Life Processes in Living Organisms Part 2 — Maharashtra (SSC) Class 10 Science Solutions (Free)

Blood composition: blood is roughly 55% plasma (the liquid part — mostly water, carrying dissolved proteins, nutrients, hormones and waste), and 45% formed elements: red blood cells (RBCs, which carry oxygen bound to haemoglobin), white blood cells (WBCs, which provide immunity against infection), and platelets (which trigger clotting to seal wounds). Heart structure: the human heart has four chambers — two upper atria and two lower, more muscular ventricles. The right side of the heart handles deoxygenated blood, and the left side handles oxygenated blood; these two sides are kept completely separate by an internal wall (the septum), which is exactly why a hole in this septum (a congenital heart defect) causes oxygenated and deoxygenated blood to mix, reducing the efficiency of oxygen delivery to the body. Blood flow pathway: deoxygenated blood from the body enters the right atrium, passes to the right ventricle, and is pumped to the lungs via the pulmonary artery. After picking up oxygen in the lungs, it returns to the left atrium, passes to the left ventricle, and is pumped to the whole body via the aorta. One-way valves (tricuspid, bicuspid/mitral, and the semilunar valves) sit between chambers and at the exit of each ventricle, snapping shut after each heartbeat to prevent blood flowing backward. Normal blood pressure readings — systolic 120 mmHg (pressure when the heart contracts) over diastolic 80 mmHg (pressure when it relaxes between beats) — reflect the pumping cycle this structure carries out roughly 70-80 times a minute.

Humans have DOUBLE circulation, meaning blood passes through the heart twice for every one complete round trip around the body, via two separate circuits: Pulmonary circulation: right ventricle → pulmonary artery → lungs (releases CO₂, picks up O₂) → pulmonary veins → left atrium. This circuit is entirely dedicated to gas exchange with the lungs. Systemic circulation: left ventricle → aorta → body tissues (delivers O₂, picks up CO₂) → vena cava → right atrium. This circuit delivers oxygen to every organ in the body and collects waste CO₂. Why this is more efficient than single circulation (as in fish, where blood passes through the heart only once per full circuit): because the two circuits are separated by passing back through the heart between them, blood can be pumped at a HIGH pressure for the long systemic journey around the whole body, while being pumped at a much GENTLER, lower pressure for the short trip through the lungs' delicate capillaries — which would otherwise be damaged by high pressure. A single-circulation fish heart has to compromise on one pressure for both jobs, which limits how much oxygen its blood can deliver at a high metabolic rate. (In the developing fetus, this same double circulation exists but is temporarily bypassed at two points — the foramen ovale and the ductus arteriosus — since the fetus's own lungs aren't yet functioning and oxygen instead comes from the placenta.)

Excretion is the process by which the body removes metabolic waste products — chiefly nitrogenous waste (urea, uric acid), excess ions, and excess water — that would otherwise build up to toxic levels. Structure: each of the two bean-shaped kidneys contains roughly one million nephrons, the microscopic functional units where filtration actually happens. Each nephron consists of: the Bowman's capsule (which surrounds a knot of capillaries called the glomerulus, where filtration begins), the proximal tubule (where most reabsorption happens), the loop of Henle (which uses a countercurrent arrangement to reabsorb water efficiently and concentrate the urine), the distal tubule (fine-tuning the composition), and the collecting duct (which carries the final urine toward the bladder). The three stages of urine formation: 1. Ultrafiltration — blood pressure in the glomerulus forces small molecules (water, glucose, salts, urea) out into the Bowman's capsule, while blood cells and large proteins are too big to pass through and stay in the blood. 2. Selective reabsorption — as the filtrate flows along the tubule, useful substances the body needs back (essentially all the glucose, most water, and most salts) are actively reabsorbed into the surrounding blood capillaries. 3. Secretion — additional waste substances, including excess H⁺ ions and certain drugs circulating in the blood, are actively added INTO the tubule fluid at this stage, on top of what ultrafiltration already removed. The final urine that results is roughly 95% water and 5% dissolved waste, mainly urea and salts.

ADH (antidiuretic hormone), released from the pituitary gland: when the blood's osmotic pressure rises (meaning the blood has become too concentrated, for example after sweating heavily without drinking enough water), the pituitary releases more ADH. ADH acts on the collecting duct of the nephron, opening aquaporin water channels that increase how much water is reabsorbed back into the blood — this dilutes the blood back toward normal and produces a smaller volume of more concentrated urine. When the body already has plenty of water, ADH release drops, less water is reabsorbed, and a larger volume of dilute urine is produced instead. Aldosterone, released from the adrenal cortex: triggered when blood sodium (Na⁺) levels are low, aldosterone increases the active reabsorption of Na⁺ in the distal tubule and collecting duct. Since water tends to follow reabsorbed sodium osmotically, this indirectly increases water reabsorption too, helping maintain both salt balance and blood pressure. What happens when ADH is deficient — diabetes insipidus: without enough ADH, the collecting duct cannot reabsorb water properly, so the body continuously produces a very large volume of dilute urine, regardless of how hydrated the person actually is. This forces the person to drink excessive amounts of water to keep up (excessive thirst, or polydipsia) — a very different condition from diabetes mellitus (high blood sugar), despite the similar name, since it involves water regulation rather than glucose regulation.

The nervous system is organised into two main divisions: Central Nervous System (CNS) — the brain and spinal cord, which process incoming information and generate the appropriate response. Peripheral Nervous System (PNS) — the network of nerves (31 pairs of spinal nerves and 12 pairs of cranial nerves) that connect the CNS to the rest of the body. The PNS is further divided into the somatic nervous system (controlling voluntary movements, like deliberately picking something up) and the autonomic nervous system (controlling involuntary functions like heart rate and digestion, itself split into the sympathetic "fight or flight" and parasympathetic "rest and digest" branches). The reflex arc — a deliberately shortened pathway: A reflex is an automatic, involuntary response to a stimulus that happens WITHOUT waiting for the brain to consciously process the situation. The pathway is: stimulus → sensory neuron carries the signal to the spinal cord → the signal crosses a synapse directly to a motor neuron within the spinal cord itself → the motor neuron carries the response signal straight to the muscle. Why this bypasses the brain, and why that matters: routing the response through the spinal cord rather than sending it all the way up to the brain and back saves valuable time. The classic example — touching a hot surface causes the hand to jerk away before the brain has even registered the sensation of pain — exists precisely because tissue damage from heat accumulates within milliseconds, and the extra travel time to and from the brain could mean the difference between a minor and a serious burn.

A synapse is the narrow gap — around 20 nanometres wide — between the axon terminal of one neuron (the presynaptic neuron) and the dendrite of the next (the postsynaptic neuron). Neurons do NOT physically touch at a synapse, so the signal must convert from an electrical form to a chemical form to cross the gap, then back to electrical on the other side. The sequence of events: 1. An electrical signal (action potential) travels along the presynaptic neuron's axon and arrives at the axon terminal. 2. This triggers calcium ions (Ca²⁺) to flow into the terminal, which causes tiny vesicles containing neurotransmitter chemicals (such as acetylcholine, dopamine, or GABA) to fuse with the cell membrane and release their contents into the synaptic gap. 3. The neurotransmitter diffuses across the gap and binds to specific receptor proteins on the postsynaptic neuron's membrane. 4. This binding triggers a new electrical change in the postsynaptic neuron — either excitatory (depolarisation, making the neuron more likely to fire its own signal, as with acetylcholine) or inhibitory (hyperpolarisation, making it less likely to fire, as with GABA). Why transmission only goes one way: the neurotransmitter-filled vesicles exist only in the presynaptic neuron's terminal, and the receptors exist only on the postsynaptic membrane — there is no matching machinery to send a signal in reverse, which is why nerve impulses always travel in a single, predictable direction through a neural circuit. Once the signal is passed on, the neurotransmitter is either broken down by enzymes or reabsorbed by the presynaptic neuron, clearing the synapse so it's ready for the next signal.

The endocrine system consists of ductless glands that secrete hormones — chemical messengers — directly into the bloodstream, producing slower but longer-lasting effects across the body compared to the fast, localised signals of the nervous system. Major glands and their roles: - Pituitary gland — often called the "master gland"; its anterior lobe secretes growth hormone, ACTH, TSH and gonadotropins (which in turn regulate other glands), while its posterior lobe releases ADH and oxytocin. - Thyroid — secretes thyroxine, which regulates the body's overall metabolic rate and growth. - Parathyroid — secretes parathyroid hormone (PTH), which regulates blood calcium levels. - Adrenal glands — the cortex secretes cortisol (stress response, metabolism), while the inner medulla secretes epinephrine (adrenaline, for the rapid "fight or flight" response). - Pancreas — secretes insulin (lowers blood glucose by promoting its uptake into cells) and glucagon (raises blood glucose by triggering its release from storage) — a matched pair that keeps blood sugar within a narrow range. - Testes / Ovaries — secrete testosterone, or estrogen and progesterone respectively, regulating development and reproduction. Negative feedback — how homeostasis is maintained: most hormone systems work like a thermostat. When a hormone's effect pushes a condition (like blood glucose) back to its normal range, this normal level itself signals the gland to REDUCE further hormone release — for example, as insulin brings blood glucose down toward normal, the pancreas correspondingly reduces insulin secretion, preventing blood glucose from being pushed too low instead. This self-correcting loop, rather than a one-off signal, is what keeps hormone-controlled conditions stable over time despite constantly changing internal and external conditions.