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.

A heart rate that is persistently greater than 100bpm is termed tachycardia. A heart rate that is persistantly lower than 60 pulse per min  is termed bradycardia. Let's examine some factors that could cause a change in heart rate:

• Increased heart rate can be caused by:
  • Increased output of the cardioacceleratory center. In other words, greater activity of sympathetic nerves running to the heart and a greater release of norepinephrine on the heart.
  • Decreased output of the cardioinhibitory center. In other words, less vagus nerve activity and a decrease in the release of acetylcholine on the heart.
  • Increased release of the hormone epinephrine by the adrenal glands.
  • Nicotine.
  • Caffeine.
  • Hyperthyroidism - i.e., an overactive thyroid gland. This would lead to an increased amount of the hormone thyroxine in the blood.
• Decreased heart rate can be caused by:
  • Decreased activity of the cardioacceleratory center.
  • Increased activity of the cardioinhibitory center.
  • Many others.

CNS PROTECTION

- Bones of the Skull       Frontal, Temporal, Parietal, Sphenoid, Occipital

- Cranial Meninges         Dura mater, Arachnoid Space, Pia mater

- Cerebrospinal Fluid

Secreted by Chroid Plexi in Ventricles

Circulation through ventricles and central canal

Lateral and Median apertures from the 4th ventricle into the subarachnoid space

Arachnoid villi of the superior sagittal sinus return CSF to the venous circulation

Hydrocephalic Condition, blockage of the mesencephalic aqueduct, backup of CSF, Insertion of a shunt to drain the excess CSF

The Body Regulates pH in Several Ways

• Buffers are weak acid mixtures (such as bicarbonate/CO₂) which minimize pH change
  • Buffer is always a mixture of 2 compounds
    • One compound takes up H ions if there are too many (H acceptor)
    • The second compound releases H ions if there are not enough (H donor)
  • The strength of a buffer is given by the buffer capacity
    • Buffer capacity is proportional to the buffer concentration and to a parameter known as the pK
  • Mouth bacteria produce acids which attack teeth, producing caries (cavities). People with low buffer capacities in their saliva have more caries than those with high buffer capacities.
• CO₂ gas (a potential acid) is eliminated by the lungs
• Other acids and bases are eliminated by the kidneys

 Pain, Temperature, and Crude Touch and Pressure

General somatic nociceptors, thermoreceptors, and mechanoreceptors sensitive to crude touch and pressure from the face conduct signals to the brainstem over GSA fibers of cranial nerves V, VII, IX, and X.

The afferent fibers involved are processes of monopolar neurons with cell bodies in the semilunar, geniculate, petrosal, and nodose ganglia, respectively.

The central processes of these neurons enter the spinal tract of V, where they descend through the brainstem for a short distance before terminating in the spinal nucleus of V.

Second-order neurons then cross over the opposite side of the brainstem at various levels to enter the ventral trigeminothalamic tract, where they ascend to the VPM of the thalamus.

Finally, third-order neurons project to the "face" area of the cerebral cortex in areas 3, 1, and 2 .

Discriminating Touch and Pressure

Signals are conducted from general somatic mechanoreceptors over GSA fibers of the trigeminal nerve into the principal sensory nucleus of V, located in the middle pons.

Second-order neurons then conduct the signals to the opposite side of the brainstem, where they ascend in the medial lemniscus to the VPM of the thalamus.

 Thalamic neurons then project to the "face" region of areas 3, I, and 2 of the cerebral cortex.

 Kinesthesia and Subconscious Proprioception

Proprioceptive input from the face is primarily conducted over GSA fibers of the trigeminal nerve.

The peripheral endings of these neurons are the general somatic mechanoreceptors sensitive to both conscious (kinesthetic) and subconscious proprioceptive input.

Their central processes extend from the mesencephalic nucleus to the principal sensory nucleus of V in the pons

The subconscious component is conducted to the cerebellum, while the conscious component travels to the cerebral cortex.

Certain second-order neurons from the principal sensory nucleus relay proprioceptive information concerning subconscious evaluation and integration into the ipsilateral cerebellum.

Other second-order neurons project to the opposite side of the pons and ascend to the VPM of the thalamus as the dorsal trigeminothalamic tract.

Thalamic projections terminate in the face area of the cerebral cortex.

The Lymphatic System

Functions of the lymphatic system:

1) to maintain the pressure and volume of the extracellular fluid by returning excess water and dissolved substances from the interstitial fluid to the circulation.

2) lymph nodes and other lymphoid tissues are the site of clonal production of immunocompetent  lymphocytes and macrophages in the specific immune response.
 

Filtration forces water and dissolved substances from the capillaries into the interstitial fluid. Not all of this water is returned to the blood by osmosis, and excess fluid is picked up by lymph capillaries to become lymph. From lymph capillaries fluid flows into lymph veins (lymphatic vessels) which virtually parallel the circulatory veins and are structurally very similar to them, including the presence of semilunar valves.

The lymphatic veins flow into one of two lymph ducts. The right lymph duct drains the right arm, shoulder area, and the right side of the head and neck. The left lymph duct, or thoracic duct, drains everything else, including the legs, GI tract and other abdominal organs, thoracic organs, and the left side of the head and neck and left arm and shoulder.

These ducts then drain into the subclavian veins on each side where they join the internal jugular veins to form the brachiocephalic veins.

Lymph nodes lie along the lymph veins successively filtering lymph. Afferent lymph veins enter each node, efferent veins lead to the next node becoming afferent veins upon reaching it.

Lymphokinetic motion (flow of the lymph) due to:

1) Lymph flows down the pressure gradient.

2) Muscular and respiratory pumps push lymph forward due to function of the semilunar valves.

Other lymphoid tissue: 

        1. Lymph nodes: Lymph nodes are small encapsulated organs located along the pathway of lymphatic vessels. They vary from about 1 mm to 1 to 2 cm in diameter and are widely distributed throughout the body, with large concentrations occurring in the areas of convergence of lymph vessels. They serve as filters through which lymph percolates on its way to the blood. Antigen-activated lymphocytes differentiate and proliferate by cloning in the lymph nodes. 

        2. Diffuse Lymphatic Tissue and Lymphatic nodules: The alimentary canal, respiratory passages, and genitourinary tract are guarded by accumulations of lymphatic tissue that are not enclosed by a capsule (i.e. they are diffuse) and are found in  connective tissue beneath the epithelial mucosa. These cells intercept foreign antigens and then travel to lymph nodes to undergo differentiation and proliferation. Local concentrations of lymphocytes in these systems and other areas are called lymphatic nodules. In general these are single and random but are more concentrated in the GI tract in the ileum, appendix, cecum, and tonsils. These are collectively called the Gut Associated Lymphatic Tissue (GALT). MALT (Mucosa Associated Lymphatic Tissue) includes these plus the diffuse lymph tissue in the respiratory tract. 

        3. The thymus:   The thymus is where immature lymphocytes differentiate into T-lymphocytes. The thymus is fully formed and functional at birth. Characteristic features of thymic structure persist until about puberty, when lymphocyte processing and proliferation are dramatically reduced and eventually eliminated and the thymic tissue is largely replaced by adipose tissue. The lymphocytes released by the thymus are carried to lymph nodes, spleen, and other lymphatic tissue where they form colonies. These colonies form the basis of T-lymphocyte proliferation in the specific immune response. T-lymphocytes survive for long periods and recirculate through lymphatic tissues.

        The transformation of primitive or immature lymphocytes into T-lymphocytes and their proliferation in the lymph nodes is promoted by a thymic hormone called thymosin.  Ocassionally the thymus persists and may become cancerous after puberty and and the continued secretion of thymosin and the production of abnormal T-cells may contribute to some autoimmune disorders.  Conversely, lack of thymosin may also allow inadequate immunologic surveillance and thymosin has been used experimentally to stimulate T-lymphocyte proliferation to fight lymphoma and other cancers. 

        4. The spleen: The spleen filters the blood and reacts immunologically to blood-borne antigens. This is both a morphologic (physical) and physiologic process. In addition to large numbers of lymphocytes the spleen contains specialized vascular spaces, a meshwork of reticular cells and fibers, and a rich supply of macrophages which monitor the blood.  Connective tissue forms a capsule and trabeculae which contain myofibroblasts, which are contractile.  The human spleen holds relatively little blood compared to other mammals, but it has the capacity for contraction to release this blood into the circulation during anoxic stress. White pulp in the spleen contains lymphocytes and is equivalent to other lymph tissue,  while red pulp contains large numbers of red blood cells that it filters and degrades.

    The spleen functions in both immune and hematopoietic systems. Immune functions include: proliferation of lymphocytes, production of antibodies, removal of antigens from the blood. Hematopoietic functions include: formation of blood cells during fetal life, removal and destruction of aged, damaged and abnormal red cells and platelets, retrieval of iron from hemoglobin degradation, storage of red blood cells.