Maintenance of Homeostasis

The kidneys maintain the homeostasis of several important internal conditions by controlling the excretion of substances out of the body. 

Ions. The kidney can control the excretion of potassium, sodium, calcium, magnesium, phosphate, and chloride ions into urine. In cases where these ions reach a higher than normal concentration, the kidneys can increase their excretion out of the body to return them to a normal level. Conversely, the kidneys can conserve these ions when they are present in lower than normal levels by allowing the ions to be reabsorbed into the blood during filtration. (See more about ions.)
 
pH. The kidneys monitor and regulate the levels of hydrogen ions (H+) and bicarbonate ions in the blood to control blood pH. H+ ions are produced as a natural byproduct of the metabolism of dietary proteins and accumulate in the blood over time. The kidneys excrete excess H+ ions into urine for elimination from the body. The kidneys also conserve bicarbonate ions, which act as important pH buffers in the blood.
 
Osmolarity. The cells of the body need to grow in an isotonic environment in order to maintain their fluid and electrolyte balance. The kidneys maintain the body’s osmotic balance by controlling the amount of water that is filtered out of the blood and excreted into urine. When a person consumes a large amount of water, the kidneys reduce their reabsorption of water to allow the excess water to be excreted in urine. This results in the production of dilute, watery urine. In the case of the body being dehydrated, the kidneys reabsorb as much water as possible back into the blood to produce highly concentrated urine full of excreted ions and wastes. The changes in excretion of water are controlled by antidiuretic hormone (ADH). ADH is produced in the hypothalamus and released by the posterior pituitary gland to help the body retain water.
 
Blood Pressure. The kidneys monitor the body’s blood pressure to help maintain homeostasis. When blood pressure is elevated, the kidneys can help to reduce blood pressure by reducing the volume of blood in the body. The kidneys are able to reduce blood volume by reducing the reabsorption of water into the blood and producing watery, dilute urine. When blood pressure becomes too low, the kidneys can produce the enzyme renin to constrict blood vessels and produce concentrated urine, which allows more water to remain in the blood.

The Cardiac Cycle: the sequence of events in one heartbeat.

systole - the contraction phase; unless otherwise specified refers to left ventricle, but each chamber has its own systole.

diastole - the relaxation phase; unless otherwise specified refers to left ventricle, but each chamber has its own diastole.

1) quiescent period - period when all chambers are at rest and filling. 70% of ventricular filling occurs during this period. The AV valves are open, the semilunar valves are closed.

2) atrial systole - pushes the last 30% of blood into the ventricle.

3) atrial diastole - atria begin filling.

4) ventricular systole - First the AV valves close causing the first heart sound, then after the isovolumetric contraction phase the semilunar valves open permitting ventricular ejection of blood into the arteries.

5) ventricular diastole - As the ventricles relax the semilunar valves close first producing the second heart sound, then after the isovolumetric relaxation phase the AV valves open allowing ventricular filling.

A small fraction of cardiac muscle fibers have myogenicity and autorhythmicity.

Myogenicity is the property of spontaneous impulse generation. The slow sodium channels are leaky and cause the polarity to spontaneously rise to threshold for action potential generation. The fastest of these cells, those in the SA node, set the pace for the heartbeat.

Autorhythmicity - the natural rhythm of spontaneous depolarization. Those with the fastest autorhythmicity act as the 1. heart's pacemaker.

Contractility - like skeletal muscle, most cardiac muscle cells respond to stimuli by contracting. The autorhythmic cells have very little contractility however. Contractility in the other cells can be varied by the effect of neurotransmitters.

Inotropic effects - factors which affect the force or energy of muscular contractions. Digoxin, epinephrine, norepinephrine, and dopamine have positive inotropic effects. Betal blockers and calcium channel blockers have negative inotropic effects 

Sequence of events in cardiac conduction: The electrical events in the cardiac cycle.

1) SA node depolarizes and the impulse spreads across the atrial myocardium and through the internodal fibers to the AV node. The atrial myocardium depolarizes resulting in atrial contraction, a physical event.

2) AV node picks up the impulse and transfers it to the AV Bundle (Bundle of His). This produces the major portion of the delay seen in the cardiac cycle. It takes approximately .03 sec from SA node depolarization to the impulse reaching the AV node, and .13 seconds for the impulse to get through the AV node and reach the Bundle of His. Also during this period the atria repolarize.

3) From the AV node the impulse travels through the bundle branches and through the Purkinje fibers to the ventricular myocardium, causing ventricular depolarization and ventricular contraction, a physical event.

4) Ventricular repolarization occurs.

Membrane Potential

• Membrane potentials will occur across cell membranes if
  • 1) there is a concentration gradient of an ion
  • 2) there is an open channel in the membrane so the ion can move from one side to the other

The Sodium Pump Sets Up Gradients of Na and K Across Cell Membranes

• All cells have the Na pump in their membranes
  • Pumps 3 Nas out and 2 Ks in for each cycle
  • Requires energy from ATP
    • Uses about 30% of body's metabolic energy
  • This is a form of active transport- can pump ions "uphill", from a low to a high concentration
  • This produces concentration gradients of Na & K across the membrane
  • Typical concentration gradients:

•  
• The ion gradients represent stored electrical energy (batteries) that can be tapped to do useful work
• The Na pump is of ancient origin, probably originally designed to protect cell from osmotic swelling

Inhibited by the arrow poisons ouabain and digitalis

The endocrine system along with the nervous system functions in the regulation of body activities.  The nervous system acts through electrical impulses and neurotransmitters to cause muscle contraction and glandular secretion and interpretation of impulses.  The endocrine system acts through chemical messengers called hormones that influence growth, development, and metabolic activities

A rise in blood pressure stretches the atria of the heart. This triggers the release of atrial natriuretic peptide (ANP). ANP is a peptide of 28 amino acids. ANP lowers blood pressure by:

• relaxing arterioles
• inhibiting the secretion of renin and aldosterone
• inhibiting the reabsorption of sodium ions in the collecting ducts of the kidneys.

The effects on the kidney reduce the reabsorption of water by them thus increasing the flow of urine and the amount of sodium excreted in it (These actions give ANP its name: natrium = sodium; uresis = urinate). The net effect of these actions is to reduce blood pressure by reducing the volume of blood volume in the system.

The Heartbeat

During rest, the heart beats about 70 times a minute in the adult male, while pumping about 5 liters of blood.

The stimulus that maintains this rhythm is self-contained. Embedded in the wall of the right atrium is a mass of specialized heart tissue called the sino-atrial (S-A) node. The S-A node is also called the pacemaker because it establishes the basic frequency at which the heart beats.

The interior of the fibers of heart muscle, like all cells, is negatively charged with respect to the exterior. In the cells of the pacemaker, this charge breaks down spontaneously about 70 times each minute. This, in turn, initiates a similar discharge of the nearby muscle fibers of the atrium. A tiny wave of current sweeps over the atria, causing them to contract.

When this current reaches the region of insulating connective tissue between the atria and the ventricles, it is picked up by the A-V node (atrio-ventricular node). This leads to a system of branching fibers that carries the current to all parts of the ventricles.

The contraction of the heart in response to this electrical activity creates systole.

A period of recovery follows called diastole.

• The heart muscle and S-A node become recharged.
• The heart muscle relaxes.
• The atria refill. 

The Electrocardiogram

The electrical activity of the heart can be detected by electrodes placed at the surface of the body. Analysis of an electrocardiogram (ECG or EKG) aids in determining, for example, the extent of damage following a heart attack. This is because death of a portion of the heart muscle blocks electrical transmission through that area and alters the appearance of the ECG

Control of the Heart

Although the A-V node sets the basic rhythm of the heart, the rate and strength of its beating can be modified by two auxiliary control centers located in the medulla oblongata of the brain.

• One sends nerve impulses down accelerator nerves.
• The other sends nerve impulses down a pair of vagus nerves

Accelerator Nerves

The accelerator nerves are part of the sympathetic branch of the autonomic nervous system, and  like all post-ganglionic sympathetic neurons  release noradrenaline at their endings on the heart.

They increase the rate and strength of the heartbeat and thus increase the flow of blood. Their activation usually arises from some stress such as fear or violent exertion. The heartbeat may increase to 180 beats per minute. The strength of contraction increases as well so the amount of blood pumped may increase to as much as 25-30 liters/minute.

Vigorous exercise accelerates heartbeat in two ways;

• As cellular respiration increases, so does the carbon dioxide level in the blood. This stimulates receptors in the carotid arteries and aorta, and these transmit impulses to the medulla for relay  by the accelerator nerves  to the heart.
• As muscular activity increases, the muscle pump drives more blood back to the right atrium. The atrium becomes distended with blood, thus stimulating stretch receptors in its wall. These, too, send impulses to the medulla for relay to the heart.

Distention of the wall of the right atrium also triggers the release of atrial natriuretic peptide (ANP) which initiates a set of responses leading to a lowering of blood pressure

The Vagus Nerves

The vagus nerves are part of the parasympathetic branch of the autonomic nervous system. They, too, run from the medulla oblongata to the heart. Their activity slows the heartbeat.

Pressure receptors in the aorta and carotid arteries send impulses to the medulla which relays these  by way of the vagus nerves  to the heart. Heartbeat and blood pressure diminish.