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

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.

Bile contains:

• bile acids. These amphiphilic steroids emulsify ingested fat. The hydrophobic portion of the steroid dissolves in the fat while the negatively-charged side chain interacts with water molecules. The mutual repulsion of these negatively-charged droplets keeps them from coalescing. Thus large globules of fat (liquid at body temperature) are emulsified into tiny droplets (about 1 µm in diameter) that can be more easily digested and absorbed.

• bile pigments. These are the products of the breakdown of hemoglobin removed by the liver from old red blood cells. The brownish color of the bile pigments imparts the characteristic brown color of the feces.

The hepatic portal system

The capillary beds of most tissues drain into veins that lead directly back to the heart. But blood draining the intestines is an exception. The veins draining the intestine lead to a second set of capillary beds in the liver. Here the liver removes many of the materials that were absorbed by the intestine:

• Glucose is removed and converted into glycogen.
• Other monosaccharides are removed and converted into glucose.
• Excess amino acids are removed and deaminated.
  • The amino group is converted into urea.
  • The residue can then enter the pathways of cellular respiration and be oxidized for energy.
• Many nonnutritive molecules, such as ingested drugs, are removed by the liver and, often, detoxified.

The liver serves as a gatekeeper between the intestines and the general circulation. It screens blood reaching it in the hepatic portal system so that its composition when it leaves will be close to normal for the body.

Furthermore, this homeostatic mechanism works both ways. When, for example, the concentration of glucose in the blood drops between meals, the liver releases more to the blood by

• converting its glycogen stores to glucose (glycogenolysis)
• converting certain amino acids into glucose (gluconeogenesis).