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.        

Cardiac Control: The Cardiac Center in the medulla.

Outputs:

The cardioacceleratory center sends impulses through the sympathetic nervous system in the cardiac nerves. These fibers innervate the SA node and AV node and the ventricular myocardium. Effects on the SA and AV nodes are an increase in depolarization rate by reducing the resting membrane polarization. Effect on the myocardium is to increase contractility thus increasing force and therefore volume of contraction. Sympathetic stimulation increases both rate and volume of the heart.

The cardioinhibitory center sends impulses through the parasympathetic division, the vagus nerve, to the SA and AV nodes, but only sparingly to the atrial myocardium, and not at all to ventricular myocardium. Its effect is to slow the rate of depolarization by increasing the resting potential, i.e. hyperpolarization.

The parasympathetic division controls the heart at rest, keeping its rhythm slow and regular. This is referred to as normal vagal tone. Parasympathetic effects are inhibited and the sympathetic division exerts its effects during stress, i.e. exercise, emotions, "fight or flight" response, and temperature.

Inputs to the Cardiac Center:

Baroreceptors in the aortic and carotid sinuses. The baroreceptor reflex is responsible for the moment to moment maintenance of normal blood pressure.

Higher brain (hypothalamus): stimulates the center in response to exercise, emotions, "fight or flight", temperature.

Intrinsic Controls of the Heart:

Right Heart Reflex - Pressoreceptors (stretch receptors) in the right atrium respond to stretch due to increased venous return. The reflex acts through a short neural circuit to stimulate the sympathetic nervous system resulting in increased rate and force of contraction. This regulates output to input

The Frank-Starling Law - (Starling's Law of the Heart) - Like skeletal muscle the myocardium has a length tension curve which results in an optimum level of stretch producing the maximum force of contraction. A healthy heart normally operates at a stretch less than this optimum level and when exercise causes increased venous return and increased stretch of the myocardium, the result is increased force of contraction to automatically pump the increased volume out of the heart. I.e. the heart automatically compensates its output to its input.

An important relationship in cardiac output is this one:

Blood Flow =  D Pressure / Resistance to Blood Flow      

• There Are 12 Pairs of Cranial Nerves

• The 12 pairs of cranial nerves emerge mainly from the ventral surface of the brain
• Most attach to the medulla, pons or midbrain
• They leave the brain through various fissures and foramina of the skull

• Nerve	Name	Sensory	Motor	Autonomic Parasympathetic
  I	Olfactory	Smell
  II	Optic	Vision
  III	Oculomotor	Proprioception	4 Extrinsic eye muscles	Pupil constriction Accomodation Focusing
  IV	Trochlear	Proprioception	1 Extrinsic eye muscle (Sup.oblique)
  V	Trigeminal	Somatic senses (Face, tongue)	Chewing
  VI	Abducens	Proprioception	1 Extrinsic eye muscle (Lat. rectus)
  VII	Facial	Taste Proprioception	Muscles of facial expression	Salivary glands Tear glands
  VIII	Auditory (Vestibulocochlear)	Hearing, Balance
  IX	Glossopharyngeal	Taste Blood gases	Swallowing Gagging	Salivary glands
  X	Vagus	Blood pressure Blood gases  Taste	Speech Swallowing Gagging	Many visceral organs (heart, gut, lungs)
  XI	Spinal acessory	Proprioception	Neck muscles: Sternocleidomastoid Trapezius
  XII	Hypoglossal	Proprioception	Tongue muscles Speech

   

• Many of the functions that make us distinctly human are controlled by cranial nerves: special senses, facial expression, speech.

• Cranial Nerves Contain Sensory, Motor and Parasympathetic Fibers

   

Water: comprises 60 - 90% of most living organisms (and cells) important because it serves as an excellent solvent & enters into many metabolic reactions

• Intracellular (inside cells) = ~ 34 liters
• Interstitial (outside cells) = ~ 13 liters
• Blood plasma = ~3 liters

40% of blood is red blood cells (RBCs)

plasma is similar to interstitial fluid, but contains plasma proteins

serum = plasma with clotting proteins removed

intracellular fluid is very different from interstitial fluid (high K concentration instead of high Na concentration, for example)

• Capillary walls (1 cell thick) separate blood from interstitial fluid
• Cell membranes separate intracellular and interstitial fluids

• Loss of about 30% of body water is fatal

Ions = atoms or molecules with unequal numbers of electrons and protons:

• found in both intra- & extracellular fluid
• examples of important ions include sodium, potassium, calcium, and chloride

Ions (Charged Atoms or Molecules) Can Conduct Electricity

• Giving up electron leaves a + charge (cation)
• Taking on electron produces a - charge (anion)
• Ions conduct electricity
• Without ions there can be no nerves or excitability
  • Na⁺ and K⁺ cations  
  • Ca²⁺ and Mg²⁺ cations  control metabolism and trigger muscle contraction and secretion of hormones and transmitters

Na+ & K+ are the Major Cations in Biological Fluids

• High K+ in cells, high Na+ outside
• Ion gradients maintained by Na pump (1/3 of basal metabolism)
• Think of Na+ gradient as a Na+ battery- stored electrical energy
• K+ gradient forms a K+ battery
• Energy stored in Na+ and K+ batteries can be tapped when ions flow

• Na+ and K+ produce action potential of excitable cells

GENERAL SOMATIC AFFERENT (GSA) PATHWAYS FROM THE BODY

Pain and Temperature

Pain and temperature information from general somatic receptors is conducted over small-diameter (type A delta and type C) GSA fibers of the spinal nerves into the posterior horn of the spinal cord gray matter .

Fast and Slow Pain

Fast pain, often called sharp or pricking pain, is usually conducted to the CNS over type A delta fibers.

Slow pain, often called burning pain, is conducted to the CNS over smaller-diameter type C fibers.

Touch and Pressure

Touch can be subjectively described as discriminating or crude.

Discriminating (epicritic) touch implies an awareness of an object's shape, texture, three-dimensional qualities, and other fine points. Ability to recognize familiar objects simply by tactile manipulation.

The conscious awareness of body position and movement is called the kinesthetic sens

Crude (protopathic) touch,  lacks the fine discrimination described above and doesn't generally give enough information to the brain to enable it to recognize a familiar object by touch alone.

Subconscious Proprioception

Most of the subconscious proprioceptive input is shunted to the cerebellum.

Posterior Funiculus Injury

Certain clinical signs are associated with injury to the dorsal columns.

 As might be expected, these are generally caused by impairment to the kinesthetic sense and discriminating touch and pressure pathways.

 They include

 (1) the inability to recognize limb position,

 (2) as­tereognosis,

 (3) loss of two-point discrimination,

 (4) loss of vibratory sense, and

 (5) a positive Romberg sign.

Astereognosis is the inability to recognize familiar objects by touch alone. When asked to stand erect with feet together and eyes closed, a person with dorsal column damage may sway and fall. This is a posi­tive Romberg sign.

Heart Failure : Heart failure is inability of the heart to pump the enough amount of blood needed to sustain the needs of organism .
It is usually called congestive heart failure ( CHF) .

To understand the pathophysiology  of the heart failure ,  lets compare it with the physiology of the cardiac output :
Cardiac output =Heart rate X stroke volume

Stroke volume is determined by three determinants : Preload ( venous return ) , contractility , and afterload    (peripheral resistance ) . Any disorder of these factors will reduce the ability of the heart to pump blood .

Preload : Any factor that decrease the venous return , either by decreasing the intravenous pressure or increasing the intraatrial pressure will lead to heart failure .

Contractility : Reducing the power of contraction such as in  myocarditis , cardiomyopathy , preicardial tamponade ..etc , will lead to heart failure .

Afterload : Any factor that may increase the peripheral resistance such as hypertension , valvular diseases of the heart may cause heart failure.

Pathophysiology : When the heart needs to contract more to meet the increased demand , compensatory mechanisms start to develope to enhance the power of contractility  . One of these mechanism is increasing heart rate , which will worsen the situation because this will increase the demands of the myocardial cells themselves . The other one is hypertrophy of the cardiac muscle which may compensate the failure temporarily but then the hypertrophy will be an additional load as the fibers became stiff  .

The stroke volume will be reduced , the intraventricular pressure will increase and consequently the intraatrial pressure and then the venous pressure . This will lead to decrease reabsorption of water from the interstitium ( see microcirculation) and then leads to developing of edema ( Pulmonary edema if the failure is left , and systemic edema if the failure is right) .