NEET MDS Lessons
Physiology
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.
|
Damage to Spinal Nerves and Spinal Cord |
||
|
Damage |
Possible cause of damage |
Symptoms associated with innervated area |
|
Peripheral nerve |
Mechanical injury |
Loss of muscle tone. Loss of reflexes. Flaccid paralysis. Denervation atrophy. Loss of sensation |
|
Posterior root |
Tabes dorsalis |
Paresthesia. Intermittent sharp pains. Decreased sensitivity to pain. Loss of reflexes. Loss of sensation. Positive Romberg sign. High stepping and slapping of feet. |
|
Anterior Horn |
Poliomyelitis |
Loss of muscle tone. Loss of reflexes. Flaccid paralysis. Denervation atrophy |
|
Lamina X (gray matter) |
Syringomyelia |
Bilateral loss of pain and temperature sense only at afflicted cord level. Sensory dissociation. No sensory impairment below afflicted level |
|
Anterior horn and lateral corticospinal tract |
Amyotrophic lateral sclerosis |
Muscle weakness. Muscle atrophy. Fasciculations of hand and arm muscles. Spastic paralysis |
|
Posterior and lateral funiculi |
Subacute combined degeneration |
Loss of position sense. Loss of vibratory sense. Positive Romberg sign. Muscle weakness. Spasticity. Hyperactive tendon reflexes. Positive Babinski sign. |
|
Hemisection of the spinal cord |
Mechanical injury |
Brown-Sequard syndrome |
|
Below cord level on injured side |
||
|
Flaccid paralysis. Hyperactive tendon reflexes. Loss of position sense. Loss of vibratory sense. Tactile impairment |
||
|
Below cord level on opposite side beginning one or two segments below injury |
||
|
Loss of pain and temperature |
||
The nephron of the kidney is involved in the regulation of water and soluble substances in blood.
A Nephron
A nephron is the basic structural and functional unit of the kidneys that regulates water and soluble substances in the blood by filtering the blood, reabsorbing what is needed, and excreting the rest as urine.
Its function is vital for homeostasis of blood volume, blood pressure, and plasma osmolarity.
It is regulated by the neuroendocrine system by hormones such as antidiuretic hormone, aldosterone, and parathyroid hormone.
The Glomerulus
The glomerulus is a capillary tuft that receives its blood supply from an afferent arteriole of the renal circulation. Here, fluid and solutes are filtered out of the blood and into the space made by Bowman's capsule.
A group of specialized cells known as juxtaglomerular apparatus (JGA) are located around the afferent arteriole where it enters the renal corpuscle. The JGA secretes an enzyme called renin, due to a variety of stimuli, and it is involved in the process of blood volume homeostasis.
The Bowman's capsule surrounds the glomerulus. It is composed of visceral (simple squamous epithelial cells; inner) and parietal (simple squamous epithelial cells; outer) layers.
Red blood cells and large proteins, such as serum albumins, cannot pass through the glomerulus under normal circumstances. However, in some injuries they may be able to pass through and can cause blood and protein content to enter the urine, which is a sign of problems in the kidney.
Proximal Convoluted Tubule
The proximal tubule is the first site of water reabsorption into the bloodstream, and the site where the majority of water and salt reabsorption takes place. Water reabsorption in the proximal convoluted tubule occurs due to both passive diffusion across the basolateral membrane, and active transport from Na+/K+/ATPase pumps that actively transports sodium across the basolateral membrane.
Water and glucose follow sodium through the basolateral membrane via an osmotic gradient, in a process called co-transport. Approximately 2/3rds of water in the nephron and 100% of the glucose in the nephron are reabsorbed by cotransport in the proximal convoluted tubule.
Fluid leaving this tubule generally is unchanged due to the equivalent water and ion reabsorption, with an osmolarity (ion concentration) of 300 mOSm/L, which is the same osmolarity as normal plasma.
The Loop of Henle
The loop of Henle is a U-shaped tube that consists of a descending limb and ascending limb. It transfers fluid from the proximal to the distal tubule. The descending limb is highly permeable to water but completely impermeable to ions, causing a large amount of water to be reabsorbed, which increases fluid osmolarity to about 1200 mOSm/L. In contrast, the ascending limb of Henle's loop is impermeable to water but highly permeable to ions, which causes a large drop in the osmolarity of fluid passing through the loop, from 1200 mOSM/L to 100 mOSm/L.
Distal Convoluted Tubule and Collecting Duct
The distal convoluted tubule and collecting duct is the final site of reabsorption in the nephron. Unlike the other components of the nephron, its permeability to water is variable depending on a hormone stimulus to enable the complex regulation of blood osmolarity, volume, pressure, and pH.
Normally, it is impermeable to water and permeable to ions, driving the osmolarity of fluid even lower. However, anti-diuretic hormone (secreted from the pituitary gland as a part of homeostasis) will act on the distal convoluted tubule to increase the permeability of the tubule to water to increase water reabsorption. This example results in increased blood volume and increased blood pressure. Many other hormones will induce other important changes in the distal convoluted tubule that fulfill the other homeostatic functions of the kidney.
The collecting duct is similar in function to the distal convoluted tubule and generally responds the same way to the same hormone stimuli. It is, however, different in terms of histology. The osmolarity of fluid through the distal tubule and collecting duct is highly variable depending on hormone stimulus. After passage through the collecting duct, the fluid is brought into the ureter, where it leaves the kidney as urine.
Neural Substrates of Breathing
A. Medulla Respiratory Centers
Inspiratory Center (Dorsal Resp Group - rhythmic breathing) → phrenic nerve→ intercostal nerves→ diaphragm + external intercostals
Expiratory Center (Ventral Resp Group - forced expiration) → phrenic nerve → intercostal nerves → internal intercostals + abdominals (expiration)
1. eupnea - normal resting breath rate (12/minute)
2. drug overdose - causes suppression of Inspiratory Center
B. Pons Respiratory Centers
1. pneumotaxic center - slightly inhibits medulla, causes shorter, shallower, quicker breaths
2. apneustic center - stimulates the medulla, causes longer, deeper, slower breaths
C. Control of Breathing Rate & Depth
1. breathing rate - stimulation/inhibition of medulla
2. breathing depth - activation of inspiration muscles
3. Hering-Breuer Reflex - stretch of visceral pleura that lungs have expanded (vagal nerve)
D. Hypothalamic Control - emotion + pain to the medulla
E. Cortex Controls (Voluntary Breathing) - can override medulla as during singing and talking
Oxygen Transport
In adult humans the hemoglobin (Hb) molecule
- consists of four polypeptides:
- two alpha (α) chains of 141 amino acids and
- two beta (β) chains of 146 amino acids
- Each of these is attached the prosthetic group heme.
- There is one atom of iron at the center of each heme.
- One molecule of oxygen can bind to each heme.
The reaction is reversible.
- Under the conditions of lower temperature, higher pH, and increased oxygen pressure in the capillaries of the lungs, the reaction proceeds to the right. The purple-red deoxygenated hemoglobin of the venous blood becomes the bright-red oxyhemoglobin of the arterial blood.
- Under the conditions of higher temperature, lower pH, and lower oxygen pressure in the tissues, the reverse reaction is promoted and oxyhemoglobin gives up its oxygen.
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:
|
|
In mM/L |
Out mM/L |
Gradient orientation |
|
Na |
10 |
150 |
High outside |
|
K |
140 |
5 |
High inside |
- 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
Exchange of gases:
- External respiration:
- exchange of O2 & CO2 between external environment & the cells of the body
- efficient because alveoli and capillaries have very thin walls & are very abundant (your lungs have about 300 million alveoli with a total surface area of about 75 square meters)
- Internal respiration - intracellular use of O2 to make ATP
- occurs by simple diffusion along partial pressure gradients