Each hormone in the body is unique.  Each one is different in it's chemical composition, structure, and action.  With respect to their chemical structure, hormones may be classified into three groups: amines, proteins, and steroids.

 Amines- these simple hormones are  structural variation of the amino acid tyrosine.  This group includes thyroxine from the thyroid gland and epinephrine and norepinephrine from the adrenal medulla.

Proteins- these hormones are chains of amino acids.  Insulin from the pancreas, growth hormone from the anterior pituitary gland, and calcitonin from the thyroid gland are all proteins.  Short chains of amino acids are called peptides.  Antidiuretic hormone and oxytocin, synthesized by the hypothalamus, are peptide hormones.

Steroids- cholesterol is the precursor for the steroid hormones, which include cortisol and aldosterone from the adrenal cortex, estrogen and progesterone from the ovaries, and testosterone from the testes.

AdenosineTriphosphate (ATP)

• Animal cells cannot directly use most forms of energy
  • Most cellular processes require energy stored in the bonds of a molecule, adenosine triphosphate (ATP)
  • ATP is referred to as the energy currency of the cell

It is a nucleotide, formed from:

• the base adenine (the structure with 2 rings),
• the 5 carbon sugar deoxyribose (one ring)
• 3 phosphates

Energy is stored in the bonds between the phosphates and is released when the bonds are broken

Blood Pressure

Blood moves through the arteries, arterioles, and capillaries because of the force created by the contraction of the ventricles.

Blood pressure in the arteries.

The surge of blood that occurs at each contraction is transmitted through the elastic walls of the entire arterial system where it can be detected as the pulse. Even during the brief interval when the heart is relaxed — called diastole — there is still pressure in the arteries. When the heart contracts — called systole — the pressure increases.

Blood pressure is expressed as two numbers, e.g., 120/80.

Blood pressure in the capillaries

The pressure of arterial blood is largely dissipated when the blood enters the capillaries. Capillaries are tiny vessels with a diameter just about that of a red blood cell (7.5 µm). Although the diameter of a single capillary is quite small, the number of capillaries supplied by a single arteriole is so great that the total cross-sectional area available for the flow of blood is increased. Therefore, the pressure of the blood as it enters the capillaries decreases.

Blood pressure in the veins

When blood leaves the capillaries and enters the venules and veins, little pressure remains to force it along. Blood in the veins below the heart is helped back up to the heart by the muscle pump. This is simply the squeezing effect of contracting muscles on the veins running through them. One-way flow to the heart is achieved by valves within the veins

Exchanges Between Blood and Cells

With rare exceptions, our blood does not come into direct contact with the cells it nourishes. As blood enters the capillaries surrounding a tissue space, a large fraction of it is filtered into the tissue space. It is this interstitial or extracellular fluid (ECF) that brings to cells all of their requirements and takes away their products. The number and distribution of capillaries is such that probably no cell is ever farther away than 50 µm from a capillary.

When blood enters the arteriole end of a capillary, it is still under pressure produced by the contraction of the ventricle. As a result of this pressure, a substantial amount of water and some plasma proteins filter through the walls of the capillaries into the tissue space.

Thus fluid, called interstitial fluid, is simply blood plasma minus most of the proteins. (It has the same composition and is formed in the same way as the nephric filtrate in kidneys.)

Interstitial fluid bathes the cells in the tissue space and substances in it can enter the cells by diffusion or active transport. Substances, like carbon dioxide, can diffuse out of cells and into the interstitial fluid.

Near the venous end of a capillary, the blood pressure is greatly reduced .Here another force comes into play. Although the composition of interstitial fluid is similar to that of blood plasma, it contains a smaller concentration of proteins than plasma and thus a somewhat greater concentration of water. This difference sets up an osmotic pressure. Although the osmotic pressure is small, it is greater than the blood pressure at the venous end of the capillary. Consequently, the fluid reenters the capillary here.

Control of the Capillary Beds

An adult human has been estimated to have some 60,000 miles of capillaries with a total surface area of some 800–1000 m². The total volume of this system is roughly 5 liters, the same as the total volume of blood. However, if the heart and major vessels are to be kept filled, all the capillaries cannot be filled at once. So a continual redirection of blood from organ to organ takes place in response to the changing needs of the body. During vigorous exercise, for example, capillary beds in the skeletal muscles open at the expense of those in the viscera. The reverse occurs after a heavy meal.

The walls of arterioles are encased in smooth muscle. Constriction of arterioles decreases blood flow into the capillary beds they supply while dilation has the opposite effect. In time of danger or other stress, for example, the arterioles supplying the skeletal muscles will be dilated while the bore of those supplying the digestive organs will decrease. These actions are carried out by

• the autonomic nervous system.
• local controls in the capillary beds

Cardiac Output:

Minute Volume = Heart Rate X Stroke Volume

Heart rate, HR at rest = 65 to 85 bpm  

Each heartbeat at rest takes about .8 sec. of which .4 sec. is quiescent period.

Stroke volume, SV at rest = 60 to 70 ml.

Heart can increase both rate and volume with exercise. Rate increase is limited due to necessity of minimum ventricular diastolic period for filling. Upper limit is usually put at about 220 bpm. Maximum heart rate calculations are usually below 200. Target heart rates for anaerobic threshold are about 85 to 95% of maximum.

Terms:

End Diastolic Volume, EDV - the maximum volume of the ventricles achieved at the end of ventricular diastole. This is the amount of blood the heart has available to pump. If this volume increases the cardiac output increases in a healthy heart.

End Systolic Volume, ESV - the minimum volume remaining in the ventricle after its systole. If this volume increases it means less blood has been pumped and the cardiac output is less.

EDV - ESV = SV

SV / EDV = Ejection Fraction The ejection fraction is normally around 50% at rest and will increase during strenuous exercise in a healthy heart. Well trained athletes may have ejection fractions approaching 70% in the most strenuous exercise.

Isovolumetric Contraction Phase - a brief period at the beginning of ventricular systole when all valves are closed and ventricular volume remains constant. Pressure has risen enough in the ventricle to close the AV valves but not enough to open the semilunar valves and cause ejection of blood. 

Isovolumetric Relaxation Phase - a brief period at the beginning of ventricular diastole when all valves are closed and ventricular volume is constant. Pressure in the ventricle has lowered producing closure of the semilunar valves but not opening the AV valves to begin pulling blood into the ventricle.

Dicrotic Notch - the small increase in pressure of the aorta or other artery seen when recording a pulse wave. This occurs as blood is briefly pulled back toward the ventricle at the beginning of diastole thus closing the semilunar valves.

Preload - This is the pressure at the end of ventricular diastole, at the beginning of ventricular systole. It is proportional to the End Diastolic Volume (EDV), i.e. as the EDV increases so does the preload of the heart. Factors which increase the preload are: increased total blood volume, increased venous tone and venous return, increased atrial contraction, and the skeletal muscular pump.

Afterload - This is the impedence against which the left ventricle must eject blood, and it is roughly proportional to the End Systolic Volume (ESV). When the peripheral resistance increases so does the ESV and the afterload of the heart. 

The importance of these parameters are as a measure of efficiency of the heart, which increases as the difference between preload and afterload increases

Reflexes

A reflex is a direct connection between stimulus and response, which does not require conscious thought. There are voluntary and involuntary reflexes.

The Stretch Reflex:

The stretch reflex in its simplest form involves only 2 neurons, and is therefore sometimes called a 2-neuron reflex. The two neurons are a sensory and a motor neuron. The sensory neuron is stimulated by stretch (extension) of a muscle. Stretch of a muscle normally happens when its antagonist contracts, or artificially when its tendon is stretched, as in the knee jerk reflex. Muscles contain receptors called muscle spindles. These receptors respond to the muscles's stretch. They send stimuli back to the spinal cord through a sensory neuron which connects directly to a motor neuron serving the same muscle. This causes the muscle to contract, reversing the stretch. The stretch reflex is important in helping to coordinate normal movements in which antagonistic muscles are contracted and relaxed in sequence, and in keeping the muscle from overstretching.

Since at the time of the muscle stretch its antagonist was contracting, in order to avoid damage it must be inhibited or tuned off in the reflex. So an additional connection through an interneuron sends an inhibitory pathway to the antagonist of the stretched muscle - this is called reciprocal inhibition.

The Deep Tendon Reflex:

Tendon receptors respond to the contraction of a muscle. Their function, like that of stretch reflexes, is the coordination of muscles and body movements. The deep tendon reflex involves sensory neurons, interneurons, and motor neurons. The response reverses the original stimulus therefore causing relaxation of the muscle stimulated. In order to facilitate that the reflex sends excitatory stimuli to the antagonists causing them to contract - reciprocal activation.

The stretch and tendon reflexes complement one another. When one muscle is stretching and stimulating the stretch reflex, its antagonist is contracting and stimulating the tendon reflex. The two reflexes cause the same responses thus enhancing one another.

The Crossed Extensor Reflex -

The crossed extensor reflex is just a withdrawal reflex on one side with the addition of inhibitory pathways needed to maintain balance and coordination. For example, you step on a nail with your right foot as you are walking along. This will initiate a withdrawal of your right leg. Since your quadriceps muscles, the extensors, were contracting to place your foot forward, they will now be inhibited and the flexors, the hamstrings will now be excited on your right leg. But in order to maintain your balance and not fall down your left leg, which was flexing, will now be extended to plant your left foot (e.g. crossed extensor). So on the left leg the flexor muscles which were contracting will be inhibited, and the extensor muscles will be excited

Sensory pathways include only those routes which conduct information to the conscious cortex of the brain. However, we will use the term in its more loosely and commonly applied context to include input from all receptors, whether their signals reach the conscious level or not.