Physiology No 6

‫بســـم الله الرحمن الرحيم‬

‫المحاضرة‬ ‫السادسة‬ ‫صـــفحة ‪106‬الى صــفحة ‪109‬‬

‫(ماعدا صفحة ‪)108‬‬ ‫‪ +‬صفحة ‪114‬‬ ‫‪ +‬صفحة ‪ 117‬الى صفحة ‪118‬‬ ‫‪ +‬صفحة ‪ 122‬الى صفحة ‪130‬‬

.Tubular selective reabsorption -2 About 90% of the filtrate is reabsorbed into the blood through the renal tubules (leaving only about 1.5 liters to be excreted as urine /24 hours), where substances that are useful to the body are reabsorbed. Lining epithelial cells have: 1- Microvilli (giving brush border appearance) which increase the surface of absorption. 2- Numerous mitochondria providing energy for cellular pump. 65% of the filtrate is reabsorbed through the proximal convoluted tubules such as glucose, amino acids, vitamins, ions including Na, K, Cl &, HCO3-). NB: Ions are reabsorbed by active transport or by diffusion. Reabsorption continues at the loop of Henle and distal convoluted tubule. Reabsorption depends on the tubular transport maximum (Tm) and the renal threshold concentration in the plasma of the substance. Tubular transport maximum (Tm): It is the maximum rate at which a substance can be reabsorbed e.g. Tm for glucose averages 320 mg/min for adult human. When a substance exceeds its renal threshold, the portion not reabsorbed is excreted in the urine e.g. It is 180mg of glucose /100ml blood. 65% of Na+ is reabsorbed by active transport at proximal convoluted tubules which is associated with passive transport of Cl- & HCO3- (+H2O). 27% of Na+ is reabsorbed by active transport at ascending limb of the loop of Henle. 8% of Na+ is selectively reabsorbed at the distal convoluted tubule under the control of aldosterone hormone in exchange with K+. The decrease in the Na level in the blood (also, the increase of K level in the blood) → stimulates the release of aldosterone hormone (adrenal cortex hormone). H2O is reabsorbed mainly in at the collecting ducts by osmosis.

Tubular secretion -3 It occurs mainly in the region of the distal convoluted tubules. Potassium, hydrogen & ammonium ions (and certain drugs such as penicillin) are secreted from the blood into the filtrate. NB: When k concentration is too high → nerve impulses are not effectively transmitted → the strength of muscle contraction decreases. So, the heart can be weakened and even fail. Secretion of potassium results from: 1- Direct effect of K ions on the tubules due to its high concentration. 2- A high K ions concentration stimulates the secretion of aldosterone hormone which further stimulates the secretion of K ions. Secretion of H+ . is an important homeostatic mechanism for regulating the pH of the blood

Urine volume is regulated by the antidiuretic hormone (ADH) Osmotic withdrawal of H2O from the collecting ducts is regulated by the permeability of its wall to H2O. The permeability of the wall of the collecting ducts is under the control of ADH (a hormone secreted from the posterior lobe of pituitary gland). ◙ When the body needs to conserve H2O as when fluid intake is low or during dehydration → a decrease in blood volume → salt concentration in the blood becomes greater → increase in the osmotic pressure → stimulates special receptors in the hypothalamus → ADH is secreted → more H2O is reabsorbed at the collecting ducts → production of small volume of concentrated urine. ◙ Also, deydration stimulates the thirst center in the hypothalamus → stimulates the increase of fluid intake.

Muscular system

Single muscle fiber ))cell

2

4

1

)dark band(

)light band(

Thick Thin filaments filaments ))myosin ))actin

3

The sliding filament model

Relaxed muscle

Contracted muscle

Fully Contracted muscle

Sarcomere Dark band

Diagram illustrating the mechanism of filament sliding

)1( ATP binds to a myosin which is released from an actin filament. )2( Hydrolysis of ATP cocks the myosin head. )3( The myosin head attaches to an actin binding site with the help of calcium. )4( The power stroke slides the actin )thin( filament.

:So, the process of muscle contraction is summarized as follows Nerve impulse arrives at nerve ending Acetylcholine is released into synaptic cleft Plasma membrane of the muscle cell is depolarized T-system is depolarized Ca2+ is released from sacoplasmic reticulum Ca2+ causes uncovering of the active sites on actin filaments Myosin attaches to active sites-cross bridges form Cross bridges flex and reattach to new active site energy (ATP)

ATP hydrolysis

When needed energy transferred to myosin-ATP Filaments are pulled past Muscle fiber shortens one another

ADP + P Cellular respiration Energy transferred to CP

Nervous system The communication between cells and organs in multicellular organisms may be neural and /or hormonal. The neural mechanism is relrelatively rapid and involves propagated electrochemical changes in the cell membranes. On the other hand, the hormonal mechanism is relatively less rapid or long-term.

Nervous system Central nervous system (CNS) Brain

Spinal cord

(contained within the skull)

(contained within the vertebral column)

Peripheral nervous system (PNS) Afferent division (conveys

Efferent division

information from receptors to CNS)

(more complicated than afferent)

Somatic nervous system (innervate skeletal muscles)

Autonomic nervous system (innervates smooth muscles, cardiac muscles & glands)

The functional unit of the nervous system is the nerve cell (neuron). It receives and sends information by producing electrochemical signals called nerve impulses. The 2nd type to the nervous system is the glial (neuroglial) cells. The neuron: The functional unit of the nervous system. The neuron: Nucleus Nucleus of Schwann cell

Signal direction

Synaptic kn

Cell body

Node of Ranvier Dendrites

Axon

Signal direction

Processes

@ The dendrites are short, numerous, branched & receive information i.e. receptive apparatus. @ The axon is a long, single, carries nerve impulse away from the cell body which may covered with myelin sheath.

Types of neurons: 1) Afferent (sensory) neurons: They are connected to receptors, which function to convert some environmental stimuli into nerve impulses. These impulses are carried into the CNS. They lie mostly out side the CNS. 2) Efferent (motor) neurons: They carry nerve impulses from the CNS to effectors such as muscles or glands. They lie mostly outside the CNS. 3) Interneurons: They are neither sensory nor motor but connect neurons with neurons. They lie entirely within the CNS (99% of the human nerve cells). @ Axons (nerve fibers) are usually bundled together in a well-formed wrapping of connective tissue to form a nerve. @ The cell bodies are located either in the CNS or in ganglia.

Neuroglial cells: The supporting cells of the nervous system. Types of neurglial cells inside the CNS: 1) Oligodendrocytes: They are responsible for the myelination of axons in the CNS. 2) Astrocytes: They have an important role in the formation of the blood-brain barrier. They can take up, release and store neurotransmitters. They participate in neuronal guidance following injury and during development. 3) Ependymal cells: They line the cavities of the brain & spinal cord. 4) Microglia: They transform to phagocytic cells in response to tissue injury. Types of neuroglail cells inside PNS: 1) Schwann cells: They form myelin sheath around peripheral nerves. 2) Satellite cells: They supporting cells in nerve ganglia.

The resting potential

There is approximately 10 times more Na+ outside than in and 10 times more K+ inside than out. This ionic inbalance is brought by: 1) Efficient sodium-potassium pump that actively transport Na+ out of the cell and K+ into the cell. For every 3 Na+ pumped out, 2 K+ are pumped in. 2) The ease of passage through ion channels varies according to the type of ion. In the resting neuron, the membrane is up to 100 times more permeable to potassium than sodium.

‫الى‬ ‫اللقــــــــــــــــــ‬ ‫ـاء‬ ‫أ‪ .‬د‪ .‬شــــــبل‬ ‫شـــــــــعلن‬