Typical Concentration Gradients and Membrane Potentials in Excitable Cells

The Na Pump is Particularly Important in the Kidney and Brain

• All cells have Na pumps in their membranes, but some cells have more than others
• Over-all Na pump activity may account for a third of your resting energy expenditure!
• In the kidney the Na pump activity is very high because it is used to regulate body salt and water concentrations
  • Kidneys use enormous amounts of energy: 0.5% of body weight, but use 7% of the oxygen supply
• Pump activity is also high in the brain because Na and K gradients are essential for nerves
  • The brain is another high energy organ; it is 2% of body weight, but uses 18% of the oxygen supply

In the Resting State Potassium Controls the Membrane Potential of Most Cells

• Resting cells have more open K channels than other types
• More K+ passes through membrane than other ions- therefore K+ controls the potential
• Blood K+ must be closely controlled because small changes will produce large changes in the membrane potentials of cells
  • Raising K will make the membrane potential less negative (depolarization)
• High blood K+ can cause the heart to stop beating (it goes into permanent contraction)

During an Action Potential Na Channels Open, and Na Controls the Membrane Potential

• Whichever ion has the most open channels controls the membrane potential
• Excitable cells have Na channels that open when stimulated
• When large numbers of these channels open Na controls the membrane potential

Tubular secretion:

Involves transfer of substances from peritubular capillaries into the tubular lumen. It  involves transepithelial transport in a direction opposite to that of tubular absorption.

Renal tubules can selectively add some substances that have not been filtered to the substances that already have been filtered via tubular secretion.

Tubular secretion mostly function to eliminate foreign  organic ions, hydrogen ions ( as a contribution to acid base balance ), potassium ions ( as a contribution to maintaining optimal plasma K+ level to assure normal proceeding of neural and muscular functions), and urea.
Here we will focus on K+ secretion and will later discuss H+ secretion in acid base balance, while urea secretion will be discussed in water balance.

K+ is filtered in glomerular capillaries and then reabsorbed in proximal convoluted tubules as well as in thick ascending limb of loop of Henley ( Na-2Cl-K symporter)

K+ secretion takes place in collecting tubules (distal nephron) . There are two types of cells in distal nephron:

- Principal cells that reabsorb sodium and secrete K+ .
- Intercalated cells that reabsorb K+ in exchange with H+.

Mechanism of secretion of K+ in principal cells : Two steps

- K+ enters tubular cells by Na/K ATPase on the basolateral membrane.
- K+ leaves the tubular cells via K+ channels in apical membrane.

Aldosterone is a necessary regulatory factor.

If there is increased level of K+ in plasma,excessive K+ is secreted , some of which is reabsorbed back to the plasma in exchange with H+ via the intercalated cells.        

Characteristics of Facilitated Diffusion & Active Transport - both require the use of carriers that are specific to particular substances (that is, each type of carrier can 'carry' one type of substance) and both can exhibit saturation (movement across a membrane is limited by number of carriers & the speed with which they move materials

An anti-diruetic is a substance that decreases urine volume, and ADH is the primary example of it within the body. ADH is a hormone secreted from the posterior pituitary gland in response to increased plasma osmolarity (i.e., increased ion concentration in the blood), which is generally due to an increased concentration of ions relative to the volume of plasma, or decreased plasma volume.

The increased plasma osmolarity is sensed by osmoreceptors in the hypothalamus, which will stimulate the posterior pituitary gland to release ADH. ADH will then act on the nephrons of the kidneys to cause a decrease in plasma osmolarity and an increase in urine osmolarity.

ADH increases the permeability to water of the distal convoluted tubule and collecting duct, which are normally impermeable to water. This effect causes increased water reabsorption and retention and decreases the volume of urine produced relative to its ion content.

After ADH acts on the nephron to decrease plasma osmolarity (and leads to increased blood volume) and increase urine osmolarity, the osmoreceptors in the hypothalamus will inactivate, and ADH secretion will end. Due to this response, ADH secretion is considered to be a form of negative feedback.

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.

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