation. The medial olfactory tract projects to the anterior perforated sub­stance, the septum pellucidum, the subcallosal area, and even the contralateral olfactory tract.

Both the medial and lateral olfactory tracts contribute to the visceral reflex pathways, causing the viscerosomatic and viscerovisceral responses.

Bronchitis = Irreversible Bronchioconstriction
 .    Causes - Infection, Air polution, cigarette smoke

a.    Primary Defect = Enlargement & Over Activity of Mucous Glands, Secretions very viscous
b.    Hypertrophy & hyperplasia, Narrows & Blocks bronchi, Lumen of airway, significantly narrow
c.    Impaired Clearance by mucocillary elevator
d.    Microorganism retension in lower airways,Prone to Infectious Bronchitis, Pneumonia
e.    Permanent Inflamatory Changes IN epithelium, Narrows walls, Symptoms, Excessive sputum, coughing
f.    CAN CAUSE EMPHYSEMA

Blood Transfusions

• Some of these units ("whole blood") were transfused directly into patients (e.g., to replace blood lost by trauma or during surgery).
• Most were further fractionated into components, including:
  • RBCs. When refrigerated these can be used for up to 42 days.
  • platelets. These must be stored at room temperature and thus can be saved for only 5 days.
  • plasma. This can be frozen and stored for up to a year.

safety of donated blood

A variety of infectious agents can be present in blood.

• viruses (e.g., HIV-1, hepatitis B and C, HTLV, West Nile virus
• bacteria like the spirochete of syphilis
• protozoans like the agents of malaria and babesiosis
• prions (e.g., the agent of variant Crueutzfeldt-Jakob disease)

and could be transmitted to recipients. To minimize these risks,

• donors are questioned about their possible exposure to these agents;
• each unit of blood is tested for a variety of infectious agents.

Most of these tests are performed with enzyme immunoassays (EIA) and detect antibodies against the agents. blood is now also checked for the presence of the RNA of these RNA viruses:

• HIV-1
• hepatitis C
• West Nile virus

• by the so-called nucleic acid-amplification test (NAT).

Red blood cell cycle:

RBCs enter the blood at a rate of about 2 million cells per second. The stimulus for erythropoiesis is the hormone erythropoietin, secreted mostly by the kidney. RBCs require Vitamin B12, folic acid, and iron. The lifespan of RBC averages 120 days. Aged and damaged red cells are disposed of in the spleen and liver by macrophages. The globin is digested and the amino acids released into the blood for protein manufacture; the heme is toxic and cannot be reused, so it is made into bilirubin and removed from the blood by the liver to be excreted in the bile. The red bile pigment bilirubin oxidizes into the green pigment biliverdin and together they give bile and feces their characteristic color. Iron is picked up by a globulin protein (apotransferrin) to be transported as transferrin and then stored, mostly in the liver, as hemosiderin or ferritin. Ferritin is short term iron storage in constant equilibrium with plasma iron carried by transferrin. Hemosiderin is long term iron storage, forming dense granules visible in liver and other cells which are difficult for the body to mobilize.

Some iron is lost from the blood due to hemorrhage, menstruation, etc. and must be replaced from the diet. On average men need to replace about 1 mg of iron per day, women need 2 mg. Apotransferrin (transferrin without the iron) is present in GI lining cells and is also released in the bile. It picks up iron from the GI tract and stimulates receptors on the lining cells which absorb it by pinocytosis. Once through the mucosal cell iron is carried in blood as transferrin to the liver and marrow. Iron leaves the transferrin molecule to bind to ferritin in these tissues. Most excess iron will not be absorbed due to saturation of ferritin, reduction of apotransferrin, and an inhibitory process in the lining tissue.

Erythropoietin Mechanism:

Myeloid (blood producing) tissue is found in the red bone marrow located in the spongy bone. As a person ages much of this marrow becomes fatty and ceases production. But it retains stem cells and can be called on to regenerate and produce blood cells later in an emergency. RBCs enter the blood at a rate of about 2 million cells per second. The stimulus for erythropoiesis is the hormone erythropoietin, secreted mostly by the kidney. This hormone triggers more of the pleuripotential stem cells (hemocytoblasts) to follow the pathway to red blood cells and to divide more rapidly.

It takes from 3 to 5 days for development of a reticulocyte from a hemocytoblast. Reticulocytes, immature rbc, move into the circulation and develop over a 1 to 2 day period into mature erythrocytes. About 1 to 2 % of rbc in the circulation are reticulocytes, and the exact percentage is a measure of the rate of erythropoiesis.

Plasma:  is the straw-colored liquid in which the blood cells are suspended.

Plasma transports materials needed by cells and materials that must be removed from cells:

• various ions (Na⁺, Ca²⁺, HCO₃⁻, etc.
• glucose and traces of other sugars
• amino acids
• other organic acids
• cholesterol and other lipids
• hormones
• urea and other wastes

Most of these materials are in transit from a place where they are added to the blood

• exchange organs like the intestine
• depots of materials like the liver

to places where they will be removed from the blood.

• every cell
• exchange organs like the kidney, and skin.